Maintenance Of Transformer

Chapter 1

Maintenance of Transformer Maintenance of Transformer Oil in Service 1. Transformer Oil Transformer oil is mineral insulating oil derived from crude petroleum. It is a mixture of various hydrocarbons. It consist partly aliphatic compounds (open chain compounds) with the general formula C n H2n+2 and Cn H2n. Many oils also contain certain aromatic compounds (closed chain or ring compounds) related to benzene, naphthalene and derivatives of these with aliphatic chains. Good transformer oil must insulate and prevent flash over of the exposed parts within the equipment and it must effectively transform the heat from the core to the radiating surface. The characteristics of new transformer oil is given in I S 335/1983. There is no single test to judge all the qualities of the oil. Each test is significant within its limits and provides only a general information about the conditions of the oil. If the dielectric strength is low, the oil is unfit for use regardless of any other condition. 2. Tests to determine the qualities of Transformer oil 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

Dielectric Strength Water Content Neutralisation Value Interfacial Tension Dielectric Dissipation Factor Test for Corrosive Sulphur in Oil Test for Oxidation Stability Specific Resistance (Resistivity) Flash Point Pour Point Viscosity Sludge Test Dissolved Gas Analysis (DGA)

Dielectric Strength The transformer oil under test is subjected to an AC electric field with continuously increasing voltage till the oil breaks down. The test cell shall be of glass or plastic and transparent and non-absorbent (Eg. Methyl Methacrylate vessel) with effective volume between 300 to 500 ml. The electrodes must be Copper, Brass, Bronze or Stainless steel and well polished having spherical shape with dia 12.5 mm to 13 mm. It shall have a spherical front as shown in the figure. The electrodes shall be mounted horizontally in the cell and the axis shall be immersed at a depth of approximately 40 mm. The electrode gap shall be 2.5 + 0.1 mm. 1

Before carrying out the test, the cell shall be cleaned by rinsing with the test oil. The sampling vessel containing the test oil shall be gently agitated to have homogeneous distribution of impurities and air bubbles to escape. The oil temperature at1he time of testing shall be around 27°C. (15°C to 35°C preferable) An increasing AC voltage of rated frequency is applied to the electrodes, approximately at the rate of 2 kV / Sec, starting from zero upto the value producing breakdown. The test kit will have provision for automatic switching off of the supply voltage within 0.02 second.

Alternative Shape Electrodes BDV Test Cell The test shall be repeated six times on the same cell filling and the arithmetic mean of the six of in results is noted as the electric strength or BDV (Break Down Voltage) of the oil under test. The test shall be conducted in a dry place free from dust and voltage applied every time after disappearance of any air bubbles. The time intervals shall be five minutes, if the disappearance of air bubble cannot be observed.

2

2. Sampling of oil Method of sampling of oil from a transformer may be as specified in I S : 6855/1973. General precautions: When sampling oil, care shall be taken not to contaminate the oil. Outdoor sampling shall be done only during fair weather conditions. Before use, the container shall be rinsed with the oil being sampled. The operator shall not permit his hands to come into contact with the sample or internal part of the sampling equipment. The sample shall be protected from light radiation during transportation. 3. Handling of oil Oil shall be handled with utmost care. Oil drums shall be kept under cover. Drums shall be clearly marked to show whether the oil contained is clean or dirty. Clean oil shall be tested and treated before use. Transfer of oil to transformer shall preferably be through a transformer oil filter plant. 4. Examination of oil in service The oil in the transformer and tap changer shall be inspected periodically as suggested in the Table below, unless otherwise specified by the suppliers. If rapid deterioration of oil is observed, the last value may be confirmed by further tests frequently and the fact may be referred to manufacturer for advice.

Table Sl

Characteristics

No

Equipment

Permissible To be

To be Reclaimed

IS

Voltage

Value

Reconditioned

>170 KV 70 to 170 KV <70 KV All voltages

>50 KV >40 KV >30 KV Above 10 x 1012

If less than Permissible Value Between 1 x 1012 and 10 x 1012

>170 KV <170 KV All voltages All voltages

<20 ppm <30 ppm

If more

All voltages All voltages

.02 or more 0.015 to 0.02

Below 0.015 6104/1971

140 or more Nondetectable

125 to 140

Below 125

1448/1970

Sediment

Precipitable sludge

1866/1978

Replaced 1

BDV

2 3

Specific resistance Ω-cm at 27°C Water Content

4

Tan at 90°C

5 6

Neutralisation value mg. KOH/g of oil IFT.N/m at 27°C

7

Flash point °C

8

Sludge

All voltages

Below 1 x 1012 -

6792/1972 6103/1971 2362/1973

0.01 or less Above 0.01 up to 0.1

Above 0.1

6262/1971

0.5 or less

Above 1.0

1866/1978

3

Above 0.5

First three characteristics of the above Table shall be tested before energising the transformer, then after 3 months and after every year. Characteristics 4 to 8 shall be checked before energising and after every year thereafter. Items 1,2 and 5 can be checked at site or laboratory, where all other characteristics can be tested only at laboratories. All the recommendations are not strictly applicable for transformers of 33 kV class and below and less than 1 MVA capacity, unless very high reliability is required. Specific resistance for other temperature shall be referred to the graph.

Characteristics of New Transformer Oil Sl. No 1.

Characteristics

Requirement

Appearance

Clear and transparent, and free from any suspended matter

2.

Density at 27°C, Max.

0.89 g / cc

3.

Kinematic viscosity at 27°C, Max.

27cSt.

4.

Interfacial tension at 27°C, Min.

O.04 N/m

5.

Flash point, Pensky-Marten (closed), Min.

140°C

6.

Pour point, Max. *

-9°C

7.

Neutralisation value a. Total acidity, Max

0.03 mg. KOH/gm

b. Inorganic acidity I alkalinity

Nil

8.

Corrosive Sulphur

Non-corrosive

9.

Electric Strength (Breakdown voltage) Min. a. New unfiltered oil

30 kV (rms)

b. After filtration

50 kV (rms)

10.

Dielectric dissipation factor (tanδ) at 90°C, Max.

0.005

11.

Specific resistance (resistivity)

12.

13.

a. At 90°C, Min.

30 x 1012 ohm-cm

b. At 27°C, Min.

500 x 1012 ohm-cm

Oxidation stability a. Neutralisation value after oxidation Max.

0.40 mg. KOH/g

b. Total sludge after oxidation, Max.

0.10 percent by weight.

Coefficient of expansion

0.000781°C ** 4

14.

Permittivity

2.2 (-0.001) **

15.

Specific Heat

2.06 kJ I Kg °C (0.0038) **

16.

Thermal conductivity

0.12 W/m°C **

* Pour point 9°C -Sufficient for tropical countries like India as against -30°C by IEC, for class 1 oil. ** The values are relating to 60°C. The figures in bracket are approximate temperature coefficient for Degree Celsius. 5. Transformer Oil Treatment Centrifugal separators can be used to remove finely divided solid particles and free water from the oil. But vacuum edge type filters are generally used for transformer oil treatment. It is a combined process of filtration, dehydration and deaeration in filter units comprising edge type filter elements, oil heaters and vacuum chamber. Direct purification of oil is used in small transformers and switch gear. The dirty oil is removed, equipment cleaned and purified oil is filled in through the filter plant. Purification of dirty oil is done outside. In the case of large transformers, oil is circulated through the purifying plant, oil being taken from the bottom and redelivered at the top. The circulation shall be preferably with the electrical equipment dead. 6. Reconditioning and Reclamation of oil Heavily contaminated oil cannot be brought back to the original condition of new oil by simple treating in the purifier plant. Reclamation can be done with treating the oil with Korvi-Fuller's Earth. It is essentially an aluminium silicate clay occurring in natural state. The Korvi Earth removes the acids and other polar compounds present in the contaminated oil by absorption. The activated Korvi-Fuller's Earth should be heated at 70°C for 2 or 3 hours to drive out the absorbed mixture. This drying should be one just before use. The dried earth, 250 gm/litre of oil to be reclaimed shall be mixed in the oil drum containing dirty oil, and the drum agitated to ensure complete mixing of earth. It should be allowed to stand for 10 hours. The earth along the sludges would by then settled down. Then the oil is separated and filtered in a suitable filter plant. Activated alumina in sphere form can be used in oil reclamation plants in place of Korvi Earth. It is expensive but convenient to use, as it does not disintegrate while in the sphere form, which eliminates added filtration. Activated alumina requirement is given in the figure given below.

5

7. Inhibitors Inhibiting an oil means adding a substance to delay oxidation. Inhibitors are now commonly used in both new and used transformer oil. Ditertiary Butyl Para Cresol (DBPC) is a generally used inhibitor, in proportion 2.5 to 3 parts per 1000 parts of oil by weight. Even if the inhibitors are completely exhausted, there would be no unpleasant consequences. The useful life of transformer oil can be prolonged by three or four times by inhibiting the oil. The detection of DBPC in oil can be done by infrared spectroscopy, thin layer chromatography or gas chromatography. New oils can be mixed with each other if they satisfy the same standard specifications. Inhibited oils can be mixed with each other, provided they contain the same inhibitor. Inhibited oils can be mixed with un-inhibited oils. Some Common inhibitors for transformer oil are, Inhibitor

Trade Name

Ditertiary butyl para cresol

-DBPC, Ional Topanol

8-Hydroxy Quinoline

-Oxine

Bis-methane

-lonex 220, AN 2246, AN 6

Azo-bis- Iso-butyronitrate

-Abin

8. Transformer Oil Filters The transformer oil filter plant is a self contained unit to upgrade the quality of oil. The duty cycle begins from pumping the dirty oil from the equipment (or container) to a heater chamber. After heating the oil to a predetermined temperature, it is then filtered through edge type filter or filter press to remove all suspended impurities. It is then sprayed into a vacuum chamber where the dissolved gases and water are vapourised and separated. 6

The heaters of the filter plant shall be controlled such that the hot point temperature of oil shall not exceed 90°C, as excessive heating will cause deterioration of oil. Improved designs of filter plants employ heating just to reduce the viscosity and to allow it to spread in a thin film, and high vacuum to remove the dissolved gas and moisture. Edge type filter elements are made up of special paper discs under end pressure. Oil passes through the infinitesimal interstices between the discs, leaving solid impurities at the paper edges. Non-hygroscopic graded filters can be used in place of edge filters, but they are much costlier. The vacuum pump shall be capable of producing a vacuum of 2 torr or less, for reducing the water content to a value of 10 ppm. For reducing acidity, absorption device using activated alumina should be incorporated in the operating cycle.

9. Hot Oil Circulation Process The contaminated oil in a transformer can be treated and the impregnated paper insulation of the transformer can be upgraded by employing closed circuit continuous circulation of heated dehumidified and filtered oil in the transformer. The contaminated oil is extracted from the bottom of the transformer, treated in the filter plant by heating, de-sludging and degassing in vacuum and the filtered oil is readmitted into the transformer at the top. Four or five such changes of complete quantity of oil through the filter plant extracts the moisture from the solid insulation of the transformer and improves the BDV of the oil. 7

For filtering power transformers, good quality filter plants are necessary. Single pass filters, which are capable to upgrade the contaminated oil from BDV 20 kV to 60 kV by one pass, are the desired type. With such plants transformers can be dried out within a limited time without impairing the quality of oil. The desired specification of transformer oil filter is as follows. 1.

Type -Single pass to improve BDV from 20 kV to 60kV

2

Vacuum -1 Torr

3. Vacuum pump -1000-1500 litres /minute capacity, ultimate vacuum 0.1 Torr, Rotary type, oil sealed, direct drive. 4.

Feed pump -Vacuum tight gear pump

5.

Heating system -Designed to avoid localised heating, made of material Nichrome 80/20/Kanthal wire, Temperature controlled thermostatically.

6.

Edge type filter -Specially treated, and with simple cleaning facility.

7.

Discharge pump -High suction, centrifugal

8.

Valves -Vacuum tight, rigorously tested for leak rate.

9.

Instruments -

(i)

Temperature indicators for oil

(ii)

Vacuum gauges

(iii)

Pressure gauges

(iv)

Moisture content meters (if necessary)

(v)

Gas content meters (if necessary)

10.

Annunciation -Visual and audible alarm for indicating mal-operations.

11.

Oil loss -Less than 2% of initial volume.

12. Acidity -Mild steel columns, structured Absorption device, Absorbent packing (if required) 13.

Flow rate –0 - 5000 litre I hour

Dissolved Gas Analysis (DGA) of Transformer Introduction Gases may be formed in oil-filled transformer due to natural ageing, but also to a much greater extent as a result of internal fault. The principal mechanisms of gas formation includes oxidation, insulation-decomposition, oil-break- down and electrolytic action. In the case of a fault, its type and its severity may often be inferred from the composition of the gases and the rate at which they are formed. In the case of incipient fault, the gases formed remain partly dissolved in the liquid insulation, free 8

gases will be found only in special cases. The dissolved gases divide between the gaseous and liquid phases by diffusion. Diffusion and achievement of saturation both take time, during which serious damage to the equipment can occur undetected. Periodic analysis of oil samples for dissolved gases forms a method of detecting incipient faults. The study of DGA, therefore may help in taking predictive/preventive maintenance of the transformers. Each and every type of electrical faults in an oil filled transformer give rise to certain types of gases. In the initial stages these gases will be absorbed in the transformer oil. This process will go on till the saturation level of a particular gas in oil is attained. Gas operated relays are provided in transformers to check the generation of gases. But this will be effective only when excessive gases are released. Excessive gases are released only when incipient faults are developed in to major faults. Since transformers are vital and costly, we cannot leave it till serious damages are caused to the insulation structure. Now techniques are developed to extract the gases dissolved in transformer oil and analyse them to pin point the nature of incipient faults at a very early stage. The process of DGA involves sampling of oil, labelling of samples, extraction of gases by Gas- chromatography and analysis of results DGA is a powerful diagnostic technique for on-line monitoring of the internal condition of large transformer. DGA enables us to detect defects in the incipient stage itself. Buchholz relay is never meant to be a diagnostic device. Enough gas must be generated to saturate the oil fully. After saturation only the gas will come out and operate Buchholz relay. Often, by the time the Buchholz relay detect the gas the damage has already been done. But DGA detects gas in parts per million (ppm) of the oil while it is still dissolved in the oil. The technique of dissolved gas analysis involves the detection and identification of faults in the incipient stage by the extraction of dissolved gas from the oil effecting a separation of each gases and quantitative analysis by the Gas Chromato Graph. By DGA we can detect incipient defects such as 1. 2. 3. 4. 5.

Arcing or high current break down Low energy sparking Partial discharges Severe local over heating or hot spot Sustained over loading

9

In practices these processes take places with varying participation of the individual insulating materials such as oil, paper, press board, resin-bonded paper, wood etc. Therefore the gases evolved due to the decomposition of these materials vary considerably. Oil sampling and Gas extraction The procurement of representative sample of oil from a transformer is very important. The sample should be collected and transported in such a way that the gases dissolved in the oil are not subjected to any changes.

1. Oil Sampling valve on Transformer 2. Plastic Tube 3. Container 4. Sampling Bottle Steel or Dark Glass with two high vacuum stop clocks

Oil Sampling

In order to obtain reliable and repeated results it is found that the interval between sampling and analysis should be minimised. Improper sampling procedures can give rise to totally erroneous results. 10

Dissolved gases are extracted from oil samples by expansion of the oil sample in a pre-evacuated known volume. The vacuum expanded gas is then compressed to the atmospheric volume. For degasification, vacuum of the order of 1 x 10 -3 torr or less is applied by double stage vacuum pumps. The degassing flask is heated by immersing it in hot oil and the oil in the degassing flask is stirred with the help of a magnetic stirrer. The dissolved gasses are drawn out with the help of a gas tight syringe and then introduced in to the gas chromatograph. Principle Of Dissolved Gas Analysis Gas chromatography is basically a technique for effecting a separation of the various constituents of the gas mixture. At a particular temperature each gas will have a particular natural velocity, that means, if we allow it to pass through a narrow tube, each gas will reach the farther end at different times, called the retention time of that gas. As usually the gas extracted is very small, an inert carrier gas is used to carry the gas through the column. As each separated constituents comes out of the column, it is identified by an appropriate detector; whose output is recorded on a chart in the form of a trace with series of peaks. Each peak representing a different constituent of the mixture. Gases are identified by either Thermal conductivity detector or by flame ionisation detector. Thermal conductivity detector (TCD) Thermal conductivity of the gas is measured using wheat stone bridge principle. A plot of thermal conductivity against time will give different peak of conductivity corresponding to different retention times. Each of these peaks will thus represent the presence of a particular gas. Area under the peak will give a measure of the magnitude of the gas present. Calibration of the equipment is initially done using known pure gases and from this the gases present in the mixture and its relative magnitude can be identified. This can be used to identify hydrogen, Oxygen, Nitrogen Carbon dioxide and Carbon monoxide.

Chromatogram of Hydrogen, Oxygen, Nitrogen by TCD method.

11

Flame ionisation detector (FID) For hydro carbons thermal conductivity detector may not give sufficiently accurate results. In the flame ionisation detector, the gas emerging from the column is burnt in a hydrogen flame between two electrodes, which measure the conductivity of the flame. The increase in current caused by the burning of organic vapour is measured and the charge is fed to a recorder. FID is almost 100 times more sensitive than TCD. Since transformer oil in service may contain both the above types of gases, both TCD and FID are necessary for the Chromatograph.

Chromatogram of Hydro Carbons by FID method.

12

Chromatogram of Carbon Monoxide and Carbon Dioxide by FID method. Gases to be analysed ;-The gas samples extracted from the oil sample are analysed by gas chromatography .The gases to be determined are : 1) 2) 3) 4) 5) 6) 7) 8) 9)

Hydrogen Oxygen Nitrogen Methane Eathane Ethylene Acetylene Carbon dioxide Carbon monoxide

H2 O2 N2 CH4 C2H6 C2H4 C2H2 CO2 CO TABLE GASES TO BE ANALYSED

1 .

Gases to be analysed normally

O2, N2, H2, CO, CO2, CH4, C2H2, C2H4, C2H6

2 .

Gases to estimate abnormality

H2, CH4, C2H2, C2H4, C2H6,

3 .

Gases to estimate

CO, CO2, CH4

TABLE 13

GAS CONTENT IN OIL BY FAULTS

1. Over heat of oil

CH4, C2H4, H2

2. Arcing in oil 3. Over heat of oil and paper combination 4. Arcing of oil and paper combination 5. Over heat of solid insulating materials

H2, C2H2 CH4, C2H2, H2, CO, CO2 H2, C2H2, CO, CO2, CO2, CO

For effective implementation of DGA technique to a particular transformer, it is necessary to have the correct characteristic of the original oil filled in the transformer at the time of commissioning Typical hydro carbon concentration for a good new oil after vacuum filtration would be within 5 ppm. DGA is repeated at regular intervals after commissioning say monthly, bimonthly or quarterly; and results compared with the original. If the level of any particular gas is found increasing on successive measurements the possible reason can be assessed from the rate of increase of the constituent gases. The technique of diagnosing the internal condition of power transformer by DGA has proved to be an invaluable tool for monitoring of large transformers. It enables most transformer faults to be detected in the incipient stage and permits timely remedial action to be taken to avert a failure The most advantageous aspect of this analysis is that it does not require shut down or disassembly or the transformer. This can be performed while the transformer is in service. TABLE Permissible Concentrations of Dissolved Gases in the Oil of a Healthy Transformer

14

15

Chapter 2

MANAGING SAFETY Introduction The risk of accident is a basic characteristic of Electricity Industry. Damages and injuries caused to public as well as to the workmen due to defective equipment and unsafe practice of work is very high in the KSE Board. It is essential to guard against risk of accidents while working on any electrical equipments. The Indian Electricity Rules 1956 are framed for this purpose. 1. Permit To Work System Statutory regulations for carrying out work on electrical installation are laid down in the Indian Electricity Act 1910, Electricity (Supply) Act 1948 and Indian Electricity Rules 1956. The Power Utility or the State Electricity Board can frame their own safety rules and instructions, generally known as standing orders for the guidance of the staff employed in connection with the execution of the work on electrical equipments and installations. “It shall be the responsibility of the person-in-charge to interpret and explain correctly the rules and regulations to all the staff concerned and to ensure that the staff thoroughly understands the same” (Quoted from BIS 5216 (Part-I)). So, all electrical works should be carried out under supervision of competent person, as laid down in Rule-45 of IER 1956. A permit to work is issued by the person-in-charge of the operation to the person-in-charge of the men who are to carry out work of specified categories. Such permit-to-work is to be returned to the issuer on completing the work. The permit shall be issued to staff of other departments, contractors, Engineers, etc. who might be required to work adjacent to live electrical mains and apparatus (BIS 5216 (Part-I) 1982). All messages and instructions related to the operation and switching should invariably be recorded in a register exclusively maintained for the purpose. 2. Handling Of Electric Supply Lines And Apparatus The relevant statutes as per the rule are given below:1. Before commencing any work the equipment or line should be earthed by suitable means. (Rule 36, IER 1956) 2. Any person or assistant who carry out work on any installation should be authorised in that behalf by the Inspector or understanding orders (Rule 36, IER1956) 16

3. Instructions in English or Hindi and in the local language for the restoration of persons suffering from electric shock shall be affixed in every generating stations, enclosed substations, enclosed switch station and in every factory as defined in Factories Act 1948 in which electricity is used. (IE Rule 44) 4. It should be ensured that all authorised persons are acquainted with and are competent to apply the instruction in clause (iii) (I.E. Rule 44) 5. In every high voltage and extra high voltage generating stations, sub stations and switch stations, artificial respirator shall be provided and kept in good working condition. (I.E.R. ’44) 6. If any accident in connection with the Generation, Transmission, Supply or use of energy, which results in or likely to have resulted in loss of human or animal life or any injury to human being or animal shall be reported to the Electrical Inspector. If the accident is of fatal nature, a telegraphic report is to be sent to the Inspector within 24 hours of the knowledge of the occurrence of the accident and written report in the prescribed form within 48 hours if the accident is non-fatal. (I.E.R. 44A) Statutory clearance to be maintained for bare conductor and live terminals (I.E.Rule64) Voltage Class Ground Clearance Sectional Clearance meter meter Not exceeding 11 kV 2.75 2.6 -do33 kV 3.7 2.8 -do66 kV 4.0 3.0 -do132 kV 4.6 3.5 -do220 kV 5.5 4.3 -do400 kV 8.0 6.5 Clearance above the ground of the lowest conductor of overhead lines including service lines erected across road/street. i)

Low and medium voltage line

-

5.8 m

ii)

High voltage line

-

6.1 m

Clearance above ground of overhead lines, including service lines erected elsewhere, other than along or across street. i) For low, medium and high voltage up to & including 11 kV (If bare) 4.6 m ii) iii)

For low, medium and high voltage line up to and including 11 kV insulated

-

4.0 m

For high voltage line above 11 kV

-

5.2 m

iv) For EHV, clearance shall not be less than 5.2 m plus 0.3 m for every33 kV or part thereof by which the voltage exceeds 33 kV, provided the clearance along or across a street shall not be less than 6.1 m (IER-77) 17

3. Clearance from building of low and medium voltage lines and service lines. i) Where a low or medium voltage overhead line passes above or adjacent to or terminates on any building the following minimum clearances from any accessible point, on the basis of maximum sag, shall be observed. a) For flat roof: i)

When the line passes above the building a vertical clearance of 2.5 m from the highest point of the building.

ii)

When the line passes adjacent to the building a horizontal clearance of 1.2 m from the nearest point.

b) For pitched roof: i)

When the line passes above the building, vertical clearance of 2.5 m immediately below the line.

ii)

When the line passes adjacent to the building, the horizontal clearance should not be less than 1.2 m ( IE Rule 79).

When the clearance is less than as specified above, the conductor shall be insulated. 4. Clearance from buildings for extra-high voltage line. a) Vertical clearance: i) ii)

For voltage up to and including 33 kV

-

3.7 m

For EHV line

-

3.7 m Plus 0.30 m for every additional 33 kV or part thereof

For high voltage lines up to and including 11 kV

-

1.2 m

For high voltage line above 11 kV up to 33 kV

-

2.0 m

For extra high voltage line

-

b) Horizontal clearance i) ii) iii)

18

2.0 m plus 0.30 m for every 33 kV or part thereof ( IE Rule 80)

Minimum Clearance In Meters Between Lines When Crossing Each Other Sl No 1 2 3 4 5 6

Normal system voltage Low & Medium 11 – 66 kV 110 – 132 kV 220 kV 400 kV 800 kV

11-66 kV

110-132 kV

220 kV

400 kV

2.44 2.44 3.05 4.58 5.49 7.94

3.05 3.05 3.05 4.58 5.49 7.94

4.58 4.58 4.58 4.58 5.49 7.94

5.49 5.49 5.49 5.49 5.49 7.94

800 kV 7.94 7.94 7.94 7.94 7.94 7.94 IER – 87

5. Switching Operations For switching operations on electrical apparatus the following conditions shall be observed. a) Switches used in isolating apparatus for giving clearance shall have contacts that are visible or the positions of which can be positively determined by inspection. All phases of such switches, irrespective of the type, shall be inspected to make sure that they are all open. b) If remote electrically operated or mechanically operated switches are used, they shall be locked or blocked or a portion of the mechanism shall be removed to prevent accidental closure. c) Switching operations in unattended stations and line sectionalising points for purpose of clearance shall be done by persons authorised for this purpose. d) All switching and other operations, requiring engineering knowledge or skill, shall be carried out by authorised persons or competent persons acting under the immediate supervision of authorised persons. e) Except for an agreed routine switching, or switching required in cases of emergency, no high voltage switching shall be carried out without the sanction of the senior authorised person. f) No high voltage earthing switch shall be operated, or circuit main earth connection attached or removed except under the instructions of the senior authorised person. Where there are feed back possibilities in sub stations, care should be taken to see that isolators have been kept open and earthed wherever necessary to avoid feed back of power. 6. Safety Precautions To Be Observed During Switching Operations a) Persons performing switching operations on high voltage apparatus shall do so using rubber gloves or standing on insulated stools, platforms or rubber mats. b) When low or medium voltage fuses, which are not in series with circuit breakers, are to be operated, the attendants shall use rubber gloves, insulated platforms or rubber mats. Where there is a possibility of arcing in the switching operations the operator shall use goggles or eye shields and keep his body as far as possible away from the switch.

19

c) When replacing a low voltage fuse which is in series with the switch, the switch shall first be opened. d) Where isolators are in use with circuit breakers, the breakers shall always be opened before opening the isolators, and inversely when the circuit is being closed, the circuit breakers shall be closed last. e) Any abnormality in the condition or operation of any switch shall be reported to the person in charge. f) Where there is interlock system to guard against irregular sequence of operation in switching, the failure of interlock shall not be taken as an excuse for incorrect operation. Following incidents and events shall be recorded promptly and accurately in the sub station log book or the register maintained for this purpose: a) All switching operations on high voltage switches and their timings with an explanation or reasons thereof; b) All clearance orders (i.e. permit-to-work, sanction-for test) received and issued; c) Particulars relating to telephonic messages in connection with the operations on high voltage switches; and d) Any other event that the undertakings may prescribe. 7. Working In Area Containing Exposed Live HV Conductors a) A permit to work or sanction for test shall be obtained for all work in areas containing exposed live high voltage conductors. b) Adjustments, cleaning and painting of earthed metal enclosures and of structures may be carried out from ground-level by competent persons provided specified clearances are maintained. Permit to work shall be obtained for such work when it is to be done from above the ground level. c) Isolation of electrical apparatus shall include i) ii) iii)

Isolation from all points from which it is possible for the apparatus to become alive (i.e. voltage and auxiliary transformers, common neutral earthing equipment). Locking of circuit breakers, isolators, spout shutters, control handles and safety devices, wherever such arrangements exist, in guaranteed position. Locking of all enclosures leading in to live sections from the work area to avoid wrong opening of doors. 20

d) The section made dead for working shall be only that required for the execution of the work and it shall be defined by use of barriers, screens and danger boards, etc, so that the minimum clearance are maintained. The section shall be bounded by red flags by day and red lights by night. e) When working on or near exposed live conductors, to the place of work (i.e. ground level, platform or access way, which may be required to be used) shall be : Sl No 1 2 3 4 5 6

Rated Voltage 240 / 440 V Not exceeding 66 kV Exceeding 33 kV but not exceeding 66 kV Exceeding 66 kV but not exceeding 33 kV Exceeding 66 kV but not exceeding 132 kV Exceeding 132 kV but not exceeding 275 kV

Clearance 61.0 cm 2.5 m 2.75 m 2.98 m 3.43 m 4.57 m

f) No material or tools shall be carried on the shoulders and long materials and tools shall be carried in a horizontal position and in a manner to maintain the clearance as above. 8. Work On Out Door Structures And Busbars In isolating the point of work from supply, care shall be taken to disconnect right points in case of sectionalised, and mesh schemes of bus bars. Isolators/switches closing on the section of bus bars on which work is to be carried out shall be locked in open position and the closing mechanism rendered inoperative. While working on the outdoor structure at a height more than 3 metres from the ground level, safety equipment such as safety belts, hand line, etc, should be used. While changing the parallel groove of tee clamps of the jumpers between the top and the bottom bus, a cradle formed out of (3/4”) 20 mm manila or nylon rope should be used and the safety belt connected to a tie rope passed over pulley block, the other end of the rope being held by at least two persons at the ground level. No persons shall stand directly below the place of work when the work is in progress in the outdoor structure to avoid any tool or bolts or nuts or clamps etc, falling on their heads. PVC helmets should be invariably used while working on the outdoor structures, both by the men stationed at the ground and those on the structures.

21

9. Work On Power Transformers: A permit-to-work or sanction-for test shall be obtained for all work on sub station transformers. When work is to be carried out on a power transformer, for isolation purposes, both the primary and secondary voltage switches and isolators shall be opened. Similarly when isolating transformers to which voltage transformers are connected, the voltage transformers shall be isolated and the low voltage fuses withdrawn to prevent the possibility of the transformers being made live through the synchronising or voltmeter plug. Transformers shall be isolated from all common neutral earthing equipment from which it may become alive. This does not require the disconnection of solidly earthed neutrals or equipment connected solely to the transformer on which work is to be done. Before commencing any work on a transformer, the transformer winding should be discharged to ground. In case the transformer is isolated from the supply by a single point of disconnection, eg. Fuse disconnects, the transformer shall be safeguarded by shorting the phase terminals together and connecting to the ground. The transformer’s neutral should never be accepted in place of grounding of phase terminals as required above. Transformers without conservators shall be treated as if the space above the oil level contains highly explosive gas and, therefore, the space shall be suitably ventilated before entering the tank. Open flames or inadequately protected portable lamps shall be kept away from the manhole and smoking shall not be permitted when working on or in the transformers. Persons working in the transformers shall not carry any loose articles like key bunches in their pockets and persons working in or on the top of transformers shall not keep any loose tools with them. Whenever transformers are replaced, the new transformer shall be checked carefully for voltage, polarity, and phase rotation. 10. Works On Circuit Breakers: For isolation purposes it shall be ensured that : a) Disconnecting switches on both sides, control switches, or control fuses, relay trip blocking switches and compartment doors are open. b) Mechanical blocking, when it is necessary to prevent unauthorised movement of the mechanism, is installed. c) In cases, where there are no disconnecting switch between the transformer terminals and the circuit breaker, the transformer should be isolated. 22

d) In oil circuit breakers trip-free feature should be blocked. Following additional precautions shall be taken in relation to work on minimum-oil type circuit breakers: a) With the exception of control cabinet, all parts of the circuit breaker shall be considered as alive. b) As the operating springs are under tension in both the open and close position of the breaker, extreme care shall be taken when adjusting the operating mechanism to avoid accidental operation. c) Where possible, when working on contacts of these breakers, the spring tension should be completely released and the control circuit opened at the breaker. d) Where breakers are operated hydraulically, care should be taken to see that motor circuit is kept open so that the hydraulic pressure does not build up by operation of the hydraulic pump, coupled to the driving motor. The D.C. supply to the marshalling boxes shall be cut off so that closing circuit remains inoperative due to absence of D.C. supply. In case the circuit breaker is not closing electrically by remote operation due to some fault in the wiring, manual closing may be resorted to, after making sure that the fault has been cleared. 11. Works On Instrument Transformers: The body of all instrument transformers shall be earthed. In handling instrument circuit the secondary of a current transformer shall not be opened while it is alive. Before any work is carried out on an instrument or other device in a current transformer secondary circuit, the device shall be bridged with jumpers so that the circuit cannot be opened at the device. The circuit shall never be opened at meter connection until it has been bridged elsewhere. Potential transformer’s secondaries shall never be short circuited. Low voltage windings of the potential transformer shall always have one side permanently and effectively earthed. 12. Work On Metal Clad Switchgear And Control Panel : While working on manually-operated, panel-mounted, circuit breakers, when the operating handle is on the front and the circuit breaker is on the rear of switchgear or on another panel, a danger notice shall be placed on the handle. When the work is to be carried out on bus bar-spouts the following operations shall be carried out; a) The section of bus bars, on which the work is to be carried out, shall be made dead and shall be isolated from all points of supply. b) The isolating arrangements and the shutters of live spouts shall be locked so that they cannot be operated. c) Where duplicate switches in one tank or on load bus bar isolators are installed and it is impossible to isolate them from all points of supply, then all switches and selectors that could be closed on the bus bars on which work is to be carried out, shall have their mechanism locked in the open position and the closing mechanism shall be made inoperative. 23

d) The bus bar shall be earthed with approved earthing equipment at a panel other than at which work is to be done on the isolated section of the bus bars . 13. Works On Lightning Arresters: No work shall be done on lightning arresters including the earth wire unless it is disconnected from the live circuit and earthed at both the line and earth terminals. The body or shields of oxide film lightning arresters must never be touched while arresters are energized. High voltage and extra-high voltage lightning arresters, which are accessible shall be provided with suitable screens or fences against possible contact while the arresters are alive. The gate of the screens shall be kept locked and the keys kept under safe custody with the operator on duty . 14. Works On Storage Batteries : While preparing electrolyte, always add acid to water and not water to acid. Smoking, open flame or the use of tools or any other devices that are liable to cause sparks, shall be avoided in storage battery rooms. While handling acid or batteries workmen shall use proper tools and lifters. Workmen shall always wear gloves and rubber aprons. Electric storage battery jars and cells, unless composed of glass, hard rubber or other insulating material shall be mounted on insulating supports. 15. Work On Underground Cables : For isolation of cables open at least one set of disconnecting switches or fuses in every source through which the cables can be made alive including leads to the cables of potential transformers then discharge the cable to earth. Cable route indicators should be provided and cable route record maintained to assess correctly the particulars of all underground cables in the vicinity of the faulty cable. Use of sharp-edged crow bars or pick axes should be avoided while excavating the earth to locate the faulty cable. All cables in the vicinity of the fault area shall be exposed and identified to establish the identity of the faulty cable. Before a high voltage or extra high voltage cable is cut, the senior authorised person shall make definite checks to identify the cable to ensure that the cable has been made dead and earthed. He shall then spike the cable in an approved manner at a point where the cut is to be made. Before any high voltage joint or chamber is to be opened in circumstances where it is not desirable to spike the cables entering the joint or chamber, the senior authorised person shall satisfy from cable-route-records, and if necessary by approved tests, that the joint or chamber, associated with the particular cable, has been made dead and on which it is safe to work. 24

Employees shall not step on live cables even though they are insulated and enclosed in a lead sheath. Tools and material shall not be rested against the sheath of the cable. When work is to be carried out on a cable in proximity to another live circuit or cable having fully insulated metallic sheath, special precautions should be taken to prevent danger from induced voltage. 16. Work On High/Medium/Low Voltages Fuses : No work on high voltage fuses shall be taken up when the circuit is alive. Authorised persons shall do the replacement of high voltage fuses only. Before starting work check shall be made with approved apparatus to ensure that the fuse contacts are dead and the isolators are in fully opened position on all the three phases. Work shall not be started unless: i) ii)

All the three phases are shorted and earthed on either side of the fuse. Proper barriers are erected against adjacent live equipment.

Following precautions should be taken while working on low/medium voltage fuses : a) Persons checking up the work on fuses shall wear approved gloves while handling fuses. Eyes shall be protected against possible flash by wearing goggles, or by shielding eyes and face. b) In case where the fuse is in series with the disconnecting switch, the switch shall be opened before replacement of the fuse. c) Where necessary, the neutral line shall be withdrawn after all phase fuses have been withdrawn and replaced first. Then all the phase fuses put in. 17. Works On Pole Mounted Substation (Distribution Transformers) The following precautions shall be observed in case of carrying out work on the pole mounted sub stations (ie distribution transformers) a) The work shall be carried out under a permit-to-work. b) Before changing or replenishing oil, or painting all exposed live parts transformers shall be disconnected. c) While working on poles, which have lightning arresters installed on them, the workmen shall avoid touching lightning arresters and the lightning arresters ground wire. d) Open flame shall not be brought near an open transformer. ******************* 25

Chapter 3

Maintenance of Sub stations The areas of maintenance in Sub station are: 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)

Power Transformers Circuit Breakers Bus bar and insulators Current Transformers and Potential Transformers Lightning arresters Earthing and Lightning shield protection Columns, beams and other supporting structures Relay and protection systems Station battery 11 kV cables and control cables Auxiliary power supply to the station Lighting Sub station building Fire fighting equipment Security fencing

Maintenance of Power Transformers Objective of maintenance The main objective of maintenance of transformers is to maintain the insulation in good condition. Moisture, dirt and excessive heat in contact with oxygen cause for deterioration of insulation. The causes of breakdown of transformers shall be detailed as follows:- , (i)

Faulty design or construction

(ii)

Improper insulation

(iii)

Over loading

(iv)

Wear and tear, neglect, accidents etc.

(v)

Failure of auxiliary equipment

In a power system, the ratio of installed capacity of generators to installed capacity of transformers is 1: 4. This makes the power transformer the costliest single item in a sub station. The transformers are really the heart of a Transmission and Distribution Net work. According to Electricity (Supply) Act 1948, a life of 35 years is expected for a Transformer above 100 KVA capacity and 25 years for others (i.e. below 100 KVA capacity). Premature death of a transformer can definitely be attributed to lack of maintenance. 26

Extensive studies have established that 85% of the transformer failures is due to the failure of the insulating system of the transformers. Ageing or deterioration of the insulating system in any transformer starts with the very moment of commissioning of the transformer. By maintenance of transformer we mean (i) assessing the rate of ageing/deterioration taking place in the transformer (ii) identification and elimination of factors contributing to the faster rate of ageing of insulating systems (iii) taking measures to compensate for the deterioration. Analysis of the results of the following tests on transformers will give indication on the condition of the transformers. i)

Insulation Resistance of Transformer.

ii)

Polarisation Index

iii)

Testing oil for the Physical, Electrical and Chemical characteristic of transformer oil according to standards.

Guide lines to longer life for power transformers:i)

A strict oil maintenance schedule may be followed

ii)

Ensure sludge free operation for oil (sludges are formed in oil due to chemical contamination of oil)

iii)

Establish 600 C as the maximum top oil temperature.

iv)

Conduct all tests periodically

v)

Conduct analysis of dissolved gases in oil every year.

vi)

Excessive moisture, oxygen, heat and acidity will damage the insulating system-eliminate such conditions

vii)

Test and calibrate all protective relays periodically

viii)

Keep the gaskets, valve seats etc intact so that they are properly sealed against infiltration of moisture and gases from the atmosphere.

ix)

Clean the insulators and bushing periodically

x)

Inspect and check all connections regularly 27

The life of the transformers are effected by the following: (i) Moisture: Transformer oil absorbs f moisture from the air and consequently the dielectric strength of oil and paper deteriorates. (ii) Oxygen: The oxygen in the air trapped inside the transformer reacts with the cellulose of the insulation and the decomposition products cause for formation of organic acids soluble in oil and sludges which blocks the free circulation of oil. (iii) Solid Impurities: If the transformer oil contains solid impurities like particles of insulating materials etc., it may accelerate deterioration. (iv) Varnishes: Some oxidising varnishes react with transformer oils and precipitate sludge on the windings. Synthetic varnishes, which have acid-inhibiting properties generally, delay the natural formation of acid and sludge in the oil. (v) Slackness of windings: Transformer windings may become slack during transportation, due to coil displacements under load conditions and due to momentary short circuits which cause electrical and magnetic unbalance. Inspection and Maintenance of Transformers Transformers shall be inspected and maintained under safe conditions. Maintenance schedules for power transformers are as given. 1. Oil: Oil level should be checked frequently and if any excessive leakage is there, it should be stopped by suitable repairs. Oil taken from a switchgear shall not be used for topping up transformer. Dielectric test of oil shall be conducted regularly. In addition chemical tests shall be done periodically to determine the deterioration of oil. If acidity, sludge or corrosion is evident, the oil shall be treated for reclamation. Simple filtration may not be sufficient. 2. Transformer Body: It shall be ensured that the body of transformer does not get rusted. Transformer body shall be painted periodically. Leaking joints can be rectified by tightening the bolts. Rollers shall be greased regularly. 3. Core and winding: At specified intervals core and windings shall be inspected after opening the tank, taking all precautions. 4. Magnetic oil gauge: The oil level indicator shall be always kept clean. Oil level in the conservator tank shall be properly maintained. 5. Silica gel breather: The silica gel breather shall always be in blue colour. When they are saturated with moisture, they turn pink. Such silica gel shall be heated and reactivated by extracting moisture. 6. Buchholz Relay: Routine inspection and operation tests should be conducted on Buchholz relays. 8. Explosion vent and Gaskets: The diaphragm at the end of the explosion vent shall be regularly checked for any crack or damage. Gaskets get shrunk due to ageing. If leaks cannot be stopped by tightening the nuts, old gaskets shall be replaced with new ones. 28

9. Pipe works, coolers, fans etc. : Pipe works should be checked for leakages and joints made leak proof periodically. The outer surface of the radiator type coolers shall be cleaned. Dust from the fans of the coolers shall be removed and bearings lubricated. 10. Temperature indicators: Oil level in the pockets holding thermometer shall be checked and replenished during annual maintenance. Capillary tubings shall be fastened properly and dial glasses cleaned regularly. Temperature indicators shall be calibrated periodically. 11. On-load Tap changers: Diverter switch shall be serviced annually by cleaning the contacts, checking oil level and replacing the contaminated oil. Motor driving mechanism shall be checked and motor maintained properly. Loading guide for oil immersed transformers Table shown below gives the guide lines in loading typical power transformers (IS 6600-1972). Typical permissible overloading calculation 1000 kVA transformer has a load of 500 kVA through out the day except for a period of 2 hours. To find the permissible over load for a duration of two hours. The weighted average annual ambient temperature of the cooling medium is 32°C. Guide Lines on loading of Power Transformers K1 a

0.25 32 °C

0.5

40°C

32°C

0.7

40°C

h

32°C

0.8

40° C

0.9

1.0

32º C

40 ºC

32º C

40 ºC

32º C

40º C

Values of K2

0. 5

1.95

1.86

1.88

1.78

1.78

1.67

1.72

1.58

1.62

1.33

1.00

1.00

1

1.72

1.63

1.66

1.56

1.58

1.48

1.53

1.40

1.45

1.19

1.00

1.00

2

1.47

1.39

1.43

1.34

1.38

1.28

1.34

1.23

1.29

1.07

1.00

1.00

4

1.26

1.18

1.24

1.16

1.22

1.13

1.20

1.10

1.16

0.99

1.00

1.00

6

1.17

1.09

1.16

1.08

1.14

1.06

1.13

1.04

1.11

0.96

1.00

1.00

8

1.12

1.04

1.11

1.03

1.10

1.02

1.10

1.00

1.08

0.96

1.00

1.00

12

1.06

0.99

1.06

0.98

1.06

0.97

1.04

0.97

1.04

0.97

1.00

1.00

24

1.00

0.92

1.00

0.92

1.00

0.92

1.00

0.92

1.00

0.92

1.00

1.00

Note : In normal cyclic duty, the value of K2 should not be greater than 1.5 K1

-

Initial load as a fraction of rated KVA

K2

-

Permissible load as a fraction of rated KVA

h

-

Duration of K2 in hours

θa

-

Temperature of cooling medium (weighed average)

Therefore permissible overload for 2 hours is K2 x Rated kVA 29

ie. 1.43 x 1000 = 1430 kVA Effects of Overloading Paper or press board used as insulation in transformers, when heated beyond certain limits under oil for long period of time, will loose mechanical strength, but the dielectric strength is hardly affected until the paper is actually charged to the point, when the free carbon causes conduction. The lower mechanical strength of paper may not be able to resist short circuit forces and break down occurs. When Transformer trips on acting Buchholz Relay Whenever the Buchholz relay of the transformer acts and the transformer is tripped. the following procedures may be adopted before energising the transformer again. i)

Isolate the transformer from the system and examine for any external visible damage.

ii)

Examine the colour of the gas accumulated in the Buchholz relay casing. The colour gives an indication of the fault. White gases emanate from the destruction of paper, yellow from the destruction of wooden materials and black or grey colour from oil decomposition.

iii)

The rate of collection of gas is an indication of the severity of fault. The gas collected may be air, Carbon Monoxide (CO) resulted from the destruction of paper and wooden materials, or acetylene obtained as a result of oil decomposition. The gas collected may be analysed using the following method.

(a). The gas may be bubbled through freshly prepared 3% silver nitrate solution. If no precipitate is found within 30 minutes, the gas collected is air. If a white precipitate is found within 30 minute and turns black, the gas collected is carbon monoxide. If the white precipitate does not change in colour, the gas collected is acetylene. This test is possible only if there is sufficient bubbling of the gas as seen through the glass window of the Buzhholz relay. (b). The gas collected can also be analysed in the following way. Allow the gas to escape by opening the cock placed above the casing and verify by means of a match, whether it is inflammable. If the gas is not inflammable, it can be concluded as air. If the gas burns with a blue flame, the gas collected is CO. If the gas burns with yellow flame, it is acetylene. If the gas collected is air and if there is no visible damage, the transformer can be test charged again. If the gas collected is CO or acetylene, the transformer should not be test charged again before checking the core and winding thoroughly. On no account, the gas collected in the chamber should be let off without carrying out the aforesaid tests.

30

Recommended Minimum Insulation Resistance for Transformer Winding Rated Voltage ( kV)

IR value in meg-ohms (using 2 kV Megger) 300C

400C

500C

600C

66 and above

600

300

150

75

33

500

250

125

65

11 & 6.6

400

200

100

50

Below 6.6

200

100

50

25

Note: As a rough guide, the megger values should be 2 meg. Ohms for every rating at 600C for 11 KV and above. The megger values double itself for every 10 0C fall in temperature. Recommended maintenance schedule for Transformers below 1 MVA Items to be inspected (a) Load (amperes) (b) Voltage (c) Dehydrating

Inspection notes Check against rated figures Check that the air passage is clear, check the colour of active agent

Frequency Hourly

Action required

Daily

(d) Oil level in transformer

Check transformer oil level

Monthly

(e) Bushings

Examine for cracks and dirt deposits

Quarterly

If silica gel is pink, change by spare charge. The old charge may be reactivated for use again If low top up with dry oil. Examine transformer for leak. Clean or replace

(f) Conservator

Check for moisture under cover Check for dielectric strength and water content. Check for acidity. Examine relays and alarm contacts, their operation, fuses etc.

Half yearly

Internal inspection above core. Overall inspection, lifting of core and

Yearly

(g) Oil in transformer

(h) Earth resistance, relays, alarms their circuits etc.

(i) Conservator

31

Yearly

Yearly

Improve ventilation. Check oil. Take suitable action to restore quality of oil (Filtration of oil if required) Take suitable action, if earth resistance is high. Clean the components and replace contacts and fuses if necessary. Change the settings if necessary. Check relay accuracy etc. Filter oil regardless of condition. Wash by hosing down with clean dry oil

windings

32

33

34

35

36

37

38

Circuit Breakers Introduction The circuit breakers in the electrical installations are very important equipments, which have to operate on no load conditions, load conditions, or on short circuit conditions. When an electric contact is open, an arc is formed and this arc has to be quenched at the earliest instant to protect the equipment and the operating personnel. Depending upon the different medium of arc quenching and various voltage levels different types of Circuit Breakers are used. Any maintenance work on the Breaker shall be undertaken only after switching off, isolating, and earthing the equipment. Also all precautions shall be taken against any accidental operation of the breaker during the maintenance work. Proper preventive maintenance will result in the efficient and economical operation of the breaker for a long period and will avoid costly break downs and outages of the system. The maintenance works shall be done as per a proper schedule of work. A team of welltrained workers, supervisors and officers is essential for the proper and systematic maintenance work. Required tools and spares should be available before taking up the work. Required safety equipments also should be provided. Maintenance work of different breakers are detailed below: Types of Circuit Breakers 1) 2) 3) 4)

Oil Circuit Breakers (Bulk and Minimum) Air Blast Circuit Breakers SF6 Circuit Breakers Vacuum Circuit Breakers

The maintenance programme will vary for each type: It is desirable to schedule the maintenance according to recommendations of the manufacturers.

Maintenance Of Circuit Breakers Bulk Oil Circuit Breaker ( O C B ) : The OCB comprises three single pole contact assemblies housed in specially shaped welded steel tanks and a sheet steel tank housing carrying the operating mechanism. The whole assembly is supported on angle iron frame work. The breaker shall be of solenoid or spring closing with arrangement for emergency hand operation. a) Fixed and Moving Contacts For normal service operating conditions, the contacts shall be inspected once in an year. The contact surfaces can be cleaned up with a smooth paper if it is pitted. If the operation is frequent, the inspection of contacts shall be at shorter intervals. After opening under abnormal conditions such as short circuits, it is necessary to examine 39

the contacts as soon as the breaker can be isolated. Particles of fused metal should be removed by using smooth file and if the burning is severe, the contacts may have to be replaced. b) Cross Jet Pot assembly: To examine the cross jet pot assembly it must be removed from the circuit breaker. Blackening due to the passage of burnt oil and gases need not be considered as deterioration, but if the passage is badly charred, the damaged plates should be renewed. c) Contact Lifting Mechanism The mechanism should be inspected at intervals not exceeding 12 months. The lock nuts of all adjustments should be checked for tightness, after checking that the mechanism functions correcting. d) Operating Mechanism: The mechanism should be inspected at intervals not exceeding 12 months and bearing and sliding surfaces lubricated with good quality machine oil. The lock nuts of all adjustments should be checked for tightness. e) Closing Contactor: The contactor for the closing solenoid should be inspected periodically. Indoor Type Minimum Oil Circuit Breaker ( MOCB) The MOCB switch gear is of single bus bar system using horizontally isolated withdraw able minimum oil circuit breakers. Each cubicle is independent and fully separated from its adjacent cubicle by sheet steel enclosure all round except for the opening in the bus bar chamber. a) Maintenance of Cubicle and Bus Bars 1)

Monthly cleaning of switch board

2)

Checking of L.T. circuits

3)

Verify the healthy conditions of indicating lamps, etc

4)

Clean the support insulators of bus bars and jumpers

5)

Replace the support insulators if necessary

6) Lubricate periodically (once in a year), mechanical joints of shutter operating levers and cams. 40

Schedule of Maintenance for Oil Circuit Breakers: a)

Monthly Cleaning bushings, checking oil levels, releasing air etc.

b)

Quarterly

i)

Cleaning mechanism box, lubricating pivots, checking tripping/closing coils, auxiliary contacts etc.

ii)

Testing insulation resistance

iii)

Testing dielectric strength of oil

iv)

Checking tightness of connections

c)

Half Yearly

i)

Examination of contacts, dressing or replacement of contacts

ii)

Check the porcelain of insulators for cracks or damages

d)

Yearly

i)

Close and trip the breaker by hand to ensure that its mechanism functions perfectly.

ii)

Re-condition/replace the oil. If it is observed that the oil is contaminated, earlier replacement of oil may be done

iii)

Complete overhauling of the breaker once in a year is desirable

iv)

Check the contact resistance, closing time and opening time for EHV class MOCB.

Air Blast Circuit Breaker (ABCB) ABCBs are designed for outdoor use and are based on the multiple interruption principal using compressed air as insulation and medium for arc quenching. The operating mechanism also works on compressed air. Maintenance is confined to periodical cleaning and lubricating of the moving parts and renewal of damaged contacts and gaskets. Schedule Of Maintenance a) Daily: 1. Check the compressed air pressure in the switch cubicle (hourly readings taken) 2. Check for any air leakage (audible sound) 3. Drain condensate from pressure vessels 4. Check operation of compressors 41

b)

Monthly: 1. External cleaning of insulators, tank, etc 2. Tightening of power connections 3. Lubricate the moving parts in control block, switch cubicle and control valves

c) a)

Yearly: For ABCBs the opening and closing time may be noted once in every year. It is also to be checked that all phases operate simultaneously The air compressor plant is part of the ABCB. Constant attention is essential to maintain the air compressor plant in good condition. The moving and fixed contacts to be renewed when the tip of contacts to be renewed when the tip of contacts is cracked or it has been eroded by about 2 mm (relative to new contact) Inspect and check the operation of Valve cartridge of control block (Control valves for opening and closing, resetting device and reloading device)

b) c) d) D. 1.

Every five years ( General Overhaul ) Arcing chamber Clean or replace contacts Check or replace packing for valve disc

2.

Hollow insulator column

-

Inspect, clean and dry inside of hollow column

3.

Air Receiver

-

Inspect, clean and dry inside of air receiver Renewal of gasket of end cover

4.

Ventilation cartridge

-

Check for air flow replace if required

5.

Back signalling device

-

Dismantle into main parts Clean and lubricate piston and cylinder wall

6.

Control Block

-

Dismantle in to principal parts Clean and lubricate Replace packing rings

7.

Main valves

-

Dismantle, clean and replace packing Lubricate piston assembly

8.

Switch Cubicle

-

Check renewal of packing Lubricate moving parts Calibration of pressure switches Check operation of back control apparatus (discrepancy trip) Clean filters

42

Dos and Don’ts FOR ABCB: Dos: 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)

Use only specified lubricants For sticking gaskets bare minimum solution (hylomer / maxfix) shall be used All gaskets and ‘O’ rings are replaced by new ones after a removal for inspection and assembly The valve connecting the breaker tank and air supply piping should always be kept open when the breaker is energised Heaters provided in the switch cubicle and control blocks should always be switched on When control block is overhauled, ensure that the matching of various levers is as per the instructions, check the engagements and settings Outside surfaces of insulators to be cleaned periodically Ensure that the wiring connections are tightly secured If oil is found leaking through the condensers replace it by new one. Periodic painting of metal parts shall be done as required Keep the breaker in closed position after opening the isolators at both sides to avoid air leakages,

Don’ts: 1) 2) 3) 4) 5) 6) 7)

Do not interchange ventilation gaskets, particular cartridge with particular punch mark shall be used in specific places only. Do not leave the control blocks uncovered any time Control block settings should not be altered Do not leave the switch cubicle door open Pressure settings of pressure switches should not be disturbed Do not disturb safety valve settings Do not use spare parts other than supplied by the firm

SF-6 Gas Circuit Breaker (GCB) General Information On Sf-6 Gas SF-6 is colourless, odour less non-flammables, non-corrosive non-toxic and chemically inert gas at normal temperature. It is much heavier than air (5 times). Dielectric strength is about 3 times that of nitrogen (air). Arc quenching ability and thermal stability are excellent. It has the property that disassociated molecules recombine rapidly after the source of arcing is removed. Sf-6 is supplied in steel cylinders as liquefied gas (40 kg). During arc interruptions gaseous or powdery secondary products may be formed. This powdery decompositions can be seen inside the interrupting chambers as while deposits and chemically affect the human skin and materials.

43

How To Handle Secondary Products: 1. 2. 3. 4. 5. 6.

Assure adequate ventilation Avoid direct contact with components exposed to SF-6 gas. Wear protective gloves, goggles, mask, etc Do not stir up powdery deposits, but remove them using a vacuum cleaner After work, wash hands thoroughly Dispose of secondary products, cleaning materials, as well as absorption material according to local environmental code and operating instruction of the firm. For re-assembly always use new gaskets and seals

7.

In GCB SF-6 gas is used as insulation and as arc quenching medium. The operating mechanism works on compressed air pressure or charged spring. Single break and double break units are available depending on voltage level and design. Generally the breaker is maintenance free. General overhauling is due after 10 years of operation. Topping up of SF-6 gas is required if ‘low gas pressure’ alarm is obtained. Maintenance a)

Daily:

a)

Check SF6 gas pressure and temperature

b)

Drain condensate water from air compressor and beaker air tank

c)

Check compressed air pressure reading

d)

Maintain space heaters in the control boxes and control cubicle

e)

Check vibration during operation

b)

Monthly

a) b) c) d) e)

Check the local /remote operation of the Breaker Clean the porcelain, control box, control cubicle and air compressor Check the indications of the auxiliary switch contacts Maintain space heaters in the control boxes and control cubicle Check the automatic starting and stopping of air compressor

c)

Half Yearly

a)

Check the insulation resistance value of GCB with 2kV Megger Across contact – Breaker open condition Breaker terminal to ground – Breaker closed condition Check the SF6 gas pressure low alarm and lock out by shorting the contacts of gas density detector. Check the air pressure alarms

i) ii) b) c)

44

d)

Yearly

a)

Check the opening and closing timings and compare with commissioning report. Check contact resistance of main contacts and compare with test report. Check the safety valve blow out and reset pressure of air compressor. Lubricate the operating mechanism as per the directions given in the manufactures maintenance manual.

b) c) d) e)

Special Maintenance

Satisfactory operation of the breaker can be ensured only by proper maintenance. Table given below shows the types of inspection and its schedule applicable for TELK SF6 Breakers TABLE Type of Inspection

Inspection interval

Remark

Patrolling inspection

Every week

Check the breaker operation for any abnormality

Special Inspection

Every 5 years or every 500 operations

Shut down required

Detailed Inspection

Every 10 years or every 2000 operations

Shut down required. Overhauling of equipments.

Table given below shows schedule for contact inspection TABLE Interrupting Current In kA

Number of Operations

40

20

15 to 20

50

5

700

Rated current - 2

3000

Items of Inspection Appearance 45

1 Check porcelains for any damage 2 Check Main terminal 3 Check earthing 4 Check breaker position indicator 5 Check foundation bolt tightness 6 Drain water from local air receiver 7 Record no. of breaker operations (Counter Reading) Gas System 1 Record the gas pressure (If leak observed, fill gas) 2 Gas leak test 3 Check G.D. setting Air system 1 Record the air pressure 2 Check for any audible sound 3 Retighten all union nuts 4 Calibrate air pressure gauge 5 Check pressure switch settings Interrupter 1 Stationary contact inspection 2 Moving contact inspection 3 Inspection of nozzle 4 Replace absorbent Basic Mechanism 1 Conform the stroke 2 Inspect the horizontal links for any undue wear 3 Check oil level in the dash pot 4 Supply new oil to dash pot 5 Apply Hitalube to moving parts like link, pin joint Control Box 1 Electromagnetic valve: 2 Auxiliary switch: 3 Operating cylinder: 4 Hook assembly: 5 Check resistance of close and trip coils Control Cubicle

-

1 Check electrical contactors 46

Clean hook using turbine oil Check Contact at terminal block Apply Hitalube to piston rod Apply Hitalube to hook roller Compare with test report

2 Check wiring connections for any looseness 3 Measure IR value 4 Check compressor for any abnormality GENERAL CONDITIONS: (FOR ROUTINE AND DETAILED INSPECTION) 1. Open the line isolators of both sides of the breaker and ground both the terminals of the breaker . 2. Turn off the control circuit power supply and remove the air from local air receiver through its drain valve. 3. Outdoor work on a rainy day should be prohibited. 4. Do not dismantle shaft seal assembly . 5. Replace the absorbent when inspection of interrupting unit is carried out. This should .be done just before evacuation. 6. If a gas joint is repaired, perform leak test. Materials required for detailed inspection 1. SF6 gas 2. Absorbent 3. Hitalube (grease) 4. Set of O-ring 5. Male contact 6. Female contact 7. .Nozzle set 8. Oil for dashpot 9. Spring for tulip contact

-

One cylinder with mass 39 kg 2kg 2 kg 1 set 3 nos. 3 nos. 3 nos. 5 litre transformer oil 6 nos.

Tools and accessories required for detailed inspection The following is the list of main items. 1. Mobile crane or derrick to lift max. 6000kg to a height of 9 m. 2. Vacuum pump suitable for evacuating a volume of 200 litres to l mm of mercury. 3. Gas feeding hose. 4. Maintenance Jig. (in case spring is to disassembled) 5. Set of spanners 6. Vernier callipers , depth gauge, feeler gauge 7. Vinyl sheet 8. Nylon wire rope, nylon hose ( l m) approx. dia. l0mm

47

48

49

50

51

52

Vacuum Circuit Breaker ( VCB ) The VCB comprises one or more sealed vacuum – interrupter units per pole. The moving contact in the interrupter is connected to insulated operating rod, linked with the operating mechanism. The contact travel is of the order of a few millimetres (15 mm) only. The movement of the contacts within the sealed interrupter unit is permitted by metal-bellows. a) Vacuum Interrupter: 1) 2) 3) 4)

Verify contact force Contact resistance The Vacuum integrity of the interrupter may be tested using a high voltage testing kit every six months. The Vacuum Interrupter is a sealed unit and it cannot be repaired or opened at site.

After the permissible number of fault operation at given level the following items shall be examined and depending on the results of examination or the manufacturer’s recommendation the breaker is overhauled. 1. 2. 3. 4.

Insulation Isolating contact Vacuum interrupter Isolating and earthing switches

Bus, Connections and Isolators: Inspection for broken or cut strands of bus, tightness of connections, condition of string insulators and hardware may be done once in every six months. Lightning Arresters a)

Monthly:

i) ii) iii) iv)

Cleaning of insulators. Checking connections Measurements of Insulation Resistance. Check the readings on the surge counter

It is important that the lightning arrester should be effectively connected to earth. Earth resistance may be measured and remedial measures taken if value is found to be above permissible levels.

53

Earthing and Lightning Shield Protection Earth resistance shall be taken monthly and records should be maintained. If earth resistance is not found satisfactory, take remedial measures to improve the value. Check all connections of the lightning shield protection once a year and rectify defects noticed. Columns, Beams and Supporting structures Detailed inspection for fatigue, rusting, deformation of members should be conducted once a year. Take remedial measures if required. Paint all non-galvanised iron parts once in two year. Relay and Protection System. All relays are to be tested every six months for their operational worthiness. Replacement or rectification of defects should be done. Station Battery: a)

Daily: Inspect battery for general condition, level of electrolyte, measure voltage and specific gravity of pilot cell, note ambient temperature and keep record of these observations.

b)

Weekly: Inspect battery, clean all dust and direct from the battery, check for plate buckling, collections of sediments at the bottom.

c)

Fortnightly : Top up distilled water if required.

d)

Quarterly : Check specific gravity and voltage of all cells, check connection, apply petroleum jelly to terminals connections, etc.

e)

Yearly:

i)

Paint the racks and battery room with acid resistant paint.

ii)

Carry out discharging of battery as per the instructions given in the maintenance manual. Boost charge the Battery after discharging.

iii)

Procedure for Battery Maintenance Initial Charging .i) Filling-in specific gravity: 1.190 + 0.005 at 27°C. ii) Rest Period 12-18 hours. iii) Charging may be commenced at any rate between the starting and finishing rates. 54

iv) v)

vi)

Once cell voltages reach 2.35V, reduce current to finishing rate and continue charging, till the cells are fully charged. If during any time of charging, temperature exceeds 50°C, suspend charging. Allow temperature to come down to 40°C and continue charging at finishing rate. If however, the time taken for the cell to cool down to 40°C is inordinately long, recharging may be started at 45°C. Cells are considered to be fully charged once three successive hourly readings of cell voltage and electrolyte gravity are found to be constant. All cells should also gas freely. The voltage of each cell should be around 2.75V on top of charge condition. However, the minimum total Ah input, as mentioned in the table must be provided to the cells even if the voltages and specific gravities are observed to be constant before that. On completion of charge, adjust acid level to 'Maximum' after correcting specific gravity of electrolyte to 1.200 + 0.005 at 27°C. Technical Specification of Exide Plante Battery TABLE Capacity when discharged at 10 hour rate to 1.85 volt in Ah

Startin g rate in Amps

Finishing rate in Amps

YKP 9

100

12

6

400

6.3

80

240

YKP17

200

24

12

800

7.3

160

480

YKP25

300

36

18

1200

10

240

720

YKP33

400

48

24

1600

12.8

320

960

Type of Cell

Charging Current

Total minimum input during initial charging in Ah

Approximate Trickle quantity of charge acid 1.19 sp. current gr. in Min. Max. litre mA mA

Float/Trickle Charge In standby application Exide Plante cells are to be maintained within a Float Voltage range of 2.18 to 2.25 V per cell. Trickle charging currents should be so adjusted, anywhere between the maximum and minimum allowed levels given in the table, such that individual cells remain fully charged. 55

Quick Recharge Exide Plante cells after a deep discharge can also be recharged quickly by applying the Starting Rates mentioned in the table. However, currents will have to be reduced to the Finishing Rate once individual cells attain a voltage level of 2.35 volts. Care will also have to be taken that electrolyte temperature does not exceed the maximum of 50°C in which case the charging will be discontinued until the temperature drops down to 40°C, or at least to 45°C. Charging may be resumed at the finishing rate from this point. Float/Trickle Charge In standby application Exide Plante cells are to be maintained within a Float Voltage range of 2.18 to 2.25 V per cell. Trickle charging currents should be so adjusted, anywhere between the maximum and minimum allowed levels given in the table, such that individual cells remain fully charged. Quick Recharge Exide Plante cells after a deep discharge can also be recharged quickly by applying the Starting Rates mentioned in the table. However, currents will have to be reduced to the Finishing Rate once individual cells attain a voltage level of 2.35 volts. Care will also have to be taken that electrolyte temperature does not exceed the maximum of 50°C in which case the charging will be discontinued until the temperature drops down to 40°C, or at least to 45°C. Charging may be resumed at the finishing rate from this point. Equalizing charge Periodical equalizing charge at finishing rate is recommended. Recharge Instructions All plante cells should normally be floated at a mean float voltage of 2.23 + 0.02 volts per cell. A battery system consisting of 55 cells (110V) can be charged with a float charging voltage of 122 Volts (max). Maintenance of Cables: a)

Measure insulation resistance of 11 KV U.G. cable quarterly.

b)

Check tightness of Cable terminations periodically

c) Inspect cable trenches and ducts periodically and carry out rectification work of defects noticed.

56

Auxiliary Power Supply: a)

Inspect Station Auxiliary Transformer, switch gear, cables etc regularly and carry out rectification and maintenance work.

b)

Inspect all L.T. Switch gear, L.T. Cables and rectify defects noted

c)

Renew wiring, switch boards etc. as and when found necessary

Station and Yard Lighting: Station and yard lighting also requires constant attention. Repairs, replacements and renewals have to be done when found necessary. Sub station Building: a)

Carry out annual maintenance

b)

Keep the sub station building clean and neat

c)

Dampness in the Sub station building may badly affect the indoor installations like 11 kV Breakers, bus-bars, relays etc. Prevent dampness inside the Sub station.

Station Switchyard: Clear over growth of grass and vegetation from the yard. Fire Fighting Equipment: a)

Maintain register for the fire fighting equipments

b)

Weigh the CO2 cylinders monthly and keep record of weighing.

c)

Refill cylinders, when found necessary

d)

Paint CO2 Cylinders, Fire buckets and other equipments annually.

Security Fencing: a)

Carry out Annual maintenance to Security fencing.

b)

If the fencing is by Chain Link or barbed wire, check the earthing connections annually.

c)

Paint all non-galvanised metallic parts annually. ***************** 57

58

59

60

Chapter 5

Substation Earthing Introduction Earthing is a general term broadly representing grounding of electrical power system and touching of non-current carrying metallic bodies of equipments to grounded electrodes. Earthing is a system feature or parameter that attach and influences, system stability, voltage balance, voltage rise during abnormal operating conditions, fault current, harmonics, telecommunication interferences, relaying, installation cost and safety in general. Earthing associated with current carrying power conductors, usually the neutral conductor is normally essential to the security of the system and is generally known as system earthing, while, earthing of non-current carrying metal works of equipment bodies is essential for the safety of human life, of animals and of property and is generally known as safety equipment earthing. 1.1.

Statutory Provisions

Earthing system design, installation, testing and maintenance are based on the basic requirements as envisaged in the relevant provisions of Indian Electricity Rules 1958. The applicable IE Rules are Rules 32, 51, 61, 61A, 62, 67, 69, 88 (2) and 90. In addition to these rules the revised IEEE guidelines, 3.5.3043/87 and National Electric Code describe the design methods and installation procedure of earthing system for better safety. 1.2.

Power System Earthing

There are two ways in which three-phase system can be operated. They are: (i ) With isolated or free neutral (ii ) With earthed neutral However these days isolated neutral system is being rarely used, as at the time of an earth fault high transient voltages of several times the normal value will be produced and this may puncture the insulation of the system at some other location than the point of fault. This may result in the damage of the associated equipment and interruption of the supply of the system. 1.2.1. Advantages of Power System Earthing 1.

Reduction in surge voltages and consequent reduction in insulation levels.

2.

Reduced power frequency voltage of the phase conductor to the frame or body of the equipment – less insulation even for power frequency voltage – possibility of graded insulation. Possibility of easy and effective ground fault relaying – less equipment damage as the first insulation failure itself is detected and cleared. Early detection of faults reduces period and extent of outages.

3. 4.

61

Disadvantages of Power System Earthing 1. 2. 3. 4. 5.

Damages to life and property by way of electric shock and fire accidents. High magnitude of destructive earth fault current and the necessity for interruption within a few cycles. Third harmonics and its multiples drive currents through grounding conductors and give rise to unnecessary heating. Telecommunication interference due to fault and harmonic current return through ground. Low earth fault and leakage currents in L.V and M.V. system necessitate the use of low set sensitive earth fault relays and sensitive ELCBs for ensuring safety.

Protective Safety Earthing Protective safety earthing of equipments is to ensure safety of life and apparatus against the harmful effects of earth faults. The earth connection improves service continuity and minimizes damage to equipments and danger to human life. The objective of an earthing system is to provide as nearly as possible a surface under or around a station which shall be at a uniform potential and as nearly zero or absolute earth potential as possible. Further, it is to ensure effective operation of the protective gear in the event of leakage through such metal parts, the potential of which with respect to neighbouring objects may attain a value, which would cause danger to life or risk of fire. Earth Electrodes Earth electrodes are provided to dissipate fault current in case of earth fault and to maintain the earth resistance to a reasonable value so as to avoid excessive rise of potential of the earthing grid. The general practice is to design the earth electrode system for the appropriate thermal withstand capacity assuming the total fault current to be passing through the earth electrodes to the mass of the earth. This is true in the case of an earthing system which is not interconnected with neutral earthing and safety earthing are interconnected, the fault current will return to the transformer neutral particularly through the grid and particularly through the mass of the earth depending upon the value of earth resistivity. Studies in this matter show that this division is varying from 10 % to 90 %. The earth electrode system need be designed only for the portion of the prospective fault current returning through the mass of the earth. The current density permissible at the earth electrodes as per I.S. 3043 is i Where i ρ t

7.57 X 103 A/m2 √ρt = current density (in A/m) = resistivity of the soil (in Ω–m) = duration of the earth fault (in s) =

Experience indicator that this formula is appropriate for plate electrodes. 62

Types of earth electrodes 1.

Plate Electrodes The approximate resistance to earth of a plate can be calculated from R

=

ρ A

π A

ohms

where ρ - resistivity of the soil ( in Ω – m ) A - area of both sides of the plate ( in m2 ) Where the resistance of a single plate is higher than the required value, two or more plates may be used in parallel and the total resistance is inversely proportional to the number employed, provided that each plate is installed outside the resistance area of any other. The size of cast iron electrode is 1.2 m X 1.2 m in area and not less than 12 mm in thickness. The area available for dissipation is 2.88 sq.m including both sides. The earth connections should be joined to the plate at not less than two separate points. Plate electrode when made of GI or steel, shall be not less than 6.3 mm in thickness. Plate electrodes of Cu shall be not less than 3.15 mm in thickness. Plate electrodes shall be of the size of least 60 cm X 60 cm.

63

The current loading capacity of a 1.2 m X 1.2 m plate is of the order of 1600 A for 2 s and 1300 A for 3 s. Plate electrodes shall be buried such that its top edge is at a depth not less than 1.5 m from the surface of the ground. 2.

Rod or Pipe Electrodes The resistance of a pipe or rod electrode is given by :

R = 100 ρ log c 4 l ohms 2πl d Where l = Length of rod or pipe (in cm) d = diameter of rod or pipe (in cm) ρ = resistivity of the soil (in Ω.m) The diameter of the pipe has a relatively minor effect in the resistance and the resistance diminishes rapidly with the first few foot of driving, but less so at depths greater than 2 to 3 m in soil of uniform resistivity.

64

A number of rods or pipes may be connected in parallel and the resistance is then practically proportional to the reciprocal of the number employed so long as each is situated outside the resistance area of any other. In practice, this is satisfied by a mutual separation equal to the driven depth. Pipes may be of cast iron of not less than 100 mm diameter, 2.5 to 3 m long and 13 mm thick. Such pipes cannot be driven satisfactorily and may, therefore, be more expensive to install than plates for the same effective area. Alternatively, mild steel water–pipes of 38 to 50 mm diameter are sometimes employed. These can be driven but are less durable than copper rods. 3.

Strip or conductor electrodes

These have special advantages where high resistivity soil underlies shallow surface layers of low resistivity. The minimum cross-sectioned area of strip electrodes shall be not less than 25 mm2, or of mechanical protection is not provided. (Refer section 12.1.1 of IS 3043). If round conductors are used as earth electrodes, their cross sectioned area shall not be less than the sizes recommended for strip electrodes. The resistance R is given by R = 100 ρ loge 2l2 2πl wt Where ρ = resistivity of the soil (in Ω.m) l = length of the strip in cm; w = depth of burial of the electrode in cm; and t = width ( in the case of strip) or twice the diameter ( fir conductors) in cm. If several strip electrodes are required for connection in parallel in order to reduce the resistance, they may be installed in parallel lines or they may radiate from a point. In the former case, the resistance of two strips at a separate of 2.4 m is less than 65 percent of the individual resistance of either of them. Other types of earth electrodes are : 1)

Water Pipes

2)

Cable Sheaths

3)

Structural steel work

4)

Reinforcement of piles

5)

Cathodically Protected structures

65

Recommended values of earth resistance System / Installation

Acceptable limit of earth resistance

Domestic

<10 ohms

Medium voltage installation

< 5 ohms

HT consumers

< 2 ohms

Small EHT S/S and generating stations

< 1 ohms

Large EHT S/S and generating stations

< 0.5 ohms

Measurement of Earth Resistance The earthing resistance of an electrode is made up of (a) (b) (c)

Resistance of the electrode Contact resistance between the electrode and the soil Resistance of the soil

The first two factors are very small fractions of an ohm and can be neglected for all practical purposes. But the measurement of earth resistivity is done for designing the earthing system and subsequently for checking its effectiveness. Soil resistivity is generally measured with “Null detector” or Megger earth tester by four electrode method Four electrodes are driven into the earth as in Fig.1 along a straight line at equal intervals. A current is passed through the two outer electrodes and the voltage between the inner electrodes are measured. The resistivity will be proportional to the ratio of voltage and current. The resistivity of the soil can be computed as follows : ρ =

4 π S R_________ 2S _ __2 S___ 2 2 √ S + 4d √ S 2 + 4d2 Where ρ = resistivity of the soil (in Ω.m) S - distance between two successive electrodes in metres R - resistance reading or V/I in ohms d - depth of burial of electrodes in metres 66 1+

Usually d will be negligible compared to the spacing S and hence the equation is simplified as, ρ=2π SR Megger

o c1

c2 o

o p1

p2 o

S

S

S

The derivation of the above equation is based on the assumption that soil resistivity is uniform. While actually measuring earth resistivity for a sub station yard, note the following: a) Readings along the periphery and diagonals should be taken. b) Readings with inter-electrode spacing of 10m, 15 m and 20m may be taken. c) The average value of the above readings may be considered for design of earthing system d)

Resistivity measurement should include temperature data, dry or moist condition of the soil and type of soil etc Design of Earth Mat

In designing the earth mat following factors are to be considered: a) b) c) d) e)

Magnitude of fault current Duration of fault current or fault clearing time Soil resistivity of the area Resistivity of the surface material Material of the earth electrodes

The area of earth mat conductor is given by A =

K x I x √t K for various materials K Material Steel Copper Aluminium

Welded Joint 0.01222 0.0047 0.0084 67

Bolted Joint 0.0157 0.0058 0.0120

where, A = I

A area in mm2

= Fault current in amps.

t = Duration of fault current in seconds (usually taken as short time rating of the switchgear). The permissible values of step and touch potentials are given by the formulae: 165 + ρs Volts …….. (3) T E touch = 165 + 0.25 ρs Volts …….. (4) T Where ρs soil resistivity in ohm metres just beneath the feet of person (usually taken as 3000 ohm metres for crushed rock) E Step

T

=

= Fault clearing time in seconds

The resistance of the earth mat is given by R

=

ρ

ρ

+

4r

………………………(5)

L

Where R = Resistance in ohms

ρ

=

r =

Soil resistivity in ohm metres Equivalent radius of the substation area in metres

L = Total length of buried conductor in metres Another important point to be examined is whether the design is safe for sustained ground current which should be below the let go value of the body current (taken as 9 milli amperes) which gives the value of E touch as follows : E touch ( Sustained ) = (Rk + 3/2 ρs) x 9 / 1000 Volts

……..(6)

Where , Rk = Body resistance taken as 1000 ohms

ρs

=

soil resistivity below the surface of the feet.

The mesh potential of the grid should be less than the E touch ( Sustained ) E mesh =

Km Ki

ρI

Volts

……….(7)

L 68

Km - Factor depending upon the size, spacing, depth and number of parallel grid conductors. Ki - Irregularity factor ( 0.65 + 0.172 n) n being number of parallel conductors. In all the substations provisions are made for earthing the following preferably by duplicate earth connections. 1. 2. 3. 4. 5. 6.

Equipment framework and other non-current carrying parts All the metallic structures and supports Neutral point of each transformer or separate system Lightning arresters. These are usually provided with independent earth grid which in turn is connected to main grounding grid. Substation fence earthed at regular intervals Transformer rail track

It is common practice to cover the area of switchyard with about 10 cm (4 in.) of gravel or crushed rock which helps in making the area safe for operating personnel against hazard due to shocks. Installation of Earth Mat Construction Earth mat is generally designed with the following sizes of MS rods. 400 kV Substations 40 mm dia 220 kV Substations 40 mm /32 mm dia 110 kV Substations 32 mm /25 mm dia Conductor above ground level for earthing equipment structures, columns and other auxiliary structures shall be galvanised flats. Rod electrodes shall be of mild steel of same diameter as earth conductor and of length as required in the design. The earth conductors should be laid within the substation area and up to 2 metres beyond the fencing, in parallel lines in both directions, at the nominal spacing as per design. The earthing conductors may be suitably re-routed in case they foul with column foundations and equipment foundation. It is better to install the mat immediately after site levelling, but after clearly setting out the locations of columns, equipment and cable trenches. Earthing conductor shall be buried at least 600 mm below the finished ground level. Maximum spacing and minimum spacing requirements as per design shall be maintained. Wherever earthing conductors cross cable trenches, underground service ducts, pipes, tunnels, rail tracks etc, they shall be laid at a minimum depth of 300 mm below the structures. Earth risers shall be provided near the equipment foundations and for future connections. Earthing conductor around the building shall be buried in earth at a minimum distance of l500 mm. from the outer boundary of the building. In case high 69

temperature is encountered at some locations, the earthing conductor shall be laid minimum 1500 mm away from such locations. Earthing conductors embedded in the concrete shall have minimum 50 mm concrete cover. Earthing conductor crossing the roads shall be laid at greater depth to avoid mechanical stress. All ground connections shall be made by direct current electric arc welding. All welded joints shall be allowed to cool down gradually to atmospheric temperature before putting any load on them. Artificial cooling shall not be resorted. All arc welding with large diameter / thick conductor shall be done with low hydrogen electrodes. Bending of large diameter rod/thick conductor shall be done preferably by gas heating. Welding of ground connections demand extreme care, as once laid the mat and joints cannot be practically inspected. Earthing Connections Neutral points of systems of different voltages metallic enclosures, frame works associated with all current carrying equipment and extraneous metal works associated with the electric system shall be connected to the single earthing system. Steel structures columns etc. shall be connected to the nearest earthing grid by two earthing leads to the ground conductor in two directions., Metallic pipes, conduits, cable tray sections metallic stairs, hand rails shall be bonded to ensure electrical continuity and connected to the earthing conductors at regular intervals. Rail tracks within the switchyard area shall be bonded across fishplates and connected to earthing grid at several locations. At the points where the track leaves the yard area. the rail ends shall be provided with insulation, to avoid hazards due to possible transferred potential. Similarly insulated pipe sections may be inserted in the pipelines leaving the station premises. Every alternative post of the switch yard fencing shall be connected to the earthing grid. Flexible earthing connectors shall be provided where flexible conduits are connected to rigid conduits. At the connection points of earthmat with equipment earthing leads, the welds shall be treated with red-lead and afterwards thickly coated with bitumen compound to prevent corrosion. Earthmat comprising closely spaced (150 mm x 150 mm x 300 mm deep) conductors shall be provided below the operating handles of the isolators. Operating handles shall be directly connected to the earthing mat through flexible copper braids.

70

Conductors of the lightning protection system shall not be connected with the conductors of the safety earthing system above ground level. Earthing terminals of each lightning arrester and capacitor voltage transformers shall be directly connected to the rod electrodes, which in turn shall be connected to the station earth grid. Metallic sheaths and armour of all multi core power cables shall be earthed at both equipment and switch gear ends. Sheaths and armour of single core power cables shall be earthed at switch gear end only. Each earth lead from the neutral of the power transformers shall be directly connected to two rod electrodes in treated earth pits. They shall in turn be connected to the station earthing grid. Separate earthing conductor shall be provided for earthing of lightning fixtures, receptacles, switches, junction boxes, lightning conduits etc. Low voltage neutrals may be isolated from the station earth grid. This is necessary to avoid hazards due to transferred potential on low voltage feeders and secondary circuits, which serve outside the station area. But such neutrals of low voltage system separately earthed shall be treated as a live conductor. Such earthing should be located so as to minimise the danger of being contacted by personnel.

Welding Details Details of Welding to be done during earthing and grid installation are shown above . 1

71

Details of Earthing Conductor Crossing The Trench

*********************

Chapter 6 72

Transformer Accessories 1. Buchholz Relay Gas actuated Buchholz Relay is provided, in the pipe leading oil from the conservator to the main tank. The relay comprises a cast housing which contains two pivoted aluminium floats or buckets, each being counter balanced. When a slight fault occurs in the transformer, small bubbles of a gas will be generated and these will attempt to escape to the conservator. The gas will be trapped in the Buchholz relay housing, pushing down the oil level in it. Then top float or bucket which will be above the oil level will move down due to its extra weight of oil in it, and make an alarm contact. If the gas produced in the transformer is more, then the oil will be further pushed down leaving the second float also out of oil. The second float or bucket will then make trip contacts close and the transformer tripping circuit will be energised, isolating the transformer from service. An inspection window provided in the relays casting will indicate the oil level in the relay. Separate single float Buchholz relay shall be provided for the tap changer conservator, for getting alarm indication. 2. Bushings High voltage connections from transformer terminals to lines need bushings. The simplest bushing is the moulded high quality glazed porcelain insulator with a conductor through its centre. These are used only up to 33 kV level. Bushings for higher voltages are oil filled capacitor type. The bushing is constructed with layers of resin bonded paper interleaved with layers of metal foil or with paper, impregnated with conducting material. This is contained in a two part porcelain container together with an oil expansion chamber at the top. High Voltage condenser hermetically sealed. A test tap is provided for measuring tan delta and capcitance. Where bushing CTs are to be provided, suitable arrangements are provided at the lower end. The arrangement will be such that, the bushing can be removed without disturbing the current transformers. 3. Arcing Horns HBushing V Oil filled Condenser arcing horn gap setting shall be as follows. Bushing 73

Transformer Winding BIL (in kVp)

Arcing Horn setting (in mm.)

325

380

550

635

650

800

950

1250

1050

1400

4. Other accessories Other accessories of transformer include the following. a. Marshalling kiosks accommodating cable terminals, W T I and O T I., b. Valves for filtering, filling oil, draining Conservators, draining oil from tank, oil sampling etc. c. Stop valves for main conservator and diverter switch Tap changer conservator. d. Jacking pad for transformer lifting and movement during transportation and erection.I e. Bi-directional flanged wheels. f. Lifting hooks, pulling eyes etc. g. Butterfly valves for radiator shut off. h. Manholes with cover for inspection inside during major maintenance works. i. OLTC control kiosk. j. Rating plate. k. Remote Tap Changer Control Cubicle ( RTCC) 5. On-Load Tap Changers (O L T C) :-The On-Load Tap Changer consists of a high speed resistor transition Diverter Switch, Tap Selector, Driving Mechanism and external Driving shaft .The tap lead wires from the tap winding of the transformer are brought and terminated at the fixed contacts of the tap selector. Tap selector do not break load current. In order to maintain a circuit connection, the diverter switch introduces an impedance which temporarily bridges the selected adjacent contacts whilst the Tap Selector connection is transferred from the operating tap to the pre selected tap. During this operation, circulating current flows, around the bridging impedance in addition to the load current carried by the winding. Since operation of the diverter switch, involves arc interruption, the oil in this compartment becomes contaminated with carbonised particles and must be kept separate from the oil in both the main tank and the selector compartment Ref. Figure given below the following electrical sequence of operations applies in changing taps from Tap 1 to Tap 2. One selector, S 1, is on Tap 1 and the other, S2, is on Tap 2 with the diverter switch S3 connecting Tap 1 to the neutral point. Diverter contacts ‘a’ and ‘b’ are closed and load current is carried from Tap 1 through contact ‘b’. This is the running position for Tap 1. On triggering the driving mechanism, the energy stored in the spring operating

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mechanism causes the contact system to commence its travel from one side of the diverter to the other and contact ‘b’ opens. At this point the load current is carried from Tap 1, through resistor R 1 and contact ‘a’. As the moving contact system continues its travel, contact is made with ‘d’ at this point resistors R 1 and R 2 are connected in series across taps 1 and 2. There will thus be a circulating current between taps, in addition to the load current that will be carried through the mid-point of the resistors. The moving contact system continues on further to cause contact ‘a’ to open. At this point the load is transferred to Tap 2, through resistor R2 and contact ‘d’. As the moving contact system reaches the other side of the diverter switch, contact ‘c’ closes and resistor R2 is shorted out. The load current is thus carried from Tap 2 through contact ‘c’ This is the running position for Tap 2. The tap change from Tap 1 to Tap 2 as described above involves no Tap Changer Connections movement of selectors but, if a further tap change in the same direction from Tap 2 to Tap 3 is initiated, the selectors must first move from Tap 1 to Tap 3 before the diverter switch operates. Thus for a tap change in the same direction as the previous movements, the selectors move first followed by a change over of the diverter switch. However, for a tap change in the reverse direction to the previous movement, i.e. from Tap 2 to Tap 1 following the last of above actions, the selectors remain stationary and the tap change will be completed by the movement of the diverter switch. The time for a complete tap change operation, i.e., from the instant of initiation to completion, varies from 6 to 10 seconds, depending on the tap changer manufacturer and the speed of operation of the drive mechanism. 75

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Ref. Figure in page No. 200 for a TELK, O L T C (1) Mechanism Case (3) Explosion Vent. (5) Shield Ring Selector (7) Insulation Drain Pipe Switch. (9) Diverter Switch Contacts Compartment. Assembly. (11) Diverter Switch Moving Contacts. Gear Box. (13) Tap Selector Stationary Contact. Cylinder. (15) Tap Selector Moving Contacts (17) External Driving Shaft (19) Bevel Gear Box. (21) Gas and Oil Operated Relay (23) Drain Valve (25) Gas Vent (27) Tap Changer Conservator

Diverter Switch Assembly

(2) Spring Mechanism (4) Outer Insulation Cylinder (6) Insulation Shaft for Tap (8)

Insulation Shaft of Diverter (10) Transition Resistor

(12) Tap Selector Intermittent (14) Tap Selector Insulation (16) (18) (20) (22) (24) (26)

Change Over Selector. External Driving Shaft Driving Mechanism. Stop Valve. Oil Level Gauge. Air Breather .

Tap Selector Assembly

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Sequence Diagram of Tap – Changing Operations .

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Three Winding Transformers A transformer may have additional windings apart from the two conventional main windings depending upon the particular application and type of connection of the main windings. In a three winding transformer, the third winding is normally called tertiary winding and it is provided to meet one or more of the fallowing requirements: a. For an additional load which for some, reason must be kept isolated from that of secondary, b. To supply phase compensating devices, such as condensers, operated at some voltage not equal to primary or secondary with some different connection. In star/star connected transformers, to allow sufficient earth fault current (zero sequence component current) to flow for operation of protective gear to suppress harmonic

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