Maintenance Of Sub Station Equipment.ppt

Maintenance of Sub Station Equipment

Er K. V. Surya Prakasa Rao, Former S.E.(Transco)

Objectives of Maintenance • Helps in Trouble free operation of equipment • Minimizes break downs and interruption to consumers • Reduces O & M costs (Expenditure) • Helps in achieving expected period of services • Improves creditablity

Types of Maintenance • Preventive Maintenance • Predictive Maintenance • Break down Maintenance

General Complaints at Sub Stations • • • • • • • • • • • • • • •

Burning of closing coils and tripping coils. Failure of Trip / Alarm Bells / Hooter. DC fuse blowing or DC MCB tripping. Complaints with battery chargers – Non functioning of chargers, PCB failures etc., Erosion of battery connecting strips. Oil leakage from power transformers and instrument transformers. Mechanical Problems with circuit breakers mechanism, failure of spring charge motors. Gas leakages from SF6 circuit breakers. Failure of vaccum in vaccum circuit breakers. Improper closing of Isolators, earth switches Red-hot's at connecting clamps, Isolator contacts. Burning of relay contacts. Power and control cable failures. Problems in communication system Mal operation of protective relays

Good O&M Practices Cont--At Substations • Adequate housekeeping • Adequate illumination in the station • Cable trench covers • Panel doors shut properly. • Cables & equipment in poor condition • Adequate sizing of all equipment • Protections are functional • Proper maintenance practices

5

Good O&M Practices Cont--• Load balancing • Releases and settings for air circuit breakers (ACBs) • Relay Co-ordination • Safety of men and material. • Work-to-permit • Data Logging • Predictive, preventive and break down maintenance • T&P 6

Good O&M Practices Cont--• •

• • • •

Infra red cameras for cyclic inspections of o/h lines, Substation bus bars for hot spots caused by faulty terminals. Live line washing for cleaning overhead lines and open terminal substation insulators by use of purified water with high resistance value in a fine spray from well-earthed nozzles. Regular switchgear site tests that would, in general, include: Cracking and chipping of bushings Checking of foundation bolts Tightness of terminal connections and bolted joints

7

Good O&M Practices Cont--Painting and corrosion protection Cleanliness and housekeeping Lubrication of moving parts of circuit breakers Oil levels Quality of SF6 gas for humidity, pressure or density Verification of correct operating sequence Closing and opening times Operation of auxiliary circuits Die-electric strength and contact resistance checking Earth resistance measurement Checking earth connections periodically 8

Creating Data Base • • •

• • • • •

Record keeping of faults, equipment maintenance programs, and modifications carried out and solutions applied. Record of protective and isolating devices installed and their relay settings. Record of factory tests, site tests, inspection and commissioning protocols. Framing guidelines, periodicity, check-list and computerized maintenance schedules for key substation equipment like: Circuit breakers Battery, battery room, float-cum-booster chargers Distribution transformers All the data would be formatted and accountability fixed for ensuring compliance. This data would form historical and computerized base for archives of the utility. 9

Safety • Lay down procedures for PTW (Permit to work) so as to ensure safety of the personnel and the public. • Procedure for cancellation of PTW and issue of “line clear”.       



Cable live terminals are properly insulated. Do not Provide two parallel ACBs for one transformer.. Do not Provide one isolators for two transformers. Handle to switch On/OFF the Breaker is in place Arc chute is existing Cover plate is in place. Termination of the cables is in proper condition with the provision of plate washer, socket, plate washer, spring washer, nut and self-locking check nut, in that order. Availability of all the safety equipment. Awareness creation to the workmen about safety 1

1. A. SULPHER HEXA FLOURIDE (SF6) GAS CIRCUIT BREAKER 1

SF6 density monitoring

2

Measurement of Humidity of SF6 gas

3

Acid concentration measurement of SF6 Gas

Yearly

4

Air content measurement of SF6 gas

Yearly

5

SF6 gas leakage test

Yearly

6

a. Checking of insulation of control circuit wirings b. Measurement of Insulation resistance across contacts (with breaker off) and pole to earth with breaker on Evacuation, recycling and refilling of SF6 gas Checking the Br.level with spirit level

Yearly

Check the complete breaker for SF6 gas leakage including the seal assembly of driving rod. If any leakage is detected the same should be arrested in consultation with manufacturer and after arresting the leakage the SF6 gas pressure is to be brought upto rated pressure by topping up SF6 gas Minimum 2 Mega Ohms with a 500 V megger

Yearly

To be done by 2.5 KV megger or above

7

8

Daily in each shift Yearly

5 yearly

Yearly

If appreciable change is observed compared to earlier readings, leakage check to be carried out Use Dew Point meter. If deviation from standard norms is observed, the evacuation, recycling and refilling of SF6 gas is to be carried out

This may be done whenever the humidity in SF6 gas in excess of permissible value.

B

1

2

3 4 5 6 7

8

AIR BLAST CIRCUIT BREAKER Checking of air pressure

Daily in each shift

If appreciable changes is observed, compared to earlier readings, leakage check to be carried out Draining out condensed water from HP Twice daily at 6 AM Frequency of draining may be cylinders and 6 PM increased during rainy season and winter Draining out condensed water from circuit Monthly -dobreaker tanks Visual checking of oil level in the dash pot; Monthly checking of manometer readings Capatiance measurement for Grading capacitors Measuring insulation resistance of control circuit wiring and measuring IR values

Yearly Yearly

Minimum 2 Mega Ohms with a 500V Megger

Measurement of IR values across contacts (each break) with breaker off and pole to earth with breaker ON Checking the Br. Level with spirit level

Yearly

Minimum 2.5 KV megger to be used

Yearly

C. VACCUM CIRCUIT BREAKERS 1

Measuring I.R values of the poles with breaker open (i.e., across contacts) and breaker closed (pole to earth)

Monthly

Record the value and compare with earlier values

2

High voltage test across contacts of each vacuum interrupter

Yearly

To ensure dielectric strength of vacuum interrupters

3

Measuring insulation resistance of control circuit wiring

Yearly

Minimum 2 Mega ohms with a 500 V megger

4

Anti condensation heathers

Daily

Ensure that heater is on and is working

D. MINIMUM OIL CIRCUIT BREAKER

1

Checking of oil leakage from guage glass, drain valve and other joints

2

Testing of oil samples for dielectric strength

Quarterly

3

Measuring IR values across contacts with breaker off and pole to earth with breaker on

Quarterly

4

Checking the Br. Level with spirit level

Daily If the valve is low oil should be replaced with fresh tested oil. Oil shall also be replaced after 12 trippings on fault or as per recommendations of manufacturer. Oil filling instructions given by the manufacturer s hall be followed.

NOTE : Before closing MOCB on a fault, the operator should ensure that adequate oil level is available in the breaker by observing the guage glass and if adequate oil is not available, the breaker should not be test charged. Yearly

E. GENERAL MAINTENACE (Common to all Breakers) 1

Yearly

2

Operating time including Pre-insertion resistor (wherever applicable) Contract travel

3

Speed curves

Yearly

4

Pole discrepancy (for individual pole operted)

Yearly

5

Yearly

6

Functional checks duty cycle operation including rapid reclosing Checking of all operational lock outs

7

Checking of all interlocks

Yearly

8

Contact resistance measurement

Yearly

9

Checking of pressure setting sand calibration of meters Through checking up of close and trip coils

Yearly

10

Yearly

Circuit breaker analyser may be used. Results are to be recorded and compared with the commissioning test result. If there is difference in the values, or if the values are beyond limits prescribed by the Manufacturer, the manufacturer has to be informed and all the operation checks are to be made in the presence of manufacturer & remedial measures are to be taken

Yearly

Yearly

Not to exceed 100 Micro ohms (by measuring DC voltage drop)

Resistance and pickup voltage are to be checked and compared with previous values (pre- commissioning results)

II OPERATING MECHANISM A

SPRING OPERATING MECHANISM Checking conditions of spring tension

Quarterly

Lubricating the motor bearings

Quarterly

I.R values of motor winding

Quarterly

Checking interlocking mechanism

Quarterly

B

PNEUMATIC OPERATING MECHANISM

i)

Checking of air pressure

ii)

Draining of moisture from air receiver, visual inspection cleaning

Daily/ Weekly

iii)

Checking of tightness of compressed air pipes work and coupling Checking of cumulative running hour of compressor Checking of air pressure drop during duty cycle operation

Yearly

iv) v)

vi)

Check for compressor running sound

vii)

Lubrication, greasing of moving parts of the compressor Blowing out dust in the windings of compressor motor Checking for proper functioning of safety valves in compressed air system Overhauling of compressor motors

viii) ix) x) xi) xii)

Checking interlocks low and high air pressure and correct operation Checking of various pressure switches for proper operation

Daily

Once in each shift Yearly

Monthly Yearly

Monthly yearly Yearly Yearly Yearly

Daily for central compressor unit and weekly for individual compressor unit

Action to be taken as per manufacturer’s recommendations Pressure drop in the air pressure to be checked for each sequence of operation and to be compared with data sheets

C

HYDRAULIC (ELECTRO) OPERATING MECHANISM Checking of oil pressure and level

Daily

Visual check for oil leakage

Daily

Checking for cumulative pump running hours Replacement/topping up of oil

vi)

vii) viii)

Visual checking of accumulator, main valves and operating cylinders Checking of printing pressure of nitrogen, nitrogen replenishment (if necessary) Checking operating pressure of safety valve

If appreciable change is observed as compared to previous readings, leakage check is to be carried out

Once in each shift As and when required Weekly Yearly

Yearly

Checking oil pressure drop during duty cycle operation

Yearly

Drop in pressure of oil to be checked for each sequence of operation and to be compared with data sheets.

ix)

Checking condition of oil

Yearly

If the oil is found to be contaminated, drain the oil and clean both oil tank and oil filter.

x)

Overhauling of oil pump

Yearly

xi)

Checking for tightness of oil pipe work and couplings

Yearly

MAJOR MAINTENANCE AND OVERHAUL OF CIRCUIT BREAKER

1

Evacuation, recycling and refilling of SF6 gas

2

Replacement of gaskets

3

Operating rod cleaning and inspection

4

Dismantling, cleaning operating mechanism

5

Interrupting chamber overhaul

and

reassembly

5 yearly

of

Assistance manufacturer availed

may

of be

3

Maintenance schedule for Batteries/Battery chargers/ DC Distribution system

S.No 1

Item of maintenance Cleaning of battery surface joints and all connections

Periodicity Daily

2 3 4

Daily Daily Daily Weekly Weekly

-

Weekly

-

8

Specific gravity measurement of pilot cell Voltage reading of pilot cells Visual checking of battery room ventilation and lighting Checking of electrical connections for tightness Application of petroleum jelly to joints and cell connections Checking electrolyte level and topping up with DM water Shallow discharge (10 hour rate) and recharging

Yearly

9 10

Readjustment of specific gravity Checking of healthiness of AC supply to the charger

Yearly Daily

The discharge to be done for a specified period -

5 6 7

Remarks To be done on rotation so as to cover all the cells in10 days -do-do-

11 12

Checking of float current (DC) Output voltage check

13

Charger cleaning with blower

14

Checking control chords

Yearly

15

Complete overhaul

Yearly

16

Checking for DC earth fault

Daily

17

Check of emergency lights

Daily

18

Auto-start and running up of DG set Checking DC distribution i. External

Daily

19

charging

ii. Internal 20

21

Checking of all electrical connections of charger panel for tightness and cleaning Calibration of all meters and relays in the charger and the DCB Bs

Daily

-

Daily

-

Fortnightly

-

Earth fault to be immediately Fused lights to be immediately -

Daily Quarterly

-

Quarterly

-

Yearly

-

attended replaced

S.No.

4. Maintenance Schedule for Lightning Arrestors Item of maintenance Periodicity

Remarks

1

Visual Inspection

Daily

2

Surge counter reading

Daily

3 4

Leakage current reading and analysis Earth resistance

5

Leakage current analysis

6

IR value

Yearly

7

Connections

Yearly

Compare results with those obtained previously -

8

Calibration of leakage current ammeter Cleaning of Insulator

Yearly

-

Yearly

-

9

Once for shift

If chipping / crack in the insulators is observed replacement action may be taken Should be in green zone

Quarterly

-

Quarterly

For gapless lighting arrestors only

5. Maintenance Schedule for Isolators S.No.

Item of maintenance

Periodicity

Remarks Visual inspection for cleanliness of insulation, proper alignment of contact arm blades, any abnormal noise and arcing will be carried out. Opportunity of shut down should be availed of whenever possible, the checks and measurements should be performed without disturbing the connections

1

Visual inspection

Daily

2

(a) Main contacts checking including earth switch high voltage terminal tightning contact resistance checking including cleaning and lubrication of main contacts

Yearly

2

(b) Main blades and main contacts checking of (i) alignment (ii) bolds, nuts, washers, cotterpins, terminal connectors, are in place and tight. Examine the contacts, heck to determine that they are aligned and contact surfaces bear with a firm uniform pressure, check contact surfaces, replace any pitted or burnt contacts or smoothen down the surface of the contacts with fine sand paper depending upon the condition of the contacts. See the bolts, nuts, washers, cotter pins and terminal connectors are in place and tight. If the switch in group operated inter phase linkage, line operating parts, rods, levers, bearings etc. should be cleaned and lubricated, as required to the manufacturers recommendations

Yearly

3

Operating mechanism Checking of  Linkages including transmission gears  Stopper bolts  Limit switch setting  Greasing of drive  Greasing of auxiliary switch contacts  Position and tightening of cable glands (before start of the rainy season)

Quarterly

Check the isolator operation. If the operating efforts appear to be excessive check the rotor bearings, all the linkages for the proper operation

Yearly

Opportunity of shut down should be availed of whenever possible The check should be done for any arcing marks on contacts, burning of switch housing etc.

4

Insulators MOM box cleaning and lubrication of operating mechanism hinges locks, joints on levers, etc, check all mounting booth for tightness

5

Visual check of Auxiliary contact

Quarterly

6

Checking for proper functioning of space heaters, illumination etc.

Quarterly

7

Checking of electrical/ mechanical inter locks

8

Earth-switch – checking of  Alignment of earth blade  Contact cleanliness  Correct operation of earthin switch  Alluminium/Copper flexible (if provided) Checking of earth connections

9

 

Structure MOM Box

Yearly

Yearly

Yearly

Standard Acceptance Norms for Isolator during preventive maintenance S.No

1

2

Type of Isolator and name of Manufacturer

Voltage (KV)

Current rating/ Value of contact resistance Between terminal pads

220 KV HEB  SMC  HIVELM

245 KV 245 KV

1600A/100 micro ohms 1600A/175 micro ohms

13 KV HEM  BIMCS  SSPL

145 KV 145 KV

1250A/146 micro ohms 1250A/200 micro ohms

NOTE : The values of contact resistance given above are the measured during type tests, the measured values during site tests should be within 120% of the values indicated.

S.No 1

6. Maintenance Schedule for Current Transformers Item of Maintenance Periodicity Remarks

2

Visual check Daily (Porcelain, Unusual noise, discolouration of terminals etc.) Oil leakage Daily

Visual check only

3

Oil level

To be recorded

4

Operation check

6

Space heater and Monthly lighting of marshalling box Cleaning of Half- yearly marshalling box and junction boxes All connections Yearly

7

IR value

Yearly

Compare with commissioning results

8

Earth resistance

Yearly

9

Tan Delta test

Yearly

5

Monthly

Check for looseness pretest

Compare with factory test results

S.No

Maintenance for CVTs Item of Maintenance Periodicity

Remarks

1

Oil leakage

Daily

Visual checks

2

Cattering sounds

Daily

3

Oil level

Monthly

If present, measure capacitance and compare with the designed value To be recorded

4 5

Earthing of PLCC link (in case it is Monthly not being used) H.F. Bushing Monthly

6

Spartik gap cleaning

Yearly

7

Cleaning of insulator

Yearly

8

Capacitance measurement

Yearly

9

Earthpit resistance

Yearly

Check for breakage If accessible

any

Compare with the factory test results/designed value

8. Maintenance schedule for earth pits in sub-stations S.No

Item of Maintenance

Periodicity

1

Watering of earth pits

Daily

2

Measurement of earth Half-yearly resistance of individual pits

3

Measurement of Half-yearly combined earth resistance of all the pits

4

Checking of Quarterly interconnections between earth pits and tightness of bolts and nuts

Remarks

GUIDELINES FOR SHIFT PERSONNEL TO THE EHT SUBSTATIONS

•

Special Note : Complete and comprehensive record of pre-commissioning test result for each equipment is to be maintained and periodical test values must be compared with the pre-commissioning Test results for taking appropriate action. The following are guidelines for maintaining voltage and specific gravity of the cells at a uniform value : – When the battery is to be stored for long period, before putting into service, necessary care is to be taken for proper storage, other-wise there may be possibility of oxidation which may have adverse impact on the porosity of the plates. The Cells will develop uniform poraoity only after repeated charge/discharge cycles and adjustment of specific gravity has to be made thereafter in all the Cells.

•

– Variation in the quantity of Distilled water while topping up or failure to tope up some cells may also contribute to the variations in cell voltage and specific gravity • – After installation of the battery, if voltages and specific gravities are not uniform and varying then equalizing charge shall be made by adjustment of electrolyte, ie., in addition or removal of electrolyte and/or addition of Distilled water is made until the specific gravities are uniform. This adjustment may be repeated after some cycles of operation ie., after 30 or 45 days. • – Batteries should not be kept under continuous Boost Charge. Batteries shall be on float charge only. After sufficient discharge of the Battery (as indicated by the Voltage and specific gravity of the Cell, Battery shall be kept under Boost charge for the required number of hours only to get the normal voltage and specific gravity.

•

• • •

• • • • • •

• •

Sub-station layout in detail including various equipment, particulars of Power Transformers, Breakers, CTs, PTs, Isolators, LAs, Batteries and Battery Charges, Station Transformers, Fire fighting equipment, Capacitor banks etc., In-feeding sources Outgoing feeders and important load centers connected to their sub-station Normal infeed and alternate in feeds – Comprehensive idea of the grid and location of the substation with reference to grid. A grid map showing the in-feeding sources and important load centers connected to the substation to be prepared and displayed. Importance of various equipment available in the substation including capacitor banks Thorough knowledge of normal operations and emergency operations to be carried out. Battery charger operation – change from Float to Boost and Boost to Float rates changing in each mode. Paralleling and deparalleling operation of transformers Operating instructions for all contingencies including issue and receipt of line clear. Lock-out values of gas pressures minimum air pressure where compressors shall start automatically and hydraulic fluid pressure where pump shall start automatically for different makes of Circuit breakers. A chart showing make, type, reference to P.O of power transformers, and breakers and the available and set parameters such as temperature alarms, trips Auto start and stop of cooler fans in respect of power transformers and gas pressures, air/hydraulic pressures, densities in respect of breakers shall be displayed for ready reference. CT available and adopted ratios, relay settings meaning of relay indications on all circuit breakers. Bucholtz and differential relay indications. All annunciations and meaning and consequences of each

•

• • • • •

• • •

Knowledge of all the relays and their functioning, purpose and shall be able to interpret correctly to decide whether the equipment can be charged again or to be kept isolated for further inspection/checkup by the concerned Maintenance/MRT personnel (especially when lockout relays, pressure relief relays, temperature relays, bucholtz relay sand differential relays operate) The works being done by MRT/Maintenance staff in the sub-station whenever they attend to the same shall follow the works being done by them. While taking over the shift duty, he shall go through the entire log of operations from the time last handed over to charge to know the important events happened, works done, condition of the equipment, line clears pending etc. While taking over shift duty he has to check condition of communication equipment, lines, battery charger, batteries, feeders and transformers, breakers as pressures, air pressures, healthy trip circuits, any relay indications not reset etc. Whenever any feeder/equipment is loaded beyond normal load due to emergencies, such equipment shall be kept under close observation until normalcy is restored, temperature on transformers under such overloading conditions and cooler fans operation shall be constantly monitored. In case of certain emergencies where operations started in particular shift are likely to be carried out during next shift the personnel of both the shifts shall be available till the operations are completed and normalcy is restored. Shift duties should not be handed over to successor staff in the middle of an emergency. Whenever new ADEs/AEs are posted to shift, senior personnel of other cadres in the Station shall be attached to them atleast for a fortnight, so that, new personnel get themselves acquainted with the operations and equipment Red hot joints at the substation shall not be continued even for brief periods. They shall be attended on priority. Each day during evening peak hours, the yard lighting should be switched off for a while and bad clamps joints should be identified and intimated to maintenance staff. Operating instructions shall be available in the control room on the shift incharge table. All the shift supervisory personnel shall sign in the operation instruction booklet to the effect that they have read the same and understood them completely.

CHECKS & OBSERVATIONS TO BE MADE DURING SHIFTS

•

• •

• •

•

Voltages 400KV,220 KV, 132 KV, 33 KV and 11 KV shall be observed and to be maintained nearest to the rated values by operation of tap changers of transformers. A little higher voltage is to be maintained (not exceeding 10%) so that tail end voltages are normal. Constant monitoring of capacitor banks. Healthy trips of all the breakers. Healthy trips of all breakers shall be checked every hour. If healthy trip fails, the breaker shall immediately be hand tripped, fault in the healthy trip circuit shall be immediately attended and rectified. Only after ensuring DC supply, the breaker shall be closed. Healthy trip indicating lamps shall be in working condition. Batttery voltages, DC leakage, Charger condition, AC switch to charger (once in a shift) and observe any fall in DC battery voltage. Communication equipment such as PLCC, P&T and wireless shall always be in working condition. They are to be treated on par with all other equipment at the Substations. Check the loading on the transformers and capacitor banks. Load current in all the three phases and in neutral circuit (wherever available) to be checked. Any difference in phase currents shall be brought to the notice of maintenance staff and the concerned transformer or capacitor bank shall be cut off from service for detailed examination.

• Note the oil and winding temperature and physically check-up by feeling the temperature of transformers by hand. • Note the tap position of various transformers. The tap position of the transformers in parallel shall be same i.e., the voltage corresponding to the tap shall be same. • Note the oil levels in conservator tanks • Check up for any oil leaks and red hots. • Check-up dehydrating breathers of transformers • Ensure that alarams and indications on annunciation panel are working • Check the diesel generator and trial run for 5 minutes daily once in the morning shift and note down the voltage. • Check the condition of Air compressors provided either common or separately for each breaker and drain the moisture accumulated once in a day during morning shift. During Rainy season this should be done once in each shift. Drain sufficient air to ensure auto start of compressors. • Running hours of each station compressors to be noted periodically and they shall be changed ‘stand by’ to ‘running’ to ‘stand by’. Register of running h ours of each station compressor shall be maintained.

• • • • • • •

• • • • • • • • •

• •

Check the entire yard for any unusual sounds, sparkings and red hots during evening shift. Compressed air lines to be checked for any air leakage. The oil levels in air compressors are to be checked Check oil levels and condition of breather if available for the MOCBs Check up gas pressures in SF 6 breakers. Check up for any sparking and flash over marks in the earth pits specially whenever feeders trip on faults. Watering of earth pits to be done daily. Ensure proper working of Energy meters on all CBs. If the energy meters are slow or not working, check the potential supply to the meters, fuses etc. Ensure the tripping of breakers on activation of relays whenever the LCs are issued on breakers. When LC is issued on Power Transformers, Alarms and Trips are to be invariably checked up with the assistance of MRT/maintenance personnel Check-up fire fighting equipment once in a week. OLTC counter reading to be noted. Ensure that lightening arrestor’s micro ammeter reading is in safe zone (Green) Whenever feeders trip, find out the position of breakers at other end and note down relay indications if any at other end. Whenever the relays of feeders/Transformers become due for periodical testing intimate the same to Divisional Engineer in charge of sub-station and give a message to MRT for testing the same. Check-up the marshalling boxes of breakers, CT junction boxes, PT junction boxes etc. and ensure that no creepers can enter into the same. Check up the batteries, exhaust fans in the battery room for proper operation once in every shift, and report any abnormalities to the maintenance staff immediately. Ensure that hind doors of al control and relay panels are properly closed, and the panels are properly sealed from the bottom and made vermin proof.

MAINTENANCE SCHEDULE OF E.H.V. POWER TRANSFORMERS

Sl. No .

Item of Maintenance

Periodicity

1. Checking the colour of silicagel in the breather and replacement or Daily reconditioning if colour changes from blue to pink say about 50% of the total quantity. Checking up the oil level of the oil seal (to be upto the level marked in the cup) 2. Checking of oil level in a) Main conservator b) OLTC conservator c) bushings Daily and examining for leaks of oil 3 Visual check for overheating if any at terminal connections and checking for Daily each shift unusual internal noises 4 Checking for noise and vibrations or an abnormality from oil pumps and cooling Daily fans 5 Checking up of oil and winding temperatures Hourly 6 Checking for pressure relief explosion vent diaphram from cracks

Daily

6 Forced cooling system: Checking for leakage of water into cooler (forced (a) cooling system by oil pumps) 7 Cleaning of bushings. Inspect for any cracks or chippings of the porcelain

Daily Monthly

8 Ensuring that oil comes out when air release valve is opened (of the main tank)

Quarterly

9 Measuring insulation resistance of windings with an appropriate Megger (note down oil temp.) 10 Checking up of winding and oil temperature. Bucholtz and surge relay and oil (a) level trips for correct operation

Quarterly Quarterly

Remarks

Sl.No.

Item of Maintenance

Periodicity

11

OLTC oil test for BDV and moisture content. Ensure oil level in OLTC

Quarterly

12

Main tank oil testing for BDV and moisture content

13

14 15

Half yearly

a) Checking of Bucholtz relay for any gas Quarterly or during fault collection and testing the gas collected Half yearly or during b) Checking of operation of Bucholtz relay by shutdown air injection Monthly or as when shut c) Noting the oil level in the inspection glass or down availed Bucholtz relay, arresting Tap changer a) Lubricating / greasing all moving parts

Quarterly

Checking of all connection on the transformer for tightness such as bushings, tank earth connection, etc.

Quarterly

Remarks

Sl.No.

Item of Maintenance

16

Forced cooling system a) Megger testing of motors (pump) lubricating the mechanical parts and cooling fans b) Cleaning of water jacket c) Cleaning of water jacket circuits of oil pumps and cooling fans for earth leakage overload trip alarm, etc. d) Checking of interlocks for pumps. Cooling of interlocks for pumps. Cooling Air Blast fans for Auto Start and Stop operation at correct temperature setting and for manual operation

Periodicity

Remarks

Yearly Quarterly Quarterly Quarterly

17

Oil level in oil seal and replacement

Quarterly

18

Testing of oil for dissolved gas analysis for 100 MVA and above. If the results show abnormality, frequency of DGA may be increased as per the recommendations of R&D

Half yearly

19

Pressure testing of oil coolers

Half Yearly

20

Testing of motors, pumps and calibrating pressure gauges, etc.

Half Yearly

Other Transformers such as 50 MVA, 31.5 MVA and 10/16 MVA Trs. Which are in service for more than 5 years

Sl.No.

Item of Maintenance

Periodicity

21

Overhauling of pumps, motors and cooling fans

22

Testing of oil in main tank for acidity, tan delta, IFT and resistivity

Yearly

23

Bushings testing for tan delta

Yearly

24

Calibration of oil and winding temperature indicator

Yearly

25

Measurement of excitation current at low voltage at normal tap and extreme taps

Yearly

26

Measurement of DC winding resistance

Yearly

27

Ratio test at all taps

Yearly

28

Checking the bushing CT for WTI for correct ratio

Yearly

29

OLTC

a) Inspection of contacts in diverter

30

Yearly or as and when necessary

Depending upon the no. of operations as recommended by the manufacturer

b) Driving mechanism visual check-up, overhauling if necessary

Yearly

a) Tap position indicator

Yearly

b) Checking for proper working of remote tap position indicator, remote winding test indicator

Yearly

Remarks

Sl.No.

Item of Maintenance

Periodicity

31

Operating of Bucholtz relay for alarm and trip by draining of oil and injection of air with cycle pump

Yearly

32

Checking for leakage in air cell (for transformers fitted with Air Cell)

Yearly

33

Oil level in Thermometer pocket top up if required

Yearly

34

Bushing partial discharge test and capacitance

Once in 5 years

35

General Overhaul a) Core tightening

Once in 10 years

b) De-sludging / washing of windings

c) Inspection of core and windings d) Replacement of all gaskets e) Check core bolt insulation 36

Filtration of oil

Whenever oil test results are below permissible limits

Remarks

Electrical Power Systems – Certain Standards, Do’s, Dont’s and facts • Today’s transformers shall not be overloaded even for short durations also. This is more critical in the tropical areas where atmospheric temperatures rise more than 40° C • The Guaranteed temperature rise of the transformers at rated load is 50° C above ambient temperature. • The Flash and fire point temperature of transformer oil in a new transformer as received from the manufacturer is about 120° C to 125° C. • When the transformer continues in service there is likelihood of decrease in flash and fire point temperatures of the transformer oil. • When we operate the transformer at 110 % of its rated load (10% overloading) the losses in the transformer will increase by 21% above the rated full load losses. • There by during 10% overloading, transformer oil temperature also may increase by 21%. • There by on over load especially during summer seasons the transformer oil may flash inside the tank and transformer do fail. • Transformers are not to be operated on over voltages also. Operation of Transformers on over voltages increases iron losses , leads to saturation of core and strain the insulation, causes huge humming & vibrations in the core and continuous operation of transformers on over voltage may fail the transformer.

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The maximum optimum loading on the transformer shall not be more than 80% of its rated capacity. Efficient loading on the transformer is about 50% of its rated capacity The sustaining time of the transformer at maximum rated fault current (short circuit current) is not more than 2 seconds. Frequent closing of circuit breaker of a feeder on fault will damage the winding and insulation materials etc. of the transformer. A Silicagel breather without oil/to the level , in the oil cup or without oil cup itself is not to be used. An oil cup with oil to the level of the Silicagel breather is a sort of oil seal between the atmospheric air and Silicagel container. An un cleared earth fault in the system do cause high temperatures in the transformer feeding that fault. So the thermometers to the transformer are essential to be in working condition and properly set for alarms/ tripping of circuit breakers connected to the transformer. Low oil level alarm for the conservator tank shall be in proper working condition. Oil leaks from the transformer is dangerous to the transformer since moisture in the atmospheric air outside the transformer will be absorbed by the leaking oil and pass the same moisture in to the transformer oil and winding insulation . Leaking oil comes in contact with oxygen in the air and acidity of oil may increase Permitting the moisture in to the transformer oil is a sort of injecting slow poison to a living creature.

• In an in service transformer the moisture content in the core and winding is almost 4 times to the moisture in the transformer oil. • Filtration of transformer oil is carried out generally before putting a new transformer into service or when oil condition in an in service transformer is detoriated. When filteration of the transformer oil in a transformer is carried out the IR values of the transformer as well as BDV values of the transformer oil are to be monitored during the filteration. Only When both the values are improved to the required level then only filteration is stopped and other tests are to be carried out and transformer may be charged when all the values are proper When the transformer is taken in to service after filteration continue the same atleast for 15 days in service and then IR values of transformer and BDV values of the transformer oil are to be tested and compared with the values obtained after filteration before charging the transformer. If there is substantial decrease in the BDV of oil after 15 days operation, it shows that during filteration moisture content in the core and winding is not eliminated fully. So again the transformer oil is to be filtered. The filteration of power transformer is to be carried out with high vaccum filter. During filteration the oil temperature shall be maintained at 60° C only , and the vaccum shall be maintained at 760 mm Hg.

• The oil in the “on load tap changers” are to be replaced once in a year with new, filtered tested transformer oil irrespective of the operation of the OLTC. Total maintenance of OLTC is to be done once in year or whenever 5000 operations are carried out whichever is earlier , or as per manufacturer’s guide lines. • The contact resistant of the circuit breaker, isolators in closed position shall not be more than 25 micro ohms. • Higher resistance in the above equipment will cause higher I2R losses and higher voltage drop when they carry load currents. • The measurement of contact resistance of the circuit breaker and isolators shall be done with a precession micro ohm meter.

• There are no gauges provided to the vaccum circuit breakers to measure the vaccum in the vaccum bottles. Lower vaccum/ no vaccum in the vaccum circuit breakers do cause passing of high voltage supply in the breakers even in open condition so the vaccum integrity of the VCB shall be checked periodically with a vaccum checker. Where vaccum checker is not available the circuit breaker is to be tested across each limb in open condition with a 2.5 KV megger across its contacts. The value shall be more than 3000 mega ohms. • One terminal of the secondary cores of the instrument transformer shall be earthed to safe guard the instruments (relays and meters) connected and the persons nearer to the instruments during the failure of instrument transformers. • The auxiliary contact of the circuit breaker ( NO contact) connected in series with trip coil shall not be by- passed. It is for the protection of trip coil as well as that of “final trip relay “contact. • In a 33/11 KV Sub Station with 24 Volts battery system each breaker shall have independent 24 Volts battery source ( Charger and Battery) This is more specific where ever Static / Micro processor based programmable protective relays are used. They pose a DC burden of 10VA per relay on the system. There by each relay draws 0.5 AMPS DC current at 24V as long they are in service.

• Where ever DC leakage Milli Ammeter are provided on the DC panel in the sub station they are to be monitored regularly. If DC leakage is more than 10 milli amps it is an emergency for the sub station and the DC leakage is to be rectified on war footing. • Once in a month the DC positive and negative are to be connected to the earth through test lamp, one after another, to ensure that there is no DC leakage in the system as well as DC leakage milli ammeter is working. • Continuous DC leakage in the DC system do cause reversal of polarity of the battery cells. There by during a fault on a feeders breakers may not trip. • During daily, weekly testing of battery cells AC supply to the battery charger is to be kept off and battery cells are tested on load. • During normal conditions of the Sub Station Charger will be supplying DC power to the System components and Indication system etc on the panels. Battery will be in floating condition. During tripping of circuit breakers on fault charger may not support the battery. Battery will be carrying out the automatic tripping of circuit breakers on fault without the support of the charger in general.

• If in a Sub Station the specific gravities of battery cells are falling requiring boost charging of the batteries frequently, then the DC load requirement of the protective system in the substation during service is to be compared with the charger output DC amperage setting and actual DC output during operation, if required the charger output DC amps may have to be increased to the requirement of substation DC loads. • In a 33/11 KV Sub Station the 11 KV feeders shall be provided with 2 nos over current Instantaneous, 1 no Earth fault instantaneous relays along with IDMT relays. The plug setting of the instantaneous earth fault relay is to be set as that of IDMT earth fault relay. • In a 33/11 KV Sub Station the Earth fault relay provided on the HV side of 33/11 KV Power Transformer of Delta/Star type will protect only 33 KV side up to and inclusive of delta winding in the power transformer. So zone of protection of earth fault relay on HV side of power transformer is limited to delta winding of power transformers, as such in a way it is a restricted earth fault relay. Hence the time lever setting of E/F relay on HV side of the power transformer in a 33/11 KV Sub station can be kept minimum to have proper protection to the power transformer HV winding. • From the above it is evident that in Delta/Star power transformers at the 33/11 KV Sub Stations the star winding is not protected by earth fault relay on H.V. side.. Hence the 33/11 KV Power Transformer are to be provided either with differential relays or restricted earth fault relay on LV side of the Power Transformer. • Non tripping of circuit breaker of capacitor bank on failure of incoming supply to S.S. or on tripping of L.V. breaker is a serious complaint It is to be attended immediately, if not, capacitor bank is to be isolated from busbars until defect is rectified

Distribution System – Operation – Certain Points • Low voltages and High System Loses are the two main problems in the distribution system in India. The reasons are: a) Over loading of Distribution Transformers and Distribution lines. b) Huge amount of Inductive loading due to agriculture pump sets. Improvements required to the system: a) Adopting HVDS completely in the distribution sector. b) Reconductoring:- Replacement of 11 KV & 33 KV old conductors with higher size conductor. c) Reducing the length of 11 KV lines in rural sector to 5 kilometers or less by installing more number of 33/11 KV substations. d) Providing of direct 33 KV lines from EHT substations to 33/11 KV substations

Operating 11KV and 33KV feeders nearer to unity power factor by installing required number of LT capacitors and 11KV automatic switched line capacitors. It is felt preferable to use following conductors for the various lines: • 33 KV lines – 120 sq. mm / 150 sq. mm conductor, • 11 KV lines – 80 sq. mm / 50 sq. mm conductor, •

Some Case Studies • • •

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1)Repairs of distribution transformers-testing –back to back. 2)Transformer oil reclamation for DTrs. 3)One BHEL make 100MVA Power Transformer at Ghanapur 220/132 KV S.S. tripped on Diffl. Relay due oil detoriation of oil and more moisture in the oil. After through filteration of oil in the transformer the transformer stood alright. 4)At 33/11KV Shamshabad S.S. Knockling sound in the 8MVA power transformers observed for a moment whenever 33KV supply was received at the S.S. and transformers were charged. The bottom dirty oil was replaced with filtered ,tested, new transformer oil then there was no sound from the transformers on charging. 5)Fire in 66/11 K.V. HV/LV control panel at Ramagundam power station-A. 6) Frequent D.Trs failure due to fault in consumer’s 3 Phase motor at Ramannapet in Nalgonda District. 7)Failure of 132 KV L.As 0f Ramagundam- Warangal 132 KV Feeders at Ramagundam. 8)Polarity reversal of battery cells at Ramagundam Power station-B. 9)Uncleared earth fault on one 11KV feeder from 33/11KV Sharajpet S.S- effect on power transformer. 10)Uncleared earth fault on 132 KV LCPH(NGSR) --K.M. Pally feeder- effect on power transformers at KM pally. 11)Failure of 33KV, 5 MVA ,3 phase BOOSTER and power transformerin 33/11kv s.s. due to33 KV earth fault on incoming 33 kv line, in Mahboobnagar District. 12)Open circuit developed in 33KV feeder C.T. secondary at 132 /33 KV Suryapet S.S. ( contd. )

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SOME CASE STUDIES- CONTD..

13)Open circuit developed in 11K.V. HT service C.T. at Suryavamsi spinning mill at Bhonagir. 14)Frequent loose connection at relay terminals of out door kiosk model circuit breakers at Ibrahimbagh 33/11K.V. S.S. 15)33/11KV S.S. constructed with PSCC poles. Un cleared earth fault on 11KV feeder caused fusing steel wires in the pole structures . 16)Nizamsagar hydro power station-Idle lead sheathed control cable left in cable trench burnt due to external 11KV earth fault. 17)Repairs to 132kv/33kv,31.5MVA power transformer at Shapurnagar repair bay 18) Sparing of transformer oil to R R circle by PGCL Hyderabad 19)Failure of vaccum in 11 kv feeder VCB at Allwyn 33/11kv S.S., HMT33/11kv S.S., Osmania Hospital 33/11kv S.S., HYDERABAD.

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SOME CASE STUDIES- CONTD..

13)Open circuit developed in 11K.V. HT service C.T. at Suryavamsi spinning mill at Bhonagir. 14)Frequent loose connection at relay terminals of out door kiosk model circuit breakers at Ibrahimbagh 33/11K.V. S.S. 15)33/11KV S.S. constructed with PSCC poles. Un cleared earth fault on 11KV feeder caused fusing steel wires in the pole structures . 16)Nizamsagar hydro power station-Idle lead sheathed control cable left in cable trench burnt due to external 11KV earth fault.

Analysis of Failure of Transformers S NO

Type of Damage

Probable cause

1

Top and bottom HV coils damaged

Sustained external short circuit of LT system not cleared by Fuses

2

Inter turn short ( Puncture of coils

Insulation is poor absorption of moisture by insulation *bad breather)

3

Insulation of all the three phases brittle

Continuous overloading

4

LV star point cut

Poor earthing with unbalanced Loading

5

LV coils damaged

Continuous overloading, repeated test charging on fault

6

Top coils in three phase get charred

Transformers operated with low oil level

7

Jumpers cut inside

Surge Voltages

8

Moisture deposit on core

Bad breathers, entry of atmospheric air into transformer

S. N o

Make & Capacity of Distribution Transformer

Weight of Core & Windin g

Quality of Oil

Total Weight

Remarks

1

Kirloskar 100 KVA

560 Kgs

340 Ltrs

1180 Kgs

Copper wound conventional transformer

2

ECE 100KVA

485 Kgs

320 Ltrs

1045 Kgs

Copper Wound conventional transformer

3

HACK 100 KVA BRIDGE

506 Kgs

305 Ltrs

1072 Kgs

Copper Wound conventional transformer

4

STEL 100KVA

230 Kgs

185 Ltrs

650 Kgs

Aluminum wound conventional transformer

5

STEL 100 KVA

260 Kgs

220 Ltrs

700 Kgs

Aluminum wound CSP transformer

6

TE 100 KVS

279 Kgs

240 Ltrs

685 Kgs

Aluminum wound CSP transformer

7

VE 100 KVA

275 Kgs

185 Ltrs

580 Kgs

Aluminum wound CSP transformer

8

VE 100 KVA

270 Kgs

205 Ltrs

595 Kgs

Aluminum wound conventional transformer

9

WEBER 250 KVA

505 Kgs

340 Ltrs

1145 Kgs

Aluminum wound conventional transformer

10

SDE 250 KVA

660 KGS

370 Ltrs

1355 kgs

Aluminum wound conventional transformer

Lightening Arrestors • • • • • • •

Why lightening arrestors are used at the Substations and Distribution Transformers? To protect electrical equipment from the effect of high voltages due to lightening. Lightening Arrestors are connected across phase and earth of the system. In normal conditions only 57.7% of line voltage is available across L.As., But L.As., are rated for more than 80% Line Voltages –WHY? The electrical power system can not be maintained always at rated voltages. The voltages in the system do vary from lower values to higher values. The L.As do protect the system components, also from high voltages arising in the system

• Under Voltage Relay for capacitor banks ?