Bhel Training Report Transformer-2 .pdf

MAULANA AZAD NATIONAL INSTITUTE OF TECHNOLOGY BHOPAL

SUMMER TRAINING REPORT ON TRANSFORMERS, CAPACITOR BANKS AND BUSHINGS MANUFACTURING BLOCK AT BHEL, BHOPAL

ALOK KUMAR MISHRA SCH NO: 121113003

INDEX S.No.

Contents

1

Products Range

2

Transformer Introduction

3

Electrical isolation

4

Autotransformers

5

Generation Transformer

6

Manufacturing of Transformers

7

Testing of Transformers

8

Bushings

9

Current Transformers

10

Capacitive voltage Transformers

11

Reactors

12

Capacitor Bank

TRANSFORMERS, CAPACITOR BANKS AND BUSHINGS MANUFACTURING BLOCK AT BHEL, BHOPAL Scope:

1. Power Transformers: Upto 765kV Class; 1000 MVA 3 Phase; 1500 MVA 1 Phase Bank. Recently BHEL Bhopal has built 1200/400 KV 333 MVA Transformer. 2. Gapped Core Reactors: Upto 765kV Class; 110 MVAR with Neutral Grounding Reactor. 3. Controlled Shunt Reactors: Upto 400 kV Class; 110 MVAR 4. HVDC Transformers & Smoothing Reactors: ± 500kV DC Converter Transformers: 315 MVA, 3 winding, 1 Phase ± 500kV DC Smoothing Reactors: 254 MVAR 5. Instrument Transformers: Upto 800kV Class Transformer Bushings Tap Changers - M&V Type Radiators and Air/ Water Coolers

Products: Bushings and cylinders    

52KV TO 525 KV OIP condenser Bushings up to 3150 amps. Rating 25 kV, up to 300 amps. Bushing for Indian Railways. Bushing for miscellaneous applications such as bushings, roof bushings, T.G. bushings etc. SRBP cylinders for transformer application up to 1400 mm I/D, up to 2400 mm long.

Capacitors 

  

Shunt Capacitor banks with all film dielectric impregnated with non PCB Impregnate of desired ratings from 6.6 to 400 kV complete with manual / automatic control equipment for Industrial and power system application. Pole mounted Capacitors for mounting in line with Rural Electrification scheme up to 11 KV Roof Capacitor in Traction Motor application: 4 microfarad, 2 KV DC Series capacitors for Reactive compensation and voltage regulation in power system from 33kV to 400 KV

   

Capacitor to improve power factor of traction substations suitable for Railway Electrification. Surge Capacitor for protection of Rotating Machine and Generator transformers winding 15 kV and 40 kV, 0.125 micro farad. AC filter Capacitor for Harmonic filtering suitable for HVDC application up to 500 kV. Coupling capacitors / CVT up to 800 kV class for Power line carrier communication application, meeting and protection.

Transformers     

Power Transformers upto 420kV class, 50/60 Hz 945 MVA, 3-phase Bank. Power Transformers upto 420kV class, 50/60 Hz 500 MVA, 3-phase Unit. HVDC Converter Transformers and Smoothing Reactors. 500 MVA, ± 500kVDC, 3 winding, 1- Phase. Convertor Transformer. 254 MVAR, 360mH, 1568A, ±500 kV DC 1 –Phase Smoothing Reactor.

Instrument Transformers  

Current transformers up to 400 kV. Capacitor voltage Transformer up to 1200 kV.

  

Gapped core Shunt Reactors up to 420 kV class, 125 MVAR 3 Phase Unit. Series and Neutral Grounding Reactors. Controlled Shunt Reactor up to 420 kV class, 80 MVAR 3 Phase Unit.

Reactors

Ultra High Voltage Laboratory  

The DC test plant is adequate for test levels suitable up to +800 kV HVDC transmission. Suitability for testing AC equipments up to 1200KV class.

For Power Transmission   

Control for protection Panels for substations up to 400 kV, distribution feeders and capacitors banks. OLTC control panels. Controls for Nuclear Power Plants.

Customer in India: National Thermal Power Corporation (NTPC), PGCIL, NJPC, NHPC, NLC, NPCIL, NEEPCO, APTRANSCO, APGENCO, JPPCL, TATA Power, Adani Power, Jindal Power, ALL State Electricity Utilities

Abroad: TNB Malaysia, PPC Greece, MEW Oman, OCC Oman, GECOL Libya, Trinidad & Tobago, New Zealand, Tanzania etc.

INTRODUCTION: TRANSFORMERS Transformer is a static device that transfers electrical power from one circuit at a voltage level to another circuit at another voltage level through a magnetic medium at constant frequency. Note: 1. Transformer is not a machine because it does not have any moving or rotating part. Basic principle of operation: Faraday’s law of mutual induction or Mutual Induction (in short)

Ideal transformer equations (eq.) By Faraday's law of induction . . . (1)

. . . (2)

Combining ratio of (1) & (2)

Turns ratio

By law of Conservation of Energy

(5)

Equivalent circuit

Core or iron losses is denoted by RC Magnetizing reactance is XM Rp and Rs represents coil resistances Xp and Xs represent coil inductances Vector Group notation: Vector group shows the phase displacement between HV and LV windings. It is important for parallel operation of transformers. Only transformers of same vector groups can be operated in parallel e.g. Dy11 and Yd11. D,d- Delta connected winding Y,y- Star connected winding a- Autotransformer 1. The high-voltage (HV) winding is designated with an uppercase letter, followed by

medium or low-voltage (LV) windings designated with a lowercase letter. 2. The digits following the letter codes indicate the difference in phase angle between the windings, with HV winding is taken as a reference. 3. The number is in units of 30 degrees. Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°. This notation resembles to a clock e.g. Dy11

HV coil- Delta connected, LV coil- Star connected, phase shift +30°.

Dyn11 Group: The Dyn winding vector groups do not allow zero sequence currents (commonly known as earth

fault

current)

to

pass

across

the

transformer.

Thus, the yn side of the transformer is a 'separately derived system' in which all earth fault current must flow through the neutral of this transformer. With this configuration, earth fault protection is possible and practical. Likewise, earth fault protection on the primary side is not affected by earth faults on the secondary side. The choice of Dyn11 vs Dyn1 or some other angle (5,7, etc) is simply due to user standardization.

Electrical Isolation: Electrical isolation means that there is no electrical connection between the coupled circuits. 1. Therefore the neutrals of both the circuits can have different potentials or one neutral may be grounded and other is left ungrounded. Largely isolation will prevent shock hazards. With no common reference and each device floating you cannot build up a harmful voltage difference between two independent systems to get a shock.

2. Electrical equipments protection: Equipments can be isolated using optocouplers, isolating transformers. Electrical isolation is required to prevent damage to either electrical circuit when one of them is under fault conditions. It is also done to isolate high voltage and low voltage circuits.

Let us assume that the signal input on left side is connected to a micro controller and on the right side we have a BJT connected to Vcc = 24 v. Micro-controller can handle a maximum of 5 volts. So, we cannot interface the base of the BJT with the micro-controller directly because if the collector and base regions are shorted due to some fault, the micro-controller will have 24 volts on its interfaced pins which damage the micro-controller. Note: Electrical isolation is not important at EHV and UHV systems because sufficient protection is provided by the circuit breakers and other protective equipments.

Tertiary windings: Advantages of Using Tertiary Winding in Transformer 1. It supplies power to substation transformer in grid substations. 2. Static capacitors or reactors can be connected to this winding for reactive power injection and voltage control. 3. As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral. 4. It reduces the unbalancing in the primary due to unbalancing in three phase load and also provides path for third harmonic magnetising current component.

Stabilization by Tertiary Winding of Transformer In star-star transformer comprising three single units or a single unit with 5 limb cores offers high impedance to the flow of unbalanced load between the line and neutral because in both of these transformers, there is very low reluctance return path of unbalanced flux.

The impedance offered by the return path of unbalanced load current is very high where very low reluctance return path is provided for unbalanced flux. In other words, very high impedance to the flow of unbalanced current in 3 phase system is offered between line and neutral. Any unbalanced current in three phase system can be divided into three sets of components likewise positive sequence, negative sequence and zero sequence components. The zero sequence current is actually co-phasial current in three lines. If value of cophasial current in each line is Io, then total current flows through the neutral of secondary side of transformer is In = 3.Io. This current cannot be balanced by primary current as the zero sequence current cannot flow through the isolated neutral star connected primary. Hence the said current in the secondary side set up a magnetic flux in the core. The impedance offered to the zero sequence current is very high. The delta connected tertiary winding of transformer permits the circulation of zero sequence current in it. This circulating current in this delta winding balances the zero sequence component of unbalance load, hence

prevents unnecessary development of unbalance zero sequence flux in the transformer core. Therefore, placement of tertiary winding in star - star-neutral transformer considerably reduces the zero sequence impedance of transformer. Note: In five limb core there is no third harmonics are present. Types of transformers: Substation Transformer (Auto Transformer): In Auto Transformer, one single winding is used as primary winding as well as secondary winding. But in two windings transformer two different windings are used for primary and secondary purpose.

If V1 voltage is applied across the winding i.e. in between ′A′ and ′C′.

Hence, the voltage across the portion BC of the winding, will be,

Copper Savings in Auto Transformer Weight of copper in winding is directly proportional to product of number of turns and rated current of the winding. Therefore, weight of copper in the section AC proportional to,

and similarly, weight of copper in the section BC proportional to,

Hence, total weight of copper in the winding of auto transformer proportional to,

The weight of copper in two winding transformer is proportional to,

⇒ N1I1 + N2I2 ⇒ 2N1I1

(Since, in a transformer N1I1 = N2I2)

Let's assume, Wa and Wtw are weight of copper in auto transformer and two winding transformer respectively,

∴

Saving

of

copper

in

auto

transformer

compared to two winding transformer,

Notes: 1. Mostly grid transformers are autotransformers e.g. 1200/400/33 KV, 400/220/33 KV etc. 2. Autotransformers are used in grid because:

(a) They are more economical compared to normal power transformers for turn ratio (or voltage ratio) 3:1 or less. As far as distribution or generating station transformers are concerned, autotransformers are not used there because the voltage ratio (or turn ratio) is much more than 3:1 e.g. Primary Distribution substation 33000* √3/11000, Generating station 220000/11000.

(b) They have lower losses and thus better efficiency compared to normal power transformers. (c) They have superior voltage regulation compared to normal power transformers as they have lower impedance. (d) They also have lower magnetisation current. All these advantages are present in autotransformers because a major part of power is directly conducted into the common winding. 3. Drawbacks of autotransformer: (a) Because of electrical conductivity of the primary and secondary windings the lower voltage circuit is liable to be impressed upon by higher voltage. To avoid breakdown in the lower voltage circuit, it becomes necessary to design the low voltage circuit to withstand higher voltage. (b) Since the impedance is low therefore it results into severer short circuit currents under fault conditions. (c) Only star/star connection is practically possible in autotransformer. Also the neutral points are not isolated.

Constructional Features: 1. There are four windings in autotransformer- LV, IV, HV and tap winding. LV- Low voltage (generally 33 KV) – Helical winding IV- Intermediate voltage winding (generally 220 KV or 400 KV) - Disc type winding HV- High voltage winding (generally 400/765/1200 KV) - Disc type winding

Tap winding- it is a part of HV winding.

2. The core is generally has five limbs.

The two extra limbs are used to reduce the height of the core so that it is easy to transport. If these limbs are not there then height of the core will be much higher so as to provide the necessary area for developing the required flux. 3. There are 10 bushings on autotransformer- 3-HV, 3-IV, 3-LV, 1-Neutral.

Core Design:

Generation Transformer: Generation Transformer is employed in power plant for stepping up the voltage for transmitting the power to the grid. Electrical power is generated in the power plant at lower voltages (typically generation voltage will be between 11kV to 33kV). Rating of the generation transformers will be almost equal to the rating of the generator (500MW generating unit will have generating transformer rating about 588MVA). Generator transformer will be Delta (LV side) and Star (HV side) connected with star connection is connected to earth through resistor to reduce the fault currents and protecting the transformer. Constructional Features: 1. These transformers are generally built as three single phase transformers because the size of single three phase unit would be excessive. 2. There are three windings in generating station transformer - LV, HV and tap winding. LV- Low voltage (generally 33 KV) – Helical winding HV- High voltage winding - Disc type winding Tap winding- it is a part of HV winding. 3. The core is generally has three or four limbs (single phase transformer).

Transformer Manufacturing: 1. Core Manufacturing: (a) Material Used: CRGO (Cold Rolled grain oriented) silicon steel. The permeability is increased and core losses are decreased by using CRGO. (b) For all transformers above 100 MVA core material is imported from China. (c) CRGO is laminated with mica insulation on both sides. Lamination reduces eddy current losses.

(d) Core of transformer is isolated from end plate (which is earthed), otherwise dangerous currents may flow in the core. Isolation test is performed to check this. (e) Generally 20 Step core is manufactured. The pallets are cut at 450

(f) Ceramic spacers are provided in between the pallets to create an oil duct. (g) Holes are punched in core so that it can be clamped on end plate. End Plate is made of mild steel. (h) The limbs and yokes are bound by epoxy resin tape which shrinks on heating.

Block Diagram:

Assembled Unit:

2.

Insulation: Insulation is provided on core, windings, coils, Tank etc. 

Class of insulation: F – 1500 C



Material Used: (a) Press board- Used in making insulating cylinder around coils (b) Udel Wood- Used at the top and base of the coils (c) Bakelite- Used in high temperature areas where there are chances of sparking. (d) Scrap Paper- Insulation of windings. (e) Gas cut- Prevents leakage of oil from tank cover. (f) Epoxy or Fibre glass- Provided on core for insulation.



Various machines like scarping, Milling, Gulleting, Circle cutting, bending, Grinding are used for processing insulating materials.



Top and bottom shunts are used at top and bottom of the coils for leakage flux control.



Metallised shield is also used to discharge the leakage currents in the insulator

3. Windings: Difference between coil and windings: Windings are the turns and the complete prepared section is called coil e.g. 1 HV turn is called winding and complete assembly is called H.V. coil. Types of conductors used: (a) PICC- Paper insulated copper conductor. (b) BPICC- Bunch paper insulated copper conductor (c) GBPICC- Glue bunch paper insulated copper conductor (d) CTC- Continuous transposition copper

Strands: In case of very large conductor cross sectional area, the conductor is stranded (or divided into many conductors) to reduced the eddy the current loss. It also provides mechanical strength and makes the winding flexible. Transposition: The different strands if not transposed would link different amount of flux. Therefore different voltages will be induced across different strands and this will allow a circulating current to flow in the coil as all the strands are connected to each other at the end terminals. There are two types of windings arrangement generally used (a) Helical winding: This type of winding is used in coil where there are lower numbers of turns i.e. LV winding.

(b) Disc winding: This type of winding is used in a coil where there are large number of turns i.e. HV and IV winding.

4. Coil Assembly: The separately prepared HV, tap and LV coils are assembled as a single unit.

The assembled coil is placed in oven at 110-1200C for 36 hours

Pressing to achieve required coil depth (CD)

Again heating in oven at 110-1200C for 18 hours for sterilisation

Hand over to power assembly

Process Map

Type of Transformer Testing Tests done at factory Type tests: These tests are done mainly in a prototype unit not in all manufactured units in a lot. Type test of transformer confirms main and basic design criteria of a production lot. Routine tests: These tests are mainly for confirming operational performance of individual unit in a production lot. Routine tests are carried out on every unit manufactured. Special tests: These tests are done as per customer requirement to obtain information useful to the user during operation or maintenance of the transformer. Tests done at site Pre-commissioning tests: The transformer testing performed before commissioning the transformer at site is called pre-commissioning test of transformer. These tests are done to assess the condition of transformer after installation and compare the test results of all the low voltage tests with the factory test reports. Periodic/condition monitoring tests: These tests are performed regularly for monitoring the condition of Transformer. Type tests of transformer includes 1. Transformer winding resistance measurement 2. Transformer ratio test. 3. Transformer vector group test. 4. Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test). 5. Measurement of no load loss and current (Open circuit test). 6. Measurement of insulation resistance. 7. Dielectric tests of transformer. 8. Temperature rise test of transformer. 9. Tests on on-load tap-changer. 10. Vacuum tests on tank and radiators. Routine tests of transformer include 1. Transformer winding resistance measurement. 2. Transformer ratio test.

3. Transformer vector group test. 4. Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test). 5. Measurement of no load loss and current (Open circuit test) 6. Measurement of insulation resistance. 7. Dielectric tests of transformer. 8. Tests on on-load tap-changer. 9. Oil pressure test on transformer to check against leakages past joints and gaskets. 10. Switching impulse test 11. Partial discharge test

Special Tests of transformer include 1. Dielectric tests. 2. Measurement of zero-sequence impedance of three-phase transformers 3. Short-circuit test. 4. Measurement of acoustic noise level. 5. Measurement of the harmonics of the no-load current. 6. Measurement of the power taken by the fans and oil pumps. 7. Tests on bought out components / accessories such as buchhloz relay, temperature indicators, pressure relief devices, oil preservation system etc.

Transformer Winding Resistance Measurement Transformer winding resistance measurement is carried out to (a) Calculate the I2R losses (b) Calculate winding temperature at the end of a temperature rise test (for cold resistance of the windings). (c) It is also done at site to ensure healthiness of a transformer that is to check loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings etc. The resistance is measured at ambient temperature and then converted to resistance at 75˚C

Methods: 1. Current voltage method of measurement of winding resistance: Test current is injected to the winding and corresponding voltage drop across the winding is measured. By applying simple Ohm's law i.e. Rx = V ⁄ I, we can determine the value of resistance. Measurement is done with D.C. The test current shall not exceed 15% of the rated current of the winding. Large values may cause inaccuracy by heating the winding and thereby changing its resistance.

For star connected three phase winding, the resistance per phase would be half of measured resistance between two line terminals of the transformer. For delta connected three phase winding, the resistance per phase would be 0.67 times of measured resistance between two line terminals of the transformer.

2. Bridge method of measurement of winding resistance- Kelvin bridge method of Measuring Winding Resistance. 3. Measuring winding resistance by Automatic Winding Resistance Measurement Kit.

Note: - Transformer winding resistance measurement shall be carried out at each tap.

Transformer Ratio Test No load voltage ratio of transformer is equal to the turn ratio. 1. We just apply three phase 415 V supply to HV winding, with keeping LV winding open. Then we measure the induced voltages at HV and LV terminals of transformer to find out actual voltage ratio of transformer. We repeat the test for all tap position separately. 2. Transformer turns ratio (TTR) meter method. A phase voltage is applied to the one of the windings by means of a bridge circuit and the ratio of induced voltage is measured at the bridge. 3.

This theoretical turn ratio is adjusted on the transformer turn ratio tested or TTR by the adjustable transformer as shown in the figure above and it should be changed until a balance occurs in the percentage error indicator. Magnetic Balance Test of Transformer Magnetic balance test of transformer is conducted only on three phase transformers to check the imbalance in the magnetic circuit. Procedure: Disconnect the transformer neutral from ground. Then apply single phase 230 V AC supply across one of the HV winding terminals and neutral terminal. Measure the voltage in two other HV terminals in respect of neutral terminal. Repeat the test for each of the three phases. In case of auto transformer, magnetic balance test of transformer should be repeated for LV winding also.

There are three limbs side by side in a core of transformer. One phase winding is wound in one limb. The voltage induced in different phases depends upon the respective position of the limb in the core. Left side phase

Central phase

Right side phase

AN

BN

CN

Voltage applied at left side phase

230 V

180 V

50 V

Voltage applied at central phase

115 V

230 V

115 V

Voltage applied at right side phase

50 V

180 V

230 V

Vector Group Test of Transformer The vector group of transformer is an essential for successful parallel operation of transformers. Procedure: For a YNd11 transformer. 1. Connect neutral point of star connected winding with earth. 2. Join 1U of HV and 2W of LV together. 3. Apply 415 V, three phase supply to HV terminals. 4. Measure voltages between terminals 2U-1N, 2V-1N, 2W-1N, that means voltages between each LV terminal and HV neutral.

5. Also measure voltages between terminals 2V-1V, 2W-1W and 2V-1W. For YNd11 transformer, 2U-1N > 2V-1N > 2W-1N 2V-1W > 2V-1V or 2W-1W. The vector group test of transformer for other group can also be done in similar way.

Insulation Resistance Test: This test is carried out to ensure the healthiness of overall insulation system of transformer. Procedure 1. First disconnect all the line and neutral terminals of the transformer. 2. Megger leads to be connected to LV and HV bushing studs to measure insulation resistance IR value in between the LV and HV windings. 3. Megger leads to be connected to HV bushing studs and transformer tank earth point to measure insulation resistance IR value in between the HV windings and earth. 4. Megger leads to be connected to LV bushing studs and transformer tank earth point to measure insulation resistance IR value in between the LV windings and earth.

L-Line, E-Earth, G-Guard Measurements are to be taken as follows: For auto transformer: HV-IV to LV, HV-IV to E, and LV to E.

For two winding transformer: HV to LV, HV to E, and LV to E. For three winding transformer: HV to IV, HV to LV, IV to LV, HV to E, IV to E, and LV to E. With the duration of application of voltage, IR value increases. The increase in IR is an indication of dryness of insulation. Limiting Value of IR of Transformer: 650 M Ω Insulation Resistance of Transformer Coil (Unassembled Transformer)

Transformer Coil Voltage

Megger Size

Min.IR Value Liquid Filled T/C

Min.IR Value Dry Type T/C

0 – 600 V

1KV

100 MΩ

500 MΩ

600 V To 5KV

2.5KV

1,000 MΩ

5,000 MΩ

5KV To 15KV

5KV

5,000 MΩ

25,000 MΩ

15KV To 69KV

5KV

10,000 MΩ

50,000 MΩ

IR Value of Transformers (Assembled)

Voltage

Test Voltage (DC) LV side

Test Voltage (DC) HV side

Min IR Value

415V

500V

2.5KV

100MΩ

Up to 6.6KV

500V

2.5KV

200MΩ

6.6KV to 11KV

500V

2.5KV

400MΩ

11KV to 33KV

1000V

5KV

500MΩ

33KV to 66KV

1000V

5KV

600MΩ

66KV to 132KV

1000V

5KV

600MΩ

132KV to 220KV

1000V

5KV

650MΩ

Polarization Index: It is the ratio of Insulation resistance (Meg-ohm) at the end of 10 min test to that at the end of 1 min test Guidelines for evaluating transformer insulation using polarization index values: Less than 1 Dangerous 1.0 - 1.1 Poor 1.1 - 1.25 Questionable 1.25 - 2.0 Fair Above 2.0 Good Dielectric Tests of Transformer: Dielectric test of transformer is one kind of insulation test. Separate source voltage withstand test: This dielectric test is intended to check the ability of main insulation to earth and between winding. A single phase power frequency voltage of prescribed level is applied on transformer winding under test for 60 seconds while the other windings and tank are connected to the earth and it is observed that whether any failure of insulation occurs or not during the test. The test is successful if no breakdown in the dielectric of the insulation occurs during test.

Nominal system

Highest system

Rated short duration

voltage rating

voltage rating

power frequency withstand

for equipment

for equipment

voltage

415V

1.1 KV

3 KV

11 KV

12 KV

28 KV

33 KV

36 KV

70 KV

132 KV

145 KV

230 / 275 KV

220 KV

245 KV

360 / 395 KV

400 KV

420 KV

570 / 630 KV

Induced voltage test: Here, three phase voltage, twice of rated secondary voltage is applied to the secondary winding for 60 second by keeping the primary of the transformer open circuited. The frequency of the applied voltage should be double of power frequency too. Here also if no failure of insulation, the test is successful.

Impulse Test Impulse test is performed to check the ability of insulation of transformer to withstand lightning impulse and switching impulse Lightning impulse: Standard lightning impulse Front time T1 = 1,2 μs ± 30% Time to halfvalue T2 = 50 μs ± 20% System disturbance due to natural lightning strokes can be represented by three basic wave shapes. 1) Full wave 2) Chopped wave and 3) Front of wave Switching Impulse: It is a long wave having front time 250 μs and time to half value 2500 μs Marx Generator:

The circuit generates a high-voltage pulse by charging a number of capacitors in parallel, then suddenly connecting them in series. At first, n capacitors (C) are charged in parallel to a voltage V by a high voltage DC power supply through the resistors (RC) Spark gaps have a breakdown voltage greater than V. To create the output pulse, the first spark gap is caused to break down (triggered); the breakdown effectively shorts the gap, placing the first two capacitors in series, applying a voltage of about 2V across the second spark gap. Consequently, the second gap breaks down to add the third capacitor to the "stack", and the process continues to sequentially break down all of the gaps.

Capacitive voltage dividers are used to measure the voltage generated by the impulse generator.

The wave shape and the peak value of the impulse voltage are measured by means of an Impulse Analysing System (DIAS 733) which is connected to the voltage divider. Performance of Impulse Test

The test is performed with standard lightning impulses of negative polarity. The front time (T1) and the time to half-value (T2) are defined in accordance with the standard. Connection of Impulse Test

Impulse generator is used to produce the specified voltage impulse wave of 1.2/50 micro seconds wave. For a three phase transformer, impulse is carried out on all three phases in succession.

The voltage is applied on each of the line terminal in succession, keeping the other terminals earthed. The current and voltage wave shapes are recorded on the oscilloscope and any distortion in the wave shape is the criteria for failure. BIL(Basic Insulation Level): It is the peak value of the voltage impulse that equipment can withstand. Partial Discharge Test: Partial discharge (PD) is a localised dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress, which does not bridge the space between two conductors. Partial discharges in a transformer deteriorate its insulation and can lead to failure of the transformer.

Measuring Circuit

1. Supply generator 2. Supply transformer 3. Test transformer 4. Voltage transformer and measuring circuit 5. Filter 6. Measuring impedance 7. Selective switch 8. Measuring instrument and oscilloscope qo - calibration generator After the transformer is energised for measuring operations, the partial-discharge value read at the measuring instrument is multiplied with the predefined K correction factor, and real apparent partial-discharge value for each terminal is found. q = K · qm qm – load read at the measuring instrument m K – Correction factor q – Real apparent load The test is considered to be successful if the partial-discharge value measured at the transformer’s measuring terminals is lower than 500pC. SRFA test: Sweep Frequency Response Analysis Test Since transformer coils are a complex RLC network therefore they exhibit a particular frequency response. In case of any change in coil parameters or any damage the frequency response of the transformer will not be same as the factory stage response. Therefore in SRFA test frequency response of transformer coil is checked at commissioning site to detect any damage. Tan delta test: It determines the health of the insulators. It is used for cable, winding, current transformer, potential transformer, transformer bushings.

Insulators acts a capacitor when a potential difference is present across them ratio, IR to IC is tanδ

Method: High voltage Schering Bridge is used to determine the capacitance and tanδ. Cx = (C1*R4)/R3 Rx = (C4*R3)/C1 tanδ = wC4R4

Typical permissible tanδ levels Voltage (kV)

Tan delta (%)

11 22 66 88 132 275 400

7 5 2.5 2 1.5 1 0.5

A very low frequency test voltage is applied across the equipment whose insulation to be tested. First the normal voltage is applied. If the value of tan delta appears good enough, the applied voltage is raised to 1.5 to 2 times of normal voltage, of the equipment. The tan delta controller unit takes measurement of tan delta values. During test it is essential to apply test voltage at very low frequency. Reason of applying Very Low Frequency The resistive component is nearly fixed but capacitive current becomes high at higher frequency and thus measurement is difficult and less accurate. The frequency range for tan delta test is generally from 0.1 to 0.01 Hz.

That means dissipation factor tanδ ∝ 1 / f.

Open Circuit Test on Transformer: This test is done to determine the magnetising current and core losses in transformer. A CVT/VT and CT are connected in LV side of the transformer as CVT or VT required for LV side have lower rating. The voltage (110%) at rated frequency is applied to that LV side with the help of an auto transformer. The HV side of the transformer is kept open.

Short Circuit Test on Transformer: This test is performed to determine the copper losses at full load condition and leakage reactance of the transformer. A CVT/VT and CT are connected in HV side of the transformer. The voltage at rated frequency is applied to that HV side with the help of auto transformer.

The LV side of the transformer is short circuited. Now voltage is slowly increased until the rated current flows in the HV side.

Temperature Rise Test of Transformer In this test we check whether the temperature rising limit of the transformer winding and oil as per specification or not. In this type test of transformer, we have to check oil temperature rise as well as winding temperature rise limits of an electrical transformer.

After completion of temperature rise test for top oil of transformer the current is reduced to its rated value for transformer and is maintained for one hour. After one hour the supply is switch off and short circuit and supply connection to the HV side and short circuit connection to the LV side are opened.

Then resistance of the windings are measured quickly. Then the resistances are measured at the same 3 to 4 minutes time intervals over a period of 15 minutes. Graph of hot resistance versus time is plotted, from which winding resistance (R 2) at the instant of shut down can be extrapolated.

Where, R1 is the cold resistance of the winding at temperature t1.

Transformer oil Dielectric strength Test: Dielectric strength is also known as breakdown voltage or BDV of transformer oil. Break down voltage is measured by observing at what voltage, sparking starts between two electrodes immerged in the oil, separated by specific gap. Low value of BDV indicates presence of moisture content and conducting substances in the oil. BDV measuring kit is generally available at site. In this kit, oil is kept in a pot in which one pair of electrodes are fixed with a gap of 2.5 mm (in some kit it 4mm) between them. Now slowly rising voltage is applied between the electrodes. Rate of rise of voltage is generally controlled at 2 KV/s and observe the voltage at which sparking starts between the electrodes.

Minimum breakdown voltage of transformer oil or dielectric strength of transformer oil at which this oil can safely be used in transformer, is considered as 30 KV. But recommended BDVs for new transformers with various voltage ratings are 1. <72.5 KV – 40KV 2. 72.5170KV – 60KV Polarity Test:

DC method of testing the polarity: When the switch S is closed if the secondary voltage shows a positive reading, with a moving coil meter, the assumed polarity is correct. If the meter kicks back the assumed polarity is wrong.

Short circuit withstand test: This test is performed to ensure that transformer is able to withstand the mechanical stress and temperature rise in case of actual short circuit. Short circuits may result in buckling or spiralling of windings

Icw – Rated short time withstand current. The rating is made up of 2 parts: the RMS rating in kA and the duration. There is no international standard for the timing, however times of 0.5, 1 and 3 seconds are commonly used. Icw includes the Rated Peak Withstand Current (Ipk), which is a surge of current that occurs on one phase of the system in the first full cycle, as well as the changes of electromagnetic forces that occur through the number of cycles seen during the test. Methods: (a) With pre-established short-circuit; (b) With post-established short-circuit. Test method (a) involves closing of a breaker at the source terminal to energize the previously short-circuited transformer. This means that the secondary winding is short circuited in advance and power is switched on to the primary. In this method there is a problem of uncontrolled magnetization of core. Test method (b) involves closing a breaker at the faulted terminal to apply a short-circuit to the previously energized transformer. By adapting this test method, the difficulty of uncontrolled core magnetization disappears. The transformer is taken up at no-load to rated voltage and the secondary short-circuit is then closed, at the predetermined phase angle, by means of a synchronous make-switch. The method using a post-established short-circuits should be preferred as far as possible, since it represents more closely the typical condition during the faults.

Standards For Transformers with Two Separate Windings there are three categories: (a) Category I: 25 kVA to 2 500 kVA; (b) Category II: 2 501 kVA to 100 000 kVA; and (c) Category III: above 100 000 kVA Minimum Values of Short Circuit Impedance

Short circuit apparent power of the system

Calculation of Symmetrical Short Circuit Current I

where Zs = short-circuit impedance of the system.

(equivalent star connection) where Us = rated voltage of the system, in kilovolts (kV); S = short circuit apparent power of the system, in MVA. Short method: 1000 KVA 11/0.433KV Transformer z=5 p.u. I fault = 100/z If=20 p.u. i.e. fault current is 20 times the rated current. The transformer is designed with the consideration of maximum calculated fault current and corresponding forces and temperature rise. And an additional tolerance is taken by the designer. Thus short circuit test only verifies that the transformer is actually able to withstand the short circuit on field not just on paper. The duration of the current is generally 1-2 s unless a different duration is specified.

BUSHINGS

In electric power, a bushing is an insulator that allows an electrical conductor to pass safely through a (usually) earthed conducting barrier such as the wall of a transformer or circuit breaker. When an energized conductor is near any material at earth potential, it can cause very high field strengths to be formed therefore insulation becomes necessity. NOTE: Bushings are the weakest link in all transformer related equipments. Most often bushing failure leads to failure of whole transformer. Design: A typical bushing design has a 'conductor rod', (usually of copper). It is inserted into aluminium hollow cylindrical rod. Now oil impregnated paper and aluminium foil is wrapped around this rod alternatively. This arrangement creates series capacitances and thus reduces the electric field stress on each insulating paper unit. Now this assembly is fitted into oil filled porcelain bushing.

Types of Bushings A. Composite Bushing: - A bushing in which insulation consists of two or more coaxial layers of different insulating materials. B. Compound-Filled Bushing: - A bushing in which the space between the major insulation (or conductor where no major insulation is used) and the inside surface of a protective weather casing (usually porcelain) is filled with a compound having insulating properties. C. Condenser Bushing: - In these bushings cylindrical conducting layers are arranged coaxially with the conductor within the insulating material. The length and diameter of the cylinders are designed to control the distribution of the electric field in and over the outer surface of the bushing. Condenser bushings may be one of several types:

1. Resin-bonded paper insulation; 2. Oil-impregnated paper insulation; These bushings are used for very high voltages as electric field is properly distributed by formation of capacitors. D. Oil-Filled Bushing: - A bushing in which the space between the major insulation (or the conductor where no major insulation is used) and the inside surface of a protective weather casing (usually porcelain) is filled with insulating oil. E. Oil Immersed Bushing: -A bushing composed of a system of major insulations totally immersed in a bath of insulating oil. Testing: 1. Capacitance and tan δ tests- Capacitance rise by more than 10% is considered dangerous and occurs due breakdown of capacitive layers. 2. 3. 4. 5.

Internal partial discharge test Impulse test Thermal stability test Dielectric tests with DC application, Snap back test

CURRENT TRANSFORMER Current Transformers (CT’s) are instrument transformers that provide an appropriately reduced value of high currents in power systems. This reduced value of current is then used for metering, control and protection. Generally multiple secondary coils are present in CTs.

Knee-point voltage- It is the magnitude of the secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy.

Note: Secondary winding of the CT must never be open circuited because dangerously high voltages are developed across its secondary terminals as it is a step up transformer.

Practical current transformer:

Current Transformers are defined by Accuracy Classes depending on the application. 1. Metering Accuracy CT’s are used where a high degree of accuracy is required from low-load values up to full-load of a system. An example of this application would be the current transformers utilized by utility companies for large capacity revenue billing. 2. Relaying Accuracy CT’s are used for supplying current to protective relays. In this application, the relays do not normally operate in the normal load range, but they must perform with a reasonable degree of accuracy at very high overload and faultcurrent levels which may reach twenty times the full-load amplitude. Protection (relaying) CTs Class ‘P’ CT’s represents protective current transformers. The rated output classes in IEC are 5, 10, 15, 20, and 30 VA. The preferred accuracy classes are 5P (5 percent maximum error) and 10P (10 percent maximum error). Lastly, IEC has an accuracy limit factor (ALF), which indicates the multiples of rated secondary current at which the accuracy class applies. The typical value of the ALF is 10, with values of 20 and 30 also available.

e.g. 20 VA class 5P10, signify a protective CT with less than 5 percent error at 10 times rated current, with a load output of 20 VA. 







Class PX CT's are defined by the position of the knee-point and the secondary wire resistance RCT. They have low leakage reactance and their performance can be easily calculated using available data. Class PR CT's are defined like the PX CT's but they have a low remanence; less than 10%. Note that remanence in CT's can be 60-80% that may cause quick saturation in case of a fault-current DC offset in the remanent direction. A class PX CT can't have that problem. CT's for transient response class "TP" are defined by their connected load RB, time constant TS and their over current figure KSSC. These linearised CT's have air-gaps in the core to obtain extreme high saturation voltage and current. PS is “Protection Special Class” CT. This core is used particularly where current balance is precisely required to be maintained. PS class CT's are special purpose CT's with minimal error compared with 5P20 Class and is more sensitive.

Measurement CTs 

These are aimed at measuring accurately current within their normal operating range of 0 to In.



For the protection of the measuring instruments in case of a fault current, it is favourable that for currents far above rated current In, the core is saturated and the output lowers so that the fault-current trough the meter is only a part of the expected current trough the meter. This is expressed by the Instrument Security Factor SF. The accuracy of a measurement CT is given by its accuracy class that corresponds to the error% at rated current and at 1.2 times rated current In.

 

The standard accuracy classes according IEC are class 0.2, 0.5, 1, 3 and 5. For classes 3 and 5, no angle error is specified. The classes 0.2S and 0.5S have their accuracy shifted toward the lower currents. This means that they have 5 measuring points instead of 4 (or 2 for class 3 & 5).

Testing 1. Insulation Resistance: Winding to Winding and Winding to Ground insulation check 2. CT Resistance: ‘Bridge’ or Low-resistance ohmmeter check of CT Secondary Winding. 3. Ratio Test: Check of CT to confirm proper Ratio. 4. Polarity: Confirmation of CT polarity. 5. Excitation: Confirmation of CT rating, verifies no shorted turns. 6. Burden: Check of CT’s ability to deliver current. 7. Impulse Test: To ensure lightning and switching impulse withstand capability of CT.

CAPACITIVE VOLTAGE TRANSFORMERS CVTs are modified voltage transformers. They are also instrument transformers which provide appropriately reduced voltage from high voltage power systems. This reduced voltage is then used for metering, control and protection. 1.

Revenue metering

2.

Protection for high voltage lines and substations.

3.

Transmission of high frequency signals (Coupling capacitor only).

Construction: Capacitive voltage transformers consist of a series of capacitors connected in series on top of a tank in which the electromagnetic unit (inductive transformer (5), series reactor (8) and

auxiliary elements) is housed. These capacitors form a voltage divider (2, 3) between the high voltage terminal (1) and the high frequency terminal (4). The capacitors, impregnated with high grade dielectric oil, are housed in one or more insulators. Each of them forms a hermetically sealed independent unit, with a very stable capacitance over time. The high frequency terminal (4) for the PLC signal comes out of one side through a piece of resin that separates the capacitive unit from the inductive voltage transformer. The medium voltage inductive voltage transformer is immersed in mineral oil and housed inside a hermetically sealed metallic tank. The secondary terminals are located inside a box (7) enabling connections and have space with protection elements such as fuses or circuit breakers.

SECTIONS 1. Primary terminal 2. Oil volume compensating system 3. Insulator (porcelain or silicone rubber) 4. Capacitors 5. Intermediate voltage tap 6. High frequency terminal 7. Inductive voltage transformer 8. Oil level indicator 9. Carrier accessories 10. Oil sampling valve 11. Earthing terminal 12. Secondary terminal box

There are two types of voltage transformers 3. Measuring VT: These VT’s are used for measurement and metering purpose 4. Protection VT: These VT’s are used system protection purposes.

Voltage Transformer (or CVT) Accuracy Classes Accuracy Class

Voltage Error (%)

Phase Error (Minutes)

Application

0.1

+0.1

+5

Precise Measurement

0.2

+0.2

+10

Measurement

0.5

+0.5

+20

Measurement

1.0

+1.0

+40

Measurement

3.0

+3.0

---

Measurement

3P

+3.0

+120

Protection

6P

+6.0

+240

Protection

Testing: 1. Temperature rise test 2. Impulse test 3. High voltage power frequency wet withstand test 4. Partial discharge test 5. Polarity test 6. Accuracy test

REACTORS Reactors are basically inductors used for reactive power compensation and voltage control in power system. Difference between Reactor and Transformer 1. Transformer will have at least 6 bushings while reactor will have only 3 bushings because reactor has only three single coils while transformer has six to nine coils. 2. Low reluctance path is present in the transformer core while high reluctance path is present in reactor core so that it consumes large reactive power. Since a Shunt Reactor magnetizing current is large, if it is designed with Iron alone as a Power Transformer, there will be large hysteresis loss. Air gaps in Iron core are provided in a Shunt Reactor to reduce this loss and to minimize the remanent flux in the core. Cheese core is especially developed for the reactors. Also the core hollow and is filled with an insulator. By construction, a Shunt Reactor can be oil immersed or dry type for both with and without iron core. Dry type Reactors are constructed as single phase units and are thus arranged in a fashion to minimize stray magnetic field on surrounding. When such an arrangement is difficult, some form of magnetic shielding is required. One of the advantages of dry type reactor is absence of inrush current. Oil immersed reactors can be core-less or with gapped iron core. These are either single phase or three phase design with or without fan cooling. These are installed within tanks which hold oil & act as metallic magnetic shields.

Reactors are thyristor controlled in order to adapt fast to the reactive power required. Especially in industrial areas with arc furnaces the reactive power demand is fluctuating between each half cycle. Since inductor voltage can change abruptly so the firing angle of thyristors in reactor can be smoothly varied while in case of capacitors this is not possible.

CAPACITOR BANK Capacitor banks are used for reactive power compensation (series and shunt), load power factor improvement, filters and in CVTs (capacitor units only). A capacitor bank is a grouping of several identical capacitors interconnected in parallel or in series with one another.

Construction: Basic materials used in capacitor packs are 1. Aluminium Foil- It acts as an electrode 2. Polypropylene- It acts as an dielectric medium

Discharge resistor reduces the terminal voltage to 50 V in a particular time frame when charged capacitor unit is disconnected from the power supply.

Capacitors are intended to be operated at or below their rated voltage and frequency as they are very sensitive to these values; the reactive power generated by a capacitor is proportional to both of them (kVar ≈ 2π f V 2).

Bank Configurations (a) Externally Fused An individual fuse, externally mounted between the capacitor unit and the capacitor bank fuse bus, typically protects each capacitor unit. The capacitor unit can be designed for a relatively high voltage because the external fuse is capable of

interrupting a high-voltage fault. Use of capacitors with the highest possible voltage rating will result in a capacitive bank with the fewest number of series groups. A failure of a capacitor element welds the foils together and short circuits the other capacitor elements connected in parallel in the same group. The remaining capacitor elements in the unit remain in service with a higher voltage across them than before the failure and an increased in capacitor unit current. If a second element fails the process repeats itself resulting in an even higher voltage for the remaining elements. Successive failures within the same unit will make the fuse to operate, disconnecting the capacitor unit and indicating the failed one. (b) Internally Fused Each capacitor element is fused inside the capacitor unit. The fuse is a simple piece of wire enough to limit the current and encapsulated in a wrapper able to withstand the heat produced by the arc. Upon a capacitor element failure, the fuse removes the affected element only. The other elements, connected in parallel in the same group, remain in service but with a slightly higher voltage across them. In general, banks employing internally fused capacitor units are configured with fewer capacitor units in parallel and more series groups of units than are used in banks employing externally fused capacitor units. The capacitor units are normally large because a complete unit is not expected to fail.

(c) Fuseless Shunt Capacitor Banks The capacitor units for fuseless capacitor banks are identical to those for externally fused described above. To form a bank, capacitor units are connected in series strings between phase and neutral. When the capacitor element fails it welds and the capacitor unit remains in service. The voltage across the failed capacitor element is then shared among all the remaining capacitor element groups in the series.

Overvoltage due to an element failure is not severe however successive failures of elements will lead to the removal of the bank. The fuseless design is not usually applied for system voltages less than about 34.5 kV. Another advantage of fuseless banks is that the unbalance protection does not have to be delayed to coordinate with the fuses.

END OF REPORT