TRANSFORMER TESTING VIVEK JHA
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Transformers: core of electricity supply
Transformer: most vital equipment of the supply system Types: • • • •
EHV Transformer HT Transformer LT Transformer Instrument Transformers
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NECESSITY OF TESTS •
To prove that the design meets the specified job requirements and to obtain transformer characteristics.
•
To check that the quality requirements have been met and that performance is within the tolerance guaranteed.
Tests performed for the former purpose are referred to as Type tests and that for the later purpose are referred to as Routine tests (carried out on every unit manufactured). In Addition to these two category of tests, special tests May also be performed to obtain information useful to the User during operation or maintenance of the transformer.
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Reference standard : IEC 60076
Transformer Tests
EXAMPLE: ROUTINE TESTS •
Measurement of winding resistance
•
Measurement of voltage ratio, polarity and check of voltage vector relationship
•
Measurement of no‐load loss and excitation current
•
Measurement of short‐circuit impedance and load loss
•
Measurement of insulation resistance
•
Switching impulse voltage withstand test
•
Lightning impulse voltage withstand test
•
Magnetic circuit (isolation) test EXAMPLE: TYPE TESTS
•
Meaurement of power taken by cooling circuits
•
Temp Rise Test
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ADDITIONAL TESTS •
TEST WITH LIGHTNING IMPULSE CHOPPED ON THE TAIL
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MAGNETIC CIRCUIT (ISOLATION) TEST
•
MAGNETIC BALANCE TEST ON THREE‐PHASE TRANSFORMERS
•
DISSOLVED GAS ANALYSIS ( DGA ) OF OIL FILLED IN THE TRANSFORMER
•
DETERMINATION OF CAPACITANCES AND TAN DELTA BETWEEN WINDING‐TO‐EARTH AND BETWEEN WINDINGS
MECHANICAL TESTS •
MEASUREMENT OF VIBRATION ON TRANSFORMER TANK
•
VACUUM TEST ON TRANSFORMER TANK
•
OIL PRESSURE TEST ON COMPLETELY ASSEMBLED TRANSFORMER
•
JACKING TEST AND DYE‐PENETRATION TEST
•
PRESSURE RELIEF DEVICE TEST
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Tests on Transformers
• • •
Erection tests (installation Tests) Pre‐commissioning tests Commissioning Tests
Erection tests: • Thorough checking for any damage • Verification of technical specifications • Foundation etc
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ERECTION AT SITE : INSPECTION
y Make visual inspection for any transit damage y Check nitrogen pressure. y Check various accessories for any type of transit damage. y Make internal inspection of the transformer to the extent possible for any visible discrepancy.
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OIL FILLING
y
Before filling the oil into transformer check the oil carefully for y y y
y
y
Break down voltage Moisture content Tan delta and capacitance
If the oil is not having the properties as recommended by the standards,it must be filtered with filters with built in heaters and vacuum pumps for improving the quality of oil. For transformers dispatched gas filled ,oil filling must always be done under vacuum.
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GENERAL TESTS AFTER TRAFO INSTALLATION
(Get Clearance for Testing & Commissioning activities) • Physical inspection • Fire protection system available • Oil level checking, no oil leakage • Power and control cabling checking, • Cooler fan and Pump control ckt checking • Buch. Relay correctly mounted, air released from Buch relay, • Cooling system & radiators, OLTC • Breather installation Ok, silica gel colour blue
• OTI & WTI Calibration
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Pre commissioning Tests
• • • • • • • • • • •
IR value of transformer and cables Winding Resistance Transformer Turns Ratio Polarity Test Magnetizing Current Vector Group Magnetic Balance Bushing & Winding Tan Delta (HV ) Protective relay testing Transformer oil testing Hi‐pot test
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Commissioning Tests:
• Remote Annunciation and control schemes • Power cable Identification & Tests • Charging the transformer : no load and full load operation
and Observation for any abnormality • Confirming the operation of all protection, interlocks and metering schemes
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1. Check the operation of the Buchholtz alarm and trip by injecting air through the test pet cock. 2. Test the OTI for alarm and trip. 3. Test the WTI for alarm and trip. 4. Check the working of the WTI / RTD (Resistance Temperature Device) repeaters at the control room. 5. Test the OLTC – Oil surge relay for trip. 6. Check alarm for low oil level . 7. Check various relays for current & time settings 8. Check the cooler unit for ∙ over current setting of fans and oil pumps ∙ cooler supply failure alarm ∙ fan/pump trip alarm . any mal‐ operation of the transformer Buchholtz relay when all the oil pumps are switched on simultaneously in forced oil cooled transformers
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FIELD TESTS : No Load • Winding resistance • Turns Ratio. • Polarity, • connection group and • Excitation current. On‐load • short circuit test, • Impedance measurement
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Insulation Resistance y This test has been the most usual
historically, being called to “ megger” the transformer (the term comes from the firm of the first IR systems).
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Insulation Resistance and Polarization Index
Equivalent diagram of the dielectric circuit of a transformer. Ri is the insulation resistance, Cg the geometric capacitance and the different Ra/Ca emulate the equivalent dielectric absorption circuit.
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Winding Resistance y This is nothing but the resistance measurement of the windings by applying a small
d.c voltage to the winding and measuring the current through the same y The measured resistance should be corrected to a common temperature such as
75°C or 85°C using the following formula:
where y RC is the corrected resistance, RM is the measured resistance y CF is the correction factor for copper (234.5) or aluminum (225) windings y CT is the corrected temperature (75°C or 85°C) y WT is the winding temperature (°C) at time of test
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Winding Resistance. y With this test we determinate the pure
ohmic resistance from each phase windings both in high and low voltage and each position of tap changer.
y in a first approach seems easy to measure,
its not so, because it is necessary to make flow relatively high currents to register the usual low resistance values µΩ/m Ω / Ω with the required precision. This currents must also flow through the equivalent inductance of the transformer.
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Transformer Turns Ratio (TTR): Transformer Turns Ratio (TTR): quotient between high voltage / low voltage. Must match with protocol /nameplate values. In the power transformer with tap changer, take readings for each position (giving extra information on its status and that of the On Line Tap Changer (OLTC)
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Transformer Turns Ratio: Doble kit
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Polarity Test
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Open circuit test & Excitation current y Excitation current is the current flowing into the high voltage winding
with the low voltage side open.
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FIELD TESTS : Load test y short circuit test, y Impedance measurement
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Short Circuit test
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Short Circuit test y “short circuit test” is based in the application of a voltage in one winding
(high voltage one) with the other winding short‐circuited. It is usual to register nominal and extreme positions if the transformer had an OLTC.
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Impedance measurement y Short circuit voltage: voltage required to circulate full load current. y should be near the protocol /nameplate value from the transformer. y It’s change will indicate irregularities in the magnetic core, winding,
displacement, short‐circuits, mechanical deformations…
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Frequency Response Analysis
y The main purpose of this test is to determinate the frequency response
graph of the equivalent electric / dielectric / magnetic / mechanic altogether evaluated.
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Frequency Response Analysis
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DIELECTRIC SYSTEM TESTS. Liquid dielectric (usually mineral oil) OR solid dielectric (usually paper)
Dielectric circuit tests. Insulation Resistance and Polarization Index (IR, PI). y Capacitance, Dissipation (power) factor (tan δ) and insulation losses in dielectric bushing. y Partial Discharge (PD). y
Oil sampling. Dielectric Strength. y Moisture content. y Dissolved Gases. y Furan Analysis. y
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Oil testing and maintenance There are three important purposes of the oil in a transformer: 1. Good dielectric strength 2. Efficient heat transfer and cooling 3. To preserve the core and assembly y By filling voids (to eliminate partial discharge) y By preventing chemical attack of core, copper and insulation by
having y low gas content and natural resistance to ageing.
Oil Testing
y
Visual checks
y
Oil BDV test
y
Power factor ( tan delta ) test
y
Acidity test
y
Moisture ( ppm )
y
D G A
Transformer BDV test set VIVEK JHA
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Breakdown voltage.
y Oil degradation can be easily appreciated with this parameter
tested. The test is based in the insertion of electrodes immersed in oil and increasing voltage up to breakdown. y Test is repeated six times to get a repeatable measurement. y The only disadvantage is that it is necessary to extract from the transformer a significant sample (test cell will have 350...600ml).
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IEC 156
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Transformer Oil Specifications Characteristics
Requirement as per IS:1866‐1983
Breakdown voltage
50KV(min) for >145KV 40KV(min) for 72.5KV to 145KV 30KV(min) for < 72.5KV
Water content
25ppm(max) for >145KV 35ppm (max)for < 145KV
Tan delta at 50 deg.C
0.2(max) for >145KV 1.0(max) for < 145KV
Specific resistance at 90 deg.C ohm‐cm
>0.1 X 10E12
Neutralization value mgKOH/gm
<0.5
Flash point (degree C)
>140
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Capacitance / Tan delta (Dissipation Factor)
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Capacitance / Tan delta (Dissipation Factor)
y Another usual approach when performing dielectric evaluation is tan
delta. This test uses AC and pursues to know loss angle of the tested element. y This measurement includes information of the moisture and
contamination degree and emulates the behaviour and voltage aggressions similar to service ones. y It is important to take note of transformer temperature and
environmental moisture (surface leakage).
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Vector Group Test : Dy11
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Vector Group Test : Dy11
Dy11
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Vector grp test
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Vector Group Dy1
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Vector Group Dy1
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Partial Discharge test. y Partial discharge are small discharges that appear inside of the dielectric as a sign of its
degeneration. They appear as a result of the increase of the electric field in small gaseous voids inside the oil and also inside the paper, epoxy or as a result of the presence of metallic contamination, etc. This discharge accelerate the thermal degradation effects and lead sometimes exponentially to the power transformer destruction.
y There are two usual detection systems, y y
Acoustic system uses sound mechanic manifestation (in ultrasonic range Electric systems determinates discharges and correlates them with Insulation healthiness.
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Some other Tests, normally performed at Manufacturer’s works
Temperature Rise Test Lightening Impulse Test Vacuum And Pressure Test Lightning Impulse Withstand Switching Impulse Withstand
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History/Trending
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Conclusion Testing is an art & science of managing Machine, Manpower, Material, Quality, Time y People responsible for upkeep of electrical apparatus need to cultivate a watchful attitude to y Look (eye) Listen (ear) y Smell (nose) Feel ( finger / skin) for symptoms of impending trouble y Adapt new techniques. y Keep on learning from the experiences world‐wide, avoid obsolescence y Develop Integrated Predictive Diagnostic System y Keep it System Oriented to ensure sustainability y (Avoid subjectivity & individual‐bias) VIVEK JHA
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Conclusion
y Safety First y Good Test equipment y people to have a deep knowledge y Take care of environmental conditions while comparing results y Work pressure and short downtime y Keep your mind cool
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Some References y y y y y y y y y y y y y
NATIONAL STANDARDS IS 2026 (PART‐III) IEC 60076‐5:2000. Power transformers. Part 5: Ability to withstand short circuit. IEC 60726:2003. Dry‐type power transformers. IEC 60076‐3 Test levels & test method requirement for different tests ANSI/IEEE Std 62‐1995. IEEE Guide for Diagnostic Field Testing of Electric Power Apparatus‐ Part1:Oil Filled Power Transformers, Regulators, and Reactors. IEEE Power Engineering Society‐1995. ANSI/IEEE Std C57.12.90‐1993. IEEE Standard Test Code for Liquid‐Immersed Distribution. Power and Regulating Transformers and IEEE Guide for Short‐Circuit Testing of Distribution and Power Transformers. ANSI/IEEE Std. C57.12.00‐1993. IEEE Standard General Requirements for Liquid‐Immersed Distribution, Power and Regulating Transformers. EN 60156‐1995. “Insulating liquids. Determination of the breakdown voltage at power frequency. Test method.”. EN 60814‐1997. “Insulating liquids. Oil‐impregnated paper and pressboard. Determination of water by automatic coulometric Karl Fischer Titration”. EN 60567‐1992. “Guide for the sampling of gases and of oil from oil‐filled electrical equipment and for the analysis of free and dissolved gases” . EN 60599:1999. “Mineral oil‐impregnated electrical equipment in service – Guide to the interpretation of dissolved and free gases analysis.”. EN 61620:1999. “Insulating liquids. Determination of the dielectric dissipation factor by measurement of the conductance and capacitance. Test method. EN 61198:1994. “Mineral insulating oils. Method for the determination of 2‐furfural and related compounds”.
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TESTING OF TRANSFORMERS y
LIGHTNING IMPULSE TEST
To simulate the system disturbances due to Lightning stroke Can be represented by three basic wave shapes ¾ ¾ ¾
Full wave LI Chopped wave (Chopped on tail) Chopped wave (Chopped on front)
Lightning disturbances do not always have these basic wave shapes. However, by
defining the amplitude and shape of these waves, it is possible to establish a minimum impulse dielectric strength that a transformer should meet. y
If a lightning disturbance travels some distance along the line before it reaches a transformer, its wave shape approaches that of the full wave. It is generally referred to as 1.2/50 wave. A wave traveling along the line might cause flashover across an insulator after the peak of the wave has been reached. This wave is simulated by a chopped wave which is of the same magnitude as the full wave. If a lightning stroke hits directly at or very near to a transformer terminal, the impulse voltage may rise steeply until it is relieved by a flashover, causing sudden, very steep collapse in voltage. This condition is represented by the front of wave
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TESTS ON TRANSFORMERS y
LIGHTNING IMPULSE TEST
CURRENT WAVEFORM DURING LI
1.2/50 µS LI WAVE
TYPICAL TEST LEVELS FOR LI TEST : H1 : 1300 kVp, VIVEK JHA
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TESTS ON TRANSFORMERS y
LIGHTNING IMPULSE TEST
1.2/50 µS LI WAVE CHOPPED AT 3.7 µS
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CURRENT WAVEFORM DURING CHOPPED LI
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SWITCHING IMPULSE TEST y
Switching impulse level is conducted to verify the integrity of transformer under various switching surges on AC/DC line.
y
The typical wave shape of Switching impulse is 250/2500 µS as per IEC 60060 & its magnitude is lower than the corresponding LI wave but for longer duration.
y
In view of the difficulties in generating these wave‐shapes in laboratories and more to minimize the core saturation during testing of transformers, IEC‐60076 Part 3 has specified a switching impulse wave having a virtual front time of at least 100 µs, duration above 90% of the specified amplitude of at least 200 µs and a total duration to the first zero passage of at least 1000 µs.
y
Unlike in the case of ac winding, both the terminals of valve winding experience full dc voltage of the bridge to which they are connected. Therefore valve winding is tested to prove the insulation of the winding with the respect to ground. Switching impulses are applied to the valve winding terminals connected together. The other winding terminals not under test are short‐circuited and grounded.
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SWITCHING IMPULSE TEST y
WAVE SHAPE OF SWITCHING IMPULSE AS PER IEC 60060
STANDARD 250/2500 µS SWITCHING IMPULSE WAVE
TYPICAL SWITCHING IMPULSE LEVELS : H1 : 1080 kVp, X1-X2 : 1290 kVp, X3-X4: 980 kVp
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SWITCHING IMPULSE TEST y
Switching impulse level is conducted to verify the integrity of transformer under various switching surges on AC/DC line.
y
The typical wave shape of Switching impulse is 250/2500 µS as per IEC 60060 & its magnitude is lower than the corresponding LI wave but for longer duration.
y
In view of the difficulties in generating these wave‐shapes in laboratories and more to minimize the core saturation during testing of transformers, IEC‐60076 Part 3 has specified a switching impulse wave having a virtual front time of at least 100 µs, duration above 90% of the specified amplitude of at least 200 µs and a total duration to the first zero passage of at least 1000 µs.
y
Unlike in the case of ac winding, both the terminals of valve winding experience full dc voltage of the bridge to which they are connected. Therefore valve winding is tested to prove the insulation of the winding with the respect to ground. Switching impulses are applied to the valve winding terminals connected together. The other winding terminals not under test are short‐circuited and grounded.
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SWITCHING IMPULSE TEST y
WAVE SHAPE OF SWITCHING IMPULSE AS PER IEC 60060
STANDARD 250/2500 µS SWITCHING IMPULSE WAVE
TYPICAL SWITCHING IMPULSE LEVELS : H1 : 1080 kVp, X1-X2 : 1290 kVp, X3-X4: 980 kVp
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Magnetic balance test • •
Done on TRANSFORMER to see whether the transformer core is magnetically balanced or not...AND ALSO WINDING CONDITION.... for example we r having delta HV and star winding LV side ... HV SIDE WE WIL BE HAVING THREE BUSHINGS....THOSE R 1U,1V,1W.. HV SIDE WE WIL BE HAVING FOUR BUSHINGS.....2U,2V,2W,2N... WE CAN DO THIS MBT (MAGNETIC BALANCE TEST)WITH 1PHASE SUPPLY(1PHASE AND NEUTRAL(230V)) OR WITH 2-PHASE SUPPLY(2PHASES(440V)) 1ST STEP OF MBT: GIVE POWER SUPPLY 1-PHASE OR 2 PHASE BETWEEN 1U AND 1V ,MEASURE THE VOLTAGES BETWEEN 1V-1W AND 1U-1W AND ALSO 2U-2N,2V-2N,2W-2N.. SO HERE IF THE CORE IS CORRECTLY BALANCED THEN WE WILL GET RESULTS AS FOLLOWS 1U-1V 1V-1W 1U-1W 2U-2N 2V-2N 2W-2N 440 300 140 90 50 40 THAT MEANS HV SIDE : 1U-1V = 1V-1W + 1U-1W I.E 440V = 300 + 140 AND ALSO LV SIDE : 2U-2N = 2V-2N + 2W-2N 90 = 50 + 40 STEP 2: AFTER DOING THIS ..INTERCHANGE THE CONNECTIONS BETWEEN 1V-1W AND MEASURE THE VALUES AS ABOVE... STEP 3: AFTER DOING THIS ..INTER CHANGE THE CONNECTIONS BETWEEN 1U-1W AND MEASURE THE VALUES AS ABOVE... IF ANY FAULT IS THERE IN TRANSFORMER WE CAN NOT FIND GOOD RESULTS.... IF THE WINDING IS DAMAGED .... WE WILL GET ZERO VALUES .... ex: HV SIDE 1U-1V = 1V-1W + 1U-1W 440V = 440 + 0 AND ALSO LV SIDE : 2U-2N = 2V-2N + 2W-2N 90 = 90 + 0 THIS IS THE ACTUAL APPLICATION OF MANGNETICA BALANCE TEST...
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