Testing Of Power Transformers

TESTING OF HIGH VOLTAGE POWER TRANSFORMERS

NECESSITY OF TESTS ON TRANSFORMER WHEN ALL MANUFACTURING PROCESSES HAVE BEEN COMPLETED, TESTS ARE PERFORMED ON TRANSFORMER AT THE MANUFACTURER’S WORKS TO ENSURE THE FOLLOWING PURPOSES: 

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 LATTER PURPOSE ARE REFERRED TO AS ROUTINE TESTS (CARRIED OUT ON EVERY UNIT MANUFACTURED). IN ADDITION TO THE AFORESAID TWO CATEGORY OF TESTS, SPECIAL TESTS MAY ALSO BE PERFORMED TO OBTAIN INFORMATION USEFUL TO THE USER DURING OPERATION OR MAINTENANCE OF THE TRANSFORMER.

TESTS THE FOLLOWING TESTS ARE GENERALLY PERFORMED ON THE TRANSFORMER REFERENCE STANDARD : IEC 60076 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  SEPARATE-SOURCE VOLTAGE WITHSTAND TEST  INDUCED AC OVER VOLTAGE WITHSTAND TEST WITH PARTIAL DISCHARGE MEASUREMENT  MAGNETIC CIRCUIT (ISOLATION) TEST

TYPE TESTS  TEMPERATURE RISE TEST  MEAUREMENT OF POWER TAKEN BY WATER PUMPS  DISSOLVED GAS ANALYSIS ( DGA ) OF OIL FILLED IN THE TRANSFORMER

SPECIAL TESTS  MEASUREMENT OF ACOUSTIC SOUND LEVEL  DETERMINATION OF CAPACITANCES AND TAN DELTA BETWEEN WINDING-TO-EARTH AND BETWEEN WINDINGS

MECHANICAL TESTS  VACUUM TEST ON TRANSFORMER TANK  OIL PRESSURE TEST ON COMPLETELY ASSEMBLED TRANSFORMER

MEASUREMENT OF WINDING RESISTANCE RESISTANCE MEASUREMENT HELPS TO DETERMINE THE FOLLOWING  CALCULATION OF THE I2R LOSSES.  CALCULATION OF WINDING TEMPERATURE AT THE END OF A TEMPERATURE RISE TEST.  AS A BASE FOR ASSESSING POSSIBLE DAMAGE IN THE FIELD.



  

A) MEASUREMENT IS MADE WITH DIRECT CURRENT, AND SIMULTANEOUS READINGS OF CURRENT AND VOLTAGE ARE TAKEN. B) TO MINIMIZE ERRORS OF OBSERVATION: 1) THE MEASURING INSTRUMENTS SHALL HAVE SUCH RANGES AS WILL GIVE REASONABLY LARGE DEFLECTION. 2) THE POLARITY OF THE CORE MAGNETIZATION SHALL BE KEPT CONSTANT DURING ALL RESISTANCE READINGS. THE VOLTMETER LEADS SHALL BE INDEPENDENT OF THE CURRENT LEADS AND SHALL BE CONNECTED AS CLOSELY AS POSSIBLE TO THE TERMINALS OF THE WINDING TO BE MEASURED. THIS IS TO BE AVOID INCLUDING IN THE READING THE RESISTANCE OF CURRENTCARRYINGLEADS, THEIR CONTACTS AND EXTRA LENGTH OF LEADS. READINGS SHALL NOT BE TAKEN UNTIL AFTER THE CURRENT AND VOLTAGE HAVE REACHED STEADY-STATE VALUES. READINGS SHALL BE TAKEN WITH NOT LESS THAN FOUR VALUES OF CURRENT WHEN DEFLECTING INSTRUMENTS ARE USED. THE CURRENT USED SHALL NOT EXCEED 15% OF THE RATED CURRENT OF THE WINDING WHOSE RESISTANCE IS TO BE MEASURED. LARGER VALUES MAY CAUSE INACCURACY BY HEATING THE WINDING AND

POLARITY BY ALTERNATING-VOLTAGE TEST  THE LINE TERMINAL OF HIGH VOLTAGE WINDING (1.1 ) SHALL BE CONNECTED TO THE ADJACENT LINE TERMINAL LOW-VOLTAGE LWINDING (2.1) AS SHOWN IN FIGURE BELOW 1.1

2.1

1.2

2.2

(a)

 ANY CONVENIENT VALUE OF ALTERNATING VOLTAGE SHALL BE APPLIED TO THE FULL HIGH-VOLTAGE WINDING AND READINGS SHALL BE TAKEN OF THE APPLIED VOLTAGE AND THE VOLTAGE BETWEEN THE RIGHT-HAND ADJACENT HIGHVOLTAGE AND LOW-VOLTAGE LEADS.  WHEN THE LATER READING IS GREATER THAN THE FORMER, THE POLARITY IS ADDITIVE.  WHEN THE LATER READING IS LESS THAN THE FORMER (INDICATING THE APPROXIMATE DIFFERENCE IN VOLTAGE BETWEEN THAT OF THE HIGH-VOLTAGE AND LOW-VOLTAGE WINDINGS), THE POLARITY IS SUBTRACTIVE.

Verification of vector group

1U 2U 2W 1W Dy-I

2V

1V

FIGURE FOR HV-DELTA / LV-STAR TRANSFORMER CONNECT 1U TO 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION 1W-2V= 1W-2W 1W-2V< 1W-1U 1V-2V<1V-2W 1V-2V <1U-1W

CONNECT 1U TO 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION 1W-2W = 1V-2W 1W-2V > 1V-2V 1U-N = (1U-2W)+(2W-N)

MEASUREMENT OF NO-LOAD LOSS AND EXCITATION CURRENT  NO-LOAD (EXCITATION) LOSSES ARE THOSE LOSSES THAT ARE INCIDENT TO THE EXCITATION OF THE TRANSFORMER. NO-LOAD (EXCITATION) LOSSES INCLUDE CORE LOSS, DIELECTRIC LOSS, CONDUCTOR LOSS IN THE WINDING DUE TO EXCITATION CURRENT, AND CONDUCTOR LOSS DUE TO CIRCULATING CURRENT IN PARALLEL WINDINGS. THESE LOSSES CHANGE WITH THE EXCITATION VOLTAGE.

MEASUREMENT OF SHORT-CIRCUIT IMPEDANCE AND LOAD LOSS  THE LOAD LOSSES OF A TRANSFORMER ARE THOSE LOSSES INCIDENT TO A SPECIFIED LOAD CARRIED BY THE TRANSFORMER. LOAD LOSSES INCLUDE I2R LOSS IN THE WINDINGS DUE TO LOAD CURRENT AND STRAY LOSSES DUE TO EDDY CURRENTS INDUCED BY LEAKAGE FLUX IN THE WINDINGS, CORE CLAMPS, MAGNETIC SHIELD, TANK WALLS AND OTHER CONDUCTING PARTS. STRAY LOSSES MAY ALSO BE CAUSED BY CIRCULATING CURRENTS IN PARALLEL WINDINGS OR STRANDS. LOAD LOSSES ARE MEASURED BY APPLYING A SHORT CIRCUIT ACROSS EITHER THE HIGH VOLTAGE WINDING OR THE LOW VOLTAGE WINDING AND APPLYING SUFFICIENT VOLTAGE ACROSS THE OTHER WINDING TO CAUSE A SPECIFIED CURRENT TO FLOW IN THE WINDINGS. THE POWER LOSS WITHIN THE TRANSFORMER UNDER THESE CONDITIONS EQUALS THE LOAD LOSSES OF THE TRANSFORMER AT THE TEMPERATURE OF TEST FOR THE SPECIFIED LOAD CURRENT.  THE IMPEDANCE VOLTAGE OF A TRANSFORMER BETWEEN A PAIR OF WINDINGS IS THE VOLTAGE REQUIRED TO CIRCULATE RATED CURRENT THROUGH ONE OF TWO SPECIFIED WINDINGS WHEN THE OTHER WINDING IS SHORT CIRCUITED, WITH THE WINDINGS CONNECTED AS FOR RATED VOLTAGE OPERATION. IMPEDANCE VOLTAGE IS USUALLY EXPRESSED IN PER UNIT OR PERCENT OF THE RATED VOLTAGE OF THE WINDING ACROSS WHICH THE VOLTAGE IS APPLIED AND MEASURED.

WATTMETER-VOLTMETER-AMMETER METHOD FOR LOAD LOSS AND IMPEDANCE VOLTAGE TEST 

FOR THREE PHASE TRANSFORMERS, THREE-PHASE POWER MEASUREMENT UTILIZING TWO WATTMETER IS POSSIBLE BUT CAN RESULT IN VERY LARGE ERRORS AT LOW POWER FACTORS ENCOUNTERED IN LOAD LOSS TESTS OF TRANSFORMERS. IT IS RECOMMENDED THAT THE TWO-WATTMETER METHOD SHOULD NOT BE USED FOR LOSS TESTS ON THREE-PHASE TRANSFORMERS OF SMALL RATINGS PREFERABLY BELOW 20 MVA, 66 KV CLASS.

MEASUREMENT OF INSULATION RESISTANCE 

INSULATION RESISTANCE TESTS ARE MADE TO DETERMINE THE INSULATION RESISTANCE FROM INDIVIDUAL WINDING TO GROUND OR BETWEEN INDIVIDUAL WINDINGS. THE INSULATION RESISTANCE IN SUCH TESTS IS COMMONLY MEASURED IN MEGA-OHMS, OR MAY BE CALCULATED FROM MEASUREMENTS OF APPLIED VOLTAGE AND LEAKAGE CURRENT

NOTE 

THE INSULATION RESISTANCE OF ELECTRICAL APPARATUS IS SUBJECTED TO WIDE VARIATION IN DESIGN, TEMPERATURE, DRYNESS, AND CLEANLINESS OF THE PARTS. WHEN THE INSULATION RESISTANCE FALLS BELOW PRESCRIBED VALUES, IT CAN, IN MOST CASES OF GOOD DESIGN AND WHERE NO DEFECT EXISTS, BE BROUGHT UP TO THAT REQUIRED STANDARD BY CLEANING AND DRYING THE APPARATUS. THE INSULATION RESISTANCE, THEREFORE, MAY OFFER A USEFUL INDICATION AS TO WHETHER THE APPARATUS IS IN SUITABLE CONDITION FOR APPLICATION OF DIELECTRIC TESTS.

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UNDER NO CONDITIONS, TEST SHOULD BE MADE WHILE THE TRANSFORMER IS UNDER VACUUM.

LIGHTNING IMPULSE WITHSTAND VOLTAGE TEST

T1 = 1.2 Micro Seconds ± 30 % T2 = 50 Micro Seconds ± 20 %

SWITCHING IMPULSE WITHSTAND VOLTAGE TEST

VIRTUAL FRONT TIME OF AT LEAST 100 µS TIME ABOVE 90% OF SPECIFIED AMPLITUDE 200 µS TOTAL DURATION OF AT LEAST 500 µS

SEPARATE SOURCE VOLTAGE WITHSTAND TEST  



A NORMAL POWER FREQUENCY, SUCH AS 50 HZ, SHALL BE USED AND THE DURATION OF THE TEST SHALL BE ONE MINUTE. THE WINDING BEING TESTED SHALL HAVE ALL ITS PARTS JOINED TOGETHER AND CONNECTED TO THE TERMINAL OF THE TESTING TRANSFORMER. ALL OTHER TERMINALS AND PARTS (INCLUDING CORE AND TANK) SHALL BE CONNECTED TO GROUND AND TO THE OTHER TERMINAL OF THE TESTING TRANSFORMER.

INDUCED AC VOLTAGE WITHSTAND TESTS WITH PARTIAL DISCHARGE MEASUREMENT

C B

D

A

Ustart

E

1.1Um/ 3

A = 5 min B = 5 min C = test time D = 5 min for ACSD E = 5 min

U2

and

U1

60 min

U2

1.1Um/ 3

<Ustart

PD LEVEL <500 PC

for ACLD

Time sequence for the application of test voltage with respect to earth

C

 120 

RATED FREQUENCY TEST FREQUENCY

TEMPERATURE RISE TEST  TO ESTABLISH THE TOP OIL TEMPERATURE RISE IN STEADY STATE CONDITION WITH DISSIPATION OF TOTAL PROCESS.  TO ESTABLISH THE AVERAGE WINDING TEMPERATURE RISE AT RATED CURRENT AND WITH THE TOP OIL TEMPERATURE RISE

JOB CONDITION  COMPLETED ASSEMBLED WITH COOLERS

TYPE OF COOLING  ONAN, ONAF, ONAF, OFWF  WATER COOLED TRANSFORMER

TEST CONDITION  RATED POWER  ONE WINDING SHORTED ( AS LIKE LOAD LOSS)

TEMPERATURE RISE  OIL RISE AND WINDING RISE

TOTAL LOSS INJECTION FOR DETERMINATION OF OIL RISE  FIRST THE TOP OIL AND AVERAGE OIL TEMPERATURE RISES ARE ESTABLISHED WHEN THE TRANSFORMER IS SUBJECTED TO ATEST VOLTAGE SUCH THAT THE MEASURED ACTIVE POWER IS EQUALS TO THE TOTAL LOSSES OF THE TRANSFORMER  THE TEST CURRENT WILL BE DONE AT ABOVE RATED CURRENT TO THE EXTENT NECESSARY FOR PRODUCING AN ADDITIONAL AMOUNT OF LOSS EQUAL TO THE NO-LOAD LOSS AND THE WINDING TEMPERATURE RISE WILL BE CORRESPONDINGLY ELEVATED.  THE OIL TEMPERATURE AND COOLING MEDIUM TEMPERATURE ARE MONITORED, AND THE TEST IS CONTINUED UNTIL A STEADY STATE OIL TEMPERATURE RISE ESTABLISHED

RATED CURRENT INJECTIONS  WHEN THE TOP OIL TEMPERATURE RISE HAS BEEN ESTABLISHED, THE TEST SHALL BE IMMEDIATELY CONTINUE WITH THE TEST CURRENT TO RATED CURRENT FOR THE WINDING COMBINATION CONNECTED. THIS CONDITION IS MAINTAINED FOR ONE HOUR WITH CONTINUOUS OBSERVATION OF OIL AND COOLING MEDIUM TEMPERATURE

 

STEADY STATE CONDITION SHUT DOWN

MEASUREMENT OF ACOUSTIC SOUND LEVEL 

THIS TEST SHALL BE DONE IN ACCORDANCE WITH THE CLAUSES GIVEN IN NEMA TR1 AND IEC-60076-10. THE DETAILED TEST PROCEDURE IS GIVEN IN THE ANSI/IEEE STANDARD, WHICH HAS BEEN APPROVED BY NEMA.

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Microphone Location Fan cooled surface 2m

TANK Box

Drain Plug

Reference sound producing source

Measurement surface 1m

0.3 m

2/3 Height 1/3 Height

Height

Microphone location for measuring audible sound from transformers

DETERMINATION OF CAPACITANCE AND DISSIPATION FACTOR BETWEEN WINDING TO EARTH AND BETWEEN WINDINGS CAPACITANCE AND TAN DELTA ARE USUALLY DETERMINED FOR WINDING TO EARTH AND BETWEEN WINDINGS BY BRIDGE MEASURING TECHNIQUE, SUCH AS SCHERING BRIDGE.

METHOD I TEST WITHOUT GUARD

METHOD II TEST WITH GUARD

TWO WINDING HV TO LV AND GROUND LV TO HV AND GROUND HV TO LV TO GROUND

TWO WINDING HV TO LV AND GROUND LV TO HV AND GROUND HV TO LV TO GROUND LV TO GROUND, GUARD TO HV

THREE WINDING HV TO IV ,LV AND GROUND IV TO HV ,LV AND GROUND LV TO IV ,HV AND GROUND HV AND IV TO LV AND GROUND HV AND LV TO IV AND GROUND IV AND LV TO HV AND GROUND HV, IV AND LV TO GROUND

THREE WINDING HV TO LV AND GROUND, GUARD ON IV HV TO GROUND, GUARD ON LV & IV LV TO IV & GROUND, GUARD ON HV LV TO GROUND, GUARD ON HV & IV IV TO HV & GROUND, GUARD ON LV IV TO GROUND, GUARD ON HV & LV HV & LV TO IV & GROUND HV & IV TO LV & GROUND

DISSOLVED GAS ANALYSIS (DGA) OF OIL FILLED IN THE TRANSFORMER DGA HAS BEEN USED AS A TOOL IN TRANSFORMER DIAGNOSTICS IN ORDER TO DETECT INCIPIENT FAULTS, TO SUPERVISE SUSPECT TRANSFORMERS, TO TEST A HYPOTHESIS OR EXPLANATION FOR THE PROBABLE REASONS OF FAILURES OR DISTURBANCES WHICH HAVE ALREADY OCCURRED AND TO ENSURE THAT NEW TRANSFORMERS ARE HEALTHY. THE GASSES THAT ARE OF INTEREST FOR THE DGA ANALYSIS ARE THE FOLLOWING;  H2 HYDROGEN  CH4 METHANE  C2 H4 ETHYLENE  C2H6 ETHANE  C2H2 ACETYLENE  (C3H6 PROPENE )– NOT ALWAYS MEASURE  (C3H8 PROPANE ) – NOT ALWAYS MEASURE  CO CARBON MONOXIDE  CO2 CARBON DIOXIDE  O2 OXYGEN  N2 NITROGEN  TCG TOTAL COMBUSTIBLE GAS CONTENT (= H2 + CH4 + C2H4 + C2H6 + C2H2 + CO) 

PROCEDURE THE DGA PROCEDURE CONSISTS OF ESSENTIAL FOUR STEPS: •SAMPLING OF OIL FROM THE TRANSFORMER •ANALYSIS OF THESE GASES FROM OIL •ANALYSIS OF THE EXTRACTED GAS MIXTURE IN A GAS CHROMATOGRAPHY, GC. •INTERPRETATION OF THE ANALYSIS ACCORDING TO AN EVALUATION SCHEME. OVERHEATING OF CELLULOSE CO, CO2 OVERHEATING OF OIL INCREASING TEMPERATURE C2H6 C2H4 C2H4 CH4 CH4 CH4 C2H2 PARTIAL DISCHARGES (PD) INCREASING TEMPERATURE H2 H2 C2H2 DISCHARGES C2H2 H2

 MEASUREMENT OF POWER TAKEN BY THE WATER PUMP MOTORS - THE MEASUREMENT SHALL BE DONE BY SUITABLE INSTRUMENTS AT RATED VOLTAGE

TESTS THE FOLLOWING TESTS ARE GENERALLY PERFORMED ON THE TRANSFORMER REFERENCE STANDARD : IEC 60076 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 WITHSTAND VOLTAGE TEST  LIGHTNING IMPULSE WITHSTAND VOLTAGE TEST  SEPARATE-SOURCE WITHSTAND VOLTAGE TEST  INDUCED AC OVER VOLTAGE WITHSTAND TEST WITH PARTIAL DISCHARGE MEASUREMENT

TYPE TESTS  TEMPERATURE RISE TEST  MEAUREMENT OF POWER TAKEN BY WATER PUMPS

SPECIAL TESTS  MEASUREMENT OF ACOUSTIC SOUND LEVEL  DETERMINATION OF CAPACITANCES AND TAN DELTA BETWEEN WINDINGTO-EARTH AND BETWEEN WINDINGS

ADDITIONAL TESTS  TEST WITH LIGHTNING IMPULSE CHOPPED ON THE TAIL  MAGNETIC CIRCUIT (ISOLATION) TEST  MAGNETIC BALANCE TEST ON THREE-PHASE TRANSFORMERS  DETERMINATION OF TRANSIENT VOLTAGE TRANSFER CHARACTERISTICS  DISSOLVED GAS ANALYSIS ( DGA ) OF OIL FILLED IN THE TRANSFORMER

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

FIELD TESTS  DEW POINT MEASUREMENT FOR LARGE TRANSFORMER FILLED WITH DRY AIR OR NITROGEN FILLED  WINDING RESISTANCE MEASUREMENT  VECTOR GROUP AND POLARITY  VOLTAGE RATIO TEST  MEASUREMENT OF MAGNETIZING CURRENT  MAGNETIC BALANCE TEST ON THREE PHASE TRANSFORMER  MAGNETIC CIRCUIT (ISOLATION) TEST  MEASUREMENT OF SHORT CIRCUIT IMPEDANCE AT LOW VOLTAGE  INSULATION RESISTANCE MEASUREMENT  MEASUREMENT OF CAPACITANCE AND DISSIPATION FACTOR 

DISSOLVED GAS ANALYSIS ( DGA )

 TESTS ON OIL FILLED IN TRANSFORMER AS PER IS 1866