Transformer Testing (Ratio Test and vector group test) | Vector Group Test of Power Transformer

Types Of Transformer Testing part-1

Transformer Testing part-2 (Insulation resistance )


  • There is various Test required on the Transformer to confirm its performance of the Transformer.
  • Mainly two types of transformer are done by the manufacturer before dispatching the transformer mainly (1) Type test of the transformer and (2) Routine test.
  • In addition, some other tests are also carried out by the consumer at the site before commissioning and also periodically on a regular & emergency basis throughout its life.
  • Transformer Testing mainly classified in
  • Transformer Tests are done by Manufacturer
  • (A) Routine Tests
  • (B)Type Tests
  • (C) Special Tests
  • Transformer Tests were done at the Site
  • (D) Pre-Commissioning Tests
  • (E) Periodic/Condition Monitoring Tests
  • (F) Emergency Tests

(A) Routine tests:

  • A Routine test of a transformer is mainly for confirming the operational performance of the individual unit in a production lot. Routine tests are carried out on every unit manufactured.
  • All transformers are subjected to the following Routine tests:
  • Insulation resistance Test.
  • Winding resistance Test.
  • Turns Ration / Voltage Ratio Test
  • Polarity / Vector Group Test.
  • No-load losses and current Test.
  • Short-circuit impedance and load loss Test.
  • Continuity Test
  • Magnetizing Current Test
  • Magnetic Balance Test
  • High Voltage Test.
  • Dielectric tests
  • Separate source AC voltage.
  • Induced overvoltage.
  • Lightning impulse tests.
  • Test on On-load tap changers, where appropriate.

 (B) Type tests

  • Type tests are tests made on a transformer that is representative of other transformers to demonstrate that they comply with specified requirements not covered by routine tests:
  • Temperature rise test (IEC 60076-2).
  • Dielectric type tests (IEC 60076-3).

 (C) Special tests

  • Special tests are tests, other than routine or type tests, agreed upon between the manufacturer and purchaser.
  • Dielectric special tests.
  • Zero-sequence impedance on three-phase transformers.
  • Short-circuit test.
  • Harmonics on the no-load current.
  • Power was taken by the fan and oil pump motors.
  • Determination of sound levels.
  • Determination of capacitances between windings and earth, and between windings.
  • Determination of transient voltage transfer between windings.
  • Tests intended to be repeated in the field to confirm no damage during shipment, for example, frequency response analysis (FRA).

(D) Pre-commissioning Tests

  • The Test performed before commissioning the transformer at the site is called the pre-commissioning test of the transformer. These tests are done to assess the condition of the transformer after installation and compare the test results of all the low voltage tests with the factory test reports.
  • All transformers are subjected to the following 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
  • Hipot test

Transformer Testing part-2 (Insulation resistance )

(A) Routine tests of the Transformer

(1) Insulation Resistance Test:

 Test Purpose:

  • The insulation resistance test of a transformer is essential to ensure the healthiness of the overall insulation of an electrical power transformer.

 Test Instruments:

  • For LT System: Use 500V or 1000V Megger.
  • For MV / HV System: Use 2500V or 5000V Megger.

 Test Procedure:

  • First, disconnect all the line and neutral terminals of the transformer.
  • Megger leads to be connected to LV and HV bushing studs to measure Insulation Resistance (IR) value in between the LV and HV windings.
  • 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.
  • 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.
  • NB: It is unnecessary to perform an insulation resistance test of the transformer per phase wise in a three-phase transformer. IR values are taken between the windings collectively because all the windings on HV side are internally connected together to form either star or delta and also all the windings on the LV side are internally connected together to form either star or delta.
  • Measurements are to be taken as follows:
Type of TransformerTesting-1Testing-2Testing-3
Auto TransformerHV-LV to LVHV-IV to ELV to E
Two Winding TransformerHV to LVHV to ELV to E
Three Winding TransformersHV to LVLV to LVHV to E & LV to E
  • Oil temperature should be noted at the time of the insulation resistance test of the transformer. Since the IR value of transformer insulating oil may vary with temperature.
  • IR values are to be recorded at intervals of 15 seconds, 1 minute, and 10 minutes.
  • With the duration of the application of voltage, the IR value increases. The increase in IR is an indication of the dryness of the insulation.
  • Absorption Coefficient = 1-minute value/15-second value.
  • Polarization Index = 10 minutes value / 1-minute value

 Tests can detect:

  • Weakness of Insulation.

 (2) D.C. Resistance or Winding Resistance Test

 Test Purpose:

  • Transformer winding resistance is measured
  • To check any abnormalities like Loose connections, broken strands, and High contact resistance in tap changers
  • To Calculation of the I2R losses in the transformer.
  • To Calculation of winding temperature at the end of the temperature rise test of the transformer.

 Test Instrument:

  • The Resistance of HV winding and LV winding between their terminals are to be measured with
  • Precision milliohm meter/ micro ohm meter / Transformer Ohmmeter. OR
  • Wheatstone bridge or DC resistance meter.

 Method No: 1 (Kelvin Bridge Method for measurement of winding resistance)


Test Procedure:

  • The main principle of the bridge method is based on comparing an unknown resistance with a known resistance.
  • When electric currents flowing through the arms of the bridge circuit become balanced, the reading of the galvanometer shows zero deflection that means at a balanced condition no electric current will flow through the galvanometer.
  • A very small value of resistance (in the milliohms range) can be accurately measured by the Kelvin Bridge method whereas for higher values Wheatstone bridge method of resistance measurement is applied. In the bridge method of measurement of winding resistance, the error is minimized.
  • All other steps to be taken during transformer winding resistance measurement in these methods are similar to that of current-voltage method of measurement of winding resistance of transformer

 Method No: 2 (current-voltage method of measurement of winding resistance)

Test Procedure:

  • The resistance of each transformer winding is measured using DC current and recorded at an ambient temp.
  • In this test resistance of winding is measured by applying a small DC voltage to the winding and measuring the current through the same
  • The measured resistance should be corrected to a common temperature such as 75°C or 85°C using the formula: RC=RM x ((CF+CT)/(CF+WT))
  • where
  • RC is the corrected resistance, RM is the measured resistance
  • CF is the correction factor for copper (234.5) or aluminum (225) windings
  • CT is the corrected temperature (75°C or 85°C)
  • WT is the winding temperature (°C) at the time of the test
  • Before measurement, the transformer should be kept in OFF condition at least for 3 to 4 hours so this time the winding temperature will become equal to its oil temperature.
  • To minimize observation errors, the polarity of the core magnetization shall be kept constant during all resistance readings.
  • Voltmeter leads shall be independent of the current leads to protect them from high voltages which may occur during switching on and off the current circuit.
  • The readings shall be taken after the electric current and voltage have reached steady state values. In some cases, this may take several minutes depending on the winding impedance.
  • 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 Calculating resistance, the corresponding temperature of the winding at the time of measurement must be taken along with the resistance value.

 Required Precaution:

  • According to IEC 60076-1, in order to reduce measurement errors due to changes in temperature, some precautions should be taken before the measurement is made.
  • For Delta-connected Winding: for delta-connected transformer, the resistance should be measured for each phase (i.e. R-Y, Y-B & B-R) . Delta is composed of the parallel combination of the winding under test and a series combination of the remaining winding. It is therefore recommended to make three measurements for each phase-to-phase winding in order to obtain the most accurate results.
  • For Delta connected windings, such tertiary winding of auto-transformers measurement shall be done between pairs of line terminals and resistance per winding shall be calculated as per the formula: Resistance per Winding = 1.5 X Measured Value
  • For Star connected winding: the neutral brought out, the resistance shall be measured between the line and neutral terminal (i.e. R-N, Y-N, B-N) and an average of three sets of reading shall be the tested value. For Star-connected autotransformers the resistance of the HV side is measured between the HV terminal and the IV terminal, then between the IV terminal and the neutral.
  • For Dry-type transformers: the transformer shall be at rest at a constant ambient temperature for at least three hours.
  • For Oil immersed transformers: the transformers should be under oil and without excitation for at least three hours. In the case of tapped windings, the above readings are recorded at each tap. In addition, it is important to ensure that the average oil temperature (average of the top and bottom oil temperatures) is approximately the same as the winding temperature. Average oil temperature is to be recorded. Measured values are to be corrected to the required temperatures.
  • As the measurement current increases, the core will be saturated and inductance will decrease. In this way, the current will reach the saturation value in a shorter time.
  • After the current is applied to the circuit, it should wait until the current becomes stationary (complete saturation) before taking measurements, otherwise, there will be measurement errors.
  • The values shall be compared with the original test and result which varies with the transformer ratings.

 Test Acceptance criteria:

  • DC Resistance Should be<=2% Factory Test.
  • Test Current <10% Rated Current

 The test can detect:

  • Short Turns
  • Loose Connection of bushing
  • Loose Connection or High Contact Resistance on Tap Changer.
  • Broken winding stands

(3) Turns Ratio / Voltage Ratio Test:

 Test Purpose:

  • Turns Ratio Test / Voltage Ratio Test is done in Transformer to find out Open Circuited turns, Short Circuited turns in Transformer winding.
  • The voltage ratio is equal to the turn ratio in a transformer (V1/V2=N1/N2). Using this principle, the turn ratio is measured with the help of a turn ratio meter. If it is correct, then the voltage ratio is assumed to be correct
  • This test should be made for any new high-voltage power transformer at the time it is being installed.
  • With the use of a Turns Ratio meter (TTR), the turns Ratio between HV & LV windings at various taps is to be measured & recorded.
  • The turn ratio is a measure of the RMS voltage applied to the primary terminals to the RMS Voltage measured at the secondary terminals.
  • R= Np / Ns
  • Where,
  • R=Voltage ratio
  • Np=Number of turns at primary winding.
  • Ns= Number of turns at secondary Winding.
  • The voltage ratio shall be measured on each tapping in the no-load condition.

 Test Instruments:

  • Turns Ratio meter (TTR) to energy the transformer from a low-voltage supply and measure the HV and LV voltages.
  • Wheatstone Bridge Circuit

 Method No1 Turns Ratio Testing:

 Test Procedure:

  • Transformer Turns Ratio Meter (TTR):
  • The transformer ratio test can be done by Transformer Turns Ratio (TTR) Meter. It has in built power supply, with the voltages commonly used is very low, such as 8, 10 V, and 50 Hz.
  • The HV and LV windings of one phase of a transformer (i.e. R-Y & r-n) are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector.
  • Values are recorded at each tap in case of tapped windings and then compared to the calculated ratio at the same tap.
  • The ratio meter gives an accuracy of 0.1 percent over a ratio range up to 1110:1. The ratio meter is used in a ‘bridge’ circuit where the voltages of the windings of the transformer under test are balanced against the voltages developed across the fixed and variable resistors of the ratio meter.
  • Adjustment of the calibrated variable resistor until zero deflection is obtained on the galvanometer then gives the ratio to the unity of the transformer windings from the ratio of the resistors.
  • Bridge Circuit:
  • A phase voltage is applied to one of the windings by means of a bridge circuit and the ratio of induced voltage is measured at the bridge. The accuracy of the measuring instrument is < 0.1 %
  • 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. The reading on this indicator implies the deviation of the measured turn ratio from the expected turn ratio in percentage.
  • Theoretical Turns Ratio = HV winding Voltage / LV Winding Voltage
  • % Deviation = (Measured Turn Ratio – Expected Turns Ration) / Expected Turns Ration
  • Out-of-tolerance, ratio test of the transformer can be due to shorted turns, especially if there is an associated high excitation current.
  • Open turns in HV winding will indicate a very low exciting current and no output voltage since open turns in HV winding cause no excitation current in the winding means no flux and hence no induced voltage.
  • But open turn in LV winding causes, low fluctuating LV voltage but normal excitation current in HV winding. Hence open turns in LV winding will be indicated by normal levels of exciting current, but very low levels of unstable output voltage.
  • The turn ratio test of the transformer also detects high resistance connections in the lead circuitry or high contact resistance in tap changers by higher excitation current and difficulty in balancing the bridge.

 Test Caution:

  • Disconnect all transformer terminals from the line or load.
  • Neutrals directly grounded to the grid can remain connected

 Method No 2 Voltage Ratio Testing:

  • This test is done to check both the transformer voltage ratio and the tap changer.
  • When the “Turns Ratio meter” is not available, the Voltage Ratio Test is done at various tap positions by applying 3 phases of LT (415V) supply on the HT side of the Power transformer. In order to obtain the required accuracy it is usual to use a ratio meter rather than to energy the transformer from a low-voltage supply and measure the HV and LV voltages.
  • At Various taps applied voltage and Resultant voltages LV side between various Phases and phases& neutral measured with a precision voltmeter & noted.

 Test Procedure:

  • With 415 V applied on the high voltage side, measure the voltage between all phases on the low voltage side for every tap position.
  • First, the tap changer of the transformer is kept in the lowest position and LV terminals are kept open.
  • Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied on each phase (Phase-Phase) on HV and induced voltages at LV terminals simultaneously.
  • After measuring the voltages at HV and LV terminals, the tap changer of the transformer should be raised by one position and repeat the test.
  • Repeat the same for each of the tap positions separately.
  • At other taps values will be as per the percentage raise or lower at the respective tap positions.
  • In the case of Delta/Star transformers the ratio measure between RY-rn, YB-yn, and BR-bn.
  • Being Delta/Star transformers the voltage ratio between HV winding and LV winding in each phase limb at the normal tap is 33 KV OR 33x√3 = 5.196,11 KV / √3 11
  • At higher taps (i-e high voltage steps) less number of turns are in the circuit than normal. Hence ratio values increase by a value equal to.5.196 + {5.196 x (no. of steps above normal) x (% rise per each tap)} 100
  • Similarly for lower taps than normal, the ratio is equal to 5.196 – {5.196 x (no. of steps above normal) x (% rise per each tap)}100

 Test Acceptance Criteria:

  • The range of measured ratio shall be equal to the calculated ratio ±0.5%.
  • Phase displacement is identical to the approved arrangement and transformer’s nameplate.
  • The IEEE standard (IEEE Standard 62) states that when rated voltage is applied to one winding of the transformer, all other rated voltages at no load shall be correct within one-half of one percent of the nameplate readings. It also states that all tap voltages shall be correct to the nearest turn if the volts per turn exceed one-half of one percent desired voltage. The ratio test verifies that these conditions are met.
  • The IEC60076-1 standard defines the permissible deviation of the actual to the declared ratio
  • Principal tapping for a specified first winding pair: the lesser ±0.5% of the declared voltage ratio
  • or 0.1 times the actual short circuit impedance. Other taps on the first winding pair and other winding pairs must be agreed upon and must be lower than the smaller of the two values stated above.
  • Measurements are typically made by applying a known low voltage across the high voltage winding so that the induced voltage on the secondary is lower, thereby reducing hazards while performing the test. For three phase delta/wye or wye/delta transformer, a three-phase equivalency test is performed, i.e. the test is performed across the corresponding single winding.

 The test can detect:

  • Shorted turns or open circuits in the windings.
  • Incorrect winding connections, and other internal faults or defects in the tap changer

(4) Polarity / Vector Group Test

 Purpose of Test:

  • The vector group of transformers is an essential property for the successful parallel operation of transformers. Hence every electrical power transformer must undergo through vector group test at the transformer at factory site for ensuring the customer-specified vector group of the transformer.

 Test Instruments:

  • Ratio meter.
  • Volt Meter. A Ratio meter may not always be available and this is usually the case on-site so that the polarity may be checked by a voltmeter.

 Test Circuit Diagram:


 Test Procedure:

  • The primary and secondary windings are connected together at one point.
  • Connect the neutral point of the star connected winding with the earth.
  • A low-voltage three-phase supply (415 V) is then applied to the HV terminals.
  • Voltage measurements are then taken between various pairs of terminals as indicated in the diagram and the readings obtained should be the phasor sum of the separate voltages of each winding under consideration.

Condition:(HV side R-Y-B-N and LV Side r-y-b-n)

  • R and r should be shorted.
  • Apply 415 Volt to R-Y-B
  • Measure Voltage between Following Phase and Satisfy Following Condition
Vector GroupSatisfied Following Condition

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