TESTING OF TRANSFORMERThe objective of testing is To ensure qualityTo ensure that the products manufactured have met the requirements of Customer Specification/Standard To prevent accidents which may occur if a failed product is put into service. To ensure that the product is fit for its intended use

STANDARDSIEC StandardsIEC 60076 (Part 1- Part 14)- Specification for TransformersIEEE / ANSI StandardsANSI/IEEE: C.57.12.00 -- General requirements for oil filled power & distribution transformersANSI/IEEE: C.57.12.01-- General requirements for Dry type power & Distribution TransformersANSI/IEEE: C.57.12.20 Specification for Overhead type distribution Transformers 500 kVA and smaller & HVVoltage-67000 V and below, Low Voltage-15000V and below. ANSI/IEEE: C.57.12.90 Standard test code for oil immersed power & Distribution Transformers and short circuit testing of power & distribution transformers.

SAFETY PRECAUTIONSCordoning off the test Area.Display of Danger boards.Indication Lamps.Use of Hand gloves and safety shoes.Entry of unauthorized personnel to be restricted.Discharge the transformer after high voltage test.Earth resistance.

PREPERATION FOR THE TEST Earthing connection should be rigid to the transformer. Check for oil level Release air from the Buchholz Relay and the bushings. Clean the bushings with non-fibrous dry cloth. Ensure that all connections are tight Cables and shorting links should be of sufficient cross section.Ensure that the ranges of all instruments are correctly chosen.Ensure that the connections are made as per the circuit diagram.Ensure that all instruments are properly earthed.

TESTS Type tests -- For design validation Sample: Only on Proto type Routine tests -- Performed to ensure that a product is manufactured as per the design specification and is free from manufacturing defects. Sample: 100 %

TESTSSpecial tests -- To check whether the product meets any special requirements of the customer specification and also to know the bench mark values for preventive maintenance and trouble free operation Sample: Some tests on sample quantity. Some tests on 100 % quantity.Acceptance test -- Performed during customer inspection on samples selected at random.

TESTS ON TRANSFORMERS.Type Tests.Measurement of insulation resistance test.Measurement of winding resistance Measurement of voltage/Turns ratio Vector group/polarity checkMeasurement of no-load Loss and no load currentMeasurement of impedance voltage, short circuit impedance and Load Loss.Induced over-voltage withstand test.Separate source voltage withstand test.Temperature Rise Test Lightning Impulse Voltage Withstand Test.Dynamic and thermal ability to withstand Short circuit test

TESTS ON TRANSFORMERS.Routine Tests.Measurement of Insulation Resistance.Measurement of winding resistance. Measurement of voltage/Turns ratio. Vector Group/Polarity checkMeasurement of No-load Loss and No load currentMeasurement of impedance voltage, short circuit Impedance and Load Loss.Induced Over-Voltage Withstand Test.Separate Source Voltage Withstand Test.

TESTS ON TRANSFORMERS.Special TestsMeasurement of Capacitance and Tan Delta.Measurement of Zero Sequence ImpedanceMeasurement of Acoustic Noise LevelMeasurement of Partial Discharge.Measurement of Unbalance Current.

ROUTINE TESTSMeasurement of Insulation Resistance.

Purpose:

To ensure proper dry ness of the active parts.To ensure sufficient clearance between the windings and between winding and earth.Instruments

Insulation Tester/Megger (Hand driven/Battery operated)Connecting Cables.Procedure

Check the IR Value between HV and LV.Check the IR Value between HV and earth.Check the IR Value between LV and earth.

ROUTINE TESTS Polarization IndexI = Ic + Ia + I lc + I ls I: Total leakage current seen by the meggar Ic: Charging Current of the capacitanceIa : Dielectric absorption currentI lc : Conduction leakage current through the insulationI ls : Surface leakage current. IR Value at 600th SecondPolarization Index = ------------------------------- IR Value at 60th Second

ROUTINE TESTSMeasurement of Voltage/ Turn Ratio Purpose: To ensure correct number of turns in different windings. Instruments:Single Phase Auto TransformerVoltmeter/MultimeterRatio meterConnecting cables.Procedure: 1. Voltage Method:Apply any convenient A.C Voltage say about 200 V on HV winding and measure the Voltage at LV winding.Calculate the Ratio of HV Voltage to LV Voltage and calculate % Error comparing with the specified value Specified Ratio Actual Ratio % Ratio Error = -------------------------------------- X 100 Specified Ratio2. Ratio Meter method:Use Turns Ratio meters to get the Ratio / % deviation directly.

ROUTINE TESTS Polarity testPurpose: To check the polarity of single phase transformersInstruments:Single phase Auto transformerVoltmeter / Multi meter.Connecting CablesProcedure:Short H1 of the HV winding to the adjacent terminal X1of the LV winding.Apply any convenient A.C Voltage say about 100 V on HV Winding H1 and H2Measure Voltage between H2 & X2Conditions:If H1 H2 < H2 X2 Additive Polarity.If H1 H2 > H2 X2 Subtractive Polarity.

ROUTINE TESTSVector Group VerificationPurpose: To check the Vector Group of three phase transformersInstruments:Three phase Auto transformerVoltmeter / Multi meter.Connecting Cables.Procedure:Short 1U of HV winding to the terminal 2u of LV winding.Apply any convenient 3 phase A.C Voltage say about 400 V on HV winding.Voltages 1U-2n, 1V-2n, 1W-2w, 1W-2v, 1V-2w and 1V-2v are measuredCheck whether the Voltages measured are satisfying the conditions for respective vector group.Conditions for Dyn 11:1V-2w = 1V-2v1W-2w < 1W-2v1U-2n + 1V-2n

ROUTINE TESTSMeasurement of Winding Resistance Purpose:To calculate I2 R loss of the winding.Proper crimping, brazing and other connections can be ensured.Whether conductors used in winding are as per the designed cross section or not can be verified.Instruments:Transformer Winding Resistance meter / Ohm meter.Kelvin Bridge / Wheatstone BridgeConnecting CablesProcedure:Record the top oil temperature of the transformer.Connect current cable and potential cable of the instrument to the winding under test.Switch on the instrument and record the reading displayed by the instrument.Convert the Resistance value to the reference temperatureCompare with the design value.

ROUTINE TESTSMeasurement of No load loss and No Load CurrentPurpose:To assess the efficiency of the transformer.To estimate energy loss in the transformer.High no load current is an indicative of inter turn/inter layer insulation failureInstruments:Auto TransformerVoltmeters, Ammeters, Watt meters and Frequency meter / Digital Power Analyzer.Connecting cables.Procedure:Make the connections as per the circuit diagram.Keep the HV winding open circuitedThe LV is supplied with a single/three phase, reasonably sinusoidal voltage equal to rated voltage and rated frequency.Record the no load current and no load loss.Compare with the guaranteed values.

ROUTINE TESTSMeasurement of Load Loss and Impedance.Purpose:To assess the efficiency and regulation of the transformerTo estimate the energy loss in the transformerInstruments:Auto TransformerVoltmeters, Ammeters, Watt meters / Frequency meter Thermometer / Digital Power Analyzer.Connecting cables/ Shorting linksProcedure:Make the connections as per the circuit diagram.Keep the LV winding short circuitedInject rated current on the HV winding.Record the Load loss and Impedance Voltage.Record the top oil temperature.Convert the load loss and impedance to reference temperature.Compare with the guaranteed values.Calculate efficiency and regulation for various load and power factor.

ROUTINE TESTSDIELECTRIC TESTS These tests are to be conducted only after all other tests are conducted successfully.High Voltage Power Frequency Withstand test.Purpose:To ensure sufficient clearance between the windings and between winding and earth.To check the adequacy of main insulation to earth and between windingInstruments:High Voltage Test setProcedure:Short all line terminals of winding under test.The line terminals of the other winding and the tank are connected together to earth.

ROUTINE TESTSDIELECTRIC TESTSAppropriate test voltage as required by the applicable standard is applied to the winding under test for 60 seconds. After completion of the test, the transformer is discharged.

ROUTINE TESTSDIELECTRIC TESTSInduced Over Voltage Withstand TestPurpose:To check adequacy of inter turn/inter layer insulationInstrument:High frequency Motor-Generator Set.Connecting Cables. Procedure:Make the connections as per the circuit diagram.Keep the HV winding open circuitedThe LV is supplied with a single/three phase, reasonably sinusoidal voltage equal to twice the rated voltage at twice the rated frequency for 60 seconds.

ROUTINE TESTSDIELECTRIC TESTS If the frequency is more than the double the rated frequency, the duration is reduced as per the formula given below. 120 X Rated Frequency Duration = ------------------------------ Test frequencyAfter completion of the test, the transformer is discharged.

TYPE TESTTemperature Rise TestPurpose:To confirm that the temperature rise of the windings and oil does not exceed the specified limits under normal service conditions.Instruments:Auto TransformerVoltmeters, Ammeters, Watt meters, Thermometers, Current Transformers and Potential Transformers.Connecting cables/ Shorting linksProcedure:Direct loading method One winding of the transformer is excited at rated voltage while the other is connected to a suitable load such that the rated currents flow in both the winding.

TYPE TESTTemperature Rise Test

Back to back method Two Transformers, one of which is the transformer under test, shall be connected in parallel and excited at the rated voltage of the transformer under test. By means of injected voltage, rated current shall be made to flow in the transformer under test.

TYPE TESTTemperature Rise TestShort Circuit method:

Measure cold Resistance Make the connections as per the circuit diagram.Keep the LV winding short circuitedThe HV is excited such that the total losses are fed to the transformerMaintain hourly record of ambient temperature, top oil temperature, and readings of all ammeters, Voltmeters and Watt meters and observe increase in top oil temperature rise.Maintain this condition till the top oil temperature rise stabilizes.Once the stability is reached, reduce the supply to make the current equal to rated current and maintain for one hour. Switch off the power supply and measure the hot resistance.Extrapolate the hot resistance back to zero so as to get the resistance of the winding at the time of switch off.Calculate the temperature rise of the winding and compare with the guaranteed values.

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