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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
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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.
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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.
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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.
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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 %
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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.
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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
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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.
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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.
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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.
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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
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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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