7110031-Tf-Testing[2].ppt

TESTING OF TRANSFORMER

The objective of testing is

To ensure quality

To 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

Reference Standards on transformers

Indian Standards

IS: 2026: Part 1- Part 5 -- Specification for power Transformers

IS: 1180: Part 1&2 -- Specification for out door type, 3 phase, sealed

& non sealed distribution transformers up to & including 100 kVA,

11 kV

IS: 11171 -- Specification for Dry Type transformers

IS: 6209 -- Method for Partial Discharge Measurement

IS: 2071: Part 1 - Part 3 -- Methods of High Voltage test

IS: 13964 -- Method of Sound level measurement.

STANDARDS

IEC Standards IEC 60076 (Part 1- Part 14)- Specification for Transformers

IEEE / ANSI Standards ANSI/IEEE: C.57.12.00 -- General requirements for oil filled power &

distribution transformers ANSI/IEEE: C.57.12.01-- General requirements for Dry type power &

Distribution Transformers ANSI/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 PRECAUTIONS

Cordoning 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 %

TESTS

Special 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 check Measurement of no-load Loss and no load current Measurement 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 check

Measurement of No-load Loss and No load current

Measurement 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 Index

I = 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 Transformer• Voltmeter/Multimeter• Ratio meter• Connecting 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 Ratio• 2. Ratio Meter method:• Use Turns Ratio meters to get the Ratio / % deviation directly.

ROUTINE TESTS Polarity test

Purpose:

To check the polarity of single phase transformersInstruments: Single phase Auto transformer Voltmeter / 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 H2 Measure Voltage between H2 & X2Conditions: If H1 H2 < H2 X2 Additive Polarity. If H1 H2 > H2 X2 Subtractive Polarity.

ROUTINE TESTSVector Group Verification

Purpose: To check the Vector Group of three phase transformers

Instruments: Three phase Auto transformer Voltmeter / 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 measured Check whether the Voltages measured are satisfying the conditions for respective

vector group.

Conditions for Dyn 11: 1V-2w = 1V-2v 1W-2w < 1W-2v 1U-2n + 1V-2n

ROUTINE TESTS

Measurement 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 Bridge Connecting 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 temperature Compare with the design value.

ROUTINE TESTSMeasurement of No load loss and No Load Current

Purpose: 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 Transformer Voltmeters, 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 circuited The 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 transformer To estimate the energy loss in the transformerInstruments: Auto Transformer Voltmeters, 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 circuited Inject 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 TESTS

DIELECTRIC 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

winding

Instruments: High Voltage Test set

Procedure: 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 TESTS

Appropriate 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 TESTS

Induced Over Voltage Withstand Test

Purpose: To check adequacy of inter turn/inter layer insulation

Instrument: High frequency Motor-Generator Set. Connecting Cables.

Procedure: Make the connections as per the circuit diagram. Keep the HV winding open circuited The 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 frequency

After completion of the test, the transformer is

discharged.

TYPE TESTTemperature Rise Test

Purpose: To confirm that the temperature rise of the windings and oil does not

exceed the specified limits under normal service conditions.

Instruments: Auto Transformer Voltmeters, Ammeters, Watt meters, Thermometers, Current

Transformers and Potential Transformers. Connecting cables/ Shorting links

Procedure: 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 Test

Short Circuit method:

• Measure cold Resistance • Make the connections as per the circuit diagram.• Keep the LV winding short circuited• The HV is excited such that the total losses are fed to the transformer• Maintain 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.