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Transformer

How to Test Percentage Impedance of Transformer?

  • Categories Transformer, Electrical Machines, Measurement, Power System, Substation
Transformer Percentage Impedance Testing

This article covers transformer impedance testing, one of the acceptances tests for new transformers, and a method for routine testing. The results of this test should compare directly to the manufacturer’s datasheet.

Related: Percentage Impedance of Transformer

Table of Contents

    • Application of Transformer Impedance Test
    • Tools for Impedance Test
  • Transformer Impedance Test Procedure
  • Impedance Test of Three Phase Three Winding Transformer
    • Calculation of Percentage Impedance
    • Related Articles

Application of Transformer Impedance Test

The Transformer Impedance Test locates problems within the transformer winding.

The tests in this task procedure apply to

  • Single-phase transformers
  • Three-phase two-winding transformers with and without buried tertiaries
  • Three-phase three-winding transformers
  • Three-phase autotransformers with and without buried tertiaries

The results of impedance tests performed in the field should be within 5% of manufacturer’s data values. Beyond this range should be investigated by further tests.

Tools for Impedance Test

The following tools are used to conduct Transformer Impedance Test

  • RMS voltmeter
  • RMS ammeter, or two multifunction meters with 0.5% accuracy
  • 0-105 V ac variable power supply
  • 20 A 120V, single-phase sources
  • Jumpers of various lengths

Transformer Impedance Test Procedure

As mentioned above transformer impedance testing procedure covers single-phase transformers, three-phase two-winding transformers with and without burred tertiary windings, and three-phase two-winding autotransformers.

The test procedure is explained below.

  1. Set the no-load tap changer to the nameplate voltage before proceeding with the test.
  2. Short-circuit the secondary terminals together and connect the voltmeters and ammeters to the high-side bushings.
  3. Connect the variable power supply and slowly increase the voltage to 120 volts. Watch for the overcurrent range of the ammeter, change the scale if necessary.
  4. Record the voltage, current, and the combination of high-side bushings (eg HI-H2, HI-H3, or H2-H3).
  5. Disconnect the variable power supply and move the power leads to the next high-side (or low-side) bushing combination. Repeat Steps 3, 4, and 5 until all combinations have been recorded.
  6. Calculate the percentage impedance.

Impedance Test of Three Phase Three Winding Transformer

The procedure for transformer impedance measurement of three-phase three-winding transformers and three-phase three-winding autotransformers is the same as for two-winding transformers, except that there is a total of nine sets of measurements instead of three.

For 3-phase three-winding transformers, three measurements are made by the short circuit test.

Zps = Leakage impedance measured rn the primary with the secondary short-circuited and the tertiary open.
Zpt = Leakage impedance measured in the primary with the secondary open and the tertiary short-circuited.
Zst = Leakage impedance measured in the secondary with the primary open and the tertiary short-circuited.

  1. Set the no-load tap changer to the nameplate voltage before proceeding with the test.
  2. Short-circuit the secondary terminals together and connect the voltmeters and ammeters to the high-side bushings
  3. Connect the variable power supply and slowly increase the voltage to 120 volts. Watch for the overcurrent range of the ammeter, change the scale if necessary.
  4. Record the voltage, current, and the combination of high-side bushings (eg HI-H2, HI-H3, or H2-H3).
  5. Disconnect the variable power supply and move the power leads to the next high-side (or low-side) bushing combination. Repeat Steps 3, 4, and 5 until all combinations have been recorded.
  6. Calculate the percentage Impedance. The measurements must refer to the high side and the same MVA base.

Calculation of Percentage Impedance

Z% for single phase transformers

Z for single phase transformers

Z% for 3-phase two winding transformers and auto transformers

Z for 3 phase two winding transformers and auto transformers

Z% for 3-phase three-winding transformer and auto transformers

Z for 3 phase three winding transformer and auto transformers

For Zst

For Zst

NOTE: MVA and KV11 refer to the primary side of transformers.

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Tesla is an Electrical Engineer, Physicist and an Inventor in making. He is a good writer and author of many courses and articles published in this site.

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    1 Comment

  1. Sanket
    December 2, 2021
    Reply

    Hi I want to calculate %impedance of three phase transfomer. I/P V is 208VAC (Ph-Ph), I/P Current 230A, kVA rating is 63kVA, Output is 480VAC (Ph-Ph), O/P Current 100A. What tester should I use to increase voltage 0 to required voltage to flow rated current in shorted secondary? If %impedance is 4% then I would need variac which gives me 230A at 8.32V which is hard to find. Please help me understand what tester is used in industry to do this testing.

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