How to Test Percentage Impedance of Transformer?

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.

Table of Contents

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.

Set the no-load tap changer to the nameplate voltage before proceeding with the test.
Short-circuit the secondary terminals together and connect the voltmeters and ammeters to the high-side bushings.
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.
Record the voltage, current, and the combination of high-side bushings (eg HI-H2, HI-H3, or H2-H3).
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.
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.

Z_ps = Leakage impedance measured rn the primary with the secondary short-circuited and the tertiary open.
Z_pt = Leakage impedance measured in the primary with the secondary open and the tertiary short-circuited.
Z_st = Leakage impedance measured in the secondary with the primary open and the tertiary short-circuited.

Set the no-load tap changer to the nameplate voltage before proceeding with the test.
Short-circuit the secondary terminals together and connect the voltmeters and ammeters to the high-side bushings
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.
Record the voltage, current, and the combination of high-side bushings (eg HI-H2, HI-H3, or H2-H3).
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.
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

[latex]Z\% = \frac{V_{h1h2}}{I_{sc}}\times\frac{MVA}{(kV_{11})^{2}}\times100[/latex]

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

[latex]Z\% = (\frac{V_{h1h2}}{I_{sc.h1h2}}+\frac{V_{h1h3}}{I_{sc.h1h3}}+\frac{V_{h2h3}}{I_{sc.h2h3}})\times\frac{1}{6}\times\frac{MVA}{(kV_{11})^{2}}\times100[/latex]

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

[latex]Zps\% =Zps\%= (\frac{V_{h1h2}}{I_{sc.h1h2}}+\frac{V_{h1h3}}{I_{sc.h1h3}}+\frac{V_{h2h3}}{I_{sc.h2h3}})\times\frac{1}{6}\times\frac{MVA}{(kV_{11})^{2}}\times100[/latex]

For Z_st

[latex]Zst\%= (\frac{V_{x1x2}}{I_{sc.x1x2}}+\frac{V_{x1x3}}{I_{sc.x1x3}}+\frac{V_{x2x3}}{I_{sc.x2x3}})\times\frac{1}{6}\times\frac{MVA}{(kV_{11})^{2}}\times100[/latex]

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