# How to Test Percentage Impedance of Transformer? - Link: https://studyelectrical.com/2019/08/how-to-test-percentage-impedance-of-transformer.html - Published: 2019-08-02T15:09:46+00:00 - Author: Tesla 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](https://studyelectrical.com/2018/05/percentage-impedance-of-transformer-and.html) ### 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. **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. 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](https://studyelectrical.com/wp-content/uploads/ 2019/08/Z-for-single-phase-transformers.png) **Z% for 3-phase two winding transformers and auto transformers** ![Z for 3 phase two winding transformers and auto transformers](https://studyelectrical. com/wp-content/uploads/2019/08/Z-for-3-phase-two-winding-transformers-and-auto-transformers. png) **Z% for 3-phase three-winding transformer and auto transformers** ![Z for 3 phase three winding transformer and auto transformers](https://studyelectrical. com/wp-content/uploads/2019/08/Z-for-3-phase-three-winding-transformer-and-auto- transformers.png) **For Z_(st)** ![For Zst](https://studyelectrical.com/wp-content/uploads/2019/08/For-Zst.png) NOTE: MVA and KV_(11) refer to the primary side of transformers. ### Related posts: [ ⌊Three wattmeter method of measuring three phase power⌉Measurement of Three Phase Power | Wattmeter Methods](https://studyelectrical.com/2019/01/three-phase-power-measurement-wattmeter-method.html) [ ⌊Transformer Bushing Types⌉Transformer Bushing Types : RIP Bushing vs OIP Bushing](https://studyelectrical.com/2019/03/transformer-bushing-types.html) [ ⌊Transformer testing at factory⌉Percentage Reactance in Power System (Transformer, Generator or Reactor)](https://studyelectrical.com/2023/01/percentage-reactance-in-power-system-transformer-generator-or-reactor.html) [ ⌊Generator Reactors⌉Current Limiting Reactor or Series Reactor](https://studyelectrical.com/2023/09/current-limiting-reactor-series.html) [ ⌊Delta delta connection⌉Delta-Delta (Δ–Δ) Transformer Connection: Advantages, Working, and Application](https://studyelectrical.com/2025/07/delta-delta-%ce%b4-%ce%b4-transformer-connection-advantages-working-and-application.html) [ ⌊Delta delta connection of transformer⌉Three-Phase Transformer Connections: Y–Y, Δ–Δ, Y–Δ, Δ–Y, V–V, T-T](https://studyelectrical.com/2025/07/three-phase-transformer-connections-y-y-%ce%b4-%ce%b4-y-%ce%b4-%ce%b4-y-v-v-t-t.html) [ ⌊A detailed technical infographic illustrating percentage impedance in a transformer. The left side features a bold title: "PERCENTAGE IMPEDANCE OF TRANSFORMER AND ITS CALCULATION." The right side shows a 3D isometric illustration of a power transformer with cooling fins, bushings, and internal windings, labeled "Primary (HV)" and "Secondary (LV)." Above the transformer, two calculation boxes display the formulas Z% = (Vsc / Vrated) * 100 and Z = V / Isc. Below the transformer is a detailed circuit schematic for a "Transformer under test" showing a voltage source, ammeter, voltmeters, and shorted output. The background consists of a teal blueprint schematic, power line towers, and various electrical symbols. 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