Short-Circuit Impedance Test of Power Transformers

1. Introduction

The short-circuit impedance of a power transformer is one of its most important electrical characteristics. It determines:

• the magnitude of short-circuit currents in the power system,
• the voltage drops under load,
• the parallel operation behavior between transformers, and
• the mechanical stresses on transformer windings during fault conditions.

Because of its technical significance, the short-circuit impedance test is a routine mandatory test in transformer manufacturing, commissioning, periodic condition assessment, and post-repair evaluation.

This test provides engineers with crucial information about:

• Leakage flux characteristics
• Winding deformation or mechanical displacement (possible after short-circuit events)
• Accuracy of nameplate impedance
• Copper losses derived from the resistive component
• Health of internal connections

Even small changes in short-circuit impedance can indicate internal mechanical issues that would otherwise remain hidden. Therefore, reliable measurement and repeatability of this test are vital for asset management.

2. Concept of Short-Circuit (Percent) Impedance

Short-circuit impedance, commonly expressed as %Uk or %Zsc, is defined as:

The percentage of rated line-to-line voltage that must be applied to one winding of a transformer, with the other winding short-circuited, to produce rated current in the transformer.

The measured impedance includes:

• Leakage reactance — dominant component
• Winding resistance
• Additional minor stray losses

3. Test Preparation and Safety Considerations

Before energizing the transformer under test:

1. Short-Circuiting the LV Terminals
   All low-voltage winding terminals are short-circuited using thick, low-resistance conductors with minimum possible length to avoid adding external impedance.
2. Ensuring Neutral Isolation
   The transformer neutral must not carry unintended current. If the supply is isolated, the transformer neutral must be disconnected from earth.
3. Using a Single-Phase Source
   A single-phase AC source is adequate for this test. It is applied between two-line terminals of the high-voltage winding.
4. Safety
   1. Even though reduced voltage is used, test circuits can produce high currents.
   1. All terminals must be secure and designed to withstand heating.
   1. Standard high-voltage safety protocols must be observed.

4. Test Method for a Three-Phase Transformer (Single-Phase Excitation)

Because a single-phase source is used, the impedance must be measured three times:

1. Between phase 1–2
2. Between phase 1–3
3. Between phase 2–3

Figure 1: Short-Circuit Impedance Testing of Power Transformers with ALLINA T1.

5. Calculation of Percent Short-Circuit Impedance

The percent short-circuit impedance is obtained from the combined results of the three single-phase measurements:

Where:

• E_ij – measured line-to-line voltage between phases i and j (in volts)
• I_ij – corresponding line current (in amperes)
• S_n – rated three-phase apparent power of the transformer (in kVA)
• V_n – rated line-to-line voltage (in kV) of the winding on which the voltage is measured, usually the HV side

The measured impedance can be separated into:

• short-circuit resistance Rsc, representing copper losses, and
• short-circuit reactance Xsc, representing leakage flux.

6. Acceptance Criteria

Permissible deviations:

• Difference between new test and previous test: ≤ 2%
• Difference between measured value and nameplate value: ≤ 3%

Exceeding these limits may indicate:

• Winding deformation
• Shorted turns
• Loose connections
• Mechanical displacement due to fault stresses

Such deviations require further diagnostic tests (e.g., FRA, winding resistance, or insulation tests).

Table 1: Transformer short circuit test result using Allina T1

7. Practical Engineering Notes

• Use cables with low resistance for short-circuiting LV windings to avoid artificially increasing the impedance.
• Ensure the test set is capable of accurately measuring phase angle.
• Confirm that only the intended winding is energized; others must be safely shorted or left open as required.
• The test is typically performed at currents of 2–10 A, depending on equipment ratings.

8. Conclusions

The short-circuit impedance test is an essential diagnostic and acceptance tool for power transformers. It reveals the transformer’s leakage characteristics, internal mechanical integrity, resistive and reactive behavior, and its compliance with nameplate parameters.

Regular impedance testing allows utilities to:

• Detect mechanical movement or damage inside windings
• Verify performance after transportation, faults, or repairs
• Ensure compatibility in parallel operation
• Support accurate fault-level and protection studies