Transformer Short-Circuit Test: Measurement and Analysis

PRACTICE 13: TRANSFORMER SHORT-CIRCUIT TEST MEASUREMENT

Introduction

The short-circuit test involves shorting the secondary winding terminals and applying a reduced primary voltage (Ucc) to allow rated current flow in the primary winding.

Outline

Fundamentals of the Test

This test determines nominal copper losses, as short-circuit iron losses are negligible due to the low applied voltage (Ucc). Iron losses are roughly proportional to the square of the magnetic flux density (B), which is significantly reduced at low voltage. If Ucc = 5% U_1n, then Bcc ≈ 0.05B, where:

• B = maximum flux density at nominal voltage
• Bcc = maximum flux density during the short-circuit test

Therefore, P_FECCAS = (0.05)² ⋅ P_Fe = 1/400 ⋅ P_Fe, where:

• P_Fe = nominal iron losses
• P_FECCAS = iron losses during the short-circuit test

The power (Pcc) absorbed during the short-circuit test represents the nominal copper losses. The test also allows calculation of:

• Short-circuit power factor (cos θcc = Pcc / (Ucc ⋅ I_1n))
• Resistive voltage drop (U_Rcc = Ucc ⋅ cos θcc)
• Reactive voltage drop (U_Xcc = Ucc ⋅ sin θcc)
• Relative voltage drops: U_cc% = (Ucc / U_1n) ⋅ 100; U_Rcc% = (U_Rcc / U_1n) ⋅ 100; U_Xcc% = (U_Xcc / U_1n) ⋅ 100
• Short-circuit resistance (Rcc = U_Rcc / I_1n)
• Short-circuit reactance (Xcc = U_Xcc / I_1n)

Measurements should be brief to prevent winding overheating.

Equipment Needed

A single-phase transformer, an AC power source, an AC voltmeter, and an AC ammeter.