Engineers at demiks, a manufacturer of transformer test equipment, speak of short circuit testing (also known as impedance testing or load loss testing) as an important diagnostic procedure for three-phase transformers. Full technical details are given below:

1. Purpose of the Test

• Determine copper losses (I²R losses) at rated current

• Calculate equivalent impedance (Zₛₛ), resistance (Rₛₛ), and reactance (Xₛₛ)

• Verify transformer's withstand capability under fault conditions

• Provide data for protection relay settings

2. Test Setup & Connections

mermaid复制

graph LR    A[3-Phase Variax] -->|Variable Voltage| B[Transformer HV]    B -->|Short Circuit| C[LV Terminals]    D[Power Analyzer] --> B    
E[CT/PT] --> D

Equipment Required:

• Variable AC voltage source (0-10% of rated voltage)

• Current transformers (CTs) & potential transformers (PTs)

• Power analyzer (measures P, I, V, PF)

• Temperature sensor (for resistance correction)

3. Step-by-Step Procedure

A. Pre-Test Preparations

1. Short LV windings using heavy copper busbars (ensure solid connection)

2. Connect instruments to HV side (test is typically performed from HV side)

3. Verify all safety interlocks are active

B. Test Execution

1. Gradually increase voltage until rated current flows (usually 5-8% of rated V)

2. Record at steady state:

• Input voltage (Vₛₛ)

• Current (Iₛₓ = Rated current)

• Power (Pₛₛ)

• Winding temperature (θ)

C. Key Measurements

4. Calculations

A. Impedance Parameters

1. Equivalent impedance (Zₛₛ):

   $${\mathrm{<span\; style="font-size:\; 18px;">Z</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}}<span\; style="font-size:\; 18px;">=</span>\frac{{\mathrm{<span\; style="font-size:\; 18px;">V</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}}}{{\mathrm{<span\; style="font-size:\; 18px;">I</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">r</span>}\mathrm{<span\; style="font-size:\; 18px;">a</span>}\mathrm{<span\; style="font-size:\; 18px;">t</span>}\mathrm{<span\; style="font-size:\; 18px;">e</span>}\mathrm{<span\; style="font-size:\; 18px;">d</span>}}}$$Zss=IratedVss

2. Equivalent resistance (Rₛₛ):

   $${\mathrm{<span\; style="font-size:\; 18px;">R</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}}<span\; style="font-size:\; 18px;">=</span>\frac{{\mathrm{<span\; style="font-size:\; 18px;">P</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}}}{\mathrm{<span\; style="font-size:\; 18px;">3</span>}<span\; style="font-size:\; 18px;">\times </span>{\mathrm{<span\; style="font-size:\; 18px;">I</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">r</span>}\mathrm{<span\; style="font-size:\; 18px;">a</span>}\mathrm{<span\; style="font-size:\; 18px;">t</span>}\mathrm{<span\; style="font-size:\; 18px;">e</span>}\mathrm{<span\; style="font-size:\; 18px;">d</span>}}^{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}}$$Rss=3×Irated2Pss

3. Equivalent reactance (Xₛₛ):

   $${\mathrm{<span\; style="font-size:\; 18px;">X</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}}<span\; style="font-size:\; 18px;">=</span>\sqrt{{\mathrm{<span\; style="font-size:\; 18px;">Z</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}}^{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}<span\; style="font-size:\; 18px;">-</span>{\mathrm{<span\; style="font-size:\; 18px;">R</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}}^{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}}$$Xss=Zss2−Rss2

B. Temperature Correction (to 75°C)

Rss(75°C)=Rss(θ)×235+θ235+75

Pss(75°C)=3×I2×Rss(75°C)

5. Interpretation of Results

Normal vs. Abnormal Findings

6. Safety Considerations

• Use IR goggles during energization (arc flash risk)

• Ensure proper shorting bar cross-section (≥1.5× cable area)

• Follow IEEE C57.12.90 and IEC 60076-5 standards

7. Practical Example

Test Data:

• 10MVA, 33/6.6kV Dyn11 transformer

• Vₛₛ = 1800V (5.45% of 33kV)

• Iₛₛ = 175A (rated)

• Pₛₛ = 85kW at 32°C

Calculations:

1. ${\mathrm{<span\; style="font-size:\; 18px;">Z</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}}<span\; style="font-size:\; 18px;">=</span>\frac{\mathrm{<span\; style="font-size:\; 18px;">1800</span>}}{\mathrm{<span\; style="font-size:\; 18px;">175</span>}}<span\; style="font-size:\; 18px;">=</span>\mathrm{<span\; style="font-size:\; 18px;">10.29</span>}\mathrm{<span\; style="font-size:\; 18px;">\Omega </span>}$Zss=1751800=10.29Ω (9.36% impedance)

2. ${\mathrm{<span\; style="font-size:\; 18px;">R</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}}<span\; style="font-size:\; 18px;">=</span>\frac{\mathrm{<span\; style="font-size:\; 18px;">85000</span>}}{\mathrm{<span\; style="font-size:\; 18px;">3</span>}<span\; style="font-size:\; 18px;">\times </span>\mathrm{<span\; style="font-size:\; 18px;">17</span>}{\mathrm{<span\; style="font-size:\; 18px;">5</span>}}^{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}}<span\; style="font-size:\; 18px;">=</span>\mathrm{<span\; style="font-size:\; 18px;">0.925</span>}\mathrm{<span\; style="font-size:\; 18px;">\Omega </span>}$Rss=3×175285000=0.925Ω

3. Corrected to 75°C: ${\mathrm{<span\; style="font-size:\; 18px;">R</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}<span\; style="font-size:\; 18px;">(</span>\mathrm{<span\; style="font-size:\; 18px;">75</span>}\mathrm{<span\; style="font-size:\; 18px;">°</span>}\mathrm{<span\; style="font-size:\; 18px;">C</span>}<span\; style="font-size:\; 18px;">)</span>}<span\; style="font-size:\; 18px;">=</span>\mathrm{<span\; style="font-size:\; 18px;">1.106</span>}\mathrm{<span\; style="font-size:\; 18px;">\Omega </span>}$Rss(75°C)=1.106Ω, ${\mathrm{<span\; style="font-size:\; 18px;">P</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}<span\; style="font-size:\; 18px;">(</span>\mathrm{<span\; style="font-size:\; 18px;">75</span>}\mathrm{<span\; style="font-size:\; 18px;">°</span>}\mathrm{<span\; style="font-size:\; 18px;">C</span>}<span\; style="font-size:\; 18px;">)</span>}<span\; style="font-size:\; 18px;">=</span>\mathrm{<span\; style="font-size:\; 18px;">101.6</span>}\mathrm{<span\; style="font-size:\; 18px;">k</span>}\mathrm{<span\; style="font-size:\; 18px;">W</span>}$Pss(75°C)=101.6kW

8. Comparison with Open Circuit Test

Industry Tip: Always perform short circuit test before open circuit test to avoid core remanence effects.

9. Advanced Applications

• Frequency response analysis: Detect winding displacement

• Dynamic short-circuit testing: Verify mechanical strength

• Thermal imaging: Identify hot spots during test

This test is essential for transformer test equipment nameplate data verification, efficiency calculations, and protection coordination studies. Always refer to factory test reports and IEEE/IEC standards for detailed analyses.