Short circuit test on Transformers;Why Short circuit test performed on HT side

Short-Circuit Test on Transformer

1. Purpose of Short-Circuit Test

• To determine copper (I²R) losses of the transformer at full load.

• To calculate the equivalent resistance and impedance of the transformer.

• Performed on High-Voltage (HT) side (not LT) since rated current is much lower and easy to achieve.

2. Why Conduct Test on HT Side?

• Rated current on HT side is lower than LT side.

• Applying small test voltage (~5–6% of rated voltage) on HT winding produces rated current.

• If performed on LT side, the HT winding would short to zero volts, causing dangerous high current and risk of winding damage.

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3. Principle of the Test

• Core losses are negligible (since applied test voltage is very small).

• Wattmeter reading = copper losses at rated current.

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4. Connection Diagram

• Ammeter, Voltmeter, and Wattmeter connected on HT winding.

• LT side short-circuited.

• Variac (auto-transformer) used to gradually increase applied voltage until rated current flows in HT side.

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5. Test Procedure

1. Short the LT side of the transformer.

2. Connect Ammeter, Voltmeter, Wattmeter on HT side.

3. Slowly increase applied voltage through Variac until ammeter reads rated current.

4. Record:

   • Applied voltage $V_{s.c.}$

   • Current $I_f$

   • Wattmeter reading $P_{s.c.}$

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6. Important Formulas

$$P_{s.c.} = I_f^2 \, R_{eq}$$ $$R_{eq} = \frac{P_{s.c.}}{I_f^2}$$ $$Z_{eq} = \frac{V_{s.c.}}{I_f}$$ $$Z\% = \frac{V_{s.c.}}{V_{rated}} \times 100\%$$

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7. Numerical Example (2 MVA, 415 V / 11 kV Transformer)

Rated Current Calculation (3-phase):

$$I = \frac{S}{\sqrt{3}\,V}$$

• On LT side (415 V):

$$I_{LT} = \frac{2 \times 10^6}{1.732 \times 415} \approx 2782 \, A$$

• On HT side (11,000 V):

$$I_{HT} = \frac{2 \times 10^6}{1.732 \times 11000} \approx 105 \, A$$

👉 Hence, SC test is done on HT side (105 A) instead of LT side (2782 A).

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8. Safety Notes

• Never apply test voltage on LT while shorting HT → may cause excessive current & winding burn-out.

• Always increase voltage gradually using Variac.

• Ensure instruments (A, V, W) are properly rated for transformer capacity.

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9. Key Takeaway

• Short-Circuit Test is always performed on HT side because rated current is lower and easily achievable.

• Wattmeter reading directly gives copper loss at full load.

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