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Short Circuit Test of Transformer: Purpose, Warnings, Etc

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An important function of the electrical system is to make the electrical energy usable by the consumer and deliver it to their doorstep. And this extremely important work is done by the transformer or power transfer device. So the short circuit test of the transformer or impedance test is a very important task.

What is short circuit test of transformer?

Short circuit test is done by supplying very low voltage (5% to 10% of rated voltage) of rated voltage with the help of variac and shorting one side (usually low-side) of the transformer. It is also called impedance test.

In this case, the low-voltage side is shorted by thick copper wire or ammeter. On the high-voltage side, the voltage is increased very slowly from zero with the help of a variac or voltage regulator, with the usual addition of a wattmeter, ammeter, and voltmeter. When the known rated current of the transformer flows into the ammeter, the voltage is no longer increased. In that case, the wattmeter reading is taken as copper loss entirely. Although there is a small amount of core loss, stray loss, is ignored.

Fig: Circuit diagram of short circuit test or impedance test

Why is short circuit test done? What is its purpose?

Basically short circuit test is done for the following reasons. The side of the transformer to which the measuring equipment will be connected can be determined by comparison with the side:

1. Equivalent resistance (R_e)

2. Equivalent reactance (X_e)

3. Equivalent impedance (Z_e)

4. Loss of money (full load or any load)

5. Efficiency and Regulation.

Why is shorted to low side during short circuit test?

Any side of the transformer can be shorted during this test. However, the low-side is shorted due to the following reasons-

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1. Low-range instruments are advantageous in connecting to the high side, as low current flows through the high side.

2. The full-load current on the high side is less than the full-load current on the low-voltage side.

3. In this case the applied voltage is 5% of the rated supply voltage.

4. The wattmeter measures the copper loss in both windings below 3% of rated output; So it is convenient to measure on the high side.

Why wattmeter indicates only copper loss during short circuit test?

The applied voltage of this test is limited to 5% to 10% of the rated voltage. As a result, mutual flux (Q_m) is much lower than the normal mutual flux in the core. Since the core loss is roughly (Q_m)2  the core loss is practically zero. This is why the entire wattmeter reading is considered a copper loss.

Why rheostat is required during short circuit test?

During the short circuit test, a rheostat or variac is connected to the primary side (in this case the high-side) to increase the voltage and make the rated current flow. If the rated current flows in the primary side, its rated current will normally flow in the short-circuited secondary. A rheostat or veriac is required to keep the rated current flowing in this type of test always correct.

What precautions should be taken while doing short circuit test?

1. The low-side terminals of the transformer should be shorted with thick resistance less copper wire or with an ammeter.

2. Low-range measuring instruments, such as ammeters, voltmeters, wattmeters, etc., should be connected to the appropriate high-side.

3. A variable rheostat or variac is connected to the input side i.e. high-side to gradually increase the voltage from zero so that the rated current flows in the transformer. The applied voltage is between 5% and 10% of the rated voltage. The rated current of the transformer should be calculated in advance and will be displayed on the ammeter.

4. Low-range instruments should be used.

5. The test should not be carried out for a long time during the test as the transformer heats up quickly in short conditions and this is detrimental to the insulation.

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