Short circuit test of transformer
Short circuit test of transformer … this test very important and classified under type test, routine test and Emergency test of transformer.
You can know more about transformer testing and its classifications here >>> transformer testing
Purpose of Short circuit test of transformer:
- Determine the load losses (Pcu) copper losses and the impedance voltage of the transformer.
- Verify guarantees, design calculations and manufacturing quality.
- Determine the resistances and reactance of windings (R1, R2, X1, X2) of transformer at primary and secondary sides.
- Determine percentage impedance (Z%) of transformer that used at short circuit calculation at electrical network and for selecting proper Circuit breaker that protect the transformer .
- Detection of deformation defects in windings due to shipping and transportation or due to internal short or faulty ground connections.
- And is also used in the case of an unjustified rise in temperature in the transformer where the defective phase is known if there is a higher current than the other two Phases and then the transformer is stopped and checked for reasons.
Precautions before do Short circuit test of transformer:
- Disconnect the electrical current from the transformer.
- Clean the terminals of the transformer.
Steps of Short circuit test of transformer:
- Inject voltage on the primary winding as shown in figure.
- Control this voltage from zero until we reach the current in the secondary winding to equal the full load current of the transformer.
- Take the voltmeter readings from the primary winding.
- Divide Voltmeter reading on applied voltage and multiply by 100 We get the percentage impedance as example: When testing a 33/11 kV , 5 Mega volt ampere transformer, we inject voltage at primary side as shown in figure and increase this voltage then the applied voltages reached 2280 volts to reach rated current at other side when dividing 2280/33000 = 0.069. If the result were multiplied by 100, the output would be 6.9%, which is called percentage impedance of transformer (Z%).
- We can neglect value of core resistance, reactance and core losses as the current that paths through core is very small so, we can represent equivalent circuit of transformer at this case as follows:
- Now we can determine parameters of transformer as follows :
Psc = Wsc : power load losses (known value)
Vsc : applied voltage (known value)
Isc : short circuit current (known value)
Acceptable error at all parameters is 10% of nameplate values which records on the transformer nameplate.