VOLTAGE ACROSS AUTOTRANSFORMERS BASIC INFORMATION AND TUTORIALS

Below shows the voltage relations across an autotransformer and switching contacts during a tap changing cycle using an autotransformer designed for 60% circulating current and with 100% load current at 80% power factor flowing through it.

Perfect interlacing between the autotransformer halves is assumed, and the voltage drop due to resistance of the autotransformer winding is neglected.

A study of the figure will disclose the fact that increasing the magnetizing reactance of the autotransformer to reduce the circulating current will

1. Increase the voltage across the full autotransformer winding

2. Increase the voltage to be ruptured

3. Introduce undue voltage fluctuations in the line

Since B-4 and B-3 represent the voltages appearing across the arcing contacts when the bridging position is opened at A and B, the voltage rupturing duty will increase with

1. Increase in voltage between adjacent taps

2. Increase in load

3. Decrease in power factor of the load

4. Decrease in the magnetizing current for which the autotransformer is designed

Vector relations for bridging position AB—voltage across adjacent taps; A-1 and A-2— reactance volts due to load current in only half the autotransformer winding; A-3 and A-4—induced voltage across full auto transformer winding; B-4— voltage ruptured when bridging position is ruptured

at A; B-3—voltage ruptured when bridging position is ruptured at B.