Auxiliary Transformer (Aux Transformer) Testing Procedure

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AUXILIARY TRANSFORMER TEST REPORT
AUXILIARY TRANSFORMER TEST REPORT

Auxiliary transformers are frequently found near the main power transformers. They are smaller & can withstand lesser power loads. They also help to maintain safety by isolating the main power supply from minor loads.

  • An auxiliary transformer powers equipment in power plants.
  • It supplies power to lights, control systems, & other tiny loads.
  • These transformers are necessary for the proper operation of power plants.
  • Auxiliary transformers function as power control systems.
  • Give lighting in power stations.
  • Support small equipment.
  • Ensure safety by separating the primary power.

The Auxiliary Transformer Test describes several transformer testing processes, including: 

  1. Measure insulating resistance to ensure proper configuration and record values. 
  2. Performs turn ratio & magnetizing current tests with a test kit, injecting voltages and measuring ratios and currents at various tap points. 
  3. Measure winding resistance by disconnecting circuits and discharging windings with a kit. 
  4. Magnetic balance measurement involves applying voltages in sequence and measuring voltages to ensure primary and secondary winding balance.

Tests include:

1). Insulation Resistance Test

2). Ratio Test

3). Vector Group Test

4). Winding Resistance Test

5). Magnetizing Current Test

6). Magnetic Balance Test

  • To assess the insulation level of transformer windings and oil, this test is performed.
  • Transmit the 5 KV on the primary to earth for 15 seconds and 60 seconds to determine the polarization index value.
  • Distribute 2.5KV between primary and secondary, and record the 15 and 60 second values.
  • Distribute 1 KV between secondary and earth and record the 15-second and 60-second values.
  • This test is used to identify transformation errors. 
  • Apply three-phase voltage to the primary side & measure the voltage of secondary side in TAPs 3 and 5.
  • Determine the ratio error using the actual and measured values.
  • This test is used to check the vector group given by the manufacturer.
  • For example: Dyn11 Transformer.
  • To validate, superimpose the 1R & 2R terminals & measure the voltages as per the following equation and justify.

Equation:

  • 1R 1Y = 1R 2n + 1Y 2n
  • 1Y 2B=1Y 2Y
  • 1B 2B < 1B 2Y.

This test is used to ensure that cores and phases are correctly identified, as well as to validate mutual flux inductance.

Apply voltage to one winding & measure the voltage sharing between the other windings. 

In theory, the adjacent winding would have more flux than the other. 

That is, if we apply to the 1R -1Y winding, 1Y-1B must share higher voltage than 1B-1R. Note done, and validate the magnetic balance as shown below.

This test is conducted to determine the transformer’s no-load losses. Measure the magnetizing current of each winding/phase with a tong tester or multimeter and compare to the factory report.

This test is performed to verify the manufacturer’s declared resistance value and to assess any winding damage. Apply a DC current of 10 A through the winding, and the kit will calculate the winding resistance based on the voltage drop. Compare and verify the factory test report.