Testing of A Station Service Transformer

by

Type Tests of a 33/0.415KV Station Service Transformer

There are mainly three standards we follow for testing of a Station Service Transformer. These are IS : 1180 part 3 – 2021, CEA Guideline and CBIP Manual.

Type Tests According to IS : 1180 part 3 – 2021

According to the latest version of IS 1180 (Part 3): 2021, which covers specifications for outdoor and indoor liquid-immersed distribution transformers up to 2,500 kVA and 33 kV, the type tests for station service transformers are detailed in Clause 21.3. These tests are conducted to validate the design and ensure the transformer meets the required specifications. The type tests include:

  1. Temperature-Rise Test
    This test checks whether the transformer can operate safely within the specified temperature limits when running at its rated load.
  2. Impulse Voltage Withstand Test
    This test evaluates the transformer’s insulation system to ensure it can handle high-voltage surges, such as those caused by lightning strikes.
  3. Short-Circuit Withstand Test
    This test verifies the transformer’s mechanical and thermal durability when exposed to short-circuit conditions. Here, to be noted that for transformers up to 200 kVA, this test is mandatory. For transformers above 200 kVA, the short-circuit withstand test is categorized as a special test and is conducted based on mutual agreement between the manufacturer and the purchaser.
  4. Pressure Test
    To ensure the structural integrity of the transformer tank and its components under pressure conditions.

According to CEA Guideline and CBIP Manual on Transformers (Publication No. 295)

The Central Electricity Authority (CEA) of India and Central Board of Irrigation and Power (CBIP) India both guide in same way the type testing of station service transformers. These tests are:

Type Tests for Station Service Transformers as per CEA Guidelines:

  1. Temperature-Rise Test
  2. Impulse Voltage Withstand Test
  3. Short-Circuit Withstand Test
  4. Dielectric Tests
    • Separate Source Voltage Withstand Test: A specified voltage is applied to each winding with bushing to ensure the insulation can handle the required stress.
    • Induced Overvoltage Withstand Test: This checks the transformer’s capability to endure overvoltage by applying voltages higher than normal to the windings.

These tests collectively ensure that the transformer is designed to perform reliably and safely under various operational over voltage conditions.

Routine Tests of Station Service Transformers

The routine tests, as per the three commonly used standards—IS 1180 Part 3, the CEA Guideline Transformer Manual, and the CBIP Manual—are nearly the same. Therefore, a combined list of tests based on these three standards is prepared as follows.

  1. Visual and Dimensional Inspection: Ensure the transformer is free from physical defects and conforms to specified dimensions. This test is not included in IS: 1180, CEA Guideline and CBIP Manual recommend this test.
  2. Measurement of Winding Resistance: Assessing the resistance of each winding to detect any abnormalities or manufacturing defects.
  3. Measurement of Voltage Ratio: Confirming that the voltage ratio between primary and secondary windings matches the specified values and verifying the correct phase relationship.
  4. Vector Group Verification: Ensure the transformer’s vector group corresponds to the specified configuration, confirming correct phase relationships.
  5. Measurement of Short-Circuit Impedance and Load Loss: Determining the impedance voltage and load losses to verify compliance with specified limits.
  6. Measurement of No-Load Loss and No-Load Current: Measuring the losses and current when the transformer operates without load to assess core performance.
  7. Measurement of Insulation Resistance: Measuring the insulation resistance between windings and between windings and earth to ensure adequate insulation quality.
  8. Induced Overvoltage Withstand Test: Applying higher-than-normal voltages to the windings to assess the transformer’s ability to withstand overvoltages.
  9. Separate-Source Voltage Withstand Test: Applying a specified voltage to each winding to test insulation strength. This test is also commonly known as the “Power Frequency Withstand Test”.
  10. Pressure Test: Ensuring the structural integrity of the transformer tank and its components under pressure conditions.
  11. Oil Leakage Test: Verifying that there are no leaks in the transformer filled with oil.
  12. Oil Tests: Analyze the insulating oil sample for dielectric strength, moisture content, and other properties to ensure it meets quality standards. This test is not included in IS: 1180, CEA Guideline and CBIP Manual recommend this test.

Acceptance Tests for Station Service Transformers

While any of the above-mentioned standard documents may not explicitly list acceptance tests for station service transformers, industry practices follow a standardized set of tests to ensure their quality and performance before transporting to the purchaser.

  1. Visual and Dimensional Inspection: Checks if the transformer meets design specifications and is free from physical defects.
  2. Insulation Resistance Test
  3. Winding Resistance Measurement
  4. Voltage Ratio Test
  5. Vector Group Verification
  6. Magnetic Balance Test
  7. Impedance Voltage and Load Loss Measurement
  8. No-Load Loss and Current Measurement
  9. Dielectric Tests
  10. Oil Tests (for Oil-Immersed Transformers)
  11. Temperature Rise Test
  12. Pressure and Leak Tests

These tests follow standard industry practices to confirm that the transformer meets all required specifications and is ready for delivery. For specific acceptance criteria, it is advisable to refer to the procurement documents and technical specifications provided by the CEA or the relevant regulatory authority overseeing the project.