Introduction to Common High-Voltage Tests

Wuhan Guodian Xigao Equipment Co., Ltd. introduces several common high-voltage tests for you.
1) Partial discharge test
Partial discharge testing is a non-destructive test, currently employing two methods. The first method uses power frequency withstand voltage as the pre-excitation voltage, which is then reduced to the partial discharge test voltage (typically a multiple of Um/√3—1.5 times for transformers and 1.1 to 1.2 times for instrument transformers), maintained for several minutes to measure the partial discharge quantity. The second method uses Um as the pre-excitation voltage, which is then reduced to the partial discharge test voltage and sustained for one hour to measure the discharge level. The latter method is adopted for transformers. The pre-excitation voltage simulates overvoltage conditions during operation; the partial discharges induced by this pre-excitation should not persist under the subsequent partial discharge test voltage. This means that discharges triggered by system overvoltages should not continue under normal long-term operating voltages. This approach ensures that transformers or instrument transformers exhibit no partial discharges under the long-term operating voltage of Um/√3, guaranteeing safe operation and ensuring both the inception and extinction voltages of partial discharges are above Um/√3.
Therefore, the design of transformer insulation structure, processing and treatment of insulating components, surface electric field strength at live and grounded electrodes, and the dielectric withstand field strength must all be considered to ensure that partial discharge remains below the specified limit. The occurrence of discharge in main or longitudinal insulation should not be used as a criterion.
When using power frequency withstand voltage as the pre-polarization voltage, the duration of the partial discharge test voltage is generally short, about 1 to 5 minutes. Extending the duration of the partial discharge test voltage imposes a more severe stress on insulation and may sometimes cause destructive damage. When Um is used as the pre-polarization voltage, the duration of the partial discharge test voltage is longer—typically one hour according to standards—and the maximum tolerable time depends on the impulse withstand characteristics of the insulation structure.
Partial discharge quantity is generally related to the electric field strength at the surface of the charged and grounded electrodes, but not to the power supply frequency. The background noise at the test site should be low, and the partial discharge from the power source must be isolated.
In terms of test sequence, partial discharge testing should be conducted after all insulation tests. Regarding test type, either long-duration induced voltage with partial discharge testing or short-duration induced voltage with partial discharge testing must be included as a factory test for transformers. According to current standards, partial discharge testing is required for transformers with Um ratings of 252 kV and above; however, the revised IEC 60076-3 standard will require partial discharge testing starting from Um ratings of 126 kV and above.
From the perspective of the specific core structure, transformers with a three-phase five-limb core configuration should not saturate the magnetic flux density in the upper and lower yokes during partial discharge testing. From the insulation standpoint, the design must meet the requirements for partial discharge testing using the three-phase method.
2) Chopped Wave Impulse Test
It is typically a waveform with truncated tail, which can be cut off using an IEC standard rod gap or a multi-pole ignition truncation device. When using a multi-pole ignition truncation device, a more precise truncation time can be achieved; if the difference in truncation time of the test wave exceeds 0.15 μs, the impulse test results will be questionable. With rod gap truncation, it is difficult to determine whether the test passes based solely on differences in truncation time.
When the chopped wave test voltage is 110% of the full wave test voltage, the withstand capability is the same if the chopping time is less than or equal to 3 μs. Transformers connected to GIS must consider chopped wave testing.
Chopped wave tests must be alternated with full wave tests. Negative polarity chopped waves are generally used.
3) Full-wave impact test
The revised IEC 76-3 standard has included full-wave impulse testing as a factory test item for transformers with Um ≥ 126 kV. Transformers requiring sudden short-circuit tests (one of the special test items) must undergo full-wave impulse testing after the short-circuit test.
4) Operating wave test
The revised IEC 6076-3 standard has included the chopped wave test as a factory test item for transformers with Um ≥ 252 kV. For 252 kV transformers not subjected to chopped wave testing, the interphase insulation is determined by full-wave impulse tests or long-time induced voltage tests with partial discharge measurement. When conducting chopped wave tests, the dimensions of external air insulation gaps must be based on the chopped wave test voltage, which may require larger clearances than when the chopped wave test is not required.