Can transformer PD withstand test be performed without a PD shielding room?
Partial discharge (PD) with voltage withstand test (ACLD long-duration induced voltage test with PD measurement) on power transformers can be performed without a PD shielding room.
Site acceptance tests and post-overhaul tests are permitted in non-shielded environments. However, type tests and factory arbitration tests normally require a PD shielding room.
The shielding room is used to reduce background noise and achieve high-precision pC quantification. It is not the only test site required by standards.
Test Scenarios
Factory Type Test / Factory Arbitration Test
In accordance with GB/T 7354 (IEC 60270), a PD shielding room is recommended. Extremely low background noise is required, typically ≤5 pC, and ≤1~2 pC for some stringent tests, to ensure accurate quantification of tiny PD signals. A shielding room is generally mandatory for this scenario. External electromagnetic interference will mask genuine PD signals, and the test report will not be valid for arbitration.
Site Acceptance Test / Post-overhaul Test
After transformer installation, ACLD with PD measurement can be carried out on-site at the substation without shielding. This test method is recognized by GB 50150. The allowable on-site background noise is relaxed: normally background noise ＜50 pC, and up to ＜100 pC for some projects. If background interference exceeds limits, the test is invalid and the transformer cannot be judged pass or fail.
Challenges & Anti-interference Measures for Non-shielded Test
PD measurement by pulse current method is susceptible to electromagnetic interference (broadcast signals, walkie-talkies, frequency converters, adjacent live lines, ground loops, corona on high-voltage leads). A complete anti-interference solution is required:
1.Use low-background PD-free excitation transformer and coupling capacitor; install power filter and isolated shielding transformer on the power supply side; fit corona shields on high-voltage leads to eliminate corona (the most common source of false PD signals on site).
2.Adopt single-point star earthing. Multi-point earthing which creates ground loops is forbidden. Use short, straight, large-section bare copper earthing wires to prevent long earthing cables from picking up spatial electromagnetic interference.
3.Use shielded coaxial signal cables for PD detector with single-end shielding earthing. Enable phase gating, time gating, digital filtering and polarity discrimination functions to distinguish external random pulse interference from genuine internal PD inside the transformer.
4. Shut down irrelevant live equipment nearby during testing; turn off walkie-talkies, mobile phones, welding machines and frequency converters; keep away from transmission busbars; remove floating metal parts around the test object to eliminate interference from floating potential discharge.
Key Limitations
1. Measurement limit deteriorates in non-shielded environments: it is difficult to reliably identify tiny PD below 20 pC. The test is mainly used to detect severe PD defects, not for high-precision quantification of minor PD.
2.If on-site background noise remains above limits, the test cannot proceed and no valid conclusion can be issued.
3.Transformer PD assessment adopts ACLD long-duration induced voltage test per GB 1094.3, which is different from conventional power-frequency external withstand voltage test.
Brief Summary
Site acceptance / post-overhaul acceptance: PD withstand test (ACLD+PD) can be performed without shielding room, provided background noise is controlled within specified limits and all anti-interference measures are fully implemented.
Type test and factory arbitration test: PD shielding room is generally mandatory.
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