How to Perform AC Resonant Testing with Partial Discharge (PD) Measurement
Traditional high-voltage withstand tests can only determine whether equipment has suffered a breakdown; they cannot identify latent insulation defects inside equipment such as cables, GIS, and transformers. There have been numerous instances in the field where equipment passed the withstand test but tripped due to a fault after being put into service. For example, in a 10 kV cable project for a municipal power grid, conventional withstand voltage testing showed no abnormalities, but an insulation breakdown failure occurred one week after commissioning. The root cause was traced to microscopic air voids inside the cable joint-a defect that could not be detected by a standalone withstand voltage test. In contrast, AC resonant withstand voltage testing combined with simultaneous partial discharge (PD) detection represents the current gold standard for power system handover and O&M inspections. By simulating real-world grid operating conditions, this approach accurately identifies early-stage hazards such as insulation aging, voids, and cracks, significantly enhancing the comprehensiveness and reliability of high-voltage equipment testing.
Pre-test preparation is key to ensuring data accuracy; inadequate preparation can easily lead to misjudgments. During a partial discharge (PD) test on 220 kV GIS equipment at a certain substation, failure to calibrate background noise in advance and disorganized wiring resulted in environmental interference levels exceeding the threshold. This led to a false positive indicating insulation hazards, delaying the acceptance process. Therefore, prior to testing, all external lines connected to the equipment under test must be completely disconnected, debris at the terminals must be cleared, and a safe testing distance must be maintained. To set up the test circuit, connect the variable-frequency resonance unit, excitation transformer, reactor, and voltage divider in sequence, and integrate a high-frequency current sensor or coupling capacitor into the partial discharge detection system. Simultaneously, perform on-site background noise calibration to control environmental interference within 5 pC, thereby avoiding data errors caused by electromagnetic interference and ensuring compliance with test sensitivity standards.
The formal test must follow a standardized voltage-rising procedure; proper operation is key to accurately detecting defects. During a 35 kV cable inspection project at a certain new energy power plant, technicians raised the voltage too quickly and did not allow sufficient time for stabilization, failing to activate latent defects and nearly resulting in a missed defect. The standard procedure is as follows: First, start the resonance system; the automatic frequency sweep locks onto the resonance point to ensure stable circuit operation. Next, the voltage is raised slowly and uniformly, first reaching the pre-test voltage and stabilizing for several minutes to allow internal particles in the equipment to mature and identify basic anomalies. The voltage is then increased to the standard test voltage, maintaining stability throughout the process with fluctuations controlled within ±1%. During this stage, the partial discharge system continuously collects real-time data, dynamically monitoring changes in discharge levels to accurately detect intermittent, faint discharge defects.
Test evaluation and wrap-up procedures are critical, as they directly determine the accuracy of the test results. Numerous past operation and maintenance cases have confirmed that most insulation hazards become apparent during the voltage stabilization phase: under normal operating conditions, the partial discharge level of the tested equipment must remain consistently below the standard limit, with no sustained upward trend and no abnormal discharge noises. During the testing of a 110 kV transformer at a certain factory, technicians detected a slow rise in partial discharge readings. They immediately reduced the voltage, shut down the unit, and conducted a troubleshooting investigation, ultimately identifying aging winding insulation as the cause and thereby preventing a major safety incident after the transformer was put into service. After the test meets the standards, the voltage must be reduced at a constant rate to zero, the equipment must be fully discharged, the connections must be removed, and the test data must be fully recorded and archived to ensure traceability of the inspection.
The Goldhome AC Resonance Test System is compatible with synchronous partial discharge detection. With its stable resonance output and anti-interference design, it effectively supports high-standard testing scenarios for various high-voltage equipment, providing precise and reliable technical support for the safe operation and maintenance of power grids.
