High-voltage power equipment, including switchgears, GIS, transformers, and power cables, operates under continuous high electrical stress during long-term service. Even minor internal insulation defects can trigger partial discharge (PD), a common and dangerous electrical phenomenon that gradually erodes equipment insulation. Without timely detection and maintenance, cumulative partial discharge will lead to insulation degradation, equipment breakdown, and even substation power outages, causing huge economic losses and safety hazards. Therefore, regular partial discharge testing has become a core part of power equipment predictive maintenance.
What Causes Partial Discharge (PD)
Partial discharge occurs when local electric field intensity exceeds the withstand voltage of insulating materials, resulting in localized breakdown and discharge without penetrating the entire insulation structure. The main causes include manufacturing defects, aging insulation, damp internal environment, mechanical damage, and improper equipment installation. These problems create weak points in the insulation system, inducing continuous partial discharge under normal operating voltage.
Common Partial Discharge Defect Types
floating potential discharge
Floating potential discharge is caused by ungrounded suspended metal particles or components inside high-voltage equipment. These floating conductors induce unstable electric fields, generating intermittent partial discharge signals. It is a typical hidden defect in switchgears and GIS equipment, which is difficult to detect through traditional visual inspection.
surface discharge
Surface discharge happens on the surface of insulating materials due to surface contamination, moisture, or aging. The surface insulation resistance decreases, forming creeping discharge along the insulation surface. This defect is common in outdoor high-voltage equipment and easily triggers flashover under humid and rainy weather conditions.
void discharge
Void discharge derives from tiny air voids inside solid insulation materials. The air gap has lower withstand voltage than solid insulation, so partial discharge occurs repeatedly in the void under operating voltage. Long-term void discharge will gradually expand insulation cracks and eventually cause insulation breakdown.
metal tip corona discharge
Sharp metal burrs or protruding metal parts inside equipment will produce concentrated electric fields. When the electric field strength exceeds the ionization threshold of air, corona discharge occurs. This defect is widespread in transformers and high-voltage switchgears, and continuous corona discharge will accelerate insulation aging.
Why Non‑intrusive Partial Discharge Testing for Predictive Maintenance
Traditional power equipment maintenance relies on regular power-off inspection, which is inefficient, costly, and cannot monitor equipment operating status in real time. Non-intrusive partial discharge testing enables live-line detection without cutting off power supply. It can capture subtle PD signals in the operating state, realize early warning of insulation defects, and avoid sudden equipment failures. This predictive maintenance mode greatly improves the safety and stability of power system operation and reduces unnecessary power outage losses.
Common Equipment Suffering from Partial Discharge
Partial discharge defects mainly occur in medium and high-voltage power equipment, including MV/HV switchgears, GIS closed combined electrical appliances, power transformers, high-voltage cable joints and terminals, and ring main units. These devices bear high electrical load for a long time, and insulation aging and structural defects are the main inducements of partial discharge.
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