Why on-site testing is needed
Gas-insulated switchgear is type- and routine-tested in the factory, then dismantled into transport units and re-assembled on site. Every flange joint, gas-handling operation and particle introduced during assembly is a potential defect. IEC 62271-203 therefore requires dielectric tests on the main circuit after installation.
Free particles, protrusions on conductors, floating components and void defects in spacers can all survive a withstand test yet go on to cause failure in service. Partial discharge measurement is the method used to detect them.
Measurement methods
Conventional measurement to IEC 60270 quantifies apparent charge in picocoulombs but is difficult in a large, noisy site environment and on long GIS sections.
UHF measurement (guidance in IEC TS 62478) uses internal or window couplers to detect the electromagnetic signal radiated by PD inside the enclosure. It offers high sensitivity, good noise immunity and the ability to locate a defect by time-of-flight between couplers. Acoustic methods are often used as a complementary locating tool.
Planning the test
Agree the test procedure with the manufacturer and asset owner before mobilisation: test voltage profile, coupler positions, sensitivity verification (an artificial pulse injected to confirm the system can see a defect of the required size), and the acceptance criterion.
Record background noise before applying voltage. A defensible result depends on demonstrating that the measuring system was sensitive enough — not just that no PD was observed.
Interpreting results
Phase-resolved PD patterns help distinguish defect types: free particles show characteristic bouncing patterns, protrusions are polarity-dependent, and floating electrodes produce regular, high-amplitude pulses.
Any confirmed internal PD is normally a hold point. The section is opened, cleaned or repaired and retested. Each finding and its closure should be captured in the ITP record.