Various measurement approaches are used, either standalone or in combination, to identify potential problems within a transformer’s insulation system.
These methods include:
- Bushing tap and bushing monitor PD sensors measuring PD via the bushing tap capacitances
- UHF drain valve antennas are inserted into the transformer tank in service via the drain valve
- Acoustic PD detection to determine the geometric location of PD sources within a transformer
Ultrasonic methods are particularly effective at finding surface PD and corona PD. Acoustic emissions from PD activity are normally at a frequency too high. As the PD gets worse, the frequency sometimes decreases into the audible range. Using an airborne ultrasonic microphone is the most sensitive way to detect PD where there is an air path between the source and the microphone. Contact ultrasonic sensors can be used for sealed chambers.
UHF testing is particularly useful for outdoor switchyards. UHF is an online test method, so no equipment needs to be taken out of service to perform the measurements. It can detect both surface and internal discharge so only one test is needed to find any sources of discharge within the switchyard.
