The ultimate aim of PD testing is to identify and locate incipient faults prior to failure so when is the best time to perform the testing? Ideally we want to test as soon as possible after the trigger which has sparked the PD into life.
A common cause of discharge is due to manufacturing defects or imperfect workmanship during installation. Therefore an excellent place to start PD testing is at the beginning of the HV equipment lifetime or after any work has been undertaken on it (e.g. a new cable termination).
Involved is a combination of Off-Line and On-Line PD testing. For example, a new piece of switchgear may be tested Off-Line as part of the commissioning checks and then On-Line once the final cable terminations have been made to verify the integrity of the entire system. This provides quality assurance and allows any PD issues to be corrected, often prior to commissioning. Correcting PD issues at the start is the easiest time to do so and will greatly increase the life cycle of the equipment.
Following initial PD testing there are many factors which go into determining the frequency of subsequent tests. These include previous PD levels, failure history, functionality, criticality, loadings, environmental conditions, usage and maintenance. Time recommended between tests can range from fulltime permanent monitoring to periods of months or years.
The type of equipment also has a large bearing on the frequency of testing. Rotating Machines can withstand a large amount of PD and trending history can be very helpful in determining failures so frequent testing is often recommended. Whereas any discharge on an XLPE cable can be considered serious so immediate corrective action may be advised.
To extend HV equipment life, LIVE HV recommend to PD test during installation, after any works and at intervals determined by several factors outlined above. Also remember PD testing is just like exercise, starting at any time is better than not starting at all!