When a high voltage test is carried out on a new or in-service cable, it is normally being applied as a preventative measure to investigate or find any damage that may have occurred. Also known as proof testing or withstand testing, this go/no-go test, is a diagnostic or preventative measurement for the cable being investigated. If other tests such as Tan Delta tests are implemented, the overall test becomes a high quality diagnostic/predictive measurement procedure as well.
The cable under test is deemed to be of ‘sound’ insulation if the insulation does not break down when applied with a voltage greater than the rated voltage of the cable.
High voltage testing will also check for local faults. For example, an XLPE cable will tend to turn critical water trees into electrical trees and subsequently cause a failure. Having been subjected to adverse impulses/surges during its service lifetime, testing will be necessary to find and avert imminent in-service failures in the future.
If cable testing highlights a subsequent failure, this indicates that the cable is already in a highly compromised condition. Company staff should be prepared at this stage to make any necessary repairs to the cable at the position indicated by previous tests. After repair, the cable is tested again until it passes the required withstand tests.
This testing may include a DC withstand or hi-pot (though not to be used on XLPE cables); a line frequency AC hi-pot; a 0.1 Hz AC VLF hi-pot; or a damped AC (DAC), where electrical stresses are created at a frequency ranging from around 50 Hz to several hundred Hertz depending on the test object capacitance.
A critical application of cable testing is to verify the quality of the cable’s insulation and ensure all safety requirements are met after the cable has been installed in the field. Another use is to confirm the integrity of an existing cable after repairs have been applied, or new through-joints or tee-off joints have been installed.