Dissipation Factor (Tan-Delta) is one of the most powerful offline nondestructive diagnostic tools used to monitor the condition of extruded cable insulation. Capacitance and Tan-Delta values from a new insulation are used as benchmark readings. By comparing periodic readings of the capacitance and Tan-Delta of your insulating material with the benchmark readings, you can measure the deterioration of the insulation, predict the life expectancy, and plan maintenance and repairs before an unexpected outage occurs. A significant advantage of Tan-Delta Testing is the ability to detect water trees—a major source of insulation failures in aged, extruded cables. Tan-Delta Testing can be performed during the manufacturing process. The testing frequency depends on the rate of change of the insulation quality, history of past failures, environmental conditions, etc. High levels of environmental moisture, elevated temperature, and chemicals or pollution may require more frequent testing.
2 Tan-Delta Testing
VLF AC High Potential (Hipot) Testing was developed in the early 1980s as a substitute for DC Hipot testing. DC Hipot testing is no longer an acceptable test method for field aged extruded cables because of its destructive nature and inability to determine insulation quality. Although Very Low Frequency (VLF) Testing is primarily used to perform hipot or withstand testing in cables, it is also suitable for testing transformers, switchgear, rotating machines, and other electrical assets. It is frequently used as a voltage source for offline testing. A VLF hipot test is a go/no-go test and is not a diagnostic test, but is one of the best ways to check the AC integrity of an electrical asset to determine a pass or fail result. Similar to Offline Partial Discharge Testing, the challenge of this test is that the equipment must be taken out of service.