An example of a still more specialized test, developed by EdF, France’s national power utility, is the “Dielectric Discharge Test” (DD). This test is performed with select megohmmeters and measures the current that flows during discharge of the test sample. It is especially applicable to multi-layered insulation. The test item is first charged until full absorption has taken place (10 to 30 minutes). At this time, capacitance is fully charged and the alignment of dipoles (absorption) is essentially complete. Only leakage current continues to flow. When the external voltage field ceases, molecules will “relax” and return to their original random configuration, constituting a re-absorption current. This discharge current is measured 60 seconds after the insulation test is finished. At this time, capacitance is discharged and voltage has collapsed, so that the charge stored in the dipoles can be viewed independently of the “masking” currents, such as leakage current that is dominant during an insulation test. A high re-absorption current indicates that the insulation has been contaminated, while a low current indicates that it is relatively clean. The precise definition of Dielectric Discharge is:
Current flowing after 1 minute (nA)
Test Voltage (V) x Capacitance (uF)
This calculation provides a figure of merit that indicates the condition of the insulation. In multi-layered insulation, each layer is meant to share the voltage stress equally. Upon discharge, each layer’s charge will decrease equally until no voltage remains. When a layer is faulty between good layers, its leakage resistance will decrease while capacitance is likely to remain the same. A standard insulation test will be determined by the good layers, and not likely to reveal this condition. But during dielectric discharge, the time constant of the faulty layer will mismatch the others to yield a higher DD value. A low DD value indicates that re-absorption current is decaying quickly, and the time constant of each layer is similar. A high value indicates that re-absorption exhibits long relaxation times, which may point to a problem.
Select megohmmeters will perform this test. The test will be referred to as the “DD” test. The Megger MIT series of insulation test sets sold and rented by Protec Equipment will conduct this specialized test. Contact us today to further explore you testing requirements.