Dissipation current waveforms monitoring in LDPE sheet under water tree deterioration test (Part 2)

Deterioration with water tree phenomenon is the one of typical problem on the electrical insulation for the high-voltage class power cable. As the water tree which bridges cable insulation between inner and outer screen makes the insulation resistivity low, the conventional way, such as the leakage current measurement and so on are applicable as the diagnostic technique. On the other hand, for an un-bridged one which induces the sudden breakdown due to high operating stress on high-voltage class power cable, as the healthy parts of insulation are still remains, the variation of resistivity at the insulating part is quite small. The dissipation current waveform observation by using “Displacement Current Bypassing method (DCB method)” is considered one of possible way as a diagnostic method in this case. As DCB method doesn't use an inductor to cancel the displacement current component, it can evaluate not only the fundamental component of dissipation current, but also the harmonic component by using FFT analysis. In this study, the dissipation current waveforms monitoring in LDPE sheet in 0.6 mm thick for 300 hours at 1 kHz and in another LDPE sheet in same thickness for 1,700 hours at 200 Hz were carried out. Applied voltage was 2 kV on both. Here, in order to save time on a deterioration test, the special cell has been developed. Dividing the measurement electrode into 4 divisions, it is able to grasp the 4 different experimental conditions from one sheet. From the results of these deterioration tests, the variations of the water tree's number and length with the 3
^rd
harmonic component of dissipation current were examined. It was indicated that the 3
^rd
harmonic component of dissipation current proportionally increased with the un-bridged water tree's number. However, even though the area, where the LDPE sheet's surface was scratched, is same, the water tree's number was somewhat small in the case of a scratched area distributed.