Partial Discharge Measurements for Studying Cavities in Mass Impregnated Cables
MetadataShow full item record
- Institutt for elkraftteknikk 
A hypothesis regarding temperature dependant discharge levels was the starting grounds for this student work. Based on calculations and conversations with experienced people in this field, there should be cavity-induced discharges at relatively low voltages when dealing with a paper-mass cable at room temperature.PD measurements with a paper-mass HVDC cable under AC stress have been carried out. To get an understanding on the cavity characteristics at different temperature levels, the cable has been heated to temperatures of 20, 30, 40, 50 and 60 ̊C. Discharges over a large range of magnitude, from 101 to 104 pC was observed. The largest discharges measured coincide with rather large portions of a butt gap being drained of oil.Partial discharges are a stochastic variable, thus one cannot expect values to coincide from measurement to measurement, but rather focus on finding tendencies. The maximum discharge levels showed a temperature dependency, but not as easily interpretable as expected. During the first half of the temperature cycles the maximum discharge levels varied as anticipated; small to non existing discharges at high temperatures and larger discharges at room temperature. This correlation became less clear over time. The deviation from the anticipated characteristics could be caused by: Temperature gradients over the insulation due to insufficient time to stabilize at the wanted temperature level. Axial oil flow in the test object. Irreversible deterioration of the insulation caused by large discharges mid-test. The discharge characteristics did show two clear tendencies throughout the lab work: Whenever the temperature was raised from one level to a higher level, the maximum discharge size was reduced. The inception voltage increases with increasing temperature and decreases with decreasing temperatureThe cable was installed in a test rig prior to start-up of the student project. Initially there was PD activity in one of the terminations which had to be modified to some extent to avoid interferences on the measurements. A new test object is under preparation and new terminations have been designed. The new terminations are based on the old design and the modifications made on it. Drawings for the new terminations have been made for the workshop and the fabrication process followed up. Measurements to locate the discharge sites were performed at room temperature at the end of the lab work. These measurements indicated an even distribution of the discharge sites along the length of the cable, meaning the terminations can be deemed healthy.