Space charge measurement of xlpe - a comparison of AC and DC stressing

Abstract

This thesis reports on aspects of the design (i.e. electrical, electronic, mechanical and control software) and construction of the "laser induced pressure propagation (LIPP)" system, which can be used to investigate the space charge formation and evolution within the dielectric material with different sample treatments under both dc and 50 Hz ac ageing conditions. As the space charge distribution in cables is of great interest to the manufacturers and the end users, the newly designed LIPP system was extended so as to be used for planar and coaxial cable structures. The theoretical principles of the LIPP technique for cable geometries and the corresponding data processing method have been developed. From the preliminary results from the XLPE cable samples, the calculated electric stress distribution based on the developed method showed a very good agreement with the theoretical profile. However, due to the limitation imposed by the HV dc power supply and ac transformer in the current system, the applied electric stress is low. Consequently the SNR of the measurement signals in the cable sample is low and the space charge effect was not significant. Therefore, in this project our interests are to study space charge under both dc and ac conditions in the planar sample, the space charge measurement in the cable sample will be left as further work. Several technical issues related to the LIPP technique have been addressed. These include unexpected sample breakdown within ac ageing period, the high frequency noise, offset of the point on wave voltage, effect of the semicon thickness to measurement resolution and mismatch of the acoustic properties between the semicon and HV metal electrode. In order to ensure the high accuracy and consistence of the newly designed LIPP, two correction factors i.e. variation in laser power and target efficiency, are introduced and verified. Under dc condition, space charge measurements on XLPE and LDPE plaques with two different treatments and semicon electrodes demonstrated that the degassing process reduces the dc threshold stress for space charge creation in the XLPE samples with XLPE semicons (approxiately 30% reduction), but not with LDPE samples with XLPE semicons. However, by using LDPE semicons with bulk LDPE insulation, there was no indication of space charge up to -19.2 kV/mm during the ramp rate measurements. The dc ageing measurements illustrated that the space charge performance of the LDPE samples with LDPE semicons is better than the XLPE samples with XLPE semicons and the LDPE samples with XLPE semicons. Also, the results showed that applying XLPE semicons to LDPE bulk insulation, the cross-linking byproducts from the semicons may diffuse across the XLPE/LDPE interface, and affect the space charge performance. In the case of ac, a qualitative and simple quantitative appraisal of the stress and charge profiles at the electrode interfaces (i.e. X-plots and Dev%) has been introduced and validated enabling the processing of a vast amounts of ac raw data without resorting to a complex mathematical treatise. The results confirmed the existence of space charge in the insulation material subjected to 50 Hz ac conditions and showed that space charge effects in XLPE and LDPE with XLPE semicon electrodes are significant when the applied ac stress is above 11.3 kV/mnipeak and the fast charge formation (charge injection) is more pronounced than the slow charge under the 50 Hz ac electric stress.

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