Abstract
Polymeric power cables are increasingly being adopted in HVDC interconnections owing to their perceived technical and economical advantages in association with the advancement in the materials and power electronics technology.Cable insulation behaves differently under d.c. compared to a.c. voltages. The insulation conductivity is influenced by the operating temperature of the cable at d.c. voltage which leads to temperature gradient across the insulation bulk and field inversion within the insulation. In addition, long term unipolar electric stress causes charge trapping and, hence, space charge accumulation within the cable insulation system. These phenomena in association with injected harmonics, abrupt over voltages and the polarity reversal could affect the insulation integrity and reliability. Partial discharges (PDs) and electrical trees (ETs) are readily developed under such conditions due to high intensity electric fields which could lead to premature failure of the cable.
The aim of this research is to investigate the influence of operational conditions of HVDC systems, particularly the classic scheme, and transient overvoltages on PD and ET phenomena within solid dielectrics. The process of partial discharge was modeled using finite element method under the subjection of d.c. with superimposed characteristic harmonic voltages of line-commutated converters (LCC). Furthermore, experiments were conducted on polymeric test samples considering the same stressing voltages for the purpose of comparison with the results obtained from the modeling. The insulation resistivity against electrical treeing was investigated on specimens that were prepared from the insulation of a d.c. XLPE cable.Terminal voltage of an LCC converter was reproduced using an interlinked environment of MATLAB and Lab VIEW and used in the experimental tests. In addition, scenarios of continuous and non-continuous d.c. voltages, and polarity reversal were considered in similar experiments to comparatively investigate the influence of different stressing voltages on the characteristics of electrical trees.
The test samples encompassing a void, as an internal insulation defect, showed a noticeably higher level of PD activity under d.c. superimposed with voltage harmonics, polarity reversing voltage and transient overvoltage compared to pure d.c. voltage. The PD pulses particularly happened after each polarity transition, at the peak regions of the superimposing harmonics,and the rising front of the switching impulses. Electrical trees were noticeably developed within the XLPE samples under the polarity reversing voltages compared to pure and non-pure d.c. voltages. The samples showed lower treeing resistance under positive polarity voltage than negative polarity. Finally, the results showed that PD activities and ET developments are mainly influenced by the type and the waveshape of test voltages, and the method of application.
| Date of Award | 2018 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Mohamed Emad Farrag (Supervisor), Alistair Reid (Supervisor) & Scott McMeekin (Supervisor) |