Electrical Treeing in Cable Insulation under Different HVDC Operational Conditions

Abstract

Electrical treeing (ET) can irreversibly deteriorate the insulation of polymeric power cables leading to a complete failure. This paper presents the results of an experimental investigation into the effects of unipolar and polarity reversing DC voltages on electrical tree (ET) and partial discharge (PD) behavior within high voltage direct current (HVDC) cross linked polyethylene (XLPE) cable insulation. A double needle configuration was adopted to produce non-uniform electric fields within the insulation samples, potentially leading to electrical trees. The development of trees was monitored through an optical method and the associated partial discharge signals were measured through an electrical detection technique, simultaneously. The analysis of the results shows reasonable relation between the formation of ETs and the type of the applied voltages. The polarity reversing attribute of the test voltages has a pronounced effect on formation and growth of electrical trees. This implicates an interaction between the space charges that accumulate within polymeric materials and the operational polarity reversing electric fields, which causes insulation degradation. Therefore, study of influencing HVDC operational parameters on insulation degradations can contribute to improvements in cable design and advancement in insulation diagnostic strategies applicable in HVDC systems leading to more effective asset management.

Electrical Treeing in Power Cable Insulation under Harmonics Superimposed on Unfiltered HVDC Voltages

“…A detailed discussion will be put forward in Section 5. The effect of non-continuous application of DC voltages on electrical treeing has been reported in [23] and Figure 4a,b illustrate the ET developments under the application of non-continuous positive and negative polarity voltages, respectively. where is the potential difference across the needle electrodes, d is the gap distance between electrodes, and r and R are the respective radii of the sharp and the blunt needles.…”

“…To reduce the mechanical stresses applied to the samples, they were first uniformly heated at 65 • C [22] without causing any deformation, then the needles were carefully inserted into the samples, and the points of the needles were arranged to be 1 mm from each other as shown in Figure 2b. For a more detailed sample preparation procedure one can refer to reference [23].…”

“…In order to detect the PD signals, an induction based sensor, known as High Frequency Current Transformer (HFCT), was appropriately clamped around the earth connection of the vessel. The sensor has a sensitivity of 4.3 mV/mA ± 5%, and it is capable of detecting PD signals within a wideband frequency range from around 0.2 MHz up to 19 MHz [23].…”

“…The camera was then connected to computer Unit 2 via a shielded cable for recording the images of the treeing process. The sensitivity of the camera is 1.7 V/lux-sec, and it can produce images with a maximum resolution of 2592 × 1944 pixels [23].…”

“…The sensitivity of the camera is 1.7 V/lux-sec, and it can produce images with a maximum resolution of 2592 × 1944 pixels. [23].…”