Title :
Diagnostic testing of thermally aged medium voltage XLPE cable joints
Author :
Mauseth, F. ; Halvorson, H.L. ; Hvidsten, S.
Author_Institution :
Dept. of Electr. Power Eng., Norwegian Univ. of Sci. & Technol., Trondheim, Norway
Abstract :
Current loading of wind farm distribution network is very different from that observed for traditional underground cable systems. Variations in wind power results in a significant number of current load cycles during service. The current can increase from zero to maximum rated load in a short time. This can cause high temperature gradients in the cable systems inducing significant thermo-mechanical forces during heating and cooling, and thermal oxidation at high load. Several premature service failures of cable joints in wind farm distribution networks have occurred, mainly due to the combined action of bad metallic connectors and high load current. Partial discharge measurements are today performed on-site using different frequencies and magnitudes of the test voltage. As the oxidation process causes the conductivity of the bulk and surfaces of the materials involved to strongly increase, this will also likely impact the frequency dependence of the partial discharges. This work includes laboratory high temperature ageing of rubber type heat-shrink MV XLPE cable joints including temperature gradients by application of high load currents. The ageing has been electrically characterised by variable partial discharge measurements in the frequency range of 10 mHz to 100 Hz up to 12 kV (2 U0). The joint material was examined by microscopy examinations after ageing. The results show that the partial discharges are strongly frequency and voltage dependent. For shorter ageing times of the rubber joints, the number of discharges per voltage cycle decrease to zero for frequencies below 10 Hz. At longer ageing times resulting in severe material ageing, the number of discharges per voltage cycle strongly increases by decreasing frequency. In this case the inception voltage at 0.1 Hz was equal or lower than that measured at higher frequencies. It is observed by SEM that especially the stress-control material was strongly degraded by the high temperature conditions.
Keywords :
XLPE insulation; ageing; distribution networks; failure analysis; heat sinks; insulator testing; oxidation; partial discharge measurement; power cable insulation; power cable testing; scanning electron microscopy; wind power plants; SEM; cooling; current load cycles; frequency 10 mHz to 100 Hz; frequency dependence; heating; high temperature gradients; laboratory high temperature ageing; material ageing; metallic connectors; microscopy examinations; partial discharge measurements; rubber type heat-shrink MV XLPE cable joints; service failures; stress-control material; thermal oxidation process; thermally aged medium voltage XLPE cable joint diagnostic testing; thermomechanical forces; underground cable systems; wind farm distribution network; wind power; Aging; Frequency measurement; Joints; Partial discharges; Power cables; Temperature measurement; Voltage measurement;
Conference_Titel :
Electrical Insulation and Dielectric Phenomena (CEIDP), 2012 Annual Report Conference on
Conference_Location :
Montreal, QC
Print_ISBN :
978-1-4673-1253-0
Electronic_ISBN :
0084-9162
DOI :
10.1109/CEIDP.2012.6378907