Current integrated technique for insulation diagnosis of water-tree degraded cable

The diagnosis of insulation performance in the high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) systems has always attracted many researchers in the field of electrical engineering. New diagnostic technology and condition monitoring apparatus are of prime importance for HVAC and HVDC developments. In this study, we introduce a valuable technique called direct current integrated charge (DCIC-Q(t)) for determining the degradation of insulating materials by means of charge accumulation. This work demonstrates the typical results of full-size cross-linked polyethylene (XLPE) cables by using this effective method. Firstly, we describe the basic measurement principle of DCIC-Q(t) technique and the evaluation details of the degradation. Then, the validity of this technique is confirmed by comparing with the pulsed electroacoustic (PEA) results. The similar charge behaviors of polystyrene and low-density polyethylene are presented via the DCIC-Q(t) and PEA methods, which indicates the good availability of this technique. Three underground, full-size, 6.6-kV XLPE cables were diagnosed using the DCIC-Q(t) method. Two cables were degraded by water and electric stress for a different time under controlled conditions. The degradation extent can be evaluated easily by the charge accumulation from the DCIC-Q(t) results. Furthermore, based on the results of charge dynamics related to the applied voltage, the properties of the degraded cable by water tree are discussed.