Cable Diagnostics with On-voltage Time Domain Reflectometry

In this paper a diagnostics technique is presented for localization of degraded regions along an XLPE insulated power cable. The diagnostics would help to develop a strategy for condition based maintenance of old XLPE power cables. A design of the diagnostics system and results of measurements performed on two power cables are presented. The measurements data is processed in order to localize the degraded insulation regions. The sensitivity of the diagnostics is investigated in terms of signal attenuation along the cable. INTRODUCTION Most interruptions in the power distribution grid are caused by power cable failures [1]. Diagnostics of power cables are an important technique used to detect progressive deterioration of cable insulation. The aim of using diagnostics is to achieve a strategy for condition based maintenance of old and aged cables, where good enough cables are kept but bad cables are replaced. XLPE insulated power cables suffer from the insulation degradation called watertreeing, see Figure 1. Nevertheless the water trees are regarded as an insulating material their breakdown strength is reduced [2, 3]. Dielectric spectroscopy diagnostics [4] can be used to detect and estimate the severity of water-tree degradation of the power cable insulation. The dielectric spectroscopy diagnostics provide information about the whole length power cable condition. However the water-treeing can be a local phenomenon, where only the specific cable regions are affected. In order to localize the degraded regions Time Domain Reflectometry (TDR) diagnostics for power cables is being developed. TDR is a pulse-radar similar technique. It is implemented by injecting a pulse into the cable and measuring the reflections along the cable. The reflections arise due to joints and also due to small irregularities in the cable itself. On-voltage TDR [5] diagnostics are performed on the cable energized with an external High Voltage (HV) source unit. Application of the HV is used as a differentiating parameter in the on-voltage TDR diagnostics as the HV stress affects dielectric properties of the water-trees. On-voltage TDR has a potential to be developed to an on-line TDR [6] where the diagnostics are performed on the power cable in operation. Figure 1: Water-trees in the field-aged power cable’s insulation. 1 Current affiliation at ABB AB, Corporate Research. 1 TDR Time domain reflectometry is implemented by injecting a short duration voltage pulse Vi with a fast rise time into the power cable with the characteristic impedance Z0, refer to Figure 2. Joints along the cable have different characteristic impedance than the cable, also the irregularities along the cable itself cause small local changes in the cable’s characteristic impedance. Therefore due to changes of the characteristic impedance along the cable the reflections Vr arise and are measured with a high speed oscilloscope. Figure 2: Basic TDR system. The ratio of the reflected voltage wave and the incident voltage wave is defined as the voltage reflection coefficient and can be expressed as: 0 0 L r i L Z Z V V Z Z − Γ = = + (1) The reflected voltage wave Vr will propagate back to the measuring system and will be recorded by the high speed oscilloscope after a traveling time tr. Knowing the wave propagation velocity v in the transmission line the distance to the discontinuity can be obtained as: