Feasibility of FDA method for the detection and classification of faults in extruded HVDC cables

A methodology for localizing failures in cable systems is required to increase their availability. To localize faults, the possible fault scenarios for HVDC cables must first be thoroughly researched, identified and classified. In this article, measurements for some possible faults in cable systems using different setups are presented and discussed to determine the potential of frequency domain analysis (FDA) method for the classification of fault scenarios on DC cables. Results are based on different fault scenarios on two medium voltage cables of different lengths. It is examined which fault scenarios could be detected. In the first step, reference measurements of the fault-free test objects were measured at both ends of the cable. The measurements at both ends of the cable showed only a negligible difference. With the measurement setup for the reference images, the reproducibility, the usable measuring range and the contacting at the feed-in point of the measuring signal were also examined. After the measurements of the reference images, the correspondence of calculations of the resonance points with the measurements was verified. It turned out that the calculations provide a good approximation in the low-frequency range, but the deviations increase with increasing frequency.

I. Introduction

Construction of several HVDC transmission corridors in Europe is planned by using extruded cable systems. Length of these cable systems reaches in some cases approx. 700 km; thus, the probability of failure occurrence, e.g. damages from outside or from any abnormality in system components or resulting from improper installation increases. Therefore, to maximize the availability of the cable systems, a methodology is required to classify possible type of faults and to detect and localize them in cable systems. Various fault location methods can be categorized in time and frequency domain. Up to now, there are limited experiences in fault location in such unprecedented long HVDC cable systems. Most of the fault evaluation methods can detect and localize faults only after a severe damage, e.g. a breakdown in insulation. The frequency domain analysis (FDA) uses voltage sweeps through the cable and might detect different types of weak spots [1] . However, applicability of FDA methods, especially in long HVDC cable systems, is still an area with limited experiences and needs to be more investigated and improved for real applications. The fault localization method presented below differs from the time domain reflectance measurement in the possibility of providing a status diagnosis even before occurrence of a fault [2] . It is therefore, not only a method for fault localization, but also makes it possible to monitor the cable state preventive and to initiate immediate repairs in critical conditions. The method is based on transmission line theory. The information is obtained from the complex line impedance, consisting of amplitude and phase angle, as a function of the frequency of the measurement signal.