Analysis of failure in power cables for preventing power outage in Alexandria electricity distribution company in Egypt

Underground cables are fundamental elements in any energy system, as they allow energy to be transported and distributed to consumer points. Power cables are usually subjected to electrical, thermal, mechanical, and environmental stresses on a constant basis when they are in service. These stresses, will lead to insulation degradation or other defects, such as, excessive sheath circulating current, partial discharge activity and increasing dielectric loss which can result in premature cable failure causing unplanned outages. This paper will show a better understanding of MV underground cable failure in Alexandria Electricity Distribution Company (AEDC) by analyzing data in connection with cable failures and by identifying the stress factors leading to cable defects or degradation. This will be supported by several diagnostic field examples in AEDC. In addition, the techniques adopted for preventing and solving MV underground cable failure problems are evaluated. These techniques mainly depend on the predictive maintenance (PM) program which is cost effective providing minimum downtime on one hand and improving the system reliability on the other hand, in turns reducing the amount of outages that was 926 in 2007/ 2008 and reaches 195 In 2015/2016 as a sign of improvement in the underground cable network's performance. Also, the causes of cable failure are decreased during the same period. Consequently, AEDC enhances and preserves the system reliability which will lead to a high level of continuity and quality of supply to customers specially that MV underground cable failure have been contributing 60% of annualized System Average Interruption Duration Index (SAIDI) for a number of years. INTRODUCTION In the last few decades the main focus of utility companies has been on developing diagnostic, location and pin pointing techniques for medium voltage cables. As cables and their accessories age, their propensity to fail in service increases. Experience obtained while conducting predictive diagnostic evaluations of cable demonstrates that cable deterioration manifests itself through discrete defects. Some examples of discrete cable insulation defects are electrical trees, water trees eventually leading to electrical trees, impurities, delamination of semi-conducting screens, protrusions in extruded insulations and carbonized tracking in laminated insulations. Accessories, on the other hand, typically fail because of manufacturing defects, poor workmanship, impurities, or moisture ingress along interfaces with the cable. AEDC network is very complex network and has large lengths of underground cables which mostly oil paper cables and XLPE cables. MV underground cable in AEDC suffers from an increasingly faults which occurs at the cable sealing box, the cable joints and the cable connection box. Another very common reason is the insulation degradation and ruptures in conductors. In addition, the cable may also get damaged due to vibration fatigue or overheating. Moreover, such cable failure will cause many undesirable effects and will lead to a high cost to the society as a whole specially for institutions like hospitals, airports, and train stations, power outages can be disastrous. One drawback of underground cables is that the procedure for finding the exact place of failure is harder, since no visual inspection can be performed. In addition, even when a fault is localized, the process of digging the ground to reach the cable, and also repairing the cable, is very difficult. Consequently, there is a great need for better methods to determine the condition of the in-service underground cables and their remaining useful life. In this paper, the testing devices used by AEDC in order to determine MV fault location are described. Also, the flow chart of a cable test is shown in section II. Then, AEDC experiences are analyzed in section III. In addition, the maintenance strategy for MV cables adopted by AEDC and its role in improving the amount of outages during the period 20072015 is evaluated in section IV. Finally, the concluding remarks are summarized. TESTING DEVICE The testing device used in this study is the cable test van which consists of a high voltage device, very low frequency (VLF) device with testing connection output, an impulse voltage device, Teleflex device, arc stabilizing unit, audio frequency generator, burning and testing unit, audio frequency receiver, measurement device and cable identifying generator. This is shown in the fig. (1) [1]. The testing voltage applied should be less than the allowable voltage according to the equation (1) to avoid more insulation failure [2].