A NEW DIRECTIONAL TRANSIENT RELAY FOR HIGH OHMIC EARTH FAULTS

Statistics show that earth faults constitute a large portion of grid faults. Conventional relays are designed only for low ohmic earth faults under stationary conditions. They cannot handle high ohmic earth faults, which occur especially in rural networks with overhead lines, or intermittent earth faults in compensated cable networks. As a consequence the earth fault is very often not recognized or the wrong feeder is selected to be healthy. This increases tremendously the time for the localization of the earth fault. On the other side the effective protection of the networks became more and more important in the competitive markets. In this paper, a new algorithm for detecting also high ohmic earth faults up to some kOhm and its benefits are explained in detail. Results from field tests to demonstrate the feasibility of this new method are presented. INTRODUCTION In many countries of the EC the "resonant grounding" is one of the most important options in electrical network design to obtain the optimal power supply quality. The main advantage of the treatment of the neutral point is the possibility of continuing the network operation during a sustained earth fault. As a consequence this reduces the number of interruptions of the power supply for the customer [1] With this, new power quality problems arise for the selective detection of earth faults. The conventional relays are only designed for low ohmic earth faults. Up to now, the advantage of transient relays lies in the fact that they are working almost perfect for very low ohmic earth faults with a fault-resistance less than a few ohms, independent of the type of the neutral grounding method. With the new algorithm a directional earth fault detection up to some kOhm is possible and it is independent of the ignition point of the arc respectively the starting point of the high ohmic earth fault. At present the fields of application being preferred for transient relays in the medium voltage were cable networks. With the new algorithm the relay is also applicable for rural networks, where the probability of a high ohmic earth fault is much higher. The algorithm can also be used for the detection of restriking earth faults in compensated cable networks and intermittent earth faults in isolated networks. The algorithm was extensively tested with various simulation models in Matlab/Simulink/SimPowerSystems before it was implemented into the real relay. BASICS OF THE EARTH FAULT To explain the behavior of a single pole earth fault, three different processes can be superposed [2], [3]. All three processes are starting at the same time, but their duration is different. It can be distinguished between the following processes: discharge of the faulty line over the earth charging of the two healthy lines over the earth stationary state of the earth fault The explanation of the three processes will be made by using a network with three feeders (A, B and C) and an earth fault in line 1 of feeder A according to figure 1.