New design of ground fault protection

Abstract

Ground fault protection is widely used to protect transmission and distribution lines in case of ground faults. Combined with a directional element and used in a teleprotection scheme ground fault protection can detect and isolate even high resistive ground faults which are not seen by distance protection. Today in general the directional element of ground fault protection is based on zero sequence components or negative sequence components. There is no clear advice which kind of polarization should be preferred for a special application. However there are a lot of maloperations due to incorrect result of the directional element of ground fault protection using either zero sequence or negative sequence quantities. Analyzing numerous fault records it seems obvious that these problems with the directional elements cannot be solved using either zero sequence or negative sequence because these quantities are sometimes very small or not related to the fault. One great advantage of numerical relays is that these relays measure all the voltages and currents of a three phase system. Analyzing fault records related to complicate cases for ground fault protection it can be seen that there is much more information about the fault than used by today's implementations of ground fault protection. For instance the location of the impedance in the complex plane often gives a clear indication about the direction to fault. This paper suggests a new design of ground fault protection using this additional information given by the numerical relays. The starting condition for the ground fault protection remains a threshold of zero sequence current. Once this threshold is exceeded a multi-criteria phase selector selects the faulted phase. Several criteria based on magnitudes of voltages and currents, changes in voltages and currents, symmetrical components and impedances are applied in parallel. The results of each single criterion are weighted and combined to get a final result for the selection of the faulted phase. Using the information of the faulted phase a multi-criteria directional element is suggested to estimate the direction to the fault. Different criteria based on actual voltages, memorized voltages, symmetrical components and delta quantities are applied in parallel. The final result is obtained by the multi-criteria directional element as a weighted combination of the result of each single criterion. This paper explains the new algorithm in more detail and illustrates the advantages of the proposed method using some real fault records. With the new design the ground fault protection takes a lot of advantages regarding phase selection and directional element from the distance protection. The main difference between distance protection and ground fault protection remains the different grading. For distance protection the sensitivity is limited by the resistive reach. The basic principle for grading of ground fault protection remains a simple threshold of zero sequence current.