Indices for relay protection quality in distribution network

Abstract

Distribution network structure can be changed as a consequence of faults, repair/maintenance, reliability reasons, improvement of quality of delivered electric energy, etc. From protection point of view, it is necessary to provide appropriate relay protection functionality (sensitivity and selectivity) for any of given structures. Changes can be sometimes very simple but with unexpected (hidden) effects - e.g. changes in network of one voltage level can cause loss of relay functionality in network supplied via it (due to decreasing of fault current level below the critical value for proper relay operation). In order to provide fast and efficient tool for identification of above-mentioned problems, numerical indices for relay protection quality/quantity are introduced. This is the main goal of this paper. In addition, introducing of the indices provide efficient tool for evaluation of some scenarios where some of relays are out of order or not appropriate adjusted. Also checking of back-up covering is included in the indices. Finally, all considerations are illustrated by examples from a real network.