COORDINATION OF DIRECTIONAL OVER-CURRENT RELAYS (DOCR) IN POWER SYSTEM PROTECTION

Abstract

The role of protective relays in a power system is to detect system abnormalities and to execute appropriate commands to isolate swiftly only the faulty component from the healthy system. A reliable protective system is vital to the stable operation of the system and the assurance of the customers' supply security. To safeguard against possible failures of protective devices, back-up protection is required to prevent the system from suffering catastrophic consequences. In the present study describes MATLAB/Simulink implementation of Coordination of Directional Over-Current Relays (DOCR) using 3 bus and 4 bus test models. These simulation models are developed to support and enhance Power System protection courses at the undergraduate level. This study helps students to acquire a clear understanding of this topic. This study provides the step by step procedure to develop MATLAB/Simulink models for 3 bus and 4 bus test models to test the coordination of DOCRs, for which decision variables TDS & PS are manually calculated. These two models have been tested for different fault occurring chances. This study also explains a model to realize DOCR application, which is described as a 'Model Relay' (MR) in the following sections and shown in Fig. 1. DOCRs in these models are well coordinated such that they operate for the faults occurred in their own zone except to backup a failed relay, thus achieved selective tripping. This study helps in understanding the concept with less complexity compared to the other models available in the literature. Understanding the operation of these test models leads to understanding the principle of DOCRs. The Model Relay with the same logic has been used to achieve coordination of DOCR's of an existing power system. The data of a 132KV Power network has been collected from APTRANSCO and constructed the same in MATLAB using sirnulink. Then MRs have been incorporated in the system and tested for various fault conditions and accordingly redesigned those MRs to operate for the fault currents. A relay should trip for a fault in its own zone of operation only. It should not trip for a fault outside its zone, except to back up a failed relay or circuit breaker Coordination of DOCRs is nothing but obtaining selective tripping.