Introduction Power System Protection deals with the protection of electrical network’s fault currents and to prevent such networks from the faulted area in order to minimise damages. It ensure the safety of a system and personnel and secure electrical equipments like transformers, generators, transmission lines etc. Protective device coordination is the process of determining the "best fit" timings of current interruption when abnormal electrical conditions occur. The basic principle is to co-ordinate protection so that the device nearest to the fault must operates first i.e. the relay closest to the fault has shortest operating time and so on. It should be such that if any relay nearest to the fault fails to operate, then the next up-stream relay should operate and so on in order to islolate the other part of the plant from the faulted part. Abstract The exercise is to perform protection coordination or overcurrent relays grading for a an electrical system and to consider following main points : • Measurement of Fault Currents on 11kV and 3.3kV bus bar. • Grading of P121 type over current relays by calculating operating times. • Plot of over current relays and fuse characteristicscurves. Results The graph in Fig. 2 explains the characteristic curves of over current relays as to when and which relays will operate first. Fuses curves have been plotted directly by taking the data from the data sheet just to show their comparision with relays. OVER CURRENT RELAYS GRADING Dr. Derek Pinches and Haseeb Aslam Ansari Staffordshire University, Stafford, England References Conclusion After carrying out the analysis of a given system it has been found that the relay B which has the lowest operating time will operate first if there is any fault occurs over there. If this relay fails then the next upstream relay D will operate after some interval of time and so on. Moreover Relay J which is at the top will trip the breaker when all downstream relays have failed and hence it provides protection coordination and so over current grading has been achieved. Method In order to perform over current relays grading of a system shown in Fig. 1, fault current at each level of busbars needs to be calculated using the formula : I F = MVA 1.732 X kV It has been shown (Mehta and Mehta, 2003) that an inverse time relay is one in which the operating time is approximately inversely proportional to the magnitude of the actuating quantity. At value less than the setting current, the relay never operates. Therefore to grade a relay we requried a setting current I S ,which should be taken as 10% extra of normal FLC with a reset ratio of 95% . I S = 1.1 x I F /0.95 The operating time of a particular relay can be calculated using Extremely Inverse Characteristic formula as : t = 80 x TMS , TMS= Time Multiplier Setting [I F / I S ]² -1 Note that a grading margin of 0.4s should be added in the operating time of all the relays in order to have correct co-ordination, so that the relays can have sufficient time for discrimination. Thus, the operating times of all overcurrent relays are tablulated as shown in Table 1. Mehta. V., Mehta. R. 2003, Principles of Power System, New Delhi : S. Chand Publications. Figure 2. Overcurrent Relays Characteristic Curves Table 1. Operating Times of Over Current Relays Current at 3.3 kV (from I S upto the Fault current I F ) Operating time of Relay B at TMS=0.1 Operating time of Relay D at TMS=0.18 Operating time of Relay E at TMS=0.33 Operating time of Relay G at TMS=0.43 Operating time of Relay H at TMS=0.44 Operating time of Relay J at TMS=0.45 2.66 2.66 8.8 11.46 11.73 12 0.53 1 3.3 4.3 4.4 4.5 0.22 0.53 1.76 2.29 2.34 2.4 0.12 0.33 1.1 1.43 1.46 1.72 0.08 0.23 0.75 0.98 1.38 1.5 Figure 1. Single Line Diagram