DMLF Division, Central Electricity Authority, New Delhi.
With such constraints as above, sometimes even a small tripping is likely to bring the grid to the verge of cascade tripping. So it becomes imperative that unwarranted trippings are avoided at all costs. A failure of protection that is failure of the protection and switchgear to trip the faulty section typically results in tripping of many more other sections on back-up protection resulting in more trippings which is likely to disturb the stability of power system. This entails that the protection schemes have to be very meticulously adopted by incorporating the experience of performance analysis of the history of grid collapses. The performance analysis of protection during grid collapses throws ample light on the strategy of protection to be adopted. With postmortem analysis, the strategy adopted is reviewed and any shortcomings are rectified.
A proper balance has to be maintained between protection and stability. Some times, it has been observed that over protection has affected stability. For example a minor grid incident had happened way back in Northern Indian grid which was result of unnecessary tripping due to overprotection. This particular incident happened on 16-03-1995 in which Delhi system collapsed due to unwarranted operation of LBB Protection which was wired with under frequency relay. One circuit Breaker failed to operate inspite of operation of U/F relays. As the U/F relay was wired with LBB protection, therefore, due to non tripping of the breaker, naturally the LBB protection operated resulting in tripping of generating units connected to the bus. That is, though there was no fault like Earth fault, phase fault etc. on the system yet there was unwarranted collapse of Delhi island.
