Experiences With Controlled Islanding and Relay Coordination in Lalitpur Power System, India

The formation of a power island, encompassing generating plants and nearby critical loads, is a strategy employed to maintain the operation of generating plants and the supply of power to essential services during potential blackouts. This paper presents a robust frequency stabilization strategy for such islands during blackouts in the main grid. The proposed methodology includes: (i) a three-stage under-frequency load shedding (UFLS) scheme that disconnects loads in large blocks to limit frequency decline; (ii) reconnection of shedded loads in smaller blocks in case of frequency rise after load shedding to maintain generationload balance; and (iii) a selective generator tripping scheme based on the estimated time to critical frequency, improving over traditional frequency or ROCOF-based methods. Load shedding locations are selected using voltage security criteria, and the entire protection scheme operates using only local measurements, removing reliance on communication infrastructure. The relay settings were tested in PSS/E for system data of Agra-Lalitpur region of Indian grid including renewables for varying operating conditions. It was also found that with grid supporting renewables, the frequency nadir improves with increasing renewables while frequency droop does not have much impact. The results have been shared with, and accepted by, the utility for potential implementation in the future.