FEASIBILITY OF ACHIEVING FAST ROCOF-BASED LOM PROTECTION - A CASE STUDY OF HONG KONG DISTRIBUTION NETWORK

Abstract

Distributed Energy Resources (DERs) are playing a critical role in the energy landscape to facilitate the decarbonisation of energy systems. One of the key requirements for the safe integration of DERs is to ensure the Loss of Mains (LoM) protection is sufficiently dependable and secure so that it can effectively detect islanding conditions while avoiding maloperation during other non-LOM grid disturbances (e.g. faults). This paper aims to assess the feasibility of achieving fast and reliable islanding detection using a commercially available Rate of Change of Frequency (RoCoF) based LoM relay. The study is undertaken on a section of the distribution system in Hong Kong (HK) where fast LoM protection operation (<260ms) is required due to the short time delay of an existing auto-reclose scheme. Systematic evaluation of the LoM relay performance both in terms of dependability and security is undertaken using Hardware-in-the-Loop (HiL) injection implemented on a Real-time Digital Simulator (RTDS). Based on the test results and detailed analysis, it is found that: 1) the objective of achieving fast RoCoF LoM detection (i.e. the dependability requirement) is feasible with the assumed settings but the protection security may be compromised under certain fault scenarios

2) in addition to active power, the reactive power plays an important role in the RoCoF-based LoM detection

and 3) natural frequency oscillations following the fault clearance can lead to the unwanted RoCoF operation. These factors will need to be considered during the LoM setting process to ensure the proper balance between LOM protection dependability and security. © The Institution of Engineering & Technology 2025.