On 19 December 2021, a single-line fault during energisation occurred in a grid segment in the Netherlands, which was caused by a defective earther fault that failed to disengage. During the fault, the implemented differential protection scheme failed to operate as intended, resulting in a delayed response of 189 ms.

This thesis aims to clarify the root cause of the malfunction and prevent any similar problems in the future. While the cause of the electrical fault was quickly identified, the reason behind the relay malfunction remained unclear.

The affected network segment was modelled using the Real Time Digital Simulator (RTDS), a tool capable of accurately replicating fault conditions. The RTDS surpasses conventionally used fault playback tools, as it can simulate a wide range of dynamic system behaviours. The protection relay was connected in a Hardware- in-the-Loop (HIL) setup to test its real-time response.

Testing with the RTDS revealed that tuning specific relay settings can effectively prevent the malfunction during future fault events. Nowadays, modern numerical relays offer a wide range of powerful protection functions. The intended behaviour of these functions, along with their impact on the relay’s response, must be carefully considered to prevent malfunctions.