Frequency control on a power system is a fine balancing act: the amount of power generated must exactly match the amount of power consumed. Should the power system experience a significant sudden loss of generation capacity, the network frequency will decline at a rate that is too fast to be manually controlled. An automatic Under Frequency Load Shedding (UFLS) programme is implemented via protection relays at strategic distributed points in the power system to curb this risk.

The South African power system has, since the early 1970’s, been protected by a 7-stage UFLS programme. Recent research has identified two shortcomings in this programme: (1) The frequency steps and load blocks are large, especially in the lower stages. (2) The long, and differing time delays in the lower stages may cause loads to be shed unnecessarily whilst the frequency is recovering, causing a frequency overshoot.

To avoid the above risks, the System Operator has proposed changing the UFLS programme to include 13 stages, distributed between frequencies of 49.2 and 47.8Hz, and each tripping 2% or 5% of system peak load. A notable feature of the new programme is that all stages are required to operate in 200 ms, 50% quicker than the fastest of the existing stages. This may cause UFLS relays to be more prone to maloperation under local transient conditions that are not related to a system frequency event. Some UFLS relays on the South African power system control upwards of 200MW of load each, and relay misoperations must be avoided.

This paper studies the performance of different UFLS relay types that are deployed on the South African power system, and presents case studies of network scenarios which may lead to maloperation under the existing and new programmes. Measures to stabilise the relays against such maloperations are explored, notably the implementation of under-voltage blocking, and blocking for excessively high frequency rates of change.