Under-Frequency Load Shedding (UFLS) Schemes – A Survey

The Under-frequency Load Shedding (UFLS) scheme has been used by utility companies around the world to mitigate frequency drop caused by simultaneous or cascading tripping of transmission lines and/or generators in a power system. In the effort to devise an optimal load shedding scheme, it is imperative that investigations are done on the many factors that may affect the response of the scheme in the event of a system contingency. This paper starts by analysing the implementation of UFLS in various power utility companies in Asia, Europe, Australasia, South Africa, Middle East and the Americas. It is observed that UFLS in these continents differ in terms of total load shed, number of UFLS blocks, average block size and trip frequency deviation thresholds depending on their system size, system inertia and generation mix. This paper also looked at the usage of System Frequency Response (SFR) models in the computation of UFLS and system parameters. Analysis on the SFR model showed that the impact of voltage dependence of loads was not taken into consideration in the early implementation of the model albeit load characteristics have significant influence on the dynamic behaviour of power systems during low frequency oscillation and severe faults. SFR model incorporating frequency and voltage dependence load models was proposed later in literature and used in the design of an optimal UFLS scheme. Investigation was also conducted on UFLS operating philosophies in terms of load shedding trigger, power imbalance estimation and distribution of load shedding. UFLS operating philosophies based solely on frequency parameters is inadequate to determine the stability of a power system especially for an islanded power system following severe disturbances. The power system may be susceptible to voltage collapse as well, which will lead to total system blackout within shorter time duration as compared to a frequency collapse phenomenon. Hence, trigger condition considering voltage information and voltage stability criterions were introduced and implemented in UFLS schemes. Distribution of load shedding based on power flow tracing method catering for both frequency and voltage instabilities was also introduced and proven to give optimal system response.