Isolated power systems are especially sensitive to generation-load imbalances. The small size, the relatively low inertia, and the comparatively large generation in-feeds make them more vulnerable to frequency instability than large interconnected systems. UFLS schemes play an important role in protecting the system integrity. The design of UFLS schemes must be efficient and robust. This paper presents a tool to simulate, design and analyze underfrequency load-shedding (UFLS) schemes of isolated power systems. The developed tool is able to simulate many operating and contingency (OC) scenarios quickly. Robustness is achieved by selecting a representative set of OC scenarios, whereas efficiency is achieved by tuning the UFLS scheme parameters optimally for the considered set of OC scenarios. The selection of the representative OC scenarios is carried out by means of clustering techniques such k-means, fuzzy c-means or KSOM. The optimization problem is solved by either deterministic or heuristic optimization algorithms such as Simulated Annealing or Genetic Algorithms according to considered UFLS parameters to be tuned. Further and since designs are based on certain assumptions, the tool allows analyzing the impact of varying design conditions such as varying step sizes and non-responding turbine-governor systems. The impact of varying step sizes and non-responding turbine-governor systems is studied by an MC approach, which makes use of probability density functions (pdf) to model random variables and to evaluate their impact on the simulated responses of a power system and its UFLS scheme to generator trippings.
Keywords: Frequency protection, underfrequency load shedding, isolated power systems, tool
Published: August 2018.