Underfrequency load-shedding (UFLS) schemes are a last-resort tool to protect a power system in case of a severe disturbance. This paper presents a design of UFLS schemes taking into account load variation, affecting their step sizes. Most of the UFLS schemes implemented today are conventional static and semi-adaptive schemes. To date, methods for the design of these schemes consider known and constant step sizes. Step sizes might actually vary due to feeder load variation, feeder outages or breaker failures. The proposed design is formulated as a scenario-based optimization problem which is based on different step size variation scenarios. The design is applied to a Spanish small isolated power system, exhibiting strong variations of the step sizes. A UFLS scheme designed without contemplating step size variations serves as a reference case. Three different cases are analyzed that differ in the degree step size variations are taken into account. Taking into account load variations benefits the design and performance of UFLS schemes, whereas neglecting these variations results in dangerously low frequencies.
Keywords: Frequency stability, power system protection, load shedding.
18th Power Systems Computation Conference - PSCC 2014. Wroclaw, Polonia. 18-22 Agosto 2014.
Published: August 2014.