Evaluation of the impact of variable system inertia on the performance of frequency based protection

In addition to the anticipated gradual reduction of system inertia in the future, its variability due to more flexible system control and management over time will also be an issue in future power system operation. This paper focuses on the impact of inertia reduction and variation on existing frequency-based protection during loss of large infeed contingency, with the UK power system used as a basis for the studies conducted. Two protection issues are identified by evaluating the impact of variable system inertia under different system conditions: the 1Hz/s RoCoF setting proposed by the UK distribution code review panel cannot effectively stabilise anti-islanding protection in locations relatively closer to the disturbance in future networks with the planned increase of infrequent infeed loss; underfrequency load shedding (UFLS) (based on existing settings) in a relatively weak system where load is mainly supported by naturally low inertia renewables may mal-operate more often which results in more interruptions to customers. In order to maintain system stability and integrity against aforementioned protection issues, three alternative methods are discussed with a focus on regional based adaptive protection.