Investigation into the local nature of rate of change of frequency in electrical power systems

Abstract

In recent years power systems world-wide have seen large increases in wind power penetration and this trend is expected to continue. This is having the undesirable consequence of reducing the inertia of electrical power systems, especially at times of high wind generation. Reduced inertia makes a power system more susceptible to a larger rate of change of frequency (RoCoF) following a grid disturbance, such as the sudden disconnection of a load or generator. High RoCoF events could trigger generator protective devices or anti-islanding RoCoF relays, disconnecting generation from the grid, compounding the initial grid disturbance and in extreme cases result in the cascade tripping of generators and grid blackout. The objective of this research was to investigate how RoCoF varies with location in an electrical power system and determine if there is any significant difference between local RoCoF observed at individual buses and the system RoCoF seen across the entire power system. The results show that generators closest to the disturbance are impacted the most after the loss of a generator, and if this generator has relatively low inertia it could see a local RoCoF many times greater than the system RoCoF. It was also observed that when a large portion of the total power system inertia is concentrated at one machine, the mean of the local RoCoFs is significantly larger compared to when the power system inertia is equally distributed across all machines. It was observed that by measuring RoCoF using a rolling average window of 0.5 seconds, the magnitude of the measured RoCoF is significantly reduced and the effect that the distribution of inertia has on the mean of the local RoCoFs is eliminated. However, in some scenarios the local RoCof was still many times greater than the system RoCoF. The results demonstrate that local RoCoF could be an issue that needs to be considered when operating low inertia power systems, particularly as wind power continues to displace conventional generation.