Control and stability of low inertia system by HVDC interconnectors

The power system inertia plays a significant role in slowing down the rate of change of frequency (ROCOF) when sudden disturbance happens. A high inertia value is always what we are expecting because it ensures the power system stability. However, with the fast proliferation of renewable energy source in electric power system, a relative low inertia value occurs in the system, which would badly affect the frequency performance. This dissertation provides novel frequency control methods for a low inertia power system, since the traditional frequency containment process is too slow to give a reaction.

Cooperation between European system and UK system by HVDC interconnector was chosen being mainly investigated. Different frequency control schemes such as droop control, exchange frequency containment reserve and sharing frequency containment reserve are simulated in order to compare the abilities to provide frequency support. Results illustrated that the droop control scheme is able to provide the best regulation above all the control schemes. This work assesses various factors such as minimal frequency drop, the rate of change of frequency as the criterion of frequency performance. However, some constrained conditions should be taken into consideration when dealing with case studies since the size and rate limit of each HVDC interconnector is different.

This dissertation also gives a prediction on the inertia of UK electrical power system in the year 2025. Since it is not precise to extract only one value to represent all the situations that may happen in ten years later, uncertainty analysis should be added to the system inertia to make the research results more cogent. Furthermore, the effects of the availability of frequency support of each HVDC line would be taken into considerations as well. These processes show the best analysis results on the cooperation of frequency support between two areas.