Improving Nuclear Generating Station Response for Electrical Grid Islanding

Abstract

This paper describes problems associated with the performance characteristics of nuclear generating stations which do not have their overall plant control design functions co-ordinated with the other grid controls. Some design changes to typical nuclear plant controls are presented which can result in a significant improvement in both the performance of the grid island and reduce the need to isolate nuclear units during the disturbance. Four areas of the overall unit controls and turbine governor controls which could be modified to better co-ordinate the control functions of the nuclear units with the electrical grid are discussed. Some simulation results are presented to show the performance of a typical electrical grid island containing a nuclear unit with and without the changes. By reviewing data from two islanding events and supplementing that with computer simulation studies, some conclusions are made. The first of these is that overall unit controllers which are not designed with grid disturbances and islanding requirements in mind can cause frequency control problems. On detection of grid islanding the turbine power should be regulated by the speed governor only while the overall unit controls maintain appropriate operation of the reactor and steam bypass system. As well, turbine auxiliary governors whose settings and time delays are not co-ordinated with the operation of the main governors and underfrequency load shedding relays on the system can cause frequency oscillation and limit cycles. Turbine auxiliary governor design and settings should be co-ordinated with other grid controls, including other unit governors.