Linear Electricity Spot Market Constraints for Managing Post-Separation Frequency Deviations

Electric power systems occassionally experience transmission outages that split the system into electrically separate islands. Within each island there could be a significant mismatch between the mechanical energy associated with the rotation of generator turbines and the aggregate electrical demand and losses. This results in frequency deviations in each island that are managed by a number of different control actions, including governor response and load shedding. This paper develops a set of linear constraints that can be included in an electricity spot market formulation so that resources can be dispatched in a way that prevents the violation of post-separation frequency standards. The paper shows how to derive the constraints directly from a linear time invariant power system model. The derivation proceeds in a direct and unambiguous manner which is a desirable feature within a restructured electricity industry because often engineering judgement is subject to the scrutiny of other industry decision-makers including the commercial decision makers of corporations and the centralized decision-makers of regulatory bodies. The technique for deriving the constraints can be used to implement an online decision-support tool for system and/or market operators to prevent post-contingency islands from violating frequency standards. The paper illustrates their use through a simple example.