Incorporating scheduling decisions in a security constrained active and reactive dispatch

One common function of an optimal power flow (OPF) running near real-time is to optimise the dispatch of generating units to meet system demand, and operational and security constraints. The National Grid Company plc (NGC) and Brunel University have developed an OPF which determines a combined active and reactive dispatch (known as CARD) with multiple time steps (snapshots linked by generation ramp rates) and multiple contingency capability. For longer term operational planning where the operational costs over periods of say 1 week to 1 year must be estimated, there is a need for a scheduling (i.e. unit commitment) facility, but less need to optimise over multiple time steps. In operational planning time frames, it is acceptable to approximate the scheduling process in order to identify the size, location and cost of transmission constraints. To our knowledge, CARD is unique in providing a unit commitment facility within the optimisation process that is not available in other OPF packages. This paper reports the development of CARD from being a near real-time dispatch program to become a longer-term scheduling and dispatch tool. Given a certain availability pattern of generation, this new tool is able to both schedule and dispatch only those generating units which are needed to satisfy the constraints while minimising the active power cost