Coordination of the settings of phase-shifting transformers to minimize the cost of generation re-dispatch

Belivanis, Manolis and Bell, Keith (2014) Coordination of the settings of phase-shifting transformers to minimize the cost of generation re-dispatch. In: CIGRE Session 2014, 2014-08-24 - 2014-08-30.

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Transmission System Operators (TSOs) in future will more frequently encounter power flow patterns that are more variable and give rise to transmission constraints on boundaries on the network. These can result in high constraint management costs, expressed in the acquisition of balancing services through ‘spot’ type markets or contractual agreements. As an alternative to lengthy or costly network reinforcements, TSOs often choose to install controllable devices in the form of Phase-shifting transformers (PSTs). PSTs can effectively redistribute active power flows on the network and their systematic and coordinated use could provide the TSO with the opportunity to reduce the amount of costly preventive re-dispatch of generation necessary to manage security constraints. However, to fully utilise available network capacity requires the effective coordination of PSTs. For this purpose, in the paper, a practical framework for the coordination of tap settings of multiple PSTs is presented. The framework is based on mathematical programming optimization methods and adopts a decomposed, mixed preventive - corrective strategy. Its intended use is for operational planning and for (close to) real time application and it is adapted for potential use in the GB transmission system. The framework takes advantage of the corrective capabilities of the system following a fault as well as its ability to sustain the consequences of a fault for a short period of time (as expressed in the post-fault, short-term thermal ratings of the equipment) and it is built around custom Optimal Power Flow (OPF) and Security Constrained OPF (SCOPF) modules. Through the iterative approach, each ‘critical’ contingency case is initially treated separately at the beginning of the iteration. This way, it is investigated whether it is possible to treat the consequences of the contingency using only corrective actions. If this is possible, optimal corrective actions are calculated as well. If this is not possible and preventive re-dispatch of controllable assets is necessary, constraints of this contingency are considered by the SCOPF module. Using the described approach - instead of an ‘all-in-one’ optimization approach or a heuristic, ‘black-box’ procedure - has some advantages: the output can be interpreted by the end-user more easily (i.e. the binding constraints at each stage can be identified and the reasoning behind some of the chosen actions can be provided), it allows more flexibility and it can represent the user’s preferences and security standards in a clearer manner.