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Saturday, 23 June 2018

Coordinated Control of Distributed and Bulk Energy Storage for Alleviation of Post-Contingency Overloads

Abstract:

This paper presents a novel corrective control strategy that can effectively coordinate distributed and bulk energy storage to relieve post-contingency overloads. Immediately following a contingency, distributed batteries are implemented to provide fast corrective actions to reduce power flows below their short-term emergency ratings. During the long-term period, Pumped Hydro Storage units work in pumping or generation mode to aid conventional generating units to keep line flows below the normal ratings. This problem is formulated as a multi-stage Corrective Security-constrained OPF (CSCOPF). An algorithm based on Benders decomposition was proposed to find the optimal base case solution and seek feasible corrective actions to handle all contingencies. Case studies based on a modified RTS-96 system demonstrate the performance and effectiveness of the proposed control strategy.

Keywords:
smart grid; energy storage; security-constrained optimal power flow; corrective control; overload.

Introduction:--

 Power systems have been operating in the way that requires the base case operating point can withstand an unexpected loss of components. However, the traditional preventive dispatch is conservative, very costly, and even infeasible to implement for potentially dangerous contingencies. Changing the dispatch paradigm from preventive to corrective would be useful to manage this situation,  as it can take into account the operator’s ability to take corrective actions after an outage occurs. Corrective actions can eliminate post-contingency overloads on affected transmission lines, allowing the system operates with relaxed security but lower costs in the normal state. The primary tools for achieving this goal are programs for solving Corrective Security-constrained Optimal Power Flow (CSCOPF) problems. Generation dispatch is a commonly used corrective action incorporated in the CSCOPF to remedy post-contingency overloads, in which operators send orders to generators to increase/decrease their output in the event that any single generating unit or transmission line suddenly trips.


However, this form of corrective action is only effective in the case of post-contingency power flows on lines are lower than their short-term emergency (STE) ratings, because most generators cannot adjust their output fast enough to relieve the STE overloads within several min, due to their slow ramp up/down rates. In the literature, many researchers have not considered the STE limits in their CSCOPF models, which eases the real-time computational burden, but ignoring such short-term emergency overloads may lead to cascading line tripping before the operator has dispatched generators. In current practice, if there are no fast-response corrective actions available in the system, the generating units should be dispatched out-of-merit in the pre-contingency state to make sure that no post-contingency power flows on the affected lines would surpass their STE ratings. This preventive/corrective combined measure can guarantee security during the post-contingency short-term state, whereas the preventive actions raise the operating costs.

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