AC Feasibility Restoration in Market Clearing: Problem Formulation and Improvement

For the direct current (dc) based scheduling model in market clearing procedure, there exists a clear mismatchbetween dc market solutions and alternating current (ac) feasible operation points. This mismatch is commonly covered by out-of-market corrections, which makes generators deviate from market solutions and consequently incur uplift payments. The enhanced coupling relationship of active power and reactive power makes this issue crucial. However, it has been seldom studied. This article presents the problem formulation of existing industrial practice to handle this mismatch. The physical properties of current practice are discussed in depth. The serious potential disadvantages incurred by current ad-hoc manners in ac feasibility restoration are analyzed and its influence on market results is demonstrated. Theoretical analysis shows that social welfare loss and price spikes will occur under certain circumstances, especially in on-peak load scenarios. To handle these issues, this article presents a modified ac feasibility restoration framework based on the linearized optimal power flow algorithm, which improves the rationality of the reactive power and voltage magnitudes to restore ac feasibility. A strategy to select the additional unit is proposed to avoid price spikes. The proposed method has the potential to reduce the overall operation costs. The effectiveness of the proposed method is verified in IEEE and Polish systems.