A mixed-integer formulation of OLTC dynamics to minimize distribution and subtransmission tap changes induced by wind and solar power variations

Abstract

Transformers equipped with an on-load tap-changer (OLTC) are the primary voltage regulators in sub-transmission and distribution networks. As the size of variable generation grows in respect to local feeder demand, the varying bidirectional flow of active power in the feeders interferes with the operation of the OLTC transformers. Currently most grid codes require utility-scale wind and PV plants to strictly regulate the voltage at the point of interconnection. This is typically achieved by installing dynamic reactive support in the form of STATCOM or SVC with instantaneously controllable setpoints and no switching cost. An effective and robust voltage coordination scheme would rely on voltage setpoints of these devices as the primary control input. Thus it is crucial to coordinate these new Volt/VAr controllers with the OLTC transformers to eliminate unnecessary tap changes and consequently reduce substation failure risk and maintenance cost. This paper introduces a centrally coordinated voltage control scheme that determines the optimal voltage setpoint based on variation of wind and solar generation in a given time window with the objective of reducing tap-change operations and resistive losses. Later the paper introduces an equivalent mixed-integer programming formulation that finds the same optimal control output but at a higher computational efficiency. Finally the paper discusses the results of the implementation of the MIP-formulation of the voltage control scheme on the DTE/ITC system serving Eastern Michigan.