A convex formulation for voltage unbalance compensation problem on hybrid microgrids

Abstract

Voltage unbalance is a power quality condition that arises due to the presence of unbalanced single-phase loads. The presence of voltage unbalances affects negatively a series of sensible equipment and, therefore, is undesirable. In a hybrid (AC/DC) microgrid environment, the presence of distributed generators connected by converters to the AC side can be used to tackle the voltage unbalance. This work presents a convex optimal voltage unbalance compensator that uses the converters’ capability to adjust the negative sequence voltage of their buses in order to keep the overall voltage unbalance of the network within a given range. The voltage unbalances compensation effort sharing between the converters is formulated as a quadratic constrained quadratic programming (QCQP) problem whose convexity assures the global optimality of the solution. The formulation is based on the equivalent negative sequence circuit of the network. The technique was evaluated by simulation on a study case microgrid and was able to successfully reduce the voltage unbalance to desirable levels.

Keywords

• voltage unbalance compensation
• hybrid microgrid
• quadratic constrained quadratic programming