Analysis of a microgrid with unbalanced load comparing three-phase and per-phase voltage droop control

The power electronics interfaces of distributed generation units in microgrids usually employ droop control for defining their active and reactive power injections. The voltage and current control loops can be implemented in different ways, presenting different degrees of complexity and resulting performance, in terms of harmonic distortion and voltage regulation in steady-state and transient conditions. The microgrid load is usually assumed to be balanced, what is seldom the case in real systems. This paper discusses the application of two control methods in a - b - c reference frame for two three-phase microgrid units transformer-less connected and working in the islanded and grid-connected modes. The first is based in the common assumption that the system is balanced, while the second allows the droop based reference voltages of the three phases to be different but keeps the reference frequency the same and the reference angles shifted by 120°. The choice of resistive or inductive droop and the relevance of a damping factor loop is discussed. The performance of the two control methods with balanced and unbalanced loads as well as under transient and steady state conditions are compared based on experimental results obtained in a laboratory set-up.