Virtual Flux Droop Method – A New Control Strategy of Inverters in Microgrids

The parallel operation of inverters in microgrids is mainly based on the droop method. Conventional voltage droop method consists of adjusting the output voltage frequency and amplitude to achieve autonomous power sharing without control wire interconnections. Nevertheless, the conventional voltage droop method shows several drawbacks, such as complicated inner multiloop feedback control, and most importantly, frequency and voltage deviations. This paper proposes a new control strategy in microgrid applications by drooping the virtual flux instead of the inverter output voltage. First, the relationship between the inverter virtual flux and the active and reactive powers is mathematically obtained. This is used to develop a new flux droop method. In addition, a small-signal model is developed in order to design the main control parameters and study the system dynamics and stability. Furthermore, a direct flux control (DFC) algorithm is employed to regulate the virtual flux according to the droop controller, which avoids the use of PI controllers and PWM modulators. Both the simulation and experimental results shows that the proposed flux droop strategy can achieve active and reactive power sharing with much lower frequency deviation than the conventional voltage droop method, thus highlighting the potential use in microgrid applications.