A Cooperative Control Method of Active and Reactive Power for Droop Control-Based Standalone Microgrids

Abstract

A microgrid that is operated autonomously due to its electrical isolation from the main grid (e.g., a remote island area) is called as a standalone microgrid. In grid-connected mode, the main grid forms and maintains the frequency and voltage stably whereas islanded or standalone mode cannot. So the standalone microgrid emulates the conventional power system to adopt frequency and voltage droop control. However, the standalone microgrid has characteristics of low system inertia and weak grid. Subsequently, the frequency and voltage stability of the microgrid is vulnerable to the change of load or output of intermittent renewable energy source (RES). This dissertation presents control methods of distributed generations (DGs) in a standalone microgrid in order to maintain the frequency and voltage stably during disturbances such as load change and/or output change of intermittent RES. Since the conventional frequency droop control method uses frequency deviation from its nominal value to share active power, the frequency deviation is inevitable. To overcome frequency deviation, an active power sharing method of using the state of charge (SOC) of the battery energy storage system (BESS) is proposed. The BESS forms constant frequency and voltage magnitude without any droop method and other controllable DGs are controlled to output desired level of active and reactive power. Thus, the output and the SOC of the BESS are changed if the active power load is changed. Other controllable DGs will share the active power load based on the SOC deviation. The DGs in the proximity of the BESS will measure the output of the BESS directly for primary SOC control. The DGs in the distance from the BESS will receive the SOC data via communication system with time delay for secondary SOC control. To enhance system reliability, the BESS controller is designed to be changed to the conventional frequency droop control mode if the communication system fails. For improvement of the voltage stability, a new droop method is proposed. Voltage–reactive power (V–Q) droop control method has been adopted conventionally in standalone microgrids to alleviate voltage deviation and to share reactive power. This method uses the voltage deviation from its nominal value to control reactive power. Subsequently, it cannot fundamentally prevent voltage deviation. Especially, if the output of intermittent RES fluctuates heavily, the voltage variation will be severer. To eliminate this voltage fluctuation, active power–reactive power (P–Q) droop control method, which changes the reactive power proportional to the change of active power of RES, is proposed. The droop coefficient is determined based on the sensitivity matrix. Moreover, active power–active power (P–P) droop control is also developed for low voltage network based standalone microgrid where reactive power compensation barely affects system voltage. The proposed method was modeled and simulated by MATLAB/SimPowerSy-stems. It is compared with the conventional droop control method to prove its effectiveness. The proposed method can be applied to the electrically isolated power system with high hosting capacity of RES. It can maintain the frequency and voltage steadily. Consequently, increment of the RES hosting capacity into the microgrid and prosperity of the energy self-supporting islands are looked for.