Conventional droop methods for microgrids

As the number of distributed generators (DGs) is increasing, the droop control methods are becoming more important. The droop control, which is also known as the primary control of hierarchical system in microgrid, has been widely used because it enables the stable power sharing among multiple generators in parallel operation. In particular, the droop control methods can be applied to the DGs to regulate the active and reactive powers for maintaining the grid frequency and voltage with the steady-state characteristic of generators based on the relationships of active power (P)–frequency (f) and reactive power (Q)–voltage (V). The regulation of output powers from renewable energy sources (RESs) is difficult due to their stochastic and intermittent behaviors. Therefore, the use of energy storage system (ESS) is necessary in microgrid to reduce the uncertainty and variability from RESs, particularly when it has very high renewable penetrations. In addition, the interest in DC microgrids has been recently rising because of their connectivity and high efficiency. In DC microgrid, the droop control is also used effectively like in AC microgrid. In this book chapter, the comprehensive overview of conventional droop control methods in both AC and DC microgrids will be firstly presented. Then, their different characteristics and features will be described for several DGs such as diesel generators, RESs, and ESSs while focusing on their proper applications to the DGs depending on the different operating conditions of microgrid.