Analysis of Resonance in Microgrids and Effects of System Frequency Stabilization Using a Virtual Synchronous Generator

In order to reduce greenhouse gases, distributed generators such as wind turbines and photovoltaic facilities have been adopted in many parts of the world. These sources are assumed to be connected to an infinite bus. Thus, if the total capacity of the grid-connected inverters is approximately equal to or greater than that of conventional synchronous generators (SGs), conventional methods such as simple current control cannot maintain power grid stability. In particular, this problem becomes conspicuous in isolated islands and small communities where large commercial power systems do not exist. On the other hand, if we use supervisory control, system flexibility and scalability will be reduced. Therefore, nonsupervisory autonomous control methods are desired. For this reason, many researchers have already studied, which enables an inverter to be operated as an SG. Some of them are called virtual SG (VSG) control, and the common point of them is to provide virtual inertia. In this paper, we have derived and analyzed a formula for the dynamic stability of microgrids and shown that the VSG expressed in the first-order equation can realize a stable grid without causing resonance among the generators and the loads. The results were verified in laboratory experiments and through a simulation using electro-magnetic transient program restructured version (EMTP-RV).