Kenichi Sakimoto

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).

Low voltage ride through capability of a grid connected inverter based on the virtual synchronous generator

When distributed generators are connected to the grid, it is required that the system withstand the voltage dip caused by the grid failure. This is known as the low voltage ride through (LVRT). One of the effective choices to realize the LVRT is a grid connected inverter with virtual synchronous generator (VSG). This paper present a VSG control with capability for LVRT. By using this control, the operation of inverter can be continued even if a severe voltage dip at the grid has happened.

Stabilization effect of virtual synchronous generators in microgrids with highly penetrated renewable energies

Because of the increasing use of renewable energy technology, distributed power sources and greatly varying loads can be critical factors in power system destabilization. This problem has hampered the adoption of renewable energy; thus, researchers have studied various control method types that enable an inverter to be operated as a synchronous generator (SG). Some of these methods, referred to as Virtual Synchronous Generators (VSGs), are designed to provide virtual inertia. Using VSG technology, we can decrease the introduction of fossil fuel-based power, which allows a variety of energy sources to be combined. In this study, we derived and analyzed the frequency system dynamics of microgrids, and show that a VSG expressed as a first-order equation can stabilize the frequency of a grid without causing resonance among the generators and loads. Furthermore, we derived excitation system dynamics, and observed that a VSG applying constant impedance can reduce the voltage fluctuations in the system. The results were verified in laboratory experiments and through a simulation using EMTP-RV. The simulation test results, which were generated using data acquired from an actual photovoltaic facility, indicated that the VSG could effectively suppress system deviations caused by sudden weather change.

Effects of suppressing frequency fluctuations by parallel operation of virtual synchronous generator in microgrids

When a greatly varying load is connected to a weak power system, stability of the system becomes a problem. In these days, because the use of renewable energy is accelerating, not only the loads but also the distributed generators can be critical factors in destabilizing the system. A virtual synchronous generator (VSG) control allows a static inverter to behave similar to a synchronous generator (SG). Using this VSG technology, we can increase the introduction ratio of renewable sources, and thus, we can combine various types of power sources. Simultaneously, if the combination of governor and rotor inertia is represented in a first-order lag element, the VSG can suppress the frequency fluctuations in microgrids. In this paper, the oscillation mechanism in parallel operations of generators is clarified, and we present the effects of suppressing the frequency fluctuations using the VSG. The results are verified by simulation using EMTP-RV and in laboratory experiments.

Virtual Synchronous Generator control with Double Decoupled Synchronous Reference Frame for single-phase inverter

We have proposed the Virtual Synchronous Generator control (VSG control) and have tested it using the demonstration equipment [1]. By using the VSG control, three-phase inverters of current control type are able to run both in grid-connecting operation and in grid-disconnecting operation. Furthermore, in order to control a single-phase inverter like a three-phase inverter using the VSG control, we have applied the technique called “Double Decoupled Synchronous Reference Frame” (DDSRF). In this paper, we will show you the simulation results and experimental results of the single-phase inverter using the VSG control with DDSRF.