Virtual Synchronous Machine also called VISMA [1] is a control algorithm to make an inverter operated as a conventional electromechanical synchronous machine.It is a promising solution to overcome the problems of the grid stability and quality, which have been exacerbated by increasing integration of distributed generation units into the grid.Compared to the conventional power plants, in which the synchronous machine dominate, the distributed generation units have either significantly smaller or no rotating mass and damping effect.These weaknesses can be compensated by using the VISMA concept and thus the power system quality will be improved.Furthermore, the penetration level of the DG sources won't be restricted any more.Up to now the VISMA was implemented by using a voltage-tocurrent model on a hysteresis controlled inverter [1][2][3].This method will be called VISMA-Method 1 here.Since the most products of inverters in the market are PWM controlled, the VISMA-Method 1 cannot be easily applied on these inverters.Therefore, a new method is developed to implement the VISMA by using a current-to-voltage model on the currently widely applied PWM controlled inverter.This new method is called VISMA-Method 2 in this paper and will be compared with the VISMA-Method 1 by simulation results.

The increasing integration of decentralized electrical sources is attended by problems with power quality, safe grid operation and grid stability. The concept of the Virtual Synchronous Machine (VISMA) [1] discribes an inverter to particularly connect renewable electrical sources to the grid that provides a wide variety of static an dynamic properties they are also suitable to achieve typical transient and oscillation phenomena in decentralized as well as weak grids. Furthermore in static operation, power plant controlled VISMA systems are capable to cope with critical surplus production of renewable electrical energy without additional communication systems only conducted by the grid frequency. This paper presents the dynamic properties "damping" and "virtual mass" of the VISMA and their contribution to the stabilization of the grid frequency and the attenuation of grid oscillations examined in an experimental grid set.

In this paper the investigation results of the Virtual Synchronous Machine (VISMA) in the island mode are presented. The increasing integration of the distributed generation units leads to continual growing of the amount of the inverter interface in the electrical grid. Therefore, the characteristics of inverters will more and more significantly influence the grid operation. The VISMA is a novel control method for the inverter to make it possible that an inverter-interfaced decentralized generation unit can be operated as a real electromechanical synchronous machine, which up to now as the main generation unit in the electrical grid has many advantages in consideration of the grid stability. These advantages can be implemented in the VISMA and are proved experimentally in the grid-connected operation mode. The island-mode is another important operation mode that will be necessary for a stand-alone grid without connecting of the main grid or when the main grid fails due to a disturbance. Therefore this operation mode of the VISMA is focused in this paper.

This paper presents the improvement of power quality and grid stability for distributed generation using the virtual synchronous machine (VISMA) which embodies a hysteresis controlled three phase inverter with a synchronous machine model on an embedded control computer to calculate the reference currents. Currently the conventional grid-connected inverters are predominantly designed to transmit electrical energy to the grid discounting the maintenance of frequency and voltage and also its transient stability. However, the VISMA is able to regulate both the active and reactive power separately and bidirectionally by setting the virtual torque and virtual excitation to meet the power system requirements. Furthermore, a virtual rotating mass is implemented in the VISMA in order to increase the inertia in the grid and improve the transient frequency stability in analogy to the conventional synchronous generator. Additionally, the virtual damping of the VISMA can reduce the frequency and power oscillation in the grid. All these properties mentioned above have been verified in simulations and measurements in an experimental micro grid.