Demands in the area of electrical energy generation and distribution, as a result of energy policies, are leading to far reaching changes in the structure of the energy supply, which is characterised, on the one hand, by the substitution of conventional power stations by renewable energy generation, a decision which has already been made, and, on the other hand, by the changeover from centralised to decentralised energy generation. From an electrical engineering point of view, a new situation will arise for consumers concerning security of supply and power quality, which calls for further technical measures by the grid operators to ensure that the increasingly stringent supply criteria can be met. This article describes a new power electronics based approach which allows a grid compatible integration of predominantly renewable electricity generators even in weak grids making them appear to be electromechanical synchronous machines. As a consequence, all the proven properties of this type of machine which have so far defined the grid continue to do so, even when integrating photovoltaic or wind energy. These properties include, for instance, interaction between grid and generator as in a remote power dispatch, reaction to transients as well as the full electrical effects of a rotating mass. In addition, this new development can be operated in such a way that it provides primary reserve allowing, from a grid point of view, electricity generators such as wind and PV to be regarded as conventional power stations.

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Virtual inertia control is considered as an important part of microgrids with high renewable penetration. Virtual inertia emulation based on the derivative of frequency is one of the effective methods for improving system inertia and maintaining frequency stability. However, in this method, the ability to provide virtual damping is usually neglected in its design, and hence, its performance might be insufficient in the system with low damping. Confronted with this issue, this paper proposes a novel design and analysis of virtual inertia control to imitate damping and inertia properties simultaneously to the microgrid, enhancing frequency performance and stability. The proposed virtual inertia control uses the derivative technique to calculate the derivative of frequency for virtual inertia emulation. Trajectory sensitivities have been performed to analyze the dynamic impacts of the virtual inertia and virtual damping variables over the system performance. Time-domain simulations are also presented to evaluate the efficiency of the virtual damping and virtual inertia in enhancing system frequency stability. Finally, the efficiency and robustness of the proposed control technique are compared with the conventional inertia control under a wide range of system operation, including the decrease in system damping and inertia and high integrations of load variation and renewable energy.INDEX TERMS Frequency stability, isolated microgrid, virtual inertia regulation, virtual synchronous machine.

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.

Up to now, the electromechanical synchronous machine rules the domain of electrical power generation devices and that way the public power grids. Its specific characteristics guarantee the stable highly parallel grid operation, automatic power balancing and damping. Not least the rotating mass is related to some dynamic properties and the condition for the grid short time frequency stability. The progressive integration of renewable sources is accompanied by questions concerning the loss of these features using inverters for grid feeding. The paper shows an approved concept that combines inverter technology and synchronous machine properties.