The grid-tied inverter synchronizes with the network on the basis of the instantaneous voltage phase angle. This angle is computed by the so-called synchronization algorithms. During grid disturbances, it is estimated with a certain accuracy, which varies for different disturbances and depends on the choice of algorithm. The tests presented here determine how to make an optimal selection of the synchronization algorithm. The research methods used are modeling, simulation and analysis of the results obtained. One of the most important outcomes is the determination of the root-mean-square sync error and its dynamics denotation. The research conclusions should be of particular interest to designers of distributed energy systems with a large number of inverter energy sources.

The process of synchronization of synchronous generators and power electronic converters with the power grid may take on quite different forms. This is due to their specific principles of operation and essential differences in energy conversion process. However, since synchronous generators and power converter often operate in the same utility network, coherent rules should be defined for them. Therefore, this paper aims at a formulation of the uniform and consistent interpretation of synchronization with the power grid for both types of aforementioned units. The author starts from the classic interpretation of synchronization for synchronous generators and power electronic converters, considered as micro-generators, specifies their mathematical and numerical models and then performs simulation tests. Selected synchronization algorithms are described in detail. Simulation tests are used for analysis of the elaboration of outcomes. The results of simulation tests are handled to formulate a uniform interpretation of synchronization for the micro-generation systems considered. Based on the results obtained, appropriate parallels are built between the two systems being compared. It is shown that the synchronization processes are identical regardless of the micro-generation unit considered. Nonetheless, they differ significantly due to their properties in transient states. Inverter systems have higher dynamics but their disadvantage lies in the relatively high sensitivity to disturbances and the complex selection process of the synchronization algorithm.

This paper presents an improved Virtual Flux-based Direct Power Control (VF-DPC) applied for a three-phase pulse width modulation rectifier. The proposed control approach incorporates an enhanced Virtual Flux estimator made up of a cascade second-degree low-pass filter. This latter guarantees the attenuation of the highest harmonics. The introduced control concept presented in this paper has interesting features such as reducing the current harmonics distortion. In other words, it ensures that the input current drawn from the power supply is perfectly sinusoidal whatever the state of the network voltage. The proposed method also allows to maintain the DC side capacitor voltage at the required level and assure that the input current is in phase with the respective voltage to satisfy the unity power factor function. The results obtained from the numerical simulation have proved the effectiveness of the proposed method for disturbed grid voltage and system parameters variation.