Renewable energy sources like wind, sun, and hydro are seen as a reliable alternative to the traditional energy sources such as oil, natural gas, or coal. Distributed power generation systems (DPGSs) based on renewable energy sources experience a large development worldwide, with Germany, Denmark, Japan, and USA as leaders in the development in this field. Due to the increasing number of DPGSs connected to the utility network, new and stricter standards in respect to power quality, safe running, and islanding protection are issued. As a consequence, the control of distributed generation systems should be improved to meet the requirements for grid interconnection. This paper gives an overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines. In addition, control structures of the grid-side converter are presented, and the possibility of compensation for low-order harmonics is also discussed. Moreover, control strategies when running on grid faults are treated. This paper ends up with an overview of synchronization methods and a discussion about their importance in the control

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Overview of Control and Grid Synchronization for Distributed Power Generation Systems

The recently introduced proportional-resonant (PR) controllers and filters, and their suitability for current/voltage control of grid-connected converters, are described. Using the PR controllers, the converter reference tracking performance can be enhanced and previously known shortcomings associated with conventional PI controllers can be alleviated. These shortcomings include steady-state errors in single-phase systems and the need for synchronous d-q transformation in three-phase systems. Based on similar control theory, PR filters can also be used for generating the harmonic command reference precisely in an active power filter, especially for single-phase systems, where d-q transformation theory is not directly applicable. Another advantage associated with the PR controllers and filters is the possibility of implementing selective harmonic compensation without requiring excessive computational resources. Given these advantages and the belief that PR control will find wide-ranging applications in grid-interfaced converters, PR control theory is revised in detail with a number of practical cases that have been implemented previously, described clearly to give a comprehensive reference on PR control and filtering.

Proportional-resonant controllers and filters for grid-connected voltage-source converters

Current regulators for AC inverters are commonly categorized as hysteresis, linear PI, or deadbeat predictive regulators, with a further sub-classification into stationary ABC frame and synchronous d-q frame implementations. Synchronous frame regulators are generally accepted to have a better performance than stationary frame regulators, as they operate on DC quantities and hence can eliminate steady-state errors. This paper establishes a theoretical connection between these two classes of regulators and proposes a new type of stationary frame regulator, the P+Resonant regulator, which achieves the same transient and steady-state performance as a synchronous frame PI regulator. The new regulator is applicable to both single-phase and three phase inverters.

Stationary frame current regulation of PWM inverters with zero steady-state error

Current regulators for AC inverters are commonly categorised as hysteresis, linear PI or deadbeat predictive, with a further subclassification into stationary ABC frame and synchronous DQ frame implementations. Synchronous frame controllers are generally accepted to have a better performance than stationary frame controllers do, as they operate on DC quantities and hence can eliminate steady state errors. This paper establishes a theoretical connection between these two classes of regulators and proposes a new type of stationary frame controller, which achieves the same steady state performance as a synchronous frame controller. The new controller is applicable to both single phase and three phase inverters.

Stationary frame current regulation of PWM inverters with zero steady state error

This paper discusses the evaluation of different current controllers employed for grid-connected distributed power generation systems having variable input power, such as wind turbines and photovoltaic systems. The focus is mainly set on linear controllers such as proportional-integral, proportional-resonant, and deadbeat (DB) controllers. Additionally, an improved DB controller robust against grid impedance variation is also presented. Since the paper discusses the implementation of these controllers for grid-connected applications, their evaluation is made in three operating conditions. First, in steady-state conditions, the contribution of controllers to the total harmonic distortion of the grid current is pursued. Further on, the behavior of controllers in the case of transient conditions like input power variations and grid voltage faults is also examined. Experimental results in each case are presented in order to evaluate the performance of the controllers.

Evaluation of Current Controllers for Distributed Power Generation Systems

A new current control method based on the internal model principle in control theory is proposed. It introduces a sinusoidal internal model into the control system. It does not use any coordinate transformations. The method can be used for tracking an arbitrary number of harmonics: a DC component or fundamental frequency component signal. It is applied to a single-phase pulsewidth modulation inverter and active filter. The validity is confirmed by simulation and experimental results.

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T. Yoda

Papers

A novel current-tracking method for active filters based on a sinusoidal internal model [for PWM invertors]