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 increasing penetration of distributed power generation into the power system leads to a continuous evolution of grid interconnection requirements. In particular, active power control will play an important role both during grid faults (low-voltage ride-through capability and controlled current injection) and in normal conditions (reserve function and frequency regulation). The aim of this paper is to propose a flexible active power control based on a fast current controller and a reconfigurable reference current selector. Several strategies to select the current reference are studied and compared using experimental results that are obtained during an unsymmetrical voltage fault. The results of the analysis allow selection of the best reference current in every condition. The proposed methods facilitate multiple choices for fault ride through by simply changing the reference selection criteria.

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Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults

This paper proposes a new control scheme of improved transient response for the pulsewidth-modulation ac/dc converter under generalized unbalanced operating conditions. The overall-bandwidth diminishing filter/functional block for extracting positive and negative sequence components has been avoided by employing dual current regulators in positive- and negative-rotating synchronous frames. The steady-state error due to 120-Hz ac signals has been reduced by employing a resonant-gain path in the current regulator. This paper also proposes simplified current-reference calculation scheme in the regulation of instantaneous active/reactive power. Compared to the previous works, the proposed control method has better transient response in compensation for generalized unbalanced operating conditions (unbalanced input supply and unbalanced input impedances) of wide range while still satisfying unity input power-factor correction and ripple-free dc-output-voltage regulation without adding any external hardware. The proposed control system has been analyzed and tuned based on the ac small signal perturbed model under unbalance. Simulation and experimental results confirm the proposed control method under severe unbalanced operating conditions.

A control scheme in hybrid synchronous-stationary frame for PWM AC/DC converter under generalized unbalanced operating conditions

Many control algorithms and circuits for three-phase pulse width modulation active rectifiers have been proposed in the past decades. In most of the research, it is often assumed that the input voltages are balanced or contain only fundamental frequency components. In this paper, a selective harmonic compensation method is proposed based on an improved multiple reference frame algorithm, which decouples signals of different frequencies before reference frame transformation. This technique eliminates interactions between the fundamental-frequency positive-sequence components and harmonic and/or negative-sequence components in the input currents, so that fast and accurate regulation of harmonic and unbalanced currents can be achieved. A decoupled phase-locked loop algorithm is used for proper synchronization with the utility voltage, which also benefits from the multiple reference frame technique. The proposed control method leads to considerable reduction in low-order harmonic contents in the rectifier input current and achieves almost zero steady-state error through feedback loops. Extensive experimental tests based on a fixed-point digital signal processor controlled 2 kW prototype are used to verify the effectiveness of the proposed ideas.

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Multiple Reference Frame-Based Control of Three-Phase PWM Boost Rectifiers under Unbalanced and Distorted Input Conditions

This paper proposes a unified mathematical model of a voltage-sourced converter (VSC) based on positive and negative synchronously rotating frames under both unbalanced grid supply and imbalanced input impedances. Based on the developed model, a new definition of instantaneous reactive power is introduced, and an accurate mathematical model of the instantaneous active and reactive powers at different locations of the VSC is derived. To avoid the use of band-trap filter for decomposing the positive- and negative-sequence dq components of current and voltage, a new control scheme with multifrequency proportional-resonant controller in the stationary alphabeta frame is employed to improve the steady state and dynamic response under generalized unbalanced operation conditions. Simulation and experimental studies with various factors were conducted and compared under different types of unbalanced conditions. The simulated and experimental results demonstrate that the proposed control scheme in the stationary alphabeta frame provides sufficiently steady-state tracking capability for ac input current, and a better transient response in compensation for wide-range generalized unbalanced operation conditions.

Modeling and Control of Grid-Connected Voltage-Sourced Converters Under Generalized Unbalanced Operation Conditions

Under severe fault conditions in the distribution system, not only input voltages but also input impedances must be considered as unbalanced. This paper presents a new control method for input-output harmonic elimination of the pulsewidth-modulation (PWM) boost-type rectifier under conditions of both unbalanced input voltages and unbalanced input impedances. The range of imbalance in both input voltages and input impedances, for which the proposed method is valid, is analyzed in detail. An analytical approach for complete harmonic elimination shows that PWM boost-type rectifier can operate at unity power factor under extremely unbalanced operating conditions resulting in a smooth (constant) power flow from AC to DC side. Based on the analyses in open-loop configuration, a feedforward control method is proposed. Elimination of harmonics at AC and DC side of the converter affects the cost of DC link capacitor and AC side filter. The proposed method is very useful when the PWM boost-type rectifier is subject to extreme imbalance due to severe fault conditions in the power system. In addition, by using the proposed method, the PWM boost-type rectifier can be operated from the single-phase supply in cases where three-phase source is not available. Simulation results show excellent response and stable operation of the PWM boost-type rectifier under the proposed control algorithm. Experimental and simulation results are in excellent agreement.

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A New Control Method for Input–Output Harmonic Elimination of the PWM Boost-Type Rectifier Under Extreme Unbalanced Operating Conditions

Ac 2007-1330: building a modern power electronics and electric machines laboratory

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Ke Chen

Papers

A New Control Method for Input–Output Harmonic Elimination of the PWM Boost-Type Rectifier Under Extreme Unbalanced Operating Conditions

Ac 2007-1330: building a modern power electronics and electric machines laboratory

Building A Modern Power Electronics And Electric Machines Laboratory