This paper proposes a step-by-step procedure for designing the LCL filter of a front-end three-phase active rectifier. The primary goal is to reduce the switching frequency ripple at a reasonable cost, while at the same time achieving a high-performance front-end rectifier (as characterized by a rapid dynamic response and good stability margin). An example LCL filter design is reported and a filter has been built and tested using the values obtained from this design. The experimental results demonstrate the performance of the design procedure both for the LCL filter and for the rectifier controller. The system is stable and the grid current harmonic content is low both in the lowand high-frequency ranges. Moreover, the good agreement that was obtained between simulation and experimental results validates the proposed approach. Hence, the design procedure and the simulation model provide a powerful tool to design an LCL-filter-based active rectifier while avoiding trial-and-error procedures that can result in having to build several filter prototypes.

Design and control of an LCL-filter-based three-phase active rectifier

Solid-state switch-mode rectification converters have reached a matured level for improving power quality in terms of power-factor correction (PFC), reduced total harmonic distortion at input AC mains and precisely regulated DC output in buck, boost, buck-boost and multilevel modes with unidirectional and bidirectional power flow. This paper deals with a comprehensive review of improved power quality converters (IPQCs) configurations, control approaches, design features, selection of components, other related considerations, and their suitability and selection for specific applications. It is targeted to provide a wide spectrum on the status of IPQC technology to researchers, designers and application engineers working on switched-mode AC-DC converters. A classified list of more than 450 research publications on the state of art of IPQC is also given for a quick reference.

/pdf/a-review-of-single-phase-improved-power-quality-ac-dc-2pm6h4zjxq.pdf

A review of single-phase improved power quality AC-DC converters

Many grid connected power electronic systems, such as STATCOMs, UPFCs, and distributed generation system interfaces, use a voltage source inverter (VSI) connected to the supply network through a filter. This filter, typically a series inductance, acts to reduce the switching harmonics entering the distribution network. An alternative filter is a LCL network, which can achieve reduced levels of harmonic distortion at lower switching frequencies and with less inductance, and therefore has potential benefits for higher power applications. However, systems incorporating LCL filters require more complex control strategies and are not commonly presented in literature. This paper proposes a robust strategy for regulating the grid current entering a distribution network from a three-phase VSI system connected via a LCL filter. The strategy integrates an outer loop grid current regulator with inner capacitor current regulation to stabilize the system. A synchronous frame PI current regulation strategy is used for the outer grid current control loop. Linear analysis, simulation, and experimental results are used to verify the stability of the control algorithm across a range of operating conditions. Finally, expressions for ""harmonic impedance" of the system are derived to study the effects of supply voltage distortion on the harmonic performance of the system.

Grid current regulation of a three-phase voltage source inverter with an LCL input filter

For a grid-connected converter with an LCL filter, the harmonic compensators of a proportional-resonant (PR) controller are usually limited to several low-order current harmonics due to system instability when the compensated frequency is out of the bandwidth of the system control loop. In this paper, a new current feedback method for PR current control is proposed. The weighted average value of the currents flowing through the two inductors of the LCL filter is used as the feedback to the current PR regulator. Consequently, the control system with the LCL filter is degraded from a third-order function to a first-order one. A large proportional control-loop gain can be chosen to obtain a wide control-loop bandwidth, and the system can be optimized easily for minimum current harmonic distortions, as well as system stability. The inverter system with the proposed controller is investigated and compared with those using traditional control methods. Experimental results on a 5-kW fuel-cell inverter are provided, and the new current control strategy has been verified.

A New Feedback Method for PR Current Control of LCL-Filter-Based Grid-Connected Inverter

Many grid connected power electronic systems, such as STATCOMs, UPFCs and distributed generation system interfaces, use a voltage source inverter (VSI) connected to the supply network through a filter. This filter, typically a series inductance, acts to reduce the switching harmonics entering the distribution network. An alternative filter is a LCL network, which can achieve reduced levels of harmonic distortion at lower switching frequencies and with less inductance, and therefore has potential benefits for higher power applications. However, systems incorporating LCL filters require more complex control strategies and are not commonly presented in literature. This paper proposes a robust strategy for regulating the grid current entering a distribution network from a three-phase VSI system connected via an LCL filter. The strategy integrates an outer loop grid current regulator with inner capacitor current regulation to stabilise the system. A synchronous frame PI current regulation strategy is used for the outer grid current control loop. Linear analysis, simulation and experimental results are used to verify the stability of the control algorithm across a range of operating conditions. Finally, expressions for "harmonic impedance" of the system are derived to study the effects of supply voltage distortion on the harmonic performance of the system.

In this paper, a control principle for a voltage source converter connected to the grid through an LCL-filter is presented. The dynamic performance is compared with the performance obtained with an L-filter and a dead-beat vector control system. Measured frequency responses and an active filtering operation verify the control principle. By using an LCL-filter, high attenuation of harmonics caused by the PWM and high dynamic performance can be obtained simultaneously. Different methods for active filtering are compared. It is advantageous to use a Fourier-method since it allows for compensation for time delays occurring in the control system in the frequency domain. By using the LCL-filter along with compensation for time delays in the frequency domain, active filtering can be performed at moderate switching frequencies. This is a major advantage in systems, such as a rectifier with an active filtering option.

Control of a voltage-source converter connected to the grid through an LCL-filter-application to active filtering

To reduce the number of measurements in digitally controlled converter systems, observers are often proposed. This gives complicated controller equations and to obtain sufficient sampling frequency, DSPs are often proposed. In this paper, a control principle avoiding measurements by using feedforward of steady state vectors is introduced. Only one state out of three is measured. The time efficient controllers facilitates using a low cost control system. >

Feedforward-time efficient control of a voltage source converter connected to the grid by lowpass filters

In the literature, the influence of nonlinearities occurring due to the effects of blanking time, the valves and the nonideal grid filter on the dynamic performance of current control are usually not described. In this paper, small-signal analysis is used to study the influence of nonlinearities for a vector current controlled VSC connected to a grid. To show the influence of nonlinearities, measured frequency responses are compared with responses from analytical models. Measured responses are presented for operation in rectifier and inverter operation as well as static VAr operation. As displayed, the frequency dependent losses in the grid filter affect the dynamic performance at high frequencies. A method for compensating for nonlinearities due to blanking-time and nonideal valves is implemented. The compensation method improves the performance.

M. Lindgren

Papers

Control of a voltage-source converter connected to the grid through an LCL-filter-application to active filtering

Feedforward-time efficient control of a voltage source converter connected to the grid by lowpass filters

Vector current controlled grid connected voltage source converter-influence of nonlinearities on the performance