Steffan Hansen

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

The aim of the paper is to propose a design procedure for an LCL-filter in a front-end three-phase active rectifier. The main goal is to ensure a reduction of the switching frequency ripple at a reasonable cost and, at the same time, to obtain a high performance rectifier. In this paper a step-by-step procedure for designing an LCL-filter is proposed and verified by simulations and experiments. An example of LCL-filter design has been reported and, with the obtained values, the filter has been realised and tested. The experimental results demonstrate the effectiveness of the design procedure both of the LCL-filter and of the controllers. The performance of the overall system is good both in the low and high frequency ranges. Moreover the good agreement between these results and those obtained in simulation validate the adopted model: the design procedure and the simulation model represent a powerful tool to design an LCL active rectifier without the need for the realisation of several prototypes.

Virtual flux based direct power control of three-phase PWM rectifiers

In this paper direct power control (DPC) of three-phase PWM rectifiers without line voltage sensors is presented. The new system is based on virtual flux (VF) estimation. The theoretical principle of this method is discussed. The steady state and dynamic behaviour of VF-DPC are presented that illustrate the operation and performance of the proposed system compared to a conventional DPC method. Both strategies are also investigated under unbalance and pre-distorted grid. It is shown that VF-DPC exhibits several advantages, particularly providing sinusoidal line current when supply voltage is not ideal. Some test results show the excellent performance of the proposed system.

Investigation of Active Damping Approaches for PI-Based Current Control of Grid-Connected Pulse Width Modulation Converters With LCL Filters

This paper deals with various active damping approaches for PI-based current control of grid-connected pulsewidth-modulation (PWM) converters with LCL filters, which are based on one additional feedback. Filter capacitor current, as well as voltage feedback for the purpose of resonance damping, are analyzed and compared. Basic studies in the continuous Laplace domain show that either proportional current feedback or derivative voltage feedback yields resonance damping. Detailed investigations of these two approaches in the discrete z-domain, taking into account the discrete nature of control implementation, sampling, and PWM, are carried out. Several ratios of LCL resonance frequency and control frequency are considered. At high resonance frequencies, only current feedback stabilizes the system. At medium resonance frequencies, both approaches have good performance. At low resonance frequencies, stability gets worse, even though voltage feedback offers slightly better damping properties. Measurements validate the theoretical results.

Detection is key - Harmonic detection methods for active power filter applications

This article gives a survey of the commonly used methods for harmonic detection in active power filters (APFs). The work proposes a simulation setup that decouples the harmonic detection method from the active filter model and its controllers. In this way, the selected methods can be equally analyzed and compared with respect to their performance, which helps in anticipating possible implementation issues. A comparison is given that may be used to decide the future hardware setup implementation. The comparison shows that the choice of numerical filtering is a key factor for obtaining a good accuracy and dynamic performance of an active power filter.

A new optimized space-vector modulation strategy for a component-minimized voltage source inverter

This paper presents a new space-vector modulation strategy suitable for a low-cost pulse-width-modulation (PWM) voltage-source (VS) inverter employing only four switches, four diodes, and a split-capacitor bank in the DC link. The work is motivated by the need for an efficient and flexible modulation method, which is optimized with respect to minimum machine-torque ripple. The modulation strategy is named space-vector modulation for four-switch inverter (SVMFSI), and it is realized by planning the switching patterns between four active voltage vectors on the basis of a desired flux trajectory for the stator-flux vector in the AC machine. The strategy is implemented in a single 8-bit microcontroller as a double-sided modulation strategy. Simulations of the machine-torque ripple are performed at a switching frequency of 4 kHz and indicate a torque ripple of 14% at nominal load. Finally, selected results are verified experimentally on a 1.5-kVA prototype B4 inverter. The test results indicate high-quality output-voltage spectra with no low-order voltage harmonics and a harmonic-loss factor (HLF) of 1.12% at unity modulation index.

Sensorless control strategies for PWM rectifier

In this paper two different control strategies for a PWM rectifier without line voltage sensors are compared. The direct power control (DPC) which has no need for line voltage measurements is compared to the conventional voltage oriented control (VOC) strategy in rotating coordinates with a novel line voltage estimator. The steady-state performance of both strategies is compared with regards to voltage unbalance and pre-distorted grid. Furthermore, the use of discontinuous modulation is motivated in the classical control strategy and is analyzed along with the novel line voltage estimator. It is shown that the VOC strategy with line voltage estimator exhibits several advantages compared to DPC. Some simulations and experimental results verifying the comparison are presented.

Evaluation of harmonic detection methods for active power filter applications

In the attempt to minimize the harmonic disturbances created by the non-linear loads the choice of the active power filters comes out to improve the filtering efficiency and to solve many issues existing with classical passive filters. One of the key points for a proper implementation of an active filter is to use a good method for current/voltage reference generation. There exist many implementations supported by different theories (either in time- or frequency-domain), which continuously debate their performances proposing ever better solutions. This paper gives a survey of the common used theories. Then, the work here proposes a simulation setup that decouples the harmonic reference generator from the active filter model and its controller. In this way the selected methods can be equally analyzed and compared with respect to their performance, which helps anticipating possible implementation issues. The conclusions are collected and a comparison is given at the end, which is useful in deciding the future hardware setup implementation. The comparison shows that the choice of numerical filtering is a key factor for obtaining good accuracies and dynamics for an active filter.

Adaptive Compensation of Reactive Power with Shunt Active Power Filters

This paper describes an adaptive method for compensating the reactive power with an active power filter (APF), which is initially rated for mitigation of only the harmonic currents given by a nonlinear industrial load. It is proven that, if the harmonic currents do not load the APF at the rated power, the available power can be used to provide a part of the required reactive power. Different indicators for designing such application are given, and it is proven that the proposed adaptive algorithm represents an added value to the APF. The algorithm is practically validated on a laboratory setup with a 7-kVA APF.

A new optimized space vector modulation strategy for a component minimized voltage source inverter

This paper presents a new space vector modulation strategy suitable for a low cost PWM-VSI inverter employing only four switches, four diodes and a split capacitor bank in the DC-link. The work is motivated by the need of an efficient and flexible modulation method, which is optimized with respect to minimum machine torque ripple. The modulation strategy is named Space Vector Modulation for Four Switch Inverter (SVMFSI) and is realized by planning the switching patterns between four active voltage vectors on the basis of a desired flux trajectory for the stator flux vector in the AC-machine (variable speed induction motor drive). The strategy is implemented in a single 8-bit microcontroller as a double sided modulation strategy. Simulations of the motor torque ripple are performed at a switching frequency of 4 kHz and indicate a torque ripple of 14% at nominal load. Finally, selected results are verified experimentally on a 1.5 kVA prototype B4 inverter. The test results indicate high quality output voltage spectra with no low-order voltage harmonics and a harmonic loss factor (HLF) of 1.12% at unity modulation index.<<ETX>>

PWM rectifier with LCL-filter using different current control structures

The control of a PWM rectifier with LCL-filter using a minimum number of sensors is analyzed. In addition to the DC-link voltage either the converter or line current is measured. Two different ways of current control are shown, analyzed and compared by simulations as well as experimental investigations. Main focus is spent on active damping of the LCL filter resonance and on robustness against line inductance variations.