In this paper, a simple model predictive current control strategy is proposed for four-leg inverters to deliver power to the (un)balanced/(non)linear loads. Unlike classical control schemes, the proposed method does not require current PI controllers or a complex modulation stage. The discrete-time model of the inverter, filter, and load are used to predict the future behavior of the load currents for each of the 16 possible switching states. The control method chooses a switching state that minimizes the error between the output currents and their references. The performance of the proposed method has been investigated for perturbations in the filter and load parameters. The neutral-leg switching frequency reduction algorithm is also proposed to improve the efficiency of the converter. The feasibility of the proposed method is verified in this first part, through simulation results.

Model Predictive Current Control of Two-Level Four-Leg Inverters—Part I: Concept, Algorithm, and Simulation Analysis

In this paper a voltage modulation method based on a triangular carrier wave for the three-phase four-leg voltage source converter is described. The four-leg converter can produce three output voltages independently with one additional leg. The proposed modulation method for the four-leg converter can be implemented with a single carrier by a simple but useful "offset voltage" concept. The method is equivalent to the so called three-dimensional space vector PWM method, but its implementation is much easier. The maximum magnitude of the balanced three-phase voltage and the maximum magnitude of zero sequence voltage, which can be synthesized simultaneously, are derived. The feasibility of the proposed modulation technique is verified by computer simulation and experimental results. These results show that a proposed carrier-based pulsewidth modulation (PWM) technique can be easily implemented without conventional computational burden.

A carrier-based PWM method for three-phase four-leg voltage source converters

While the classical control techniques for three-phase two-level four-leg inverters are based on pulsewidth modulation or 3-D space vector modulation, this paper presents a simple digital current control strategy without the modulation stage. The proposed controller uses the discrete nature of the four-leg inverter and filter to generate the switching states. Using a predictive cost function, the optimal switching state to be applied in the next sampling interval is selected. The proposed controller offers excellent reference tracking with less current harmonic distortion for balanced and unbalanced loading conditions. The feasibility of the proposed strategy is verified by digital implementation on a dSPACE DS1104-based rapid prototype platform.

Digital Predictive Current Control of a Three-Phase Four-Leg Inverter

Four-leg inverters have been selected as one of the preferred power converter topologies for applications that require a precise control of neutral current, like active filters. The main advantage of this topology lies in an extended range for the zero sequence voltages and currents. However, the addition of a fourth leg extends the space vectors from two to three dimensions, making the selection of the modulation vectors more complex. Most of the algorithms that deal with this problem require an /spl alpha//spl beta//spl gamma/ transformation. This paper presents a new space vector modulation algorithm using abc coordinates (the phase voltages) avoiding the /spl alpha//spl beta//spl gamma/ transformation. Thanks to the use of abc coordinates, the algorithm is much simpler and more intuitive than in /spl alpha//spl beta//spl gamma/ representation, drastically reducing the complexity of modulation algorithm and the computational load associated to it.

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Three-dimensional space vector modulation in abc coordinates for four-leg voltage source converters

This paper describes the experimental implementation of the model predictive current control algorithm for a two-level four-leg inverter operating under balanced, unbalanced, and nonlinear loading conditions. The proposed scheme is designed to predict the future behavior of the load currents for each of the 16 possible switching states of the converter. The control method chooses a switching state that minimizes the error between the output currents and their references. The algorithm is embedded using a MATLAB/Simulink software environment, and experimental results based on the dSPACE DS1103 controller are provided. These results verify the advantages of the proposed control strategy in terms of robustness under filter and load parameter variations, average neutral-leg switching frequency reduction, reference tracking error, and percentage total harmonic distortion.

Model Predictive Current Control of Two-Level Four-Leg Inverters—Part II: Experimental Implementation and Validation

The paper presents a three-dimensional space-vector-modulation scheme for three-phase four-wire active power filters. The focus is on the implementation of a fixed frequency pulse width modulation (PWM) scheme with a minimum number of switch commutations per period and maximum DC bus voltage utilisation. For three-wire applications, space vector modulation is known to provide better utilisation of the DC voltage compared to a sinusoidal PWM approach. This concept is extended to four-wire applications by employing a four-leg active power filter. The largest symmetrical region in which the active filter's voltage space vector may reside is identified. Restricting the voltage space vector to this region avoids over-modulation and thereby prevents the production of low order harmonics. A digital controller is employed to provide deadbeat current control. The combination of the digital controller and the modulation scheme gives the four-leg active power filter the capability to independently track reference current waveforms in the three phases within one switching period. The four-leg active power filter may be used for harmonic compensation, reactive power compensation, load balancing, and neutral current compensation. Experimental results obtained from a 5 kVA laboratory active power filter validate the proposed modulation scheme as well as the control design.

D. Shen

Papers

Fixed-frequency space-vector-modulation control for three-phase four-leg active power filters