Four-leg Voltage Source Converter Research Articles

Dynamic voltage restorer based on a four-leg voltage source converter

Analysis and control of a dynamic voltage restorer are described. The voltage restorer is based on a four-leg voltage source converter. The three-phase input variables are resolved into the positive, negative and zero-sequence components using a weighted, recursive, least-squares estimator. Identical controllers have been used for each sequence component. A laboratory model of the restorer has been constructed and its performance has been tested by simulation and experiments.

Current Regulation in Four-Leg Voltage-Source Converters

Three-phase four-leg voltage-source converters by virtue of the fourth leg connected to the neutral points of the wye-connected loads effectively handle the zero-sequence currents and voltages occasioned by connected load impedance imbalances and/or nonlinearity of loads. This paper develops a generalized carrier-based pulsewidth-modulation scheme for this converter which with a natural reference-frame control scheme that directly controls the actual load currents ensures current regulation irrespectively of the nature of the load. The proposed modulation and control schemes are effectively validated by computer simulation and experimental results for the converter operation in the linear and overmodulation regions.

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

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.

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

Four-leg voltage-source converters can effectively provide the neutral connection in three-phase four-wire systems. They can be used in inverter, rectifier, and active filter applications to handle the neutral current caused by the unbalanced and/or nonlinear load or unbalanced source. In this paper, three-dimensional (3-D) space vector modulation (SVM) schemes are proposed for controlling the four-leg voltage-source converters. Important issues for 3-D SVM, such as definition of 3-D vectors, identification of adjacent switching vectors in the 3-D space, and switching vector sequencing schemes and comparisons are addressed. The proposed 3-D SVM is a superset of the traditional two-dimensional (2-D) SVM, and thus it inherits all the merits of the traditional 2-D SVM. A 100 kW 5 kHz four-leg inverter and a 20 kHz four-leg rectifier prototypes are built and controlled by the proposed 3-D SVM. Experimental results are presented to validate the effectiveness of the 3-D SVM.