Two-level Three-phase Voltage Source Inverter Research Articles

An Improved Three-Phase Variable-Band Hysteresis Current Regulator

This paper presents an improved variable-band hysteresis current controller for a two-level three-phase voltage source inverter (VSI). The controller takes the average voltages of the phase-leg switched outputs as an approximation to the load back-EMF voltages, and uses these results to vary the hysteresis bands so as to maintain constant phase-leg switching frequencies. The switching frequency control process is then further refined by fine tuning the hysteresis band variations to synchronize the zero crossings of the phase-leg current errors with a fixed reference clock so as to achieve a nearest space vector switching sequence, which further ensures that the switched output spectrum has been optimized. Finally, a technique is proposed to replace the third phase-leg current regulator with a fixed-frequency open-loop pulse-width modulator, where its commanded reference is generated from the average switched output voltages of the other two phase legs. This avoids the hazard of the three independent hysteresis current regulators adversely interacting with each other in a conventional system, resulting from an overconstrained control problem with only two degrees of freedom. Additionally, this approach allows the linear modulation range to be increased by adding a common-mode third-harmonic component to the third phase-leg reference command signal.

A Simplified Control Scheme for Three-Phase Three-Level (NPC) Series Active Filter to Compensate All Voltage Perturbations

This paper presents a simplified control scheme for three-phase three-level series active power filter to compensate all load voltage perturbations such us harmonic unbalances, sags and swells. The standard configuration of three-phase series active power filter uses the two-level three-phase voltage source inverter with hysteresis controller. This configuration presents some drawbacks and is limited to low power systems. Today three-level (NPC) inverter is one of the most successfully employed topology in various industrial applications. In this paper a new scheme to increase the capability of series Active Power Filter (APF) voltage controller based on three-level inverter using simplified Pulse Width Modulation (PWM), is proposed. The control strategy adopted use instantaneous reactive power theory, easy to implement and gives good performance for all load voltage perturbations. The numerical simulation results carried with MATLAB-Simulink and SimPowerSystem Toolbox show the effectiveness of the proposed control scheme.

A New Multilevel Conversion Structure for Grid-Connected PV Applications

A novel scheme for three-phase grid-connected photovoltaic (PV) generation systems is presented in this paper. The scheme is based on two insulated strings of PV panels, each one feeding the dc bus of a standard two-level three-phase voltage-source inverter (VSI). The inverters are connected to the grid by a three-phase transformer having open-end windings on the inverter side. The resulting conversion structure performs as a multilevel power active filter (equivalent to a three-level inverter), doubling the power capability of a single VSI with given voltage and current ratings. The multilevel voltage waveforms are generated by an improved space-vector-modulation algorithm, suitable for the implementation in industrial digital signal processors. An original control method has been introduced to regulate the dc-link voltages of each VSI, according to the voltage reference given by a single maximum power point tracking controller. The proposed regulation system has been verified by numerical simulations and experimental tests with reference to different operating conditions.