Three-phase PWM Voltage Source Inverter Research Articles

Digital control of a three-phase grid connected inverter

This paper presents the design and practical implementation of a digital current controller for a three-phase PWM voltage source inverter connected to the grid via an LCL filter to produce a high quality current injected into the grid. A two-feedback loops control system is proposed, with an outer grid current loop and an inner filter capacitor current loop. The controller also incorporates a grid voltage feedforward loop to compensate for the effect of the utility voltage disturbance. The paper also discusses the effect of the sampling and computational time delay both on system stability and current quality. This time delay is shown to reduce the stability of the inner loop. However, it is also shown that it is better, from the point of view of output current THD quality, to increase the inner loop time delay to a full one sampling period, so that the mains frequency envelope (nearly ripple free) current component is sampled at the peaks and troughs of the PWM, away from the switching instances of the transistors and associated switching noise. To ensure system stability, a capacitor current observer is proposed to compensate for the computational time delay. The design of the capacitor current observer and the controller are discussed. Simulation and practical results are presented to validate the design.

Processamento eletrônico da energia solar fotovoltaica em sistemas conectados à rede elétrica

Este artigo apresenta um sistema fotovoltaico conectado à rede elétrica comercial em configuração centralizada e construído com um inversor trifásico de dois estágios capaz de extrair a máxima potência do arranjo de painéis fotovoltaicos. O algoritmo P&O é adotado como técnica de MPPT. O isolamento é realizado por um transformador de alta frequência. O conversor que compõe o estágio CC-CC dispensa a estrutura de controle, pois funciona com razão cíclica e frequência constantes ao longo de toda a faixa de operação. Isto viabiliza o uso de conversores CC-CC ressonantes, cujo rendimento é elevado em altas frequências, favorecendo a compactação da estrutura de potência. Assim, o conversor trifásico série ressonante é escolhido para compor este estágio. O MPPT é transferido para o estágio CC-CA, que, invariavelmente, possui um controlador para a corrente injetada na rede elétrica. O inversor trifásico PWM alimentado em tensão, que compõe o estágio CC-CA, é controlado e modulado vetorialmente. O controle vetorial impõe a transformação de Park as correntes de linha, de onde resulta a corrente de eixo direto. O MPPT utiliza as mesmas variáveis do controlador de corrente e maximiza a corrente de eixo direto, a qual reflete a potência extraída do arranjo fotovoltaico. Assim, nenhuma medição específica para o MPPT é realizada, resultando em economia de sensores.

Comparisons between a hybrid shunt active filter and a pure shunt active filter

This paper deals with the hybrid shunt active filter for harmonic compensation of a three-phase diode rectifier in a 480-V adjustable-speed motor drive system. The hybrid filter is formed by a three-phase LC filter tuned around the seventh harmonic frequency and a small-rated three-phase voltage-source PWM inverter. The LC filter and PWM inverter are directly connected in series. As a result, the DC capacitor voltage of the PWM inverter in the hybrid filter is much lower than that of a conventional pure shunt active filter. This results in higher efficiency, less switching ripple, and less EMI emission. Computer simulation is carried out to compare the hybrid filter with the pure filter in terms of circuit configuration and filtering performance. Simulation results indicate that the hybrid filter is superior in effectiveness and viability to the pure filter. In addition, theoretical analysis based on vector loci confirms the validity of the simulation results. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(2): 61–70, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20145

Comparisons Between a Hybrid Shunt Active Filter and a Pure Shunt Active Filter

This paper deals with a hybrid shunt active filter for harmonic compensation of a three-phase diode rectifier in a 480-V adjustable-speed motor drive system. The hybrid filter is formed by a three-phase LC filter tuned around the 7-th harmonic frequency and a small-rated three-phase voltage-source PWM inverter. The LC filter and PWM inverter are directly connected in series. As a result, the dc capacitor voltage of the PWM inverter in the hybrid filter is much lower than that of a conventional pure shunt active filter. This results in higher efficiency, less switching ripple and less EMI emission. Computer simulation is carried out to compare the hybrid filter with the pure filter in terms of circuit configuration and filtering performance. Simulation results indicate that the hybrid filter is superior in effectiveness and viability to the pure filter. In addition, theoretical analysis based on vector loci confirms the validity of the simulation results.

A Three-phase Voltage-Source PWM Inverter System Characterized by Sinusoidal Output Voltage with Neither Common-mode Voltage nor Normal-mode Voltage

This paper deals with an inverter system integrating a small-rated passive EMI filter with a three-phase voltage-source PWM inverter. The purpose of the EMI filter is to eliminate both common-mode and normal-mode voltages from the output voltage of the inverter. The motivation of this research is based on the well-known fact that the higher the carrier or switching frequency, the smaller and the more effective the EMI filter. An experimental system consisting of a 5-kVA inverter, a 3.7-kW induction motor, and a specially-designed passive EMI filter was constructed to verify the viability and effectiveness of the EMI filter. As a result, it is shown experimentally that both three-phase line-to-line and line-to-neutral output voltages look purely sinusoidal as if the inverter system were an ideal variable-voltage, variable-frequency power supply when viewed from the motor terminals. This results in completely solving serious issues related to common-mode and normal-mode voltages produced by the inverter.

Series active power filter compensates current harmonics and voltage unbalance simultaneously

An active power filter connected in series to the power distribution system is presented and analysed. The series active power filter is implemented with a three-phase PWM voltage-source inverter and operates with a resonant LC passive filter connected in parallel to the power lines. The proposed scheme is able to simultaneously compensate current harmonics, the fundamental negative and zero sequence voltage components generated by the voltage source unbalance, and also eliminates the zero sequence current harmonic components that circulate through the neutral conductor. The active power filter is discussed in terms of the principles of operation under steady-state and transient conditions. The design and implementation of the power and control circuits are reported. Key predicted results are verified experimentally on a 5 kVA prototype model.

Analysis and design of a series voltage unbalance compensator based on a three-phase VSI operating with unbalanced switching functions

Voltage unbalance in AC supply systems is typically corrected by means of a shunt connected thyristor-controlled static VAr compensator. This approach has the disadvantage of slow response, harmonic injection into the AC system, and the requirement for large passive components. The proposed system consists of a three-phase PWM voltage source inverter, connected in series with the line through a three-phase transformer. The unbalance compensation is achieved by canceling the negative sequence component of the line-to-line voltages of the source. It is also shown that by having the inverter operate with unbalanced switching functions, it is possible to balance the load voltage and to control the amplitude of the positive sequence component in order to perform load voltage regulation. A complete mathematical description of the method is presented, demonstrating that the compensation can be achieved with low kVA inverters and low harmonic injection. Implementation procedures, design equations, and a design example are also included in order to illustrate the proposed method. Experimental results of a 1.5 kVA laboratory prototype system confirm the feasibility of the technique. >