Abstract
Energy conversion efficiency and output current harmonic distortion are two key performance measures of small-scale grid-connected distributed generation (DG) systems. As the interface between renewable energy generators and the utility grid, dc-ac converters (i.e. inverters) have been increasingly used in recent years. In this paper, a novel control algorithm termed “variable switching frequency control (VSFC)” is proposed to increase the weighted efficiency of the inverter for renewable energy generators through selection of optimal switching frequencies of pulse width modulation (PWM) in real time, based on estimation of output current harmonic distortion. Both simulation and experimental results indicate the proposed VSFC algorithm ensures that the inverter can operate with maximum efficiencies at different output levels while satisfying standard requirements for grid current harmonic distortion. Furthermore, implementation of this method to improve the inverter efficiency has no impact on inverter costs or control complexity.
Highlights
D UE to increasing energy demand and pressing environmental issues, renewable energy technologies have grown substantially in recent years
Through inverter loss analysis and use of current harmonic distortion modelling, the selection of switching frequencies for the gridconnected inverter is optimized in real time by the proposed "VSFC" to ensure that the inverter operates with maximum efficiencies at all output levels
It is observed that the experimental Total harmonic distortion (THD) measurements including the current harmonics up to 51st-order frequency are consistent with the estimated values which are obtained through deducting the high frequency harmonic component from current ripples
Summary
D UE to increasing energy demand and pressing environmental issues, renewable energy technologies have grown substantially in recent years. Some researches presented in recent literature have developed variable switching frequency PWM methods in converter systems to reduce the switching losses [27]–[29]. Most of them developed variable switching frequency based PWM strategies to optimize ripple current distribution of a fundamental cycle through building current harmonic prediction models These methods normally require a considerable amount of calculation efforts either on-line or off-line which could reduce the system flexibility and robustness, when the work condition varies frequently, i.e. distributed renewable generation systems. In order to overcome all the problems mentioned above, a novel control algorithm called "variable switching frequency control (VSFC)" is developed in this paper to increase the weighted efficiency of the inverter through selection of optimal switching frequencies of PWM in real time while meeting requirements of grid interconnection standards. Through inverter loss analysis and use of current harmonic distortion modelling, the selection of switching frequencies for the gridconnected inverter is optimized in real time by the proposed "VSFC" to ensure that the inverter operates with maximum efficiencies at all output levels
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