Research On Harmonic Current Elimination Method For Single-phase PWM Rectifiers

ABSTRACT:Single-phase PWM rectifier has been widely used in the fields of ac electric drives and renewable energy distribution generations with the advantages of high power factor, low current harmonics and bidirectional energy flowing. Although the grid current harmonics are much lower using PWM rectifier than that using diode rectifier, they can not be eliminated completely and are affected by many factors. More and more attentions have been paid on improving grid current quality with PWM rectifiers widely used.Mathematical model of single-phase PWM rectifier was established first. Impact on grid current made by dc-link voltage ripple, voltage harmonics, grid current predictive error and grid parameters was analyzed. The dc-link voltage ripple can be filtered by notch filter in the sample loop to eliminate its affect on grid current. PWM rectifier input impedance can be increased by adding repetitive or resonant controller into current-loop to reduce grid current harmonics caused by both grid voltage harmonics and PWM modulation.In the field of photovoltaic power generation, single-phase PWM rectifier is mainly used for low-power photovoltaic inverter, usually in5kW power level below. Its switching frequency is often10kHz-20kHz and the control cycle is short. Single-phase photovoltaic inverter current-loop has a wide-bandwidth and computation consumption of repetitive control is small. For these reasons, dead-beat control combined with repetitive control was proposed to eliminate grid current harmonic. Impact on current-loop robustness and dynamic performance by repetitive controller was analyzed. Design method of repetitive controller and current harmonic elimination performance was discussed. It is shown that smaller repetitive controller gain can improve current-loop robust and reduce grid current distortion when grid current amplitude reference steps. However, fundamental current tracking speed will slow down at the same time.In ac electric locomotive auxiliary power supply system, single-phase PWM rectifier's power is hundreds of kilowatts and its switching frequency is typically2kHz-5kHz. Low switching frequency will decrease current-loop dynamic performance. Because of the long control cycle, harmonic compensator based on resonant control is proposed to improve PWM rectifier input impedance at harmonic frequency. The design method setting different fundamental and harmonic resonant controller coefficients was proposed to obtain high static and dynamic current-loop performance. Paralleled harmonic compensator by adding multiple harmonic resonant controllers is often used and discussed in references. However, its design method is difficult to visually configure zeros and poles in z plane. Root locus design method of cascaded harmonic compensator in z domain was proposed with the flexibility to configure the distribution of current-loop poles and zeros. In this method, it is can be make sure that current-loop has ability to eliminate harmonic voltage in steady-state and rapid fundamental current tracking without current distortion in dynamic-state.In ac electric locomotive main traction system, single-phase PWM rectifier's power is megawatt and switching frequency is typically less than1kHz. The current-loop bandwidth is further reduced. Control delay will also result in deterioration of current-loop performance. Predictive current controller is often used to obtain high dynamic performance. Grid current predictive error using traditional open-loop current predictive method will distort grid current which can not be eliminated by adding internal model into current-loop. The current predictive algorithm based on repetitive control observer was proposed to improve grid current predictive precision. Design method of grid current observer was given and the observer stability and current-loop stability were both discussed.Traction network and locomotive grid-side PWM rectifier can be regarded as a cascaded system while traction network is specifically supplying for electric locomotive. PWM rectifier current-loop can be designed to guarantee stability. However, grids short-circuit impedance and distributed capacitance will affect the stability of PWM rectifier leading to grid current oscillation and a large amount of harmonics. Impedance ratio was used to analyze the influence on the stability of the current-loop by network parameters. Current-loop controller parameters design was analyzed respectively for single and multiple paralleled operations to avoid resonance between the grid and PWM rectifier. PWM rectifier input impedance was discussed when repetitive and resonant internal model was introduced. Finally, simulation and experimental results verified that theoretical analysis is correct.