Research On Resonance Problems Of Grid-Connected Converter Under Weak Grid Condition

With the large-scale development of renewable energy power generation, distributed power generation system is usually adopted and connected to medium and low voltage distribution network, which appears some weak grid characteristics, such as high grid impedance, low short circuit capacity and rich grid background harmonic. Under this weak grid condition, harmonic resonance phenomenon may arise among the grid-connected converters and grid. In this dissertation, resonance related problems is investigated under such situation, including resonance mechanism, influence factors and counter-part measurements, with the main work listed as follow.(1) Characteristics of LCL filter and its damping method analyzingLCL filter is often adopted to suppress converter’s switching frequency harmonics, while the side-effect of resonance problem may deteriorate system performance. The up-to-date passive and active damping methods for LCL filters are investigated to deal with this three-order system and its inherent resonant frequency. The active damping method of single loop converter side current control is selected considering its simplicity and effectiveness.(2) Mechanism analyzing of grid impedance on system resonance and the effective suppression strategyBecause the original design of grid-connected inverter under ideal grid conditions may not meet the weak grid with high impedance application, grid-connected converter considering the effect of grid impedance is modeled, and system of multiple such converters operated in parallel is established. Resonance mechanism caused by grid impedance to the converter and system is analyzed using these models with frequency domain approaches. The research reveal that, higher grid impedance can increase the stability margin of high frequency harmonics, while amplify the low-order harmonic. In order to suppress the low frequency resonance caused by grid impedance, a virtual negative impedance control strategy is proposed to compensate the voltage drop caused by the grid impedance at point of common coupling (PCC).(3) Influence analyzing of background harmonic voltage and the suppression strategyLow frequency background harmonic voltage is trend to be created when multiple converters or power electronic devices connected to PCC, as they are operated asynchronously. This voltage will cause low frequency current oscillation in the grid and deteriorate power quality. Full grid voltage feed forward control is investigated, which is usually applied to eliminate the influence of background harmonic voltage, and this method turned out to be not effective due to grid impedance. So, a virtual impedance-based selective harmonic compensation (VI-SHC) strategy is proposed, which increases the harmonic impedance in low frequency area to improve the disturbance rejection ability to background harmonic voltage.The analyzing, method and control strategy present in this dissertation are validated by simulation and experimental results.