| Power electronic converter is the key equipment to realize seamless connection between distributed power sources and loads in microgrids.The three-phase PWM converter has been widely used for its low harmonic distortion,small filter capacitor,and four quadrants operation.Three-phase PWM converter in microgrids may encounter some practical problems such as interaction with weak grids,unbalanced three-phase AC voltages,unbalanced loads and much redundancy converters.Therefore,the key of the microgrid operation lies in that how to achieve power conversion and power quality enhancement simultaneously by using the three-phase PWM converter.Based on the research on the interaction mechanism between the three-phase PWM converter and the weak grid,this paper studies the operating characteristics,optimal control strategies and multi-mode operation of the three phase PWM converter under unbalanced voltage and current conditions.The main researches and innovations are as follows:(1)The mechanism of voltage collapse in three-phase pulse width modulation(PWM)rectifiers is investigated.A state space mode with non-ideal grid and the PWM converter is established.An analytical stability boundary expression is derived by analyzing the equilibrium point of the averaging state space model,which can not only accurately locate the voltage collapse boundary in the circuit parameter domain,but also reveal the essential characteristic of the voltage collapse.Results are obtained and compared with those of the trial-error method,the Jacobian method and power balance method.Based on the analysis results,the key parameters on the voltage collapse are identified,and then the system parameters can be divided into two categories.One of them affects the critical point,and other affects only the instability process.Furthermore,an effective control strategy is proposed to prevent a vulnerable system from being driven into the instability region.(2)Aiming at the problem that the performance of the three-phase PWM converter is affected by the unbalanced voltages in microgrids.A high performance three-phase PWM converter with reduced DC-link capacitor under unbalanced AC voltage conditions is proposed in this paper.The unique feature of the converter lies in that sinusoidal symmetrical AC currents and ripple free DC voltage can be achieved simultaneously without using a bulky capacitor.The control scheme is implemented in α-β frame,so the complex rotate transformations are avoided,and it does not require the extraction of positive and negative sequence currents,which simplifies the control algorithm.A two-quadrant operation active power compensator together with its controller is developed to suppress the voltage ripple in the DC-link.Compared with the passive compensation method,the compensator is equipped with a smaller capacitor to reduce the system size,weight and cost.Moreover,the compensator could be integrated with the conventional three-phase three-leg PWM converter seamlessly.(3)Aiming at the unbalanced three-phase loads in the practical microgrid,a hybrid compensation method by using DSTATCOM and passive susceptance to compensate the unbalanced three-phase loads is proposed.An equivalent admittance model of the load in three-phase four wire microgrids is obtained.According to the admittance model,a passive optimization compensation scheme based on △-type and Y-type passive susceptance is proposed.The mathematical model of the three-phase four-leg DSTATCOM is established in the rotating coordination,and the power transmission characteristics of the four-leg DSTATCOM in compensating unbalanced loads are analyzed.On this basis,a control strategy of the four-leg DSTATCOM is proposed to compensate the unbalanced loads.Aiming at the shortcomings of passive compensation and active compensation methods,a hybrid unbalanced load optimization compensation strategy by combining the advantages of passive and active compensation methods is proposed in three-phase four-wire microgrids.The proposed scheme has the advantages of both passive and active compensation method to enhance the compensation capacity and extend the operating life of the power electronic device.Therefore,it is more suitable for the practical applications.(4)In order to improve the utilization of power electronic converter and improve the power quality of the hybrid AC/DC microgrid,a multi-mode operation control strategy for the bidirectional PWM converter is proposed in this paper.Based on requirement on the power quality of the hybrid microgrid and power conversion characteristics of the bidirectional PWM converter,the operation of the converter is divided into three main modes.The operation principle of the three modes has been analyzed,and the corresponding control strategy is designed.The characteristics of the converter for unbalanced voltages compensation and the negative sequence current suppression are analyzed in detail.The proposed control strategy can improve the power quality of the hybrid microgrid by using the additional power of the bidirectional converter so as to meet the requirements of the hybrid microgrid and realize the power quality customization.The proposed control strategy can also decrease the amount of the power electronic converter.The operation characteristics and control technologies of the three-phase PWM converter in microgrid are studied in this paper.A laboratory three-phase PWM converter prototype can be operated in multi-scenarios is designed.Some experiments have been done to verify the effectiveness of the analysis and the proposed strategies in this paper.The achievements of this paper can enhance the adaptability of the three-phase PWM converter to the practical microgrid,make the three-phase PWM converter more efficiency,more flexible,and provide some auxiliary service to the microgrid to ensure its optimized operation. |
