Control Of Three-phase Four-leg Converters For Microgrids Application

As an important orientation of renewable energy power generations, micorgrids become the attractive topic due to its enhanced flexible and reliable operation. Most of distributed generations are connected to microgrids through power electronic interfacing converters, which play an important role in microgrids. In three-phase four-wire microgrids, the converters should supply both single-phase and three-phase loads. In numerous four-wire converters, the three-phase four-leg topology, which provides a zero sequence current path, has the advantages such as higher dc link voltage utilization, lower dc link capacitance requirement and extra degree of freedom by an additional fourth leg. The four-leg converter is a beneficial application in four-wire islanded microgrids, especially under unbalanced load conditions. This dissertation focuses on the control methods of three-phase four-leg converters for microgrids application. The control methods are investigated for the purpose of voltage quality enhancement under unbalanced load conditions.Firstly, the control method of a four-wire microgrid formed by a single three-phase four-leg converter is investigated. The decoupled control scheme of a four-leg converter is presented based on mathematical model in synchronous rotating frame. The four-leg converter models in positive, negative and zero sequence are derived. In order to improve the power quality caused by unbalanced loads, an improved control strategy, which could be suitable for the positive, negative and zero sequence independently is proposed. Positive and negative sequence components are extracted by a reduced order generalized integrator based sequence detection method. And a proportional resonant regulator is adopted in zero axis controller to simplify the conventional control algorithm. Zero sequence output impedance with proposed control method is analyzed. The proposed method is verified by simulation and experiments.Secondly, the control method of a three-phase four-leg converter in stationary frame and relevant issues in implementation are investigated. A quasi-proportional resonant regulator based single loop control strategy is presented and the reference tracking error due to the finite gain at fundamental frequency of the quasi proportional-resonant regulator is discussed. On this basis, the effect of regulator and filter parameters on gain term selection are also analyzed. The reference voltage feedforward method is added to improve the system reference tracking accuracy and dynamic performance. The system stability is enhanced by adopting inductor current based active damping. Finally, the reasonability of parameters analysis and the proposed strategy are validated on a four-leg converter test rig. Furthermore, the options of control strategies for four-leg converters are discussed.Thirdly, the current sharing strategy of multiple turee-phase four-leg converters in islanded microgrids is investigated. The active power-frequency/reactive power-voltage amplitude based droop control is studied. The conventional droop control method has not ensured the unbalanced current sharing accuracy. The equivalent circuits of converters in parallel in positive, negative and zero sequence are derived. A negative and zero sequence virtual impedance method, which integrated with positive sequence power droop control is proposed. Then, the zero sequence circulating current in paralled four-leg converters with separate dc power supplies is also investigated. The effect of zero sequence current regulator and virtual impedance on zero sequence circulating current are analyzed. The effectiveness of proposed method is verified by simulation and experiments.Fourthly, the control methods for power quality improvement based on four-leg converters in microgrids are investigated. The output impedance characteristics of current control mode converters is analyzed and virtual admittance method is introduced. The current control mode converter is controlled as a virtual admittance to compensate voltage unbalance. The virtual admittance is tuned dynamically according to voltage unbalance factor and the method is verified by simulation. Then, the output impedance features between current control mode and voltage control mode converters with unbalanced voltage compensation are compared. The virtual impedance compensation concept is extended to voltage control mode converters. The four-leg converters with this functionality can achieve negative and zero sequence voltage unbalance compensation as well as positive sequence power sharing. Finally, the voltage unbalance compensation strategy of voltage control mode converters is validate by simulation and experiments.