Research On Control Strategies For Multiple Inverters Dominated Microgrid Under Nonlinear Loads And Complex Grid Operating Conditions

With the increasing of renewable energy resource such as wind turbines and solar power plant, distributed generation(DG) have been widely used. microgrid is one of the most promising development for the future DG system. As an interface between the DG systems and the main grid or the local loads, voltage source inverters(VSI) of the microgrid are the most common used topologies. By controlling the inverter, the microgrid system can be ensured in stable and reliable operation state. In the face of the AC microgrid system, the control strategies of the single three-phase inverter and microgrid systems which are operate in grid-connected and islanding modes are mainly investigated in this paper. This paper is organized as follows:(1) The control strategies and parameters design guidelines of three-phase gridconnected LCL-filtered inverter are investigated. The influence of control delays are analyzed by the Nyquist stability criterion, and an accurate synchronous reference frame equivalent proportional-integral(SRF-EPI) controller in αβ stationary frame using the parallel virtual resistance-based active damping(PVR-based AD) strategy for gridinterfaced DG systems to suppress the LCL resonance is proposed. Under the non-ideal grid conditions, the grid voltage feed-forward and multiple PR controllers are integrated in the current loop to mitigate the current distortion introduced by the grid background distortion. The parameters design based on steady-state error, magnitude and phase margin are presented to analyze the system robustness. The simulation and experimental results are provided to validate the feasibility of the proposed control methods.(2) The control strategies of three-phase standalone LCL-filtered DG system are invesgated. For inverter-based standalone DG system, a new multifunctional multi-loop control methodology utilizes the PVR-based AD method is proposed. Take the actual conditions into consideration, a detailed systematic parameters design procedure and the small-signal state-space model of the the islanded DG system using an extra feedback capacitor current shaping loop in the stationary reference frame are presented. Under unbalanced and nonlinear load conditions, the inner voltage and current controllers are based on an enhanced delay compensation(EDC) scheme based on two integrators of the discrete PR structure to achieve zero steady-state error and the multi-resonant harmonic compensator plus PR controller to prevent low-order load current harmonics to distort the output voltage, ensuring the stability margins with a higher accuracy. Simulation and experimental results are provided to validate the effectiveness of the proposed strategy.(3) The enhanced power sharing control strategies for islanded multiple inverters dominated microgrid are investigated. Under the mismatched feeder impedance and unbalanced and nonlinear load conditions, an enhanced hierarchical power sharing control scheme is proposed to address the load sharing problem in islanded microgrid. In the primary control loop, the improved droop control with an additional phase-shift loop is adopted to achieve power sharing requirement and an over-damped feature. The moving average filter(MAF)-based sequence decomposition method is proposed to extract the fundamental positive/negative sequences and harmonic components accurately. The accurate power calculation and the selective virtual impedance loops which contains virtual positive- and negative-sequence impedance loops at fundamental frequency, and virtual variable harmonic impedance loop at harmonic frequencies combined with the proposed unbalance and harmonics compensation to share the reactive power, unbalance and harmonic power accurately. The proposed active damping scheme is used to damp the resonance peak and improve inner-loop stability. In the secondary control, the centralized secondary control strategy is adopted to compensate the deviations of voltage amplitude and frequency. Moreover, a small-signal model for the primary and secondary controls with additional phase-shift loop is presented, which shows an over-damped feature from eigenvalue analysis of the state matrix. The feasibility of the proposed method is verified by simulation and experimental results.(4) The operation performance of microgrid in grid-connnected mode and the smooth transition from islanded to grid-connected mode of microgrid are investigated. Based on the two parallel-connected converters, the mathematic model of the microgrid based on the droop control in grid-connected mode is build and the occurrence mechanism of output harmonic current of the interconnected microgrid is analyzed. Then, an auxiliary control loop with the selective compensation of voltage harmonics is proposed. The traditional voltage synchronizing methods are discussed, and the voltage synchronizing control strategy based on improved phase angle adjustment is adopted to achieve smooth transition. Simulation and experimental results confirm the vadility of control methods.