Research On The Key Techniques Of Multi-inverter Control System In Microgrid

Microgrid is an emerging frontier technology based on distributed generation of new energy, and is also an effective way to improve energy supply efficiency of distributed power generation for supporting each other with the grid. Microgrid has two operation modes:grid-connected mode and off-grid (island) mode. In grid-connected mode, microgrid and the low and medium voltage distribution network operate grid-connected and support each other to realize bidirectional exchange of energy. In the situation of external power fault or planned island, microgrid can be converted to off-grid operation mode and supply energy for the critical loads in microgrid. Distributed generation microsources such as photovoltaic, wind, and fuel cells and energy storage devices mostly connect through the inverters to the microgrid at the public point of coupling, so the multi-inverters are universal in microgrid. Due to the difference among the output impedance and the coupling impedance of the inverters, non-linearity of loads, difference among the control methods, and diversification of the internal structure of microgid, there are still many technical problems in the aspects of grid-connected control of microgird inverters, operation control of parallel multi-inverters and power quality active control in microgrid, which are urgent to breakthrough.This dissertation is funded by the National Natural Science Foundation of China "research of multi-inverter parallel operation and power quality control methods in microgrid", and the National Basic Research Program of China (973program)"basic research related to energy supply system of distributed generation". It carries out these researches on the inverter grid-connected control technology in grid-connected mode of microgrid, circulating current analysis and its sharing control technoque of parallel inverters in island mode of microgrid, and power quality active control technoque of microgrid inverters. It has solved the key problems of grid-connected inverter control and multi-inverter parallel operation in microgrid, and provided theoretical basis and practical guidance for demonstration and promotion of microgrid. The main technical innovations are as follows:1. In photovoltaic grid-connected control system, the delay and the filtering inductance variation affect the system response speed, stability, and the current distortion. In this paper, a robust predictive deadbeat grid-connected control method based on power feed-forward is proposed. Power feed-forward control is introduced to speed up the system response. For the grid-connected current controller, a robust predictive deadbeat control method is proposed to enhance system robustness and to reduce the current distortion due to the control delay and inductance deviation. A delayed z-domain deadbeat control model was analyzed, and the delay and inductance effects on system stability were discussed. The design of the proposed robust predictive deadbeat control was given. In z-domain, the system transfer function was derived, and the system stability was analyzed, and the control parameters were selected. Simulation and experimental results verified the validity of the proposed control method.2. By improving the grid-connected control strategy of the inverter, microgrid and distributed generation (DG) have the ability of reactive power compensation and voltage and frequency regulation, which is an effective way to solve the power quality problems of microgrid and can further reduce the threshold of distributed generation. For the single-phase photovoltaic grid-connected generation system installed at the end of the grid, a grid-connected power control method with reactive power compensation was proposed in this paper. By this way, the system can quickly provide not only active power for the grid and the local loads but also the required reactive power for the loads. Based on the analysis of the system structure, the dc component of single-phase load reactive power can be detected by constructing two-phase orthogonal current and using the ip-iq algorithm based on the instantaneous reactive current, and the dc component of active current can be obtained by the PI outer voltage loop, thus the grid-connected reference current is derived. Combined with robust predictive deadbeat control, the system need not the additional power devices and can be installed at the end of the grid, which can realize dynamic reactive power compensation of the local loads and gird-connected power generation. As a result, the power quality is improved. To solve the delay of single-phase reactive current detection, and to improve further system response and current control accuracy, a grid-connected power control method with the function of reactive power compensation without delay is proposed. The method consists of reactive current detection, the voltage and current dual control loop, power feedforward, and grid voltage feedforword. The method of constructing two-phase quadrature current by doing the derivation over the load current is proposed to realize the detection of single-phase reactive current without delay. Compared with the traditional delay method, the big delay caused by asynchronism of sampled data in traditional single-phase reactive current detection and the lag in load current tracking is solved, which has improved the system stability. The quasi-PR control is adopted in the inner current loop to realize the zero steady error control of grid-connect current and reduce the influence on the current due to the grid frequency offset. Grid voltage feed-forward was introduced to reduce the current distortion due to the distortion and disturbance of the grid voltage. The design of the grid-connected power control system is given. The effects on the system performance were analyzed under the different control parameters of the quasi-PR controller, and the proper parameters were selected. Simulation and experimental results verified the validity of the proposed control methods.3. For parallel multi-inverters in island microgrid, the difference of equivalent output impedance and line impedance affects greatly on power sharing and circulating current restraining. From power transmission characteristics of parallel inverters, the influence of power sharing among resistive inverters was analyzed in this paper, where the conventional droop control was applied. Based on the analysis of circulating current characteristics of resistive inverters, a robust droop multiple loop control method was proposed, which included the outer power loop and the inner voltage and current loop. In the outer power loop, a robust droop controller is adopted to reduce the effects on accurate power sharing due to the impedance difference. Introducing virtual complex impedance including resistive component and inductive component, the equivalent output impedance of inverters is redesigned as pure resistance. Quasi proportional-resonant (QPR) control is applied to realize zero steady-state errors control of the output voltage with wide bandwidth for parallel inverters, which will reduce further the deviation of output voltage and restrain circulating current. The output voltage feedforward control and the proportional control of the capacitor current are adopted to improve the transient response and current disturbance. The effects on the equivalent output impedance in the different control mode and parameters were analyzed comparatively, and the proper parameters were selected. Simulation and experimental results show the correctness and validity of the proposed control method.4. For parallel multi-inverters in low voltage microgrid, load mutation will lead to voltage fluctuation, and providing rapid reactive power for inverters is a necessary way to maintain the system voltage stability. Thus, an inverter using resistive-capacitive output impedance (RC-inverter) was proposed in this paper. The equivalent output impedances of RC-inverter were designed as resistive-capacitance by introducing resistive-capacitive virtual complex impedance into the feedback of the output current. RC-inverters cannot only provide rapid reactive power for low-voltage microgrid to maintain the system voltage stability, but also restrain high frequency resonance between output impedance of inverters and the grid impedance. Based on the equivalent modeling of RC-inverter, a multi-loop power sharing control method for parallel inverters was presented, which mainly includes the outer power droop control, the virtual impedance, and the output voltage control. The effects of the virtual complex impedance on parallel circulating current were analyzed, and the appropriate control parameters were selected. Both the simulation and experimental results verify the effect of the control method.