Research On Parallel Operation Of Multi-Inverters In Microgrid And Composite Control

The microgrid is an important form of power generation of distributed generation. It puts dispersed and small distributed generators together to form a small network to supply for local load, or connects to the traditional power grid by PCC, and it can work in grid-connected mode or island mode. Distributed generators mostly adopt inverter interfaces, which increase the flexibility of microgrid control but the difficulty of maintaining system stability. Distributed generation is influenced by environment easily, so its power production and power quality can not be guaranteed. Then effective control technology is needed to ensure the requirements for power quality. At the same time, parallel operation of multiple DGs helps to expand power production, but it brings the change of frequency and voltage, and unreasonable power sharing, which is not good for stability. The research trends of microgrid control technology and control strategy of parallel operation of multiple DGs at home and abroad are summarized, and several common control methods are analyzed. In order to improve the problem of unreasonable power sharing among multiple DGs of parallel operation in microgrid, the improved control method is proposed. In addition, this paper elaborates not only dynamic operation simulation and characteristic analysis of microgrid under multi-source environment but also some special problems of microgrid power quality and its composite control.A common control strategy of microgrid is generally feed-forward decoupling constant power(PQ) control based on equilibrium assumption. Through the power analysis on the microgrid control strategy and analysis of grid-connected inverter output performance using switching function, it could come to the conclusion that the output of the inverter consists of third harmonic and negative sequence components based on the control strategy of equilibrium assumption under the condition of voltage unbalance on the grid. In order to improve the performance, a control loop of negative sequence voltage is used to superimpose on the original PQ control strategy, achieving the purpose of offsetting voltage negative sequence component in the grid and maintaining three-phase balance, also, the output of third harmonic could be suppressed. The simulation and experiment results indicate that the improved microgrid unbalanced PQ control strategy may improve the survival capacity of microgrid under asymmetrical grid voltage. This paper presents the principles of Droop control, constant voltage constant frequency control (V/f control) and constant power control (PQ control) of DG and their simulation analyses, the results show that droop control can realize autonomic regulation along the drooping curve, V/f control can guarantee the constant voltage and frequency, and PQ control can ensure constant power output. Sets up the experimental platform of micro source inverter and verifies the correctness of control strategy under the grid-connected mode, grid-disconnected mode, and conversion between control algorithm each other, cutting or increasing load in islanding mode. This paper designs load-storage hybrid system (LSHS), introduces the structure, control procedures and grid-connected control strategy of LSHS, analyzes the source of LSHS’s referenced power, its application in the emergency control is explored. Simulation results show that the participation of LSHS can stabilize the fluctuation of intermittent DGs, regulate system frequency and voltage, and balance the active power of system, thus, the flexibility of microgrid control and economical efficiency of microgrid construction are improved.Distribution mechanism of parallel power of multiple microgrid inverters and the variation rule of active power and reactive power circulation between these inverters are presented, specific analyses on the impact mechanism of equivalent line impedance to power distribution and limitations of traditional droop control are given in the paper. For this reason, a improved Q-En droop control method with En restoration mechanism whose control principle is analyzed is proposed to improve reactive power sharing, and it is applied to parallel operation system of multiple DGs. The different work cases are analyzed by simulation and experiment, the results show that the method can well improve the problem of unreasonable power sharing, the designed controller has strong robust performance.This paper presents the mathematical models of micro-sources such as wind generator, solar battery and micro gas turbine etc, sets up corresponding simulation models and simulates the dynamic characteristics, verifying the validity of the control method under different operation modes. A microgrid simulation model including four micro-sources is established, Its dynamic operation characteristics is analyzed under the various conditions such as grid-connected environment change, micro-sources loss, operation mode conversion and isolated load change. Simulation results prove that the appropriate adopt of control strategy can make the microgrid controllable and dynamic performance excellent, without obvious impact on grid in grid-connected mode, and microgrid can operate smoothly in island mode.A topology of multi-functional grid-connected inverter is proposed and applied to a microgrid. It can compensate harmonic, unbalance and reactive currents of microgrid at the same time. Its mathematical model is established, its controller for tracking the output current and PI control parameter are designed based on the weighted currents feedback approach. A simple but effective algorithm is given for calculating the reference currents, which includes two parts:output power tracking and power quality compensation. Finally, PSCAD/EMTDC is used to verify and confirm the proposed methods. The simulation results validate its good performance. It can not only enhance the power quality of microgrid, but also can regulate the power flow between microgrid and utility grid, which can aid the utility grid with necessary active or reactive power. In order to let microgrid can be more easily accepted by distribution network and undertake much responsibility, it is a new idea to make microgrid and the existing power quality equipments work in coordination. Reactive voltage collaborative control system consisting of Static Var Generator (SVG) and microgrid is proposed. It uses small-capacity SVG as continuous subsystem for rapid continuous reactive power regulation and microgrid as discrete subsystem to provide large-capacity graded reactive power. Their collaborative work can realize rapid adjustment of reactive power. Simulation shows that combined running system could effectively suppress voltage fluctuation. Power supply quality is further improved and original power devices capacity is decreased. At the same time, microgrid reactive power can be real time scheduled. Microgrid takes responsibility to regulate power quality, not only provide power supply. It could be more easily accepted by distribution network.