Research On Power Coordinated Control Of Inverter In Microgrid Based On Feeder Power-flow Optimization

With the rapid development of the world economy, the contradiction between the growing demand for energy and the continuous decrease of energy reserves has been becoming more and more serious. Besides, environmental pollution problems caused by the traditional energy also need to be solved urgently. Consequently, the distributed generation technology based on renewable energy has been developed rapidly. The distributed generation has improved the flexibility and cleanliness of the grid, and ensured the security and reliability of power supply. However,distributed generations with various types and geographically dispersed location also have brought many new problems to the traditional grids. The concept of microgrid provides a good solution to the contradiction. The microgrid can operate not only in the grid-connected mode accepting the supervision and management from the TSO(Transmission System Operator) of the main grid, but also in island mode as an autonomous system. This thesis focuses on the droop control strategies for distributed generation, and studies power coordination control strategy of the microgrid on the basis of the feeder power-flow optimization. The main works of this theis are as follows.(1)The mathematical model of three-phase inverter is established and the outer voltage loop and inner current loop are designed. The power characteristics of inverter with different line impedances are analyzed. The virtual power method is used to realize active power and reactive power decoupling control for the coupling between active and reactive power in low voltage microgrid. The variable virtual impedance strategy is proposed to eliminate the influence of differences between line impedances on power distribution. Then the equations of state of the microgrid including multiple inverters are established with the small signal analysis method, and the influence of the control parameter on system stability is analyzed. The influences of different line impedances on inverter power distribution are simulated and the effectiveness of the scheme is verified on experimental platform.(2)Exploratory study for the controllable power in microgrid is carried out. The UPC(Unit Power Control) and FFC(Feeder Flow Control) are analyzed in the paper. Aiming at the microgrid with four different architecture, power and frequency characteristics are derived and compared in detail in grid-connected mode, transition mode and islanded mode. The microgrid with hybrid structure based on UPC and FFC is designed, and the operating states of the microgrid are simulated. The influences of different control modes on the microgrid operation characteristics are studied.(3)The power between the main grid and the microgrid has the characteristic of uncertainty because of the diversity of the types of distributed generations. The feeder power-flow features are analyzed and optimization scheme is given. The strategy changing the control mode is studied and its feasibility is verified by simulations. The control aim of this theis is to maintain feeder flow constant and minimum and the power coordinated control scheme considering feeder power-flow optimization is proposed and simulations are performed in different operating modes. The influences on feeder flow, frequency and output power of distributed generation are studied. The simulation results prove that power coordinated control method can maintain constant feeder flow under load variation conditions and the change of frequency in transition mode can’t exceed the limits. The experiments are accomplished to validate the effectiveness of the method in islanded microgrid. The microgrid under the power coordinated control can be seen as a variable load from the grid viewpoint, which is beneficial for the grid to carry out unified management and dispatch.(4)For the microgrid containing distributed generation,reactive circulating current is generated between the distributed generations with different line impedances and local loads. The reactive circulating current can increase line loss and damages the inverter. Besides, the reactive circulating current makes reactive power distribute unreasonably. The causes of reactive circulating current and the influence on microgrid are analyzed,and an adaptive translation suppression strategy for reactive circulating current is put forward. Simulations are carried out under different line impedances and local loads conditions, and the results show that the control strategy can restrain reactive circulating current effectively and has good stability and dynamic voltage performance. The effectiveness of the reactive circulating current suppression strategy is verified on the experimental platform.