Flexible Micro Power Grid Interconnection Control Strategy Research

Microgrid is the main application form of the distributed generation, which is based on modern power converter technology, and taking variety decentralized electric sources into a same physical network, to achieve operational control and energy management of the local small microgrid. Microgrid can operate not only as a controllable generation/consumption unit with grid connected, but also as a autonomous and independent small regional power grid,thus has wide application prospect. Research on the control and co-operation technology between the micogrid and the power grid has important theoretical significance and application value.Firstly, the paper summary the technology of operation and control for microgrid and problem of grid-connecting for microgrid, and analyzed the working principle and control method of micro sources in microgrid. The paper also established the mathematical model, the inverter control system model and electromagnetic transient simulation model of micro sources, the micro sources includephotovoltaic power system, wind turbine power system and energy storage system. The Simulation analysis and verification were made by PSCAD, at last the dynamic operating characteristics of micro sources were obtained.This paper analyzed the structure composition and control method of the typical microgrid,and established the central control model of microgrid by using master slave control strategy.In view of grid-connecting for microgrid, the main factors affected grid-connecting were analyzed. The characteristics of grid-connecting for microgrid were researched under the condition of the existence of physical quantities such as voltage, phase and frequency. The simulation platform of microgrid grid-connecting characteristics were established by PSCAD.The digital simulations were performed to microgrid operating and control characteristics and microgrid grid –connecting characteristics. For the problem of emergency grid/off-grid or unplanned grid-connecting, the microgrid grid-connecting characteristics were researched,the influence of switching power on the grid-connectingwas obtained.This paper researched the flexible grid-connectingcontrol strategy of microgrid, andanalyzed the principle of synchronization control for microgrid grid-connecting.Based on detection method of difference between the voltage vectors, the control strategy of synchronization control for microgrid grid-connecting were given.For master-slave control mode of microgrid, the adjustment method of microgrid in synchronization control process was researched, an active power allocation strategy was also proposed.The frequency and phase in microgrid grid-connecting process were corrected by dynamic allocation and flexible control unbalanced power between multiple micro sources.This control method avoided the failure of the grid-connectingwhen microgrid single regulated power conditioning capacity was limited, and reduced the energy exchange between microgrid and bulk power system.It can also damped frequency/voltage oscillations, and achieved flexible control of microgrid grid/off grid in regular and emergency situations.The validity of the method was verified by the results of control strategysimulation.