| The combination of the power grid and microgrid is considered to be an important form to save the investment, reduce energy consumption, improve power supply reliability and flexibility. Distributed generation in microgrid can be divided into synchronous DG and inverter interfaced DG, compared with the synchronous DG, inverter interfaced DG with all control power electronics is more flexible and has a tremendous performance advantages. For inverter interfaced microgrid, it can be comprehensive utilization of local resources and provide clean energy to the user, but many of the traditional large power grid power quality problems such as harmonic and voltage sag are still exist, the traditional management method is passively to install’ the APF, DVR and other power quality control device. As the inverter DG has the same structure of the main circuit with the traditional power quality control device, it can be actively used to control the power quality without affecting the power generation and make full use of the flexible control characteristics of inverter DG..In the inverter type microgrid, the control of micro sources is realized through the control of power electronic inverters. Whether the microgrid operates in grid-connect mode or island mode, the effective control of power electronic inverters in microgrid is need to keep the voltage and frequency within acceptable limits from a standard and consequently guarantee the power quality. In this paper, based on the mathematical model of two phase synchronous rotating reference frame for three-phase inverters, the PQ control and droop control are analyzed particularly and the corresponding controllers are also chosen. In addition, expression of equivalent output impedance of the inverter was deduced in view of the droop control voltage and current double closed loop control, and mapping the Byrd of the equivalent output impedance inverter. When the micro grid connected to low voltage distribution network, the traditional droop control with small equivalent output impedance is not applicable, by introducing virtual impedance; the inverter equivalent output impedance is large enough. The models of microgrid and inverter control system are established. The simulation results verified the effectiveness of PQ control model and drop control model.Secondly, the harmonic governance problem in the microgrid is studied. The structure, principle and current tracking control method of the traditional harmonic control device APF are analyzed. The basic principle of the second order generalized integrator (SOGI) is also analyzed. And the harmonic current detection method based on the second order generalized integrator is used in the control of the grid-connected inverter. The simulation model was built for the harmonic active control of the inverter interfaced microgrid. By simulation it can be seen that the active control can play a certain role in microgrid harmonic governance and the grid-connected inverter can provide all the required reactive power for the load. When the grid current still can’t meet the harmonic standard, the APF is installed for the passive control.Finally, the inverter interfaced microgrid voltage sag compensation is studied. The principle and control mode of the traditional voltage sag compensation device DVR are analyzed. The principle of multi-inverter interfaced microgrid voltage sag compensation and the reactive power distribution restrictions are also analyzed. The active control of the voltage sag compensation is realized through the improvement of Q/V control in the droop control. And at the same time, the active can realize the accuracy of reactive power distribution by capacity ratio without affecting by the line impedance. The simulation model of the active control for inverter interfaced microgrid voltage sag compensation is built. The simulation results show that under the active control, DG could compensate the PCC voltage sag at the same time supply power to the load. When the PCC voltage sag exceed a certain degree, the excessive increase of DG output reactive power is neither economic nor practical, the control of the output voltage of DG and adding DVR to load voltage sags for passive control are needed, simulation results show that combination of the active control and passive control can realize full compensation of load voltage sags. |
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