| Conventional energy shotrage, environmental pollution and the increase ofpower load have brought development opportunities to new energy distributedgeneration. Microgird is a kind of tiny power system,which is consist of differentdistributed generations and energy storage devices, near to the customer, couldoperated in standalone and grid connected modes. The advantage of microgrid is that itcan overcome the bad influence of distributed power generation to the grid, reducetransmission loss and supply reliable power according to the needs of customer.Therefore, control of microgrid has become a research hot spot in recent years.There are many sorts of distributed generation sources, grasp their characteristicis the foundation of building microgird and selecting it’s control strategies. This paperdescribed the characteristics of photovoltaic power generation, wind power, fuel cellpower generation and microturbine power generation system, and studied the powerelectronic interfaced converters through which different distributed sources connect tothe microgird.In alternating current microgrid, Distirbuted Generation(DG) are mostlyconnected to microgrid through inverters, therefore, control of inverters have asignificant inlfuence on the performance of microgrid. This paper focuses on thecontrol strategies of three-phase inverter in grid-forming and grid-connected modes.Based on mathematical model of inverter in dq coordinate, pirnciples and controller’sdesign of PQ control, double—closed-loop voltage control and droop control wereintroduced, simulations were completed to prove that the parameters was appropirate.Control strategy of load sharing was analyzed when the inverters were paralleledbased on droop control. Active load sharing were accurate，and the parameter oftransmission line does not influent active load sharing. Reactive load sharing errorscaused by the voltage droop of line impedance in traditional droop control wereanalyzed and improved through modifying the slope of V-Q curves in consideration ofline voltage drop and the minimum limitation of load voltage.This paper alsointroduced the restoration algorithm of frequence and voltage of microgird. Aim at different cases，models were builded in Psim sotfware, and simulations werecompleted.There are a few kinds of non-dispatchable distributed sources in microgird. Thispaper, taking photovoltaic(PV) power generation for example, introduced the controlstrategies of non-dispatchable distributed generation system. First, characteristic ofphotovoltaic cells，maximum power point tracking (MPPT) control strategy designwere introduced. Based on that, the three-loop-control strategy used for single stagegrid-connected PV generation system was analyzed and simulated in Psim sotfware.Also, the control strategies of two-stages PV generation system was introduced. Forthe front stage converters, we use the MPPT control, and for the back stage inverter，we use the DC bus control strategies to make sure the power from the front stage wascompletely transferred to the inverter. Second, the equivalent circuit models of thelead-acid battery and methods of charging, parameters design of bidirectionalbuck/boost converter were introduced. Finally, PV-battery power dispatchablegeneration was studied. At the same time, the system energy management strategy indifferent working conditions and the control strategies of each converter module areanalyzed. DC bus voltage control strategy is used to balance the power of the systemwhen the battery has power handling capacity. And the charging method of MPPTwith current limiting is used when the battery is charged by photovoltaic cells. Whenthe light is too much and the battery is fully charged, the photovoltaic modulestabilizes the DC bus voltage to balance the power of the system, while the battery iscut out. Based on that, simulation was completed in Psim sotfware. |
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