A Hierarchical Control Scheme Of Dc Microgrid

Due to the pressures of environment pollution and energy shortage, distributed power generation has attracted a widely attention. However, as a large number of distributed generation resources are connected to grid directly, there is a significant impact on the operation of power grid. In order to solve above problems, micro-grid is brought into being. Micro-grid which is flexible and interactive can be generalized into three types:DC, AC and AC-DC hybrid micro-grid, depending on the characteristic of dc bus current. Compared with others, DC micro-grid obtains some advantages such as:less energy conversion links, higher system efficiency, lower line losses. Because of the weak capability of anti-disturbance and limited capacity of system, the dc bus voltage flicker and voltage collapse may happen when operating condition changes. A proper energy control strstegy is necessary to maintain DC bus voltage and power quality of DC micro-grid. In this paper, a control model of DC micro-grid is built and some issues on hierarchical control strategy are studied.Firstly, the operating principle of PV is introduced and model of PV is built. Based on disturbance-and-observe algorithm, PV source can output its maxmum power even insolation and temperature change. The change and discharge model of battery is built, which can operate in change and discharge mode depending on the SOC. The decouple model of bidirectional AC/DC convertor is built, which can operate in rectification and invertion modes to control bidirectional flow of energy.Secondly, control strategis for PV source, battery, AC/DC convertor and load is built based on DC bus voltage. A scheme for PV source is designed to control PV source which operates in MPPT mode and constant voltage mode; a droop strategy for battery is built to adjust charge power or discharge power based on bus voltage; bidirectional energy flow between public grid and micro-grid can be achieved by change operate modes of AC/DC convertor. A load cut-off scheme is designed to restore DC bus voltage in a normal range when a large of loads are cut in. Simulation results show that DC bus voltage can be maintain when DC micro-grid operate in grid-connected mode, however, droop control scheme can be maintain in normal range, voltage error still exists when system operates in insland mode.Then, DC bus voltage error caused by droop control is analyzed and a voltage restore scheme is designed, which controls battery power to match up with the error between PV output and loads. Simulation results show that with the voltage restore scheme, DC bus voltage can be maintained even insolation and loads change, however, the voltage restore scheme maybe invalid and voltage error still exist when the output power of battery is limited.Finally, based on droop control and voltage restore scheme, inter-connected mode of DC micro-grid is studied, which consists of hierarchical control strategy of DC micro-grid. Simulation results of four cases especially the case of limited battery power output show that the bidirectional flow between DC micro-grids can be achieved by change the current of the link line, however, the voltage error depending on current occurs again.