A Control Method of a Small-Scale DC Power System Including Distributed Generators

This paper describes a control method of multiple distribution power generators connected to a dc micro-grid. This dc micro-grid system needs to construct a special dc transmission line. However, if its installation place is limited to a local area such as a building, a factory and a small city or town, this will not cause a big fault to the ac grid system to which the dc grid system is connected. The dc grid system takes the following advantages over the ac grid system: 1) Each power generator connected to the dc gird can be easily operated cooperatively because it controls only the dc-grid voltage. 2) When the ac grid system falls into abnormal or fault conditions, the dc-grid system is disconnected from the ac-grid system, and then it is switched to stand-alone operation in which the generated power is supplied to the loads connected to the dc grid. 3) The system cost and loss can be reduced because only a single ac-grid-connected inverter unit is needed. Fig. 1 shows a dc grid system considered in this paper. The system consists of the following five generation and control units; a solar-cell generation unit, a wind-turbine generation unit, a battery energy-storage control unit, a flywheel power-leveling unit, and an ac grid-connected inverter unit. In terms of system reliability, extendibility and maintainability, the followings are required for these power units connected to the dc grid: 1) These units can be connected to, or disconnected from, the active dc gird. 2) Other generation and control units with different power ratings can be easily connected to the dc gird in the near future. 3) Neither signal nor data communication is made among the existing units. To meet these demands, the proposed control method can be achieved by detecting only the dc-grid voltage as common information. The followings are taken into account; 1) the circulating current among the units connected to the dc gird, and 2) cooperative control between the ac-grid inverter unit and the battery unit. The circulating current may flow among the units when a voltage difference exists among them. To suppress the circulating current, the proposed method pays attention to the dc-side output characteristic of each unit. The solar-cell and wind-turbine units are controlled as current sources. The ac-grid inverter and battery units are controlled to give an equivalent impedance to the dc side. The relation between the output dc current and the dc-grid voltage in these units depends on the equivalent impedance. However, the ac-gird inverter unit and the battery unit can assign their output power. It is better to take as small power as possible from the ac-grid system. Then, the equivalent resistance characteristics in the ac-grid inverter unit and the battery unit are changeable according to the dc-gird voltage, as shown in Fig. 2. Experimental results from a 10-kW prototype system verify the validity and effectiveness of the proposed control method. Two cases are demonstrates as follows; Case 1: When the generation power from the solar generator unit increases gradually, the battery unit continues charging this power because the equivalent resistance of the storage unit is smaller than that of the ac grid inverter unit. After the battery unit reaches a fully-charged condition, the ac-gird inverter unit supplies the power generated by the solar cell unit to the ac gird. Case 2: When the ac grid falls in fault, the ac grid inverter detects the fault and is stopped for 15 sec. The surplus power remains in the dc gird. The solar-cell unit reduces the generated power. The battery unit supplies the power to the dc gird. At this time, the flywheel unit compensates for a sudden power change. As the power generated by the solar-cell unit increases, the power fed by the battery unit decreases.