Development of Power Supply System with Distributed Generators using Parallel Processing Method

We investigated various ways of designing and operating distributed generators (DGs), and microgrids, which consist of several DGs in order to find more effective ways to utilize renewable energy. The level of electric power from renewable energy sources tend to fluctuate with meteorological changes. Also, the demand power of loads has the characteristic that volume tends to vary with how they are used. In the past, when there was an inbalance between supply and demand, the utility grid absorbed the difference. However, sharp variations in power flow in the utility grid will cause variations in both frequency and voltage. To make the necessary adjustments, and thus maintain a supply-demand balance, various equipment, e.g., a power system stabilizer that uses storage batteries, or capacitors, have already been developed as a possible solution. However, this equipment is costly and customers are reluctant to install it. In the meantime, customers sometimes request that their DGs are capable of the isolate-mode. When DGs with loads shift to the utility interconnect-mode from the isolate-mode, DGs are typically restarted after a short halt to synchronize exactly with frequency of the utility grid. We propose the use of the operating-change method that involves a high-speed switchgear and thus continues without interruption. However, this will be also expensive for customers. To solve these problems, we paid attention to the constitution of Parallel Processing UPS (P.P. UPS). We then developed a new power supply system that uses the following P.P. UPS components: an ACswitch (ACSW) with mechanical switch (MS), a PWM-converter (bi-directional converter), and valve-regulated lead-acid (VRLA) batteries. Our system can be used to not only maintain a balance of supply and demand, but to also continue to supply power during disturbing at the utility grid. Figure 1 shows a configuration of the power supply system for field test in campus. The bi-directional converter and the DGs are connected to the AC-bus. The VRLA batteries through the bidirectional converter are connected to the DC-bus. Figure 2 shows waveforms of PV output and Load power (Point D). Figure 3 shows waveforms of output from the bi-directional converter (Point B) the storage batteries (Point C). Figure 4 shows a waveform of the received electric power flow of the ulitiy grid (Point A). The power flows in the AC-bus of a building, i.e., those of the utility grid, the PV output, and load consumption are shown in Fig. 2. The utility grid power increased from 21:00 to 24:00, which shows the battery charging in grid-interconnected mode. Also, the power supplies to loads continued for a whole day, which shows that state migrations are complet. Fig. 1. Configuration of the power supply system for field test