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1.3.2 Energy Storage Systems

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Energy storage systems have become a key enabling technology for a robust, high efficiency, and cost‐effective power grid. Grid level energy storage systems are used in frequency regulation, spinning reserve, peak load shaving, load leveling, and so on. Besides, energy storage systems are also introduced in distributed systems to stabilize the power output of renewable energy. The converter is the interface to connect the energy storage component with the grid. Energy storage systems require a bidirectional power flow control such as the battery energy storage system (BESS). The energy loss is also doubled during the whole energy utilization cycle by charging and discharging the energy storage component. Therefore the efficiency of the converter becomes more critical than that of the unidirectional converters. The soft‐switching technique has a potential in the energy storage applications.

The PCS for the BESS can be divided into single‐stage and double‐stage structures. For the single‐stage PCS, the battery voltage should not fluctuate widely during the discharging or charging process, which is typically related to the characteristics of the battery technology. The soft‐switching technique can be applied as PV inverters, as shown in Figure 1.18. To extend the system power capacity and improve fault tolerance, a PCS structure with paralleled three‐phase converters is adopted as shown in Figure 1.18. An auxiliary resonant circuit is installed in the common DC bus. Both converters can realize the ZVS operation with EA‐PWM.

The double‐stage PCS consists of a bidirectional DC/DC conversion stage and a DC/AC stage as shown in Figure 1.19. The DC/DC stage boosts the battery voltage to the suitable level so that the inverter stage can be directly interfaced to the grid. This type of PCS is suitable for maximum utilization of the battery stored energy, whose voltage has a wider variation during the entire SOC. With the ZVS auxiliary circuit, it can realize soft‐switching for both DC/DC stage and DC/AC stage.

The double‐stage interface has the advantage of a common DC bus line. Different energy storage units can be integrated into the system, which makes the system more expandable and fault tolerant. Figure 1.20 shows the diagram of the double‐stage interface with two boost converters interfacing to the fuel cell and super‐capacitor, respectively. All power devices can realize ZVS operation with only one auxiliary circuit in the middle DC link. Further extension to multiple energy sources system is shown in Figure 1.21 where various types of energy storage components are integrated by bidirectional DC/DC converter to the DC bus with only one auxiliary resonant circuit. All the converters connected to the DC bus can realize the soft‐switching operation with only one auxiliary resonant circuit.


Figure 1.18 Paralleled three‐phase ZVS inverter for BESS.


Figure 1.19 ZVS inverter with front boost stage for BESS.


Figure 1.20 ZVS inverter with paralleled DC/DC converters for BESS.


Figure 1.21 Multiple energy storage system with ZVS converters.

Soft-Switching Technology for Three-phase Power Electronics Converters

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