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.
1.3.3 Distributed FACTS Devices
Distributed FACTS devices are used to improve reliability, power quality, and efficiency of the distributed systems. It includes static var compensators (STATCOMs), active power filter (APF), unified power flow controller (UPFC), unified power quality controller (UPQC), dynamic voltage restorer (DVR), etc.
A shunt APF is usually installed near the nonlinear load for the compensation of the harmonic current generated by the nonlinear load. The power converter can operate with higher switching frequency with soft‐switching technique. Higher switching frequency means the APF has better dynamics and precisely cancel harmonic of the nonlinear load. Figure 1.22 shows soft‐switching APF. The same topology can also be utilized in STATCOM.
DVR can stabilize voltage of the utility to loads. It also benefits from the application of the soft‐switching technique with regard to fast control response and reduced system size. Figure 1.23 shows a DVR using the soft‐switching technique.
The UPQC combines the functions of the shunt var compensation and series var compensation. It consists of two BTB connected converters shown in Figure 1.24. The one converter is connected in series with the grid while the other is connected in parallel with the grid. Each converter can generate reactive power at its own AC output terminal. By inserting the auxiliary circuit in the middle DC link, both converters can realize ZVS operation. The dynamics or power density of the UPQC are enhanced since the switching frequency is increased due to the soft‐switching.
1.3.4 Uninterruptible Power Supply
Uninterruptible power supply (UPS) is widely used in data centers and manufacturing processes to provide continuous and higher quality power. UPS is composed of the rectifier, inverter, and neutral line control half‐bridge. Totally there are seven switch legs. By installing an auxiliary resonant circuit in the DC side as shown in Figure 1.25, all switches in these seven switching legs can realize soft‐switching. Thus the power density of the UPS can be enhanced [19].
Figure 1.22 ZVS inverter for APF/STATCOM.
Figure 1.23 ZVS converter for DVR.
Figure 1.24 ZVS converter for UPQC.
Source: Based on Shi et al. [19].
Figure 1.25 UPS with the auxiliary resonance circuit for soft‐switching.
1.3.5 Motor Drives
High speed drives such
