Soft-Switching Technology for Three-phase Power Electronics Converters. Rui Li

stage can realize soft‐switching operation. Due to the soft‐switching, the switching frequency is increased so that passives such as inductors and capacitors may be reduced. The PV inverter becomes more compact.

Figure 1.13 Active clamping converters: (a) circuit; (b) auxiliary switch state and DC bus voltage.

Three‐phase PV inverters are widely used in medium power or large power PV power generation systems. Six switch inverter circuits are commonly used due to their circuit simplicity. To reduce its switch loss, an auxiliary resonant circuit is introduced in the DC side of the converter as shown in Figure 1.15. The soft‐switching SiC MOSFET grid inverter achieves a high efficiency of 98.6% at 300 kHz switching frequency, which is about three times of the original hard‐switching counterpart with the same conversion efficiency [33]. Thus the inverter becomes more compact due to small passives. Besides, EMI noise caused by high dv/dt of SiC MOSFET is relieved due to the soft‐switching.

Another circuit used in distributed PV generation systems is the string inverter [17]. The string inverter is basically composed of two conversion stages: the DC‐DC and DC‐AC stages. The DC‐DC stage is adopted to extend the PV voltage operation region and harvest more solar energy as mentioned before. It usually has multiple DC‐DC boost converters, as shown in Figure 1.16, which are connected in parallel to increase the maximum power point tracking (MPPT) efficiency and power capacity as well. By introducing the soft‐switching technique, higher power conversion and higher power density can be obtained.

Figure 1.14 Single‐phase PV inverter for residential applications.

Figure 1.15 Three‐phase ZVS PV inverter.

Figure 1.16 Two‐stage three‐phase ZVS inverter for PV system.

Similar to PV inverters, the soft‐switching technique can also be applied to wind power systems. Two most popular wind power conversion systems (PCSs) are doubly fed induction generator (DFIG) and permanent magnet synchronous generator (PMSG). Both of them utilize a BTB converter to interface the grid side. The BTB converter for the PMSG system with the soft‐switching technique is shown in Figure 1.17. In a typical wind power system, the entire power converter is packed in a cabinet and placed in a nacelle with limited space. The soft‐switching BTB converter operates with higher switching frequency so that the volume and weight of the passive components can be significantly reduced. The reduced size and weight of the power converter can spare more room in the nacelle. Thus a step‐up transformer can be accommodated in the nacelle to reduce the cable cost and energy losses.

Figure 1.17 ZVS back‐to‐back converter for PMSG system.

      1.3.2 Energy Storage Systems

      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.