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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 as pumps or compressors are applied in the industry. The inverter of high speed drives needs to operate at very high switching frequency up to tens of kHz. Since switching loss is proportional to switching frequency, switching loss of the power semiconductor in a high speed drive is so high that the power devices have to operate at its derating state for the safety. Thus the power rating of power device is unable to be fully utilized. An auxiliary resonance circuit is introduced to the DC side of the inverter as shown in Figure 1.26. The switch loss of the inverter is significantly reduced due to soft‐switching of the power devices in the inverter.

      1.3.6 Fast EV Chargers

To reduce charging time of EV, power rating of EV chargers is becoming large and large. The EV charger shown in Figure 1.27 is composed of two power conversion stages. To increase power density of the charger, soft‐switching technique is used. The first stage is three‐phase rectifier where an auxiliary resonant circuit is introduced to realize soft‐switching for the rectifier. For the second stage, double inductors and one capacitor (LLC) resonant DC/DC converter is used. It can realize soft‐switching for all switches in the DC/DC converter. All switches in EV charger can realize soft‐switching so that its power density is enhanced.

Figure 1.26 High speed drives with auxiliary resonance circuit.

Figure 1.27 Soft‐switching EV charger.

      1.3.7 Power Supply

With development of big data, cloud computing, etc., it is more stringent than ever to require the data centers to have higher computing speed and density with less energy consumption. Compared with super junction Metal‐oxide‐semiconductor field effect transistors (MOSFETS), Gallium nitride high Electron mobility Transistor (GaN HEMT) shows significant improvement on the switching performance, but its switching frequency is still limited when hard switching is used. A ZVS totem‐pole PFC circuit with fixed switching frequency is investigated for server power supply [34]. In Figure 1.28, the right leg with switch S_2H and S_2L operates at utility frequency and silicon MOSFET is used. The left leg with S_1H and S_1L operates at 500 kHz and GaN HEMT is chosen. With the auxiliary circuit, the GaN device can operate at ZVS condition. The turn‐on loss of the GaN is eliminated and its turn‐off loss is reduced.

1.4 The Topics of This Book

      This book will focus on the soft‐switching technology for three‐phase converters or inverters and their applications. Aiming to reduce or even eliminate the voltage and current overlapping during the switching transient process, soft‐switching techniques provide a solution

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