The distinct advantage of the soft‐switching converter with TCM is that no auxiliary resonant circuit is needed. However, there are some drawbacks such as wider range of variable switching frequency, higher rms current, and turn‐off current. Recently, there are many studies about TCM control. For readers interested in this research can read the related references on IEEE Xplore.
As mentioned earlier, the DC‐side resonance converter only needs one auxiliary resonant circuit shared by three‐phase legs of the converter. It has issues such as variable switching frequency, higher voltage stress on devices, etc. To overcome these drawbacks, the ZVS‐SVM and EA‐PWM for three‐phase active clamping converters were proposed. The auxiliary power device only switches once in each switching cycle to realize ZVS for all the switches. It features fixed switching frequency and lower voltage stress of the power switch devices. The converter basically operates like the conventional PWM converter.
As an example of the soft‐switching technique, the concept of three‐phase active clamping converters with EA‐PWM is briefly explained. With EA‐PWM, the converter operates at the fixed switching frequency. An auxiliary circuit with auxiliary switch S7 is installed in the DC side of the converter in Figure 1.13a. Once a high loss switch commutations in the switch bridge happens, the auxiliary switch S7 will turn off, which starts a resonant process to create a slot with duration λ7TS on the DC bus voltage waveform vbus where λ7 is the turn‐off duty ratio of the auxiliary switch S7 and TS is the switching period in Figure 1.13b. During vbus = 0, the switches in the three‐phase switch bridge make the commutation under zero voltage on their terminals, which is known as zero voltage switching. Thus the switching loss is reduced. With EA‐PWM, all high loss commutations of the switch bridge in each switch cycle are synchronized. The auxiliary power device only switches once in a switching cycle to realize ZVS for all the switches. More detailed introduction will be given in Chapter 3.
1.3 Applications of Soft‐switching to Three‐phase Converters
1.3.1 Renewable Energy and Power Generation
The increasing applications of renewable energy and distributed power generation have become a new driving force for application of power electronics. Three‐phase converters are playing more and more important role in the grid. More efficient and more reliable power converters are required. Soft‐switching technique can be applied to converters for renewable energy integration, energy storage systems, and Flexible AC power transmission (FACTS) devices to increase power density and dynamics and reduce size of the equipment [32].
A single‐phase PV inverter is widely used for residential PV applications as shown in Figure 1.14. The front boost converter is used to extend solar energy harvesting duration each day so that the inverter can feed power to grid with a wider PV panel output voltage. By introducing an auxiliary resonant circuit and EA‐PWM control, both the boost stage and the H‐bridge inverter 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
