Читать книгу Soft-Switching Technology for Three-phase Power Electronics Converters - Rui Li - Страница 21

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.