Читать книгу Soft-Switching Technology for Three-phase Power Electronics Converters - Rui Li - Страница 23

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.