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A Two-stage High Power Density Single-phase ac-dc Bi-directional PWM Converter for Renewable Energy System

Fig. 1. Two-stage bi-directional single-phase PWM converter.
It is well-known that the single-phase ac-dc conversion requires a bulky dc-link capacitor for filtering the ripple power from the grid. Also, in order to interface a dc distributed renewable energy system to a single-phase utility grid, bi-directional power flow control and dc-bus voltage regulation are of a major concern.

By using a H-bridge in series with a bi-directional SR dc-dc converter, a novel two-stage topology as grid-interfaced single-phase PWM converter is proposed as shown in Figure 1. There is a huge dc-link capacitor value reduction range for this topology. The dc-link capacitor can be chosen based on the input power, dc-link average votlage level, the max. and the min. voltage limit of the dc-link. A minimum dc-link capcitor value can then be found in Figure 3 to be only around 110 µF at 10 kW condition. A dc LC type filter is designed to interface to the dc renewable energy system rendering fast dyanmics bus voltage regualtion performance as well as a small voltage ripple. A bi-directional control system and design process is also presented for the proposed topology under dc renewable energy system application.

This converter can be easily integrated as a compact power module with the intrinsic active short-circuit protection, small converter volume, as well as the simplification of the system-level design. A 10 kW prototype, as shown in Fig. 3, are built for the verification purpose. In the end, 7.5 kW rectifier mode test is shown in Fig. 4. As seen, dc-link bears a 125 V voltage rippe, but output bus voltage only has 10 V ripples.

Fig. 2. Min. point of dc-link capacitor.
Fig. 3. 10 kW hardware prototype.
Fig. 4. 7.5 kW rectifier mode experimental results. 240 Vrms Grid voltage [200V/DIV] (green), AC current [50A/DIV] (blue), dc-link voltage [100V/DIV] (purple), output voltage [100V/DIV].
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