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Control Loop Design of a Two-stage Bidirectional AC/DC Converter for Renewable Energy Systems

Fig. 1. Bidirectional control structure.
The full-bridge topology is widely used as a simple solution to transfer energy between the ac grid and the dc renewable energy system. However, it needs a large output capacitor and lacks the capability of short circuit protection. A two-stage topology consisting of an ac/dc stage and a dc/dc stage with a dc-link capacitor placed between them can reduce the dc-link capacitor dramatically and the dc/dc stage can limit the short circuit current if a proper topology is used.

In this paper, a unified control system is discussed for the two-stage converter to work in both rectifier and regenerative mode. Since the power stage consists of an ac/dc stage and a dc/dc stage with a dc-link capacitor placed between them, the corresponding controller design can be divided into two parts: the ac/dc stage controller and the dc/dc stage controller. The proposed bidirectional digital control system is shown in Fig. 1. Basically, the controller for the ac/dc stage is used to achieve a high power factor and loosely regulate the dc-link voltage at a high voltage (500 V). The controller for the dc/dc stage is used to regulate the dc output voltage with small ripple.

With the proposed controller, the converter can operate at these two modes and switch be-tween them seamlessly. A 10 kW prototype is used to verify the performance. The output voltage is regulated accurately at 380 V with only 2.5 V peak to peak voltage ripple.

Fig. 2. Mode transition test.
Vac (light blue) [250V/DIV], Iac (purple) [10A/DIV],
Vo (blue) [100V/DIV], Io (green) [2A/DIV].
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