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Low Frequency Common Mode Voltage Control for Systems Interconnected with Power Converters

Fig. 1. Structure of floating filter and CM voltage control loop
The use of a dc system in data centers and future homes is promising. DC systems can be interconnected with the ac system through power converters. One typical example is connecting a 380 V dc grid to the single-phase ac utility through a transformerless two-stage ac/dc converter, as illustrated in Fig. 1. In such a configuration, the ac and dc system common-mode (CM) currents are coupled through the common ground. While the high-frequency noise is filtered by passive components, the dc and low-frequency CM voltage also need to be controlled for bipolar dc systems.

In this paper, a CM duty cycle injection method is proposed to actively control the dc bus-to-ground voltage. As a result, the dc bus voltage is symmetric and the low-frequency voltage ripple is suppressed. The operation range of the proposed method is proven to be easy to satisfy. The impact of different voltage levels and asymmetric ac grounding is analyzed, and the complete CM circuit model was derived and its performance was verified by using hardware, based on which the closed-loop controller is designed. The experimental resultsverify the usefulness of the control method during steady state and transient operation. The control method was also generalized to a three-phase ac scenario.

To verify the proposed low-frequency CM voltage control method, a 10-kW bidirectional ac/dc converter is built. As illustrated in Fig. 2(a), after enabling the control loop, the dc bus voltage is adjusted to be symmetric to the ground. The low-frequency ripple is also suppressed. Fig. 2(b) and (c) show the steady state ac voltage, ac current, positive and negative dc bus voltage, with and without the CM voltage control.

Fig.2. (a) DC bus CM voltage control. Zoomed in for (b) Before CM control and (c) After CM control
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