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Low-Frequency Common-Mode Voltage Control for Systems Interconnected With Power Converters

Image of structure of floating filter and CM voltage control loop.
Fig. 1. Structure of floating filter and CM voltage control loop.
A dc system shows promise for use in data centers and future homes. It can be interconnected with an ac system through power converters. One typical case is connecting the 380 V dc grid to the single-phase ac utility through a transformerless two-stage ac-dc converter, as shown in Fig. 1. In such a configuration, the ac and dc system common mode (CM) quantities are coupled through the common ground. While the high-frequency noise is filtered by passives, 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 was proposed to actively control the dc bus to ground voltage. As a result, the dc bus voltage is symmetrical, and the low frequency voltage ripple is suppressed. The operation range of the proposed method was identified and proved to be easily met. The impact from the different voltage level and asymmetrical ac grounding was analyzed. The complete CM circuit model was derived and verified by hardware, based on the design of the closed loop controller. The experimental result verifies the control method in steady state and transient modes. The trade-off was clarified. 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 was built. As shown in Fig. 2 (a), after enabling the control loop, the dc bus voltage is adjusted to be symmetrical to the ground. The low-frequency ripple is also suppressed. Fig. 2 (b) and (c) show the steady state ac voltage, ac current, and positive and negative dc bus voltage, with and without the CM voltage control. ??
Image of dc bus CM voltage control (a). Zoom in for before (b) and after (c) CM control.
Fig. 2. Dc bus CM voltage control (a). Zoom in for before (b) and after (c) CM control.