High-Precision Sensing of DC Bias in AC Grids

This paper proposes a novel method for detecting parasitic dc components in ac grids. The method uses a parallel-connected sensing reactor and a low-cost digital signal processor. From the reactor current, the algorithm derives a sum of even harmonics, considering at the same time their phase shift with respect to the fundamental. The method is insensitive to the measurement noise and offset, and it does not require additional passive filters. Precision better than 100 μV in 0.4-kV grids is maintained over a wide range of ac voltages. This paper details the detection method, provides the guidelines for the reactor design, and includes the key implementation details related to the signal acquisition-sampling and digital processing. High precision of the new sensor is thoroughly verified on an experimental setup connected to a 0.4-kV grid.