AC/DC Current Transformer With Single Winding

Dc currents in power grids are created by induction from geomagnetic variations and newly also by transformerless power inverters installed in solar power stations. These currents may not only cause serious errors of current transformers (CTs) measuring the consumed energy but also cause overload of distribution transformers that may result in blackout. Installing dc current sensors into the grid would be very expensive. We have shown that the existing CTs in the grid can be used in the fluxgate mode to simultaneously measure dc current. Utilizing this mode, the CT ac error is inevitably increased. We analyze the sources of these errors and show techniques to reduce them. The experiments were made on the off-the-shelf 500 A/5 A CT. We show that the uncompensated 5 A dc current without compensation increases its ac amplitude error from 0.1% to 1%. We also found that the dominant additional error was caused by the increased burden. With the optimized design of the excitation circuit, it is possible to sense a dc current component with 10% accuracy and reduce the additional ac error from previously reported 1.5% to 0.1% when the dc current component is compensated. This technique can be widely used to improve the ac accuracy of the existing CTs.