An improved two-stage current transformer

A novel design of magnetic cores and windings of a two-stage current transformer which considerably improves its linearity and accuracy for a given load is discussed. The equations which describe the operation of the transformer are given, and formulas for the calculation of the error are derived. A practical technique for unloading the second core and bringing its magnetic flux near zero is given. Experimental results show that with relatively small cores and a small amount of copper an accuracy better then 20 ppm and 20 μrd is achieved. The device is passive, with no electronically enhanced technique being implemented