| Summary: | The series summation method is an important method in the calibration of the voltage coefficient (VC) of the standard voltage transformers (VTs), in which the key step is connecting a fullyinsulated VT and a semi-insulated VT in series, and comparing them with the third VT under test. Caused by the imperfect electrical shielding of the serially-wound VTs, additional errors are introduced in the calibration. By analyzing the cause of the additional errors in the serial connection of the VTs, two methods were designed in this paper to measure the additional errors. The proposed methods were harnessed to measure the additional errors for a serial connection of a 35 kV fully-insulated two-stage voltage transformer (TSVT) and a 35 kV semi-insulated TSVT. The results show that the two methods give a consistency of better than 1.0 × 10<sup>-6</sup> in ratio error and 2.0 μrad in phase displacement, respectively. The accurate measurement of the series additional errors and the correction of them significantly improve the accuracy of the VC measurement of a 110/√3 kV TSVT based on the series summation method. Furthermore, the VC measurement result is verified by the consistency of the series summation method with the series additional errors corrected and the high-voltage standard capacitor method.
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