Multifunctional Wideband Digital Sampling System for the Calibration of Conventional Transformers and Electronic Transformers

The calibration system for conventional instrument transformers does not apply to the electronic transformers (ECT/EVT) that meet the requirements of the IEC61850-9-2 digital substation protocol. In this article, a calibration system for both electronic transformers (ETs) and conventional transformers based on sampling techniques using digital multimeters (DMMs) is reported. This calibration system uses conventional instrument transformers as references, noninductance shunts for current-voltage (I/V) conversion, and an inductive voltage divider (IVD) for secondary attenuation. Having received the pulse-per-second (PPS) signal from the merging unit (MU), a function generator (FG) burst pulses to trigger the DMMs for synchronous sampling. An improved quasi-synchronization algorithm is adopted as windowed discrete Fourier transform (DFT) for ratio and phase displacement calculation. This article analyses the uncertainty contributions and makes corrections to the calibration results. Uncertainty evaluation under power frequency proposed for the calibration of CT up to 10 kA is estimated to be within 10 ppm and 15 mu rad for ratio and phase displacement and 25 ppm and 30 mu rad for voltage transformers up to 500 kV root 3 with a coverage factor k =2.