Novel Calibration systems for the dynamic and steady-state testing of digital instrument transformers

The measurement of harmonics is one of the key tasks in modern power systems, performed basically for power quality assessment, verification of planning levels compliance, immunity and compatibility purposes, and so on. Especially at medium voltage (MV) and high voltage (HV) levels, harmonics are always measured through voltage and current transformers (VTs and CTs), which very often are inductive. Recent articles show that inductive VTs and CTs, due to the intrinsic iron core nonlinearity, can introduce errors up to some percent in harmonic measurements. This article aims to investigate in-depth this phenomenon by analyzing how the accuracy of harmonic measurements performed through an inductive MV VT is affected by the presence subharmonics in its input voltage. Subharmonics are spectral components having frequencies lower than the power frequency, that is 50 Hz for European countries. Some theoretical considerations are carried out and the performances of two different inductive MV VTs are tested in various operating conditions. Experimental results show that subharmonics can worsen the harmonic measurements done through inductive VTs up to some tens of percent.