Abstract
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.
Highlights
THE measurement of harmonics is one of the key tasks in the modern power systems, performed basically for Power Quality (PQ) assessment, verification of planning levels compliance, immunity and compatibility purposes and so on [1]
The subharmonic chosen for these additional analyses is the one that, according to the results shown in Section VI, most influenced the performance of the VTs, i.e., the subharmonic characterized by 0.1 % amplitude and 0.01 Hz frequency
When the fundamental tone is 80 % of the rated value, the subharmonic influence leads to a ξ equal to 5.48 %, whereas the same quantity reaches 12.66 % if the fundamental tone amplitude increases to 120 % of the rated one
Summary
THE measurement of harmonics is one of the key tasks in the modern power systems, performed basically for Power Quality (PQ) assessment, verification of planning levels compliance, immunity and compatibility purposes and so on [1]. Several experimental tests, varying the harmonic content of input waveforms and other operating conditions, like fundamental amplitude and burden, are performed They are executed with and without the subharmonic presence, whose frequency and amplitude are chosen in the ranges previously identified. In this way, some general considerations on the degradation of accuracy of harmonic measurement performed through an inductive VT, when subharmonics are present in the input waveform, are carried out.
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