Measuring Harmonics With Inductive Voltage Transformers in Presence of Subharmonics

Abstract

The measurement of harmonics is one of the key tasks in the 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 papers show that inductive VTs and CTs, due to the intrinsic iron core nonlinearity, can introduce errors up to some percent in harmonic measurements. The paper 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 performance of two different inductive MV VTs are tested in various operating conditions. Experimental results show that subharmonics can worse the harmonic measurements done through inductive VTs up to some tens of percent.

How Instrument Transformers Influence Power Quality Measurements: A Proposal of Accuracy Verification Tests

“…At present, there are few papers in the literature dealing with metrological performances of ITs used for PQ measurement. Some authors of this paper have already shown that the metrological performances of ITs in the presence of PQ phenomena can drastically change [ 30 , 32 , 33 , 34 , 35 , 36 ].…”

Impact of DC Transient Disturbances on Harmonic Performance of Voltage Transformers for AC Railway Applications

“…There is a normative gap regarding IT performance verification in both supply systems when ITs are used in PQ measurements. Information and data on this issue are partially provided from both the literature [ 14 , 15 , 16 , 17 , 18 , 19 ] and the technical report [ 20 ]. Moreover, in this respect, a European metrology research project, EMPIR 19NRM05 IT4PQ [ 21 ], has been funded.…”

“…DC voltage and more in general, low-frequency disturbances can badly influence the behavior of voltage transformers (VTs) [ 15 ]. When a transformer is supplied with a DC waveform, a DC magnetization bias of the magnetic circuit will result.…”

The Use of Voltage Transformers for the Measurement of Power System Subharmonics in Compliance With International Standards

“…Subharmonics are particular cases of interharmonics; in fact, they are defined as components with frequencies lower and, consequently, not integer multiples of the fundamental frequency at which the supply system is designed to operate. Subharmonics are injected into the power grid by distributed generation systems, like wind farms [15], [16], hydro-powers [17] or photovoltaic plants [18], and loads, like arc furnaces and cyclo-converters [19]- [21].…”

“…However, the resolution of 5 Hz might not be sufficient for proper detection of subharmonics, especially when subharmonics with frequency not integer multiple of 5 Hz are present, since they produce spectral leakage that can badly affect the results [24]. The authors in [21] have experimentally shown how subharmonics, having realistic amplitudes and frequencies, can impact on the performance of inductive Voltage Transformers (VTs) when they are used for harmonic measurements.…”