Evaluation of Voltage Transformers’ Accuracy in Harmonic and Interharmonic Measurement

Abstract

The measurement of Power Quality (PQ), generally performed at Low Voltage (LV) level, is gaining more and more importance also at the Medium Voltage (MV) level, due to the increasing presence of switching power converters (both loads or generators) directly connected to MV grids. In this case, the use of Voltage Transformers (VTs) is unavoidable to scale voltage down to amplitudes compatible with the input ranges of PQ instruments. However, the current absence of an international standard dealing with VTs used for PQ measurements leaves the manufacturers and the users in a situation of complete uncertainty, since different products can have performance specifications tested and stated in completely different ways. This paper aims at defining an integrated approach for the evaluation of VTs accuracy used for PQ measurements, focusing on harmonics and interharmonics. The paper provides experimental results of the tests performed on a MV inductive VT according to the proposed procedure. As a result, it is demonstrated that VT accuracy in the measurement of a specific phenomenon should be evaluated with complex waveforms including the contemporary presence of different PQ phenomena.INDEX TERMS Instrument transformers, voltage transformers, harmonics, interharmonics, power system measurements, power quality, accuracy.

Adaptive Polynomial Harmonic Distortion Compensation in Current and Voltage Transformers Through Iteratively Updated QR Factorization

“…A straightforward idea to choose L(m) is progressively increasing its value staring from 0 (linear reconstruction), estimating the parameters of the corresponding compensation formula through (14) while computing the NRMSE from (15). The iterative process is stopped when:  NRMSE (L) (m) falls below a threshold NRMSEt(m).…”

“…The target of this stop condition is avoiding overfitting; when triggered, L(m) should be decreased by one. Considering the procedure summarized in Section III-C to perform the QR decomposition required by (14), it is possible to derive a computationally effective adaptive process for selecting the optimal value of L. Let us suppose that L ̂ is the number of terms currently used to consider HD, thus corresponding to the L ̂+1-th iteration step. From (10), (11), (12), Y m (L ̂) is obtained by adding a new column y m (L ̂) to Y m (L ̂1) , the regressor matrix at the previous iteration.…”

“…Conventional inductive ITs are often employed for harmonic measurements [12]- [14], albeit their metrological performances are not guaranteed. They are typically regarded as affected by severe bandwidth limitations [15], [16], but the iron core makes that VTs [17], [18] and CTs also suffer from a nonlinear inputoutput relationship.…”

The Accuracy of Evaluation of the Requirements of the Standards IEC 61000-3-2(12) with the Application of the Wideband Current Transducer

“…The assessment of current harmonics generated by equipment connected to the power system is of paramount importance in the field of electromagnetic compatibility (EMC). Efficient assessment serves as a preventive measure against the propagation of electromagnetic disturbances and safeguards the overall power quality [ 1 , 2 , 3 , 4 , 5 , 6 ]. The standards IEC 61000-3-2 and IEC 61000-3-12 define the limits for the harmonic current generated by the low voltage equipment connected to the power system [ 7 , 8 ].…”

Challenges of Accurate Measurement of Distorted Current and Voltage in the Power Grid by Conventional Instrument Transformers

“…Therefore, an increasing number of nonlinear loads connected by households and industrial customers cause current and voltage distortion in power networks. This creates the need for the transformation of nonsinusoidal voltage and current composed not only from the fundamental component but also containing higher harmonics, interharmonics, and subharmonics [1][2][3][4][5]. Many papers describe their behavior during the transformation of only sinusoidal current or voltage [6][7][8][9].…”