Inductive Voltage Transformers Research Articles

Measuring Harmonics With Inductive Voltage Transformers in Presence of Subharmonics

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.

3D Electromagnetic Field Analysis Applied to Evaluate the Accuracy of a Voltage Transformer under Distorted Voltage

Voltage transformers (VTs) are an important element of the measuring system that allows measuring the energy flow in medium and high voltage networks. Additional problems with the accuracy of the measurement introduced by the appearance of sources and nonlinear receivers cause deformation of the voltage shape in the energy system. Due to the high metrological requirements, the design of voltage transformers requires high accuracy (for class 0.2 ΔU ≤ 0.2, phase displacement ≤10 min), which is not possible with the use of analytical methods using approximate models. Therefore, only the application of numerical modeling by the finite element method, taking into account real three-dimensional phenomena, allows achieving high modeling accuracy. The article concerns the phenomenon of the influence of voltage higher harmonics of supply voltage on the accuracy (up to the 100th harmonic) of the measuring inductive voltage transformer (IVT). The applied modeling method takes into account the phenomena in the transformer core and the circuit equations resulting from the winding arrangement, which allows for the study of the deformation voltage transformation. Experimental tests on a real model to evaluate the method used were necessary. The article presents simulations for a model transformer, and results have been confirmed by experimental tests.

Harmonic Synchrophasors Measurement Algorithms With Embedded Compensation of Voltage Transformer Frequency Response

The widespread diffusion of nonlinear power electronics-based devices in distribution grids has raised the interest toward power quality monitoring. In this respect, the estimation of synchronized harmonic phasors would enable the employment of algorithms that allow identifying the sources of harmonic disturbances, which, until now, have only been tested through numerical simulations or implemented on small-scale. However, the impact of the uncertainty due to the employed voltage transformers (VTs) and to the other devices that compose the measurement chain has to be analyzed. In this framework, the present article proposes a class of harmonic synchrophasor estimation algorithms that allows compensating the frequency response function of the instrument transformer. The technique has been tested and validated under realistic conditions considering an inductive VT. Experimental results highlight its effectiveness, especially when taking into account its ease of implementation. The black-box approach allows abstracting from the operating principle of the VT.

A novel method for accurate ground parameter estimation in MV networks

This paper presents a novel method for ground parameter estimation that enables the accurate estimation of the total ground capacitance and conductance in MV networks. The novelty of the method is that the effect of the leakage impedance of the inductive voltage transformer can be significantly reduced compared to the prior-art method. The key idea of the novel method is based on dual inductive voltage transformers for resonant earthed neutral networks. Until now, it has been necessary to simplify the equivalent circuit model by neglecting the effect of the inductive voltage transformer leakage impedance to enable ground parameter estimation with the prior-art signal injection-based method. The experimental test results clearly demonstrate the significant effect of the inductive voltage transformer leakage impedance on ground parameter estimation with the prior-art method and show that the proposed method is effective at improving the accuracy of ground parameter estimation, which is beneficial for the security risk assessment of underground cables, the configuration and expansion capacity of the Petersen coil and the detection of high-impedance ground faults.

Frequency Response Test of MV Inductive Voltage Transformers for Power Quality Applications

Frequency Response Test of MV Inductive Voltage Transformers for Power Quality Applications

An Innovative Approach to Express Uncertainty Introduced by Voltage Transformers

The importance of monitoring voltage harmonics in distribution grids has raised because of the increasing diffusion of power electronics-based nonlinear devices. In this respect, the role of voltage transformers (VTs) is vital since they represent the input stage in power quality monitoring systems, and they are responsible for one of the major uncertainty contributions. Nevertheless, their metrological performances are still quantified in terms of accuracy class, namely, rating maximum ratio error and phase displacement. The aim of this article is moving from the concept of maximum error to that of uncertainty, while proposing a new approach to evaluate and express the accuracy of VTs employed for harmonic measurements. The method is applied to the calibration of a conventional inductive VT by means of a proper setup that allows injecting complex multitone waveforms. Experimental results clearly show the advantages of the proposed approach, which permits a better description of the metrological behavior with respect to the usual ratio error and phase displacement.

Impact of inductive current transformers on synchrophasor measurement in presence of modulations

Instrument Transformers (ITs) are essential devices for all the measurement tasks, f.i. metering, protection and diagnostics, in electricity grids, since they scale voltage and current down to levels compatible with measuring instruments. At the input of Power Quality (PQ) instruments and Phasor Measurement Units (PMUs), inductive voltage and current transformers (VTs and CTs) are still the most used ITs, even if, their performance in presence of disturbances is not fully assessed. This paper analyses the impact of the inductive CTs on the measurement of fundamental synchrophasors, by a PMU, in presence of a particular class of disturbances, the undesired modulations on current signals. Experimental tests, on two different CTs, show that there are critical situations, where, even compensating CT systematic ratio and phase errors, 1) CT exceeds its accuracy class limit and 2) considering the sole contribution of CT, PMU performance stays very close to the error limits of the standards.

Nonlinear Behavioral Modelling of Voltage Transformers in the Frequency Domain: Comparing Different Approaches

Instrument transformers suffer from weak nonlinearities, which may have a significant impact on harmonic measurements. Therefore, their detailed characterization should be based on a nonlinear model instead of a mere measurement of the frequency response function. Starting from this assumption, this article compares different models that can be employed to represent the behavior of voltage transformers (VTs): simplified Volterra models, frequency transfer matrixes (FTMs), and spectral linearization approximation; an inductive VT has been considered as a case study. The results highlight that the FTM approach is significantly affected by the range of variation of the fundamental component, while remarkable accuracy can be reached with the spectral linearization approximation at the expense of complexity. On the other hand, simplified Volterra models allow obtaining an effective tradeoff between the achieved accuracy and the number of coefficients.

Harmonic Distortion Compensation in Voltage Transformers for Improved Power Quality Measurements

Monitoring voltage harmonics represents one of the most important tasks in power quality assessment. In particular, the employed instrument transformer plays a key role in the achieved accuracy. Its harmonic measurement performance is typically evaluated by measuring its frequency response function. However, nonlinearities may have a nonnegligible impact on measurement uncertainty; for example, this occurs as far as the inductive voltage transformers are considered. This paper proposes a simple technique allowing the compensation of the most significant nonlinear effect that is the harmonic distortion produced by the large fundamental primary voltage. The method is first derived and introduced by means of numerical simulations and then implemented through a proper experimental setup. Results highlight remarkable accuracy improvements when realistic voltage waveforms are measured.

Detection of ferroresonance occurrence in inductive voltage transformers through vibration analysis

Steady state ferroresonances in isolated neutral electrical systems result in sustained abnormal system oscillations, which can damage inductive voltage transformers (IVTs) and other system elements. During this phenomenon, the IVT becomes saturated; consequently, the dimensions of the ferromagnetic material of the IVT change owing to magnetostriction. Because of this inherently nonlinear behaviour, the natural vibration modes of the IVT are excited. The aim of this study is to take advantage of these anomalous vibration modes to reveal the occurrence of ferroresonance in IVTs. The methodology for ferroresonance detection in IVTs through vibration analysis is described and demonstrated experimentally.

Characterization of Voltage Instrument Transformers Under Nonsinusoidal Conditions Based on the Best Linear Approximation

Voltage instrument transformers are usually tested at the rated frequency. In order to assess their performance in measuring harmonic components, typically, the frequency response function (FRF) is evaluated. Therefore, this conventional characterization does not consider nonlinear effects that may have a nonnegligible impact on the accuracy, especially when the transducer under test is represented by an inductive voltage transformer (VT). In this paper, a simple procedure for the characterization of voltage instrument transformers is presented. The method is based on the concept of best linear approximation of a nonlinear system. It requires applying a class of excitation signals that resembles the typical voltage waveforms found in power systems. Results consist of the FRF that permits the best linear compensation of the transducer response, and sample variances that allow quantifying the impact of noise and nonlinearities on the accuracy. The method is presented and explained by means of numerical simulations. After that, it has been applied to the characterization of a conventional inductive VT. Experimental results show how the accuracy of the transducer under test is heavily degraded by nonlinear phenomena when low-order voltage harmonics are considered.

Effect of voltage harmonics on pulse repetition rate of partial discharges

The objects of the research are partial discharge processes simulated under the influence of non-sinusoidal voltage. In the context of studying the effect of voltage harmonics on partial discharges, the total harmonic distortion can only be used for rough estimation. The reason for this is that the total harmonic distortion does not take into account the phase angles of the individual voltage harmonic components. Under non-sinusoidal voltage with the total harmonic distortion of 10 % and above, the intensity of the partial discharges in the insulation increases. However, at lower values of the total harmonic distortion, the harmonics can also affect the characteristics of the partial discharges. To solve this problem, it is necessary to carry out a study, increasing the number of voltage harmonics taken into account. To do this, it is possible to use the known three-capacitive equivalent circuit for a dielectric with gas cavity, making appropriate changes to the voltage source. The model was studied using voltage harmonic of the fundamental frequency together with voltage harmonics from the 2nd to the 30th order included. It was found that for the fixed amplitude the phase angle of the voltage harmonics has a decisive influence on the number of partial discharge pulses. In the presence of voltage harmonics, this number can be the same as under the influence of a pure sinusoidal voltage. It can also be 14.3 % less and 14.3 % or 28.6 % greater. In all cases, the value of total harmonic distortion remains the same. The possibility of using high-voltage reference inductive voltage transformers for voltage distortion measurements was studied experimentally. It was found that this is expedient for the region of the maximum value of the magnetic permeability of their magnetic cores, which corresponds to a range of 80–120 % of the transformer rated voltage. Mobile laboratories for checking high-voltage transformers on-site with the addition of appropriate equipment can simultaneously be used to measure a number of power quality indices.

Frequency response and harmonic distortion testing of inductive voltage transformer used for power quality measurements

International standards related to power quality measurements define methods and accuracies for the measuring instruments, but do not specify the accuracy of instrument transformers. Therefore, it is not possible to specify an overall accuracy for such measurements. Inductive voltage transformers (IVTs) which are used for power quality measurements should be tested to determine the ratio correction factors (RCFs) and phase angle errors at higher frequencies. In this paper, harmonic distortion and frequency response tests are performed to determine the level of harmonics generated by IVT and its ability to transform harmonics from high voltage (HV) to low voltage (LV) side. Complex voltages consisting of fundamental voltage and certain amount of superimposed harmonic are used to check the frequency response of IVT. RCFs and phase angle errors of the IVT are determined and can be applied to power quality monitors for compensation of errors that occur at high harmonic frequencies. Different test circuits are proposed for generation of HV consisting of fundamental voltage and harmonic voltages with amplitudes in range 5-15 % of the applied fundamental voltage. In order to improve the testing capabilities at higher voltage levels (for equipment with 123 kV≤Um≤420 kV), a compensation for both fundamental and harmonic voltages is proposed with a special connection through blocking and pass filters.

Ferroresonance conditions in wind parks

Wind parks are made up of a large number of saturable inductances (power transformers, inductive voltage transformers (IVTs)), as well as capacitors (cables, wind turbine harmonic filters, capacitor voltage transformers (CVTs), voltage grading capacitors in circuit-breakers). Therefore, they may present scenarios in which ferroresonance occurs. This paper presents the scenarios that can lead to ferroresonant circuits in doubly fed induction generator (DFIG) based wind parks. Ferroresonance conditions and their consequences are demonstrated by simulating the detailed model of the wind park in EMTP-RV. The wind park simulation model includes detailed models of wind turbines, medium voltage (MV) collector grid, high voltage/medium voltage (HV/MV) wind park substation, overvoltage, overcurrent and differential current protections, current transformers (CTs), IVTs and CVTs.

Mitigating Ferroresonance in Capacitive and Inductive Voltage Transformers with Ferroresonance Suppressing Methods

In order to restrain ferroresonance in coupling capacitor voltage transformers (CCVT), three kinds of ferroresonance suppression circuits (FSCs) have been presented. The resonant type, the fast-saturation reactor type and electronic type which are connected to secondary side of CCVTs, are proposed in this paper. Frequency domain analyses, for voltage ratio magnitude, are carried out for CCVTs to obtain preferable FSC. Furthermore, using surge suppressors such as metal oxide varistors (MOV) to improve the performance of the FSCs is investigated. Time domain simulations demonstrate that ferroresonance can be cleared in few cycles in presence of FSCs. In addition, to mitigate ferroresonanse in inductive transformers (VTs), we connect the damping resistor to the open-delta-arranged auxiliary winding.

Reference voltage divider designed to operate with oscilloscope to enable determination of ratio error and phase displacement frequency characteristics of MV voltage transformers

The goal of the paper is to demonstrate that simple and cost effective resistive voltage divider compensated with capacitance can provide high accuracy transformation of MV voltage in a wide frequencies range. Therefore, it may be applied in the measuring system with the oscilloscope as a reference voltage dividers to determine accuracy of transformation of distorted and sinusoidal voltages up to 20kHz by the 2kV/100V voltage transformers. This thesis is supported by determined in developed wattmeter-based measuring system constructed 2kV/100V voltage dividers and inductive voltage transformer ratio and phase errors frequency characteristics. Differential measurements of rms values and phases between output voltages of two produced voltage dividers with same voltage ratio were carried out in order to verify the accuracy of ratio and phase errors frequency characteristics determination.

Ferroresonance in subharmonic 3rd mode in an inductive voltage transformer, a real case analysis

This paper focuses on a ferroresonance incident in subharmonic 3rd mode captured in the electrical substation Rio Escondido (REC), in the northeast of Mexico. Bearing in mind that this kind of ferroresonance phenomenon is a sporadic event and difficult to record, the contribution of this paper consists of the analysis of this real-life ferroresonance phenomenon and to propose a practical solution that prevents these kinds of incidents. An analytical calculation to obtain the electrical parameters of a damping reactor used to damp out the ferroresonance phenomenon registered in the REC substation is reported.

Investigation the Performance of Electro-Optical AC Voltage LiNbO3 Sensor Based on Pockels Effect

In contrast to conventional voltage sensor technology (e.g., inductive voltagetransformers or capacitive voltage transformers), optical sensors have inherent andadvantageous features, such as wider bandwidth, larger dynamic range and lighter weight.The aim of this work is to implement an AC voltage sensor based on Pockels electro-opticeffect in LiNbO3 crystal. The research has been conducted in two ways .The first way is theinstallation tested uses external electrodes and a He-Ne laser as a light source, whereas thesecond way is by using the direct voltage application to the metalized sides of the LiNbO3crystal built into an optic cell. Results of both tests shows that by using the direct voltageapplication to the metalized sides of the LiNbO3 crystal built into an optic cell led to betterrepeatability of voltage measurements and corresponds to more realistic conditions.

Novel Optimization Algorithm to Demodulate a PZT-FBG Sensor in AC High Voltage Measurements

The application of optical fiber sensors in high-voltage environments has long been recognized as being useful, due to many properties of silica, such as insulation and electromagnetic induction immunity. Thus, optical voltage transducers offer many improvements on traditional inductive and capacitive voltage transformers, such as linear performance, wider dynamic range, lighter weight, smaller size, and improved safety. In this paper, we aim toward a power-line voltage transformer application by using a piezoelectric ceramic crystal stack together with a fiber Bragg grating (FBG) sensor to construct the core of a potential transformer for 13.8 kV class. For the AC demodulation process, we develop a mathematical model to identify the optimization parameters through a novel algorithm to improve sensitivity on FBG demodulation. This new scheme is simple, reduces the cost of the setup implementation and shows a solution to circumvent the temperature drift of the system.

The study of electronic voltage transducers operation in conditions of dips and interruptions of measured voltage

Abstract The aim of the paper is to present a response on dips and on interruptions of the measured voltage by means of electronic voltage transducers and inductive voltage transformers. Results of laboratory studies show high overvoltages generated in both a primary and secondary side of inductive VTs during the above mentioned conditions. Overvoltages in secondary circuits may damage measuring or protection apparatus connected to the secondary side of the VT. Using electronic voltage transducers enables users to avoid these problems.