Instrument Current Transformers Research Articles

Restoration of the current transformer secondary current under core saturation conditions based on ANN

Relay protection and emergency automation is an integral part of electric power systems. The algorithms of the relay protection and emergency automation are based on the use of analysis of the parameters of the electrical mode - current and voltage - obtained from current and voltage instrument transformers. As a rule, these transformers operate on the basis of the laws of electromagnetism. Operational experience shows that the magnetic core of current transformers under fault conditions can be saturated. As a result, the shape of the measured current is distorted which can lead to false operation of the relay protection and emergency automation system. In this paper a comparative analysis of the existing methods is presented. The advantages and disadvantages of them are described. The problem of current transformer core saturation is reduced to a regression problem. To solve the problem an artificial neural networks-based method is proposed. Computational experiments for both proposed and existing methods are performed. The results of experiments show that the efficiency of the proposed method is 1.25 times higher relative to the most effective existing method. The method of re-scaling data at real-time work applied to the restoration of the distorted shape of current is proposed.

Разработка и исследование подхода к обработке сигналов цифровых измерительных трансформаторов тока и напряжения

During the development of secondary systems of stations and substations, including relay protection, automation, control, electricity metering, etc., qualitative changes have recently occurred due to the introduction of digital technologies. The introduction of digital technologies is supported by innovative development programs such as “Digital Transformation 2030”. The introduction of digital technologies is associated with the use of digital data transmission channels according to uniform standards. Projects of “digital substations” are being actively implemented with the transmission of digital information at all levels, including the primary equipment, the transmission of information both from current and voltage measuring transformers, and from switching devices. Currently, digital instrument transformers are actively being developed and implemented, in which information about measured currents and voltages is presented in digital code. New approaches to measure primary currents and voltages, and transmit information pose new challenges, and open prospects for improving signal processing methods used for relay protection purposes. Thus, the relevant aim of the study is the development and research of approaches to digital processing of signals of digital instrument current and voltage transformers. To solve the problems posed within the framework of this study, analytical and numerical solution methods have been used. Numerical methods are applied using software products such as MathCAD and the Python programming language to generate signals and display them on graphs. To substantiate the reliability, verification of the results obtained by different methods is carried out. The authors have formulated an approach to digital processing of signals obtained using current sensors of a digital instrument transformer. The authors have studied the approach to determine synchronized current vector values by processing information received from primary current converters. The characteristics of the algorithm for obtaining a vector have been analyzed and the dependences of errors on the influence of distorting factors have been plotted. The proposed approach is a development of the existing approach to obtain a vector by getting information from current sensors operating on different physical principles. The results show the advantages of the proposed method. The proposed digital signal processing algorithm is more technically expensive since it contains at least two metrologically calibrated measuring channels instead of one. However, all other things being equal, it allows to increase the accuracy and speed of the digital processing algorithms taken as a prototype. The results obtained can be used to formulate algorithms for measuring relay protection devices.

Design and Characterization of a Combined Low Power Voltage and Current Instrument Transformer for Low Voltage Applications

Design and Characterization of a Combined Low Power Voltage and Current Instrument Transformer for Low Voltage Applications

Uncertainty in Measurements of Power and Energy on Power Networks.

The problem on measurement of power and energy in single-phase and three-phase networks is always very important. The accuracy and precision of measurement is of equal importance for both manufacturers and consumers. The general problem is to determine the uncertainty of the energy produced in a specified power station and delivered to different consumers. Measuring instruments used for energy measurement are voltage and current instrument transformers and electricity meters. The approach proposed combines the information that can be derived from the nominal accuracy specifications of the instrument transformers with the information resulting from the calibration of the electricity meters. The combination allows achieving the evaluation of uncertainty in power and energy measurement. Both instrument transformers are subject to ratio error and phase error. In the paper an expression for maximum (positive and negative) errors as a function of the relative load current is derived. A method for calibration of electricity meters, measurement instruments used in the procedure, processing and presentation of the final results are presented as well. Having the information about the accuracy of both voltage and current instrument transformers and finding the uncertainty of electricity meter (according to the method outlined), it is possible to determine the total uncertainty of energy (power) measurement in conditions of load current variations and different modes of operation. The final accuracy of energy measurement may be presented by giving the specification limits in terms of measured value.

Modern Methods of Construction Problem Solving in Designing Various Types of Instrument Transformers

Current and voltage instrument transformers are essential elements of the power system. They work like transformers but do not transmit power. Their task is to accurately transform currents or voltages during system operation, which are then used for measurement and protection. These devices are very precise, and their accuracy is defined by the IEC/CEN standards. Instrument transformers are located at all transformer stations, specialized high-voltage and high-power laboratories, and testing and teaching laboratories. Therefore, the design requirements are very different depending on their type (current, voltage, or combined), function (measuring, protection), and operating voltage (high, medium, and low). The requirements concern not only the operation but also the strength of the insulation system and, sometimes, the mechanical strength.

Optimalno podešavanje rezolucije registra električne energije pametnog brojila

Technical losses in the transmission network are: losses of power, i.e. electrical energy, which are a consequence of the consumption of power, i.e. energy for heating the elements in the transmission network due to the existence of active resistance in these elements, losses due to hysteresis, losses due to eddy currents, losses due to drain currents in insulation, losses due to corona and dielectric losses. With the most frequent use of smart meters for indirect connection in operation in a plant set up to display electricity from the secondary side, the active electricity will be read on the meter screen with an additional error. Then the transformation ratios of voltage and current instrument transformers are set in the smart meter as 1:1. In order to obtain the actual value of energy, it is necessary to multiply the measured value of electricity from the meter by the transformation ratios of instrument transformers. Another way is to adjust the actual transformation ratios of voltage and current instrument transformers in the smart meter, so that the actual value of electricity in megawatts can be read on the meter screen. In this paper, the errors of electricity reading are considered, taking into account: transformation ratios of measuring transformers, resolutions and units of electricity, methods of setting smart meters, internal constants of meters, nominal secondary currents of electric meters and the prescribed maximum energy that is registered on the screen of electric meter. It investigates how much larger the errors of secondarily set electric meters for indirect connection with the usual resolution for displaying electricity in three decimal places and the unit of measurement in kilowatt hours compared to other possible settings for reading meters. Errors in the calculation of technical losses of electricity on high-voltage transmission lines and cables and power transformers are noticeable.

Novel approach for relays tuning using detailed mathematical model of electric power system

The correct operation of relay protection (RP) devices largely determines the security of electric power systems (EPS). The key point that in turn determines the behavior of protection in various emergency modes is a determination of their settings. The existing methodologies and tools often do not allow to provide RP settings adequate to the practical conditions of their functioning. This statement is confirmed by the statistics of faults in EPSs. The main reason for this issue is the inability to reliably reproduce transients in EPS via the software programs used in practice to calculate the RP settings. The Hybrid Real-Time Power System Simulator (HRTSim), developed by the authors, allows to reliably reproduce the entire range of normal and emergency modes and processes in EPS of any scale, topology and configuration by using detailed three-phase models of all power system components. Taking into account this possibility and features of the HRTSim, the task of detailed modeling of RP devices, including instrument current and voltage transformers, becomes relevant. The implemented special tools for RP modeling and the HRTSim in general allow to develop a new methodology for RP tuning. At the same time, the typical elements – relays – can be identified for each type of RP that determine their operation algorithm. This paper presents an analysis of the main relays of the most modern microprocessor-based RP devices and proposed principles of their tuning. The developed approach allows to adapt the RP settings to the practical conditions of their functioning in EPSs and to minimize the probability of RP maloperation.

Intelligent merging unit: DSE-based online instrumentation channel error correction

This paper proposes an online instrumentation channel error correction method based on weighted least square-dynamic state estimation (WLS-DSE), which can be integrated within Merging Unit (MU). It derives the mathematical dynamic device models of both voltage instrumentation channel and current instrumentation channel, which include physically-based high-fidelity dynamic device models of instrument transformers (potential transformer or current transformer), AC cable and burden resistor. To obtain the best estimate of the instrumentation channel's primary quantities, three major modules are designed in a standard object-oriented manner to create the measurement model and construct the dynamic state estimation algorithm. With the introduction of MU, each instrumentation channel of a MU can be integrated with the proposed error correction method, so that MU is able to directly report corrected primary values, which is applicable in real-time protective relays. The performance of the proposed error correction method is validated through extensive numerical experiments under different current transformer (CT) saturation levels and different instrument transformer ratios. The results demonstrate that the proposed method is capable to accurately reproduce the primary quantity on a sample-by sample-basis.

A New Industry-Oriented Technique for the Wideband Characterization of Voltage Transformers

The constant increase of switching power converters in distribution and transmission grids pushes the need for harmonic measurements. Recent literature has shown that inductive Voltage and Current Instrument Transformers (VTs and CTs) can be used for harmonic measurements, but their accurate frequency characterization involves the use of quite costly instrumentation. In this context, this paper proposes an industry oriented technique for frequency characterization of medium voltage inductive VTs, involving instrumentation that, typically, is present in accredited laboratories or at VT manufacturers premises. In fact, it requires the generation of only sine waves and the measurement of: i) ratio and phase errors in sinusoidal rated conditions, the same prescribed by the international standards, and ii) ratio and phase errors with sine waves at 7 V from power frequency up to the first resonance frequency. The proposed technique, named SINDICOMP-LV, has been applied to medium voltage VTs and validated by the INRIM reference system for frequency characterization of VTs. Accuracy improvement with respect to the use of a conventional low voltage frequency sweep is up to one order of magnitude for the ratio error and up to 5 mrad for the phase over the considered frequency ranges, depending on the frequency value

Problems of methodological support in the verification of instrument current transformers

The article deals with the technical and methodological problems that arise during the verification of transformers at the site of operation. Devices of this type are included in the systems of commercial metering of electricity consumption; therefore, periodic verification of their characteristics is necessary, however, in most cases, verification is necessary to perform at the installation site for a number of reasons that exclude the dismantling and transportation of transformers. The article provides an analysis of the reasons that limit the possibility of using on-site verification of transformers due to the principle of their operation and design features. The possibility of solving the identified problems as a result of the use of improved equipment and measurement rules is shown. A significant dependence of the influence of the load on the secondary winding of transformers to the error of measurement results has been established, and for voltage transformers this dependence is stronger than for current transformers. It is also proposed to provide for unique measuring instruments, such as transformers, methods for determining metrological characteristics available for their implementation at the site of operation, the use of which will lead to significant savings in verification costs.

Analysis of the State of High-Voltage Current Transformers Based on Gradient Boosting on Decision Trees

This paper addresses the problem of instrument current transformers technical state assessment based on machine learning methods. The introductory parts of the paper provide a detailed analysis of modern methods and approaches for technical state assessment of high-voltage power equipment of power plants and substations as well as a review of modern software tools and the latest trends in the given field of study. Justification of the relevance of the presented research aimed at instrument current transformers technical state assessment is provided along with the motivation for machine learning methods application for improvement of the accuracy and quality of high-voltage equipment state classification. Within the framework of the study, a comparative analysis of gradient boosting on decision trees and random forest algorithms was carried out for a given mathematical problem formulation. The main stages of processing the initial dataset are proposed as a step-by-step procedure, including feature extraction, feature transformation, feature interactions, etc. The outperforming efficiency of gradient boosting on decision trees algorithm was validated for real power equipment fleet. The resulting classification quality metrics of current transformers technical state assessment, Precision and Recall, are estimated to be 87.1% and 83.7%, correspondingly.

Supplementary comparison of instrument current transformers

This report describes a supplementary comparison on current ratio at power frequencies. The traveling standard was a high precision current transformer. Frequencies of 50 Hz and 60 Hz were used with a current range between 5 A and 1000 A. Participants belong to SIM, except the national metrology institute of Germany, PTB, that belongs to EURAMET. The pilot institute was UTE, from Uruguay. The measurement results and uncertainties from all national metrology institutes, that have declared CMCs, were in agreement with them. The schedule had long delays due to customs procedures, but had no influence on the results due to the very high stability of the traveling transformer.Main textTo reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Simulation of heat exchange between a digital instrument transformer and its environment for operation in emergency and unfavorable weather conditions

Reliable functioning of electric power grids to a large extent depends on the serviceability of each unit in the given power system, subject to the impact of various unfavorable factors, namely grid operation mode and weather conditions, such as elevated ambient temperatures, insolation and no-wind. These environmental parameters have a significant negative impact on the process of heat removal from electrical equipment, which may result in its overheating and breakdown. ISPEU staff have developed and put into operation innovative design digital current and voltage instrument transformers. The purpose of the present study is to examine the impact of insolation, ambient temperature and emergency operating modes on the thermal state of transformers. A mathematical model of heat exchange for a 6(10) kV transformer was developed and verified with the results of physical experiments conducted in an environmental chamber. Heat exchange between the transformer and its environment was simulated for a July day in Sochi in nominal and emergency operating modes, and also with various values of heat conductivity of electrical insulation sealant used. It was discovered that maximum influence on transformer thermal state is exerted by self-heating of instrument resistors at elevated voltage levels.

Development of a thermal self-diagnostics system for a digital current and voltage instrument transformer

Ivanovo State Power Engineering University (ISPEU) staff have developed and put into operation innovative designs of digital current and voltage instrument transformers (DCVT) based on resistor dividers performing technical and commercial electric power accounting, and also participating in work of relay protection and automation systems. Thermal self-diagnostics systems working in real time for instrument transformers do not exist in electric power engineering, while thermal examination is often conducted according to maintenance schedules, which negatively impacts the reliability of electrical equipment functioning. Development and building of new systems of self diagnostics for instrument transformer thermal state, revealing temperatures at several points, including values for the most heated up part, in real time, is therefore a vital task, especially for controlling operation of “smart” grids. Based on the results of physical and mathematical simulation of the process of heat exchange between a transformer and its environment at different values of ambient temperature, insolation and grid voltage, methods have been developed to determine the impact of the above factors on the thermal mode of DCVT operation. Algorithm has also been developed for thermal self-diagnostics and thermal protection against overheating of instrument transformers. System of thermal self diagnostics has been developed for 6-220 kV.

Proposal for extension of routine tests of the inductive current transformers to evaluation of transformation accuracy of higher harmonics

In the paper conditions and evaluation criteria for testing transformation accuracy of inductive current transformers for distorted current are proposed. Moreover, testing procedure and developed measuring system with its calibration method are described in detail. To ensure high accuracy of results the values of ratio and phase errors at harmonic are calculated from the harmonic components of the composite error measured by the differential circuit. Necessary equations are presented. Proposed solution enables possibility for evaluation of accuracy class extension for quality metering of active and passive instrument current transformers regardless of the types of the primary connection and the primary converter. Moreover, rated values of secondary currents of tested CT and reference transducer may be different. Presented implementation of developed methodology justifies the need for extension of routine tests as for multi-range reference inductive current transformers designed for transformation of sinusoidal current of frequency 50 Hz positive results for 0.1 and 0.2 accuracy classes are obtained. In the paper short description of other measuring systems and standard requirements for testing transformation accuracy of passive and active instrument transformers are presented. Furthermore, possible conditions are discussed that may cause decrease of transformation accuracy of the inductive current transformers for higher harmonics of distorted current in relation to sinusoidal current of their rated frequency.

Application of the inductive high current testing transformer for supplying of the measuring circuit with distorted current

The increasing number of non-linear loads and renewable energy sources causes a decrease in the power quality in the power networks. Therefore, tests of current transformers should be performed in similar conditions. This requires generation by a high current testing transformer of distorted currents that rms values are from a few hundred to several thousands of amperes with a given harmonics levels. However, its frequency band of operation is limited by inductances of its windings and connected load that result from the length and parameters of the used current track and connected device under test. To ensure the constant rms value of higher harmonic of distorted secondary current with the increase of its frequency proportional increase of the rms value of this harmonic in distorted primary voltage of the high current testing transformer is required. However, the maximum permissible value of primary voltage is limited by the dielectric strength of insulation of the primary winding. The purpose of presented studies is to determine the factors that condition the frequency band of operation of a high current testing transformer and limits the range of higher harmonics while testing of the transformation accuracy of instrument current transformers for distorted currents.

Compensation of Nonlinearity of Voltage and Current Instrument Transformers

This paper aims at characterizing and improving the metrological performances of current and voltage instrument transformers (CTs and VTs) in harmonic measurements in the power system. A theoretical analysis is carried out to demonstrate that, due to the iron core nonlinearity, CT and VT output signal is distorted even when the input signal is a pure sine wave. Starting from this analysis, a new method for CT and VT characterization and compensation is proposed. In a first step, they are characterized in sinusoidal conditions and the harmonic phasors of the distorted output are measured; in the second step, these phasors are used to compensate the harmonic phasors measured in normal operating conditions, which are typically distorted. The proposed characterization and compensation techniques are called SINusoidal characterization for DIstortion COMPensation (SINDICOMP). Several experimental tests, using high-accuracy calibration setups, have been performed to verify the proposed methods. The experimental results showed that the SINDICOMP technique assures a significant improvement of CT and VT metrological performances in harmonic measurements.

Machine Learning Approach Application for High-voltage Instrument Transformers Technical State Assessment

This paper describes the possibilities of machine learning application in the tasks of technical state assessment of high-voltage instrument transformers. An analytical review of modern systems for technical state assessment of high-voltage equipment is presented, their advantages and disadvantages are described. A mathematical model of an automated system for assessing the high-voltage instrument current and voltage transformers based on gradient boosting over decision trees has been developed. The efficiency of the developed solution is proved using the example of analysis of a real distribution zone, which allows identifying the state of instrument transformers with an accuracy of 84%.

Demagnetization of instrument transformers before calibration

Abstract This paper describes the influence of magnetization of an instrument current transformer (ICT) core on ICT errors, and presents a procedure for demagnetizing an ICT. The dependence of ICT errors on the magnetization of the ICT core for different magnetic materials is given in the paper, together with a detailed procedure for ICT demagnetization. The results of experiments are summarized, and conclusions are drawn on when ICT demagnetization is necessary, and on how to prevent the destruction of an ICT due to its winding being punctured.

Determination of instrument current transformer errors in frequency range up to 1 kHz

Two procedures for determining instrument current transformer (ICT) errors in the specified frequency range are described in the paper. The basis is use of accurate, frequency independent resistors, the ratio of which corresponds to the ratio of measured currents. These resistors are included in the primary and secondary circuit of the measured ICT. The difference in voltage drops across them corresponds to the ratio error and their phase shift to the phase displacement. The evaluation of the real and imaginary component of this difference is performed using a lock-in amplifier, phase-locked at the voltage corresponding to the primary current. In the case of primary currents of 100 A to 1 000 A, the use of resistors in the primary circuit is difficult with respect to their power loss. The procedure using a current transducer is also described.