Calibration Of Current Transformers Research Articles

Active on-site calibration technology of current transformer based on non-rated frequency current injection.

This paper proposes an active on-site calibration method through background current cancellation and non-rated current injection. It can measure the error of the current transformer in service from 1% to 120% rated current percentage without power supply interruption. In order to establish the error relationship between rated frequency and arbitrary frequency, a theoretical analysis of current transformer calibration at the arbitrary frequency has been developed by means of the equivalent circuit. It describes a method to determine the phase angle and ratio errors of the measuring transformers at arbitrary frequencies on the basis of the calibrated error values at rated frequency. To prove the theoretical analysis, an experimental validation was carried out. The experimental results demonstrate that this active onsite calibration is a valid tool for the evaluation of current transformer performances. The calibration results showed that, for both cases (non-rated frequency calibration and mixing frequency calibration), the difference between mean ratio error and rated frequency ratio error was lower than 0.01%, and the difference between mean phase error and rated frequency phase error was lower than 1', which meets the requirement of the 0.2 accuracy class calibration.

Optimization Design Study of a Full-Range Charged Calibration Device for a Low-Voltage Current Transformer

According to standards, the harmonic content should be less than 5% at the time of calibration, while the calibration index proposed in this paper stipulates that the primary current amplitude of 1%~120% of the full range of the accuracy level reaches 0.05 S level. In view of the above problems and indexes, this paper adopts the active regulation technology of primary current; firstly, the harmonics of primary current are extracted, and then the opposite harmonic currents are injected by the penetrating wire to offset the harmonics, and finally, charged calibration is carried out. The inductive current transformer charged calibration device was first designed, and the particle swarm algorithm was used to optimize the offset performance of the key link of the background current offset suppression. The transfer function of the offset loop was derived, and it was theoretically analyzed that it can meet the offset requirements up to the 200th harmonic. The results show that the optimized calibration method can realize the full-range calibration of low-voltage inductive current transformers under charged conditions by making a prototype of the device and constructing a test circuit for experiments, and measuring the data and analyzing them.

Three Phase Current Transformer Calibration Without External Reference IT Using Synchrophasors: An SVD Approach

Recently a pragmatic data centric method to calibrate substation capacitive voltage transformers (CVT) by exploiting local redundancy in substation CVT PMU data has been proposed. It does not require any external reference CVT. This paper extends the above philosophy to current transformer (CT) calibration problem. Once a line terminals CVTs are calibrated, then estimate of the three phase line currents can be deduced. Currents in a bus, for each of the three phases sum to zero. Hence, the corresponding measurement matrix will be rank deficient. Singular value decomposition's low rank approximation property is used to filter the noise and then a linear least squares problem is solved to calibrate remaining substation CTs. The method then can be intrinsically extended to calibrate other CTs of the network without using other series line impedances. Results are presented with both simulated and field PMU data of 400 kV lines.

Calibration of a Digital Current Transformer Measuring Bridge: Metrological Challenges and Uncertainty Contributions

In this paper, we consider the calibration of measuring bridges for non-conventional instrument transformers with digital output. In this context, the main challenge is represented by the necessity of synchronization between analog and digital outputs. To this end, we propose a measurement setup that allows for monitoring and quantifying the main quantities of interest. A possible laboratory implementation is presented and the main sources of uncertainty are discussed. From a metrological point of view, technical specifications and statistical analysis are employed to draw up a rigorous uncertainty budget of the calibration setup. An experimental validation is also provided through the thorough characterization of the measurement accuracy of a commercial device in use at METAS laboratories. The proposed analysis proves how the calibration of measuring bridges for non-conventional instrument transformers requires ad hoc measurement setups and identifies possible space for improvement, particularly in terms of outputs’ synchronization and flexibility of the generation process.

Calibration of a Digital Current Transformer Measuring Bridge: Metrological Challenges and Uncertainty Contributions

Calibration of a Digital Current Transformer Measuring Bridge: Metrological Challenges and Uncertainty Contributions

Metrological characterisation of current transformers calibration unit for accurate measurement

<p class="Abstract"><span lang="EN-US">The manuscript presents a method for the metrological characterisation of the commercial AC comparators used to calibrate current transformers. The theoretical basis for simulating the difference between two almost identical currents has been outlined, as well as the mathematical models for both a ratio error and a phase displacement has been derived. The measurement setup, consisting of conventional measuring instruments, has been described with a detailed presentation of its parameters. The sources of uncertainty have been distinguished and analysed with determining the current phase shift which led to a significant increase of relative measurement uncertainty. The simulation of measurement results was yielded in two ways: physically using a method presented and virtually using a Monte Carlo method. The second method confirmed that evaluating the measurement uncertainty through derived sensitivity coefficients is correct enough. The simulation results in the range from 1 to 1200 parts per million for both ratio error and phase displacement motivated the use of a comparator characterised through the proposed method for accurate measurement, especially for very low errors.</span></p>

Voltage Dependence of the Reference System in Medium- and High-Voltage Current Transformer Calibrations

A sampling current ratio measurement system has been developed to calibrate the voltage dependence of ratio error and phase displacement of current transformers (CTs) used for medium-voltage (MV) and high-voltage (HV) applications. The influence of voltage-induced leakage current on the apparent ratio error and phase displacement of the reference circuit was unambiguously demonstrated using a novel measurement method. Measurement results for voltages up to 24 kV show that the observed leakage current is purely capacitive and caused by the shielded cable capacitance of the HV cable used to feed the current. The maximum ratio error and phase displacement were found to be about 100 parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> and 100 μrad for the smallest current of 100 A used for these investigations. Since these values scale linearly with the applied voltage and inversely proportional with current, the assumption of negligible voltage dependence used in other reference systems presented in literature is questionable. Using the corrected reference circuit, a commercial class 0.2S MV CT with 2500-A nominal current was calibrated at voltage levels up to 24 kV. The voltage dependence of ratio error and phase displacement for primary currents up to 500 A was demonstrated to be capacitive as well, with a maximum impact on the current ratio of almost 0.9% for a primary current of 100 A (corresponding to 3.6% at 1% of nominal current). This value is beyond the limits of in-force standards, which emphasizes the necessity of implementing voltage dependence testing in those standards.

Comparator effect on equivalence of results of calibrating current transformers

Many different high-precision systems for determining the ratio error and phase displacement of current transformers have been developed by leading specialists in the world. The latest research solutions with the use of the latest measurement tools, instantaneous sampling techniques, and analysis of sources of uncertainty have been applied in these developments. The objective difficulty is that only a limited number of specialized institutes implement such projects with the involvement of leading experts in the field of measurement and significant funds. First of all, these are the national metrological institutes of countries with high economic opportunities. At the level of conventional calibration laboratories equipped with modern facilities and highly qualified personnel, the uncertainty of measurements increases 10 times or more when calibrating precision instrument transformers. Thus, it has not been investigated to what extent the readouts of commercial comparators of different manufacturers in the calibration of instrument transformers of class 0.2S and more precise are equivalent. Determination of the equivalence level of AC comparators of different types in the day-to-day calibration of current transformers is the main objective of this research. More than 50 comparators of different types (with inductive or resistive current transducers) were investigated relative to 2 well-characterized reference current transformers. Comparison of the results obtained by two instruments with radically different principles of measurement gave a difference of 23 μA/A in ratio error and 52 μrad in phase displacement. The results of estimating the readout stability of modern comparators of serial production are also highlighted. The results of the analysis of the data obtained allow us to assume that measurement results of ratio error of about 50 μA/A have equivalence level within ±20 μA/A. The measurement results of phase displacement of about 50 μrad have equivalence level within ±15 μrad. Concerning the determination of metrological characteristics of current transformers with accuracy class 0.2S, their equivalence must be considered taking into account all exploited comparators. The results cause the question about the adequacy of the accuracy margin of the current transformer in the production to overlap the difference in the readouts of 260 μA/A and 500 μrad

The Research on the Calibration of Direct-Current Current Transformers

The Research on the Calibration of Direct-Current Current Transformers

Intelligent Calibration and Report Generation of Optical Current Transformers

With the development and advancement in power systems and smart grids our metering should be advanced and accurate. For the better protection of power systems the conventional current transformers have been replaced by OCTs and FOCT (optical and fiber optical current transformer) as comparatively they give precise and error free measurements. For analyzing errors and the accuracy of the measuring systems we need an efficient and digital calibration systems through which we can do error analysis of transformers and can check their accuracy. In this paper we have designed a LabVIEW based intelligent calibration systems for the error analysis of transformers. A simulation based which is not effected by environmental interfering, calibration system is proposed in which the accuracy of OCT is analyzed by comparing it with the standard referenced transformer.

Calibration of Current Transformers in distorted conditions

In the context of modern power systems, where there are lots of non-linear loads and generators based on switching power electronics, the accurate measurement of voltage and current harmonics is a key task for the knowledge of the actual state of the network. Voltage and current transducers play a crucial role since they are always the first part of the measurement chain. Currently, classical voltage and current instrument transformers are the most installed transducers, but their performance not always is fully characterized in the presence of distorted waveforms. Therefore, in this paper a calibration setup for the accurate characterization of current transformers with distorted waveforms is presented. System implementation and characterization is presented; then it is employed for the evaluation of the performance of a commercial current transformer in distorted conditions.

Error reduction of phasor measurement unit data considering practical constraints

Wide area measurement system relies on phasor measurement unit (PMU) data to monitor, protect, and control high-voltage transmission networks. However, errors in instrument transformers (ITs) located at the inputs of a PMU can significantly degrade its output quality. This study proposes two methodologies for voltage and current transformers calibration using PMU data. The first method calibrates ITs using one good quality voltage measurement located at a tie-line. This method tolerates errors in both the ITs (which are to be estimated) as well as the PMUs. The second method attains the same objective as the first one, with the additional constraint that some portion of the data is unusable. Thus, the second method can be used even when the incoming data is intermittent.

An AC Current Transformer Standard Measuring System for Power Frequencies

This paper describes the design and setup of the new ac current transformer (CT) calibration system at Physikalisch-Technische Bundesanstalt, which uses enhanced, actively compensated current comparators (CCs) for rated primary currents from 5 A up to 60 kA, and a new CT bridge based on the differential method. The bridge makes use of modern, high-resolution analog-to-digital converters, and makes it possible to connect traditional CTs as well as two-stage CTs with rated secondary currents from 100 mA to 5 A. The lower current ranges are integrated to enable a calibration of CTs of the highest available precision for national power standards. The current transducers and the two-channel sampling system of the bridge allow a resolution of approximately $10^{\mathrm {-9}}$ , while the estimated Type B uncertainty of the bridge is around $10^{\mathrm {-8}}$ .

A Comparison of Two Current Transformer Calibration Systems at NRC Canada

A comparison of two systems based on different methods for calibration of current transformers at several different transformation ratios, burdens, and power frequencies of 60 Hz and 50 Hz is performed at the National Research Council of Canada. The obtained discrepancies in ratio and phase errors are within $5~\mu \text{A}$ /A and $\mu $ rad, respectively, for small test currents and less for larger test currents.

Error and Uncertainty Estimations for A Passive-CC-Based AC Current Ratio Standard at High Audio Frequencies

This paper describes a development framework for alternating current ratio standard working at high audio frequencies. To realize this, first, a calibration method for frequency-dependent balancing current generators (BCGs) producing error currents to balance current comparators (CCs) is proposed. Second, the BCG error correction is made not only for the current transformer (CT) calibration but also for the CC self-calibration. Third, practical capacitive error corrections are also proposed to be introduced for the CT and CC calibrations. Finally, in consideration of the BCG error, capacitive error, and compensated CC error, a proposed way of calculating CT error and estimating its uncertainty is described.

Automatisation des comparateurs de courants électriques alternatifs pour la mesure des fortes intensités au LNE.

Two compensated current comparators with magnetic flux cancelling have been made in the 1970s by the LCIE (Laboratoire Central des Industries Electriques) as a reference for high current transformers calibration for currents up to 50 kA at industrial frequencies 50 Hz, 60 Hz and 400 Hz. They are associated with manually operated electronic equipments: an adjustable phantom burden and a ratio error set capable to provide with high accuracy the quadrature and the in-phase components of the current transformer errors. This article presents the work that has been done to entirely redefine theses devices by abandoning the idea of a direct servo system of the old equipment and replacing it by a software servo that offers high calibration stability, better incertainties and fully controllable by a remote computer via an USB connection, allowing full automation of the procedure.

On the Calibration of Direct-Current Current Transformers (DCCT)

Modern commercial direct-current current transformers (DCCT) can measure currents up to the kA range with accuracies better than 1E-5. We discuss here a DCCT calibration method and its implementation with commercial instruments typically employed in low resistance calibration laboratories. The primary current ranges up to 2 kA; in the current range below \SI{100}{\ampere} the calibration uncertainty is better than 3E-7. An example of calibration of a high-performance DCCT specified for primary currents measurement up to 900 A is discussed in detail.

Design and Calibration of Inductive Current Transformer

In this paper, the design principle of inductive current transformer is introduced at first. Then an inductive current transformer is produced by applying the structure of two-stage inductive voltage divider. And finally, the error of the sample machine is given through examine test, and the suggestion for improving the machine is given based on the data. The data from the sample machine shows that: without compensation winding, the error level of 1200 ampere-turn of inductive current transformer can reach 0.1 level, after adding the compensation winding, it can reach 0.02 level. The design and data have laid a foundation for the future design of inductive current transformer with higher level of accuracy.

A high-current calibration system based on indirect comparison of current transformer and Rogowski coil

The calibration of the protective current transformer (CT) is of particular importance, since its accuracy at high currents is crucial to the correct operation of the subsequent relay protection devices. Conventional calibration methods have been using an electromagnetic CT which contains an iron core as the standard CT. The iron core is big and difficult to manufacture for high-current measurement, and the serious residual magnetism of the iron core at high currents can lead to excessive measurement errors. This paper proposes a calibration system based on indirect comparison of CT and Rogowski coil, i.e. using an iron-core CT to correct the error of the Rogowski coil at low currents, which may be caused by the position of the current-carrying conductor and so on, and then using the calibrated Rogowski coil as the standard transformer at high currents for its good linearity and wide dynamic range, and there is no magnetic saturation. Since the output of the Rogowski coil needs to be integrated, an improved digital integrator based on direct current (dc) negative feedback is adopted, which can effectively eliminate the influences of temperature drift, time drift and dc offset caused by the analogue circuit. The measurement errors of each part of the calibration system have also been discussed, and the test results show that the accuracy of the system can reach up to the 0.05S Class and the uncertainties are 0.038% for ratio and 0.68′ for phase in the range 500 A to 50 kA.

High-Current CT Calibration Using a Sampling Current Ratio Bridge

A measurement setup is developed for the accurate ratio measurement of ac current transformers (CTs) for primary currents up to 5 kA. A unique property of the system is the use of high-quality digitizers for sampling of the secondary current signals and step-down transformers with different current ratios. This allows for accurate comparison of CTs even when their nominal ratios are not equal. The calibration results of the key components in the setup show that the sampling current ratio bridge has a ratio uncertainty of better than 2 $\mu{\rm A/A}$ in magnitude and 0.8 $\mu{\rm rad}$ in phase for CT-under-test current ratios that do not differ by more than a factor of 5 from one of the ratios in the reference CT. The linearity of the bridge for input currents changing between 1% and 120% of nominal input current is better than 4 $\mu{\rm A/A}$ in magnitude and 0.5 $\mu{\rm rad}$ in phase.