Standard Current Transformer Research Articles

Approach of obtaining the error of current transformers through the measurement of turns correction factor and impedance

The error detection platform for current transformers (CTs) typically measures CT error by using the comparison method provided by International Electrotechnical Commission (IEC) 61869–2. It uses a more accurate standard CT as reference and the error of tested CT is obtained through comparison. This paper proposes an alternative approach of determining CT error by measuring the turns correction factor and magnetizing impedance. The measurement of magnetizing impedance is essentially equivalent to the excitation current measurement method. Therefore, high current sources are not required. More importantly, the measurement process requires only an impedance measurement instrument and a millivoltmeter, eliminating the need for constructing a complex system, and on-site CT error measurement can be realized. This method has engineering practical value. This paper firstly derives formulas for CT error in terms of turns correction factor and impedance. The turns correction factor is measured through the proposed differential voltage method, while the impedance is measured by using an impedance measurement instrument. Additionally, the measurement uncertainty introduced by the nonlinearity and impedance measurement instrument is analyzed. The final CT error results indicate that the proposed method yields comparable accuracy to the comparison method.

Development of a non-intercepting weak beam current measurement electronics for Heavy Ion Accelerator Facility in Lanzhou.

Non-destructive measurements of low-intensity charged particle beams are particularly challenging for beam diagnostics. At the Heavy Ion Accelerator Facility in Lanzhou (HIRFL), beams with weak currents below 1 µA are often provided for experiments. The detection of such low beam current is below the threshold of typical standard beam current transformers. Therefore, a low-intensity monitoring system is developed by using a sensitive capacitive pick-up (PU) and low-noise electronics. This device measures beam currents by digitally analyzing the amplitude of the PU signals using a homodyne detection scheme. During lab tests, the amplitude nonlinearity is <0.5% in the operational range of 1 nA-45 µA and the amplitude resolution is 0.94 nA. At present, four measurement systems for low beam currents are installed at HIRFL for the monitoring of standard operating conditions with low beam currents below 1 µA. After an absolute calibration with a Faraday cup, it can be used for accurate beam intensity measurement with a current resolution of about 1 nA.

Study on harmonic metering characteristics of Rogowski coil current transformer

This paper introduces the harmonic requirements of a power system transformer and the principle of the Rogowski coil electronic current transformer, summarizes the simple and practical methods for harmonic measurement of the Rogowski coil electronic current transformer, and conducts harmonic measurement of the electronic current transformer through high-precision harmonic source and standard current transformer and transformer calibrator. The results show that this method has strong applicability and simple operation, which greatly enriches the harmonic measurement methods.

Design and Simulation Analysis of Wide-range Current Transformer with Adaptively Adjustable Air Gap Length

Abstract This paper proposes a wide-range current transformer structure with adaptively adjustable air gap length. The length of the air gap of the iron core of this structure can be automatically adjusted with the current harmonic ratio of the secondary side of the current transformer, so that the current transformer has good anti-saturation characteristics; on the other hand, the output voltage value of the detection winding can be adjusted after proportional calculation. The secondary side current is compensated to ensure the accuracy of its measurement. At the end of the article, the structure of the current transformer is simulated and analyzed. The calculation results show that the measurement error of the secondary side current after compensation can reach the design requirements of the 0.2-level standard current transformer during the change of the primary current from 100% to 350%. This structure can effectively broaden the range of the current transformer and improve its measurement accuracy.

Error calculation of broadband standard current transformer based on equal-ampere-turn principle

Due to the lack of corresponding devices and traceability systems for wide-band current transformer verification, the wide-band current measurement requirements of many power systems cannot be solved. The most important problem is that the wide-band standard current transformer cannot be transmitted by the superior measurement agency. This article proposes a method of self-estimating errors using the principle of equal ampere-turns. The key is to use the absolute error of the 5/5 ratio to calculate the errors of other ratios. Not only does the error extrapolate to the windings with the same ampere turns, but also expands. To windings with different ampere turns, the frequency has also expanded from power frequency to broadband. Through the analysis of the circuit model of the current transformer, the rationality of the calculation process is demonstrated, and the effectiveness of the method and the accuracy of the data are verified by actual measurement. This method is of great significance to the ongoing research of new low-frequency power grids in power systems and to solve other broadband current measurement needs.

Design and Research of 10KV Electronic Transformer Live Performance Evaluation Platform

Design and research a test platform suitable for 10KV electronic voltage transformer and 10KV electronic current transformer for long-term charging performance evaluation. The 10KV electronic transformer live test platform includes a primary boost device, a primary current boost device, a reference voltage transformer, a reference current transformer, a standard voltage transformer and a standard current transformer. The test circuit is designed to be three-phase. Under the condition that the system input voltage is 380V / 220V and the total input capacity is not greater than 100KVA, the auxiliary transformer and other auxiliary equipment are used to achieve long-term live operation and performance evaluation of the 10KV electronic transformer.

Research and application of all-fiber optic current transformer based on optical reciprocity loop

Aiming to the problems of traditional active electromagnetic transformer such as easy magnetic saturation, poor stability and anti-interference ability, and limited installation, etc., this paper designs the optical fiber current transformer to measure current by rotation angle based on Faraday magneto-optic effect; HB Spun optical fiber is used as sensing element without saturation and can be used for high current measurement. The designed transformer uses the optical reciprocity loop to eliminate the interference of temperature and optical fiber defect on the measurement of optical rotation angle, and uses reflector to enlarge the optical rotation angle to four times, which can realize accurate measurement of small current; Sensing element uses flexible fiber ring with shape variability characteristic, which helps for measurement of current in complex space. The paper compares flexible fiber ring with different loops to standard current transformer, the results show that optical reciprocity loop can eliminate the interference of temperature on the current measurement and the accuracy of all-fiber current transformer is 0.5 in the range of -5 ℃~70 ℃, which can realize the accurate measurement of small current.

Estimation Method of the Errors Limits of Electromagnetic Current Transformers

A method for estimating the limits of errors of electromagnetic current transformers is proposed. In contrast to the standard technique, the proposed method considers not only the basic factors, but also additional ones, that affect the results of measurements. The additional influencing factors are the residual magnetization of the core and the nonsinusoidality of the current under measurement. The method was developed and explored based on a known nonlinear mathematical model of the current transformer. By the example of the standard TOL-10 current transformer, it is shown how the limits of errors of the current transformers can be determined for the operating conditions. Based on the estimation of the error limits, an accuracy class can be accurately attributed to the newly manufactured transformers and the conformity of the limits of errors of the current transformers in service to their class of accuracy can be verified. The method also allows increasing the reliability and accuracy of the measured results and the efficiency of the current transformers without making any design alterations.

Development of High-voltage Standard Current Transformer with Three Phases in One Enclosure Based on Shielding and Balanced Winding

When three-phase testing method is applied to test errors of three-phase combined transformers on site, many testing devices are needed while laboratory space is limited. Therefore, narrowing down the space that testing devices occupied appears to be crucial. This article designed and developed the gas-insulated high-voltage standard current transformer with all three phases in one enclosure. Its characteristics is that double shielding and shielding combined with balancing winding are used separately to restrain the influence of leakage current and magnetic field on the error of standard current transformer. Moreover, the article carried out the effect of leakage current and magnetic field on the error of standard current transformer. The results show that the influence of leakage current on the error of standard current transformer is less than 1/10 of the error limit. The influence of adjacent phase current on standard current transformer is less than 1/10 of the error limit. The high voltage standard current transformer with three phases in one enclosure, designed and developed in this article, is capable of testing errors of 10 kV three-phase combined transformers for measurement on the field with three-phase testing method.

High Accuracy On-Line Calibration System for Current Transformers Based on Clamp-Shape Rogowski Coil and Improved Digital Integrator

To solve the problems of the existing on-site calibration methods, for instance, the methods need power off, and the standard transformers have large volume, heavy weight and small dynamic range, this paper proposes a high accuracy on-line calibration method for current transformers. By using a clamp-shape Rogowski coil as the standard current transformer, instead of traditional electromagnetic transformer, the volume and weight are reduced greatly, and the power of the line needn’t to be interrupted. The output signal of the clamp-shape Rogowski coil needs to be integrated, and to overcome the problems of analog integrator, which have temperature and zero drift, a high accuracy digital integrator is proposed in the paper to improve the accuracy and stability of the signal processing circuit. Test results indicate that the accuracy of the whole calibration system can reach up to 0.05 accuracy class. The on-line calibration system can calibrate the traditional and electronic current transformers when the line is energized.

A Wideband Current Transformer Bridge

The development and testing results of a new current transformer (CT) test set at Physikalisch-Technische Bundesanstalt are described. The operation range for the ratio based bridge is intended for frequencies from 16.7 Hz up to 20 kHz. Equal setups of magnetically shielded current comparators with primary windings for rated currents from 100 mA up to 5 A convert the secondary currents of the standard CT and of the transformer under test (TUT) into proportional voltages. The errors of the TUT are calculated from the subsequent measurement of the complex voltage ratio. This current bridge and the voltage-ratio-based method allow comparing CTs with nonequal ratios. At 50 Hz, the measuring system offers an extended measurement uncertainty of less than 2 ppm or μrad, respectively. The bridge is verified for use with uncertainties of below 200 ppm or 400 μrad at a frequency of 20 kHz.

A Method for Measurement Traceability of Calibrator for Electronic Instrument Transformer Tester

The bottleneck of the measurement traceability system of electronic instrument transformer is the calibrator, which is based on the digital micro-differential source principle and plays the role as intermediate between digital values in data frames and the analog output of the existing standard current transformer (CT) or voltage transformer (VT). A kind of working mechanism of the calibrator for electronic instrument transformer tester and related calibration method are proposed in this paper. The ratio accuracy can be calibrated by the high-precision multimeter and the phase accuracy can be calibrated by the high-speed oscilloscope, which records the waveforms of analog output and the synchronization signal, and the time offset between them can be traced to the time standard. A calibrator is designed and manufactured according to the mechanism and related experiments are conducted to verify the feasibility of the calibration method.

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.

An accurate online calibration system based on combined clamp-shape coil for high voltage electronic current transformers

Electronic transformers are widely used in power systems because of their wide bandwidth and good transient performance. However, as an emerging technology, the failure rate of electronic transformers is higher than that of traditional transformers. As a result, the calibration period needs to be shortened. Traditional calibration methods require the power of transmission line be cut off, which results in complicated operation and power off loss. This paper proposes an online calibration system which can calibrate electronic current transformers without power off. In this work, the high accuracy standard current transformer and online operation method are the key techniques. Based on the clamp-shape iron-core coil and clamp-shape air-core coil, a combined clamp-shape coil is designed as the standard current transformer. By analyzing the output characteristics of the two coils, the combined clamp-shape coil can achieve verification of the accuracy. So the accuracy of the online calibration system can be guaranteed. Moreover, by employing the earth potential working method and using two insulating rods to connect the combined clamp-shape coil to the high voltage bus, the operation becomes simple and safe. Tests in China National Center for High Voltage Measurement and field experiments show that the proposed system has a high accuracy of up to 0.05 class.

在线校验用标准电流互感器的设计

该文介绍了一种在线校验用标准电流互感器的设计。该电流互感器基于电磁感应原理，利用双铁芯工作，并引入了无源补偿，具有开合式结构，能够在线安装，抗干扰能力强，其输出为模拟小信号，能直接被采集系统采集。实验结果表明，其精度优于0.05级，并且在开合50次后，仍能满足0.05级的标准要求。 This paper introduces the design of a standard current transformer for on-line calibration. The current transformer based on the electromagnetic induction theory works with dual cores and has the passive compensation circuit. It not only can be installed on line due to the open-close structure, but also possesses a strong anti-jam capability. The output of the current transformer is small analog signal which can be picked up directly by acquisition system. The testing results show that the accuracy class of the current transformer is better than 0.05 and it still can meet the standard when opened and closed 50 times.

Traceable Technique to Calibrate Clamp Meters in AC Current From 100 to 1500 A

A measurement technique to calibrate current clamp meters in ac current in the measurement range from 100 to 1500 A at 50 Hz has been developed at the National Institute of Metrological Research (INRIM) in addition to the primary reference measurement systems for ac current calibrations at INRIM. This technique is traceable to the national standards of dc resistance, dc voltage, and ac current ratio. It is based on a standard current transformer (CT), on a 50-mΩ standard shunt, and on a 61/2-digit digital voltmeter (DVM) that measures the voltage on the standard shunt inserted in a conductor, to which also the clamp meter is applied, whose current is supplied by a CT that can generate currents up to 1500 A. The CT is controlled by a variable transformer. The clamp meter is applied to a copper wire and placed on a tilting support simulating the actual operating mode of the meter. This measurement technique is applicable in industrial metrological laboratories and is an alternative to measurement practices of accredited secondary laboratories involving digital calibrators and turn current coils. This measurement technique was also tested utilizing an 81/2-digit high-accuracy DVM. The results of the evaluation are presented and discussed. The expanded ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> = 2) uncertainties of the technique span from 1.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> at 100 A to 2.0 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> at 1500 A.

Error analysis of the rough model of the symmetric dual-optical signal detection based MOCT

Symmetric double signal analysis based magneto optical current transformer(MOCT) is usually based on a rough model to calculate the primary current, but due to the use of approximate calculation, the primary current and output voltage is not as strictly linear relationship, so the rough model introduce some errors which exceed the national standard national electronic current transformer Standards 0.2 or 0.5 class accuracy requirements when the primary current increase outside a certain range, at this time the rough model can not be used to measure the current. As the voltage RMS and current RMS usually are used in the power system measurement, this article will analyze relationship between the actual output voltage RMS and the current RMS and obtain the relationship of the approximate calculation caused error and the increasing current RMS, and obtain the current measurement range that meet the accuracy requirement of the national standard.

Development of an on-site calibration method for a current transformer testing system and its application

The ratio error and phase displacement of a current transformer (CT) are measured by using a CT testing system, which consists of a current source, a standard CT, a CT under test, a CT comparator and a CT burden. Methods for on-site calibration of the components in CT testing systems in industry have been developed recently, which do not require any component to be detached from the systems. The method utilizes several travelling transfer standards: standard CTs, non-reactive standard resistors, a calculable wide-ratio error CT and non-reactive shunt resistors. The travelling standard CT is used for absolute evaluation of a current source and a standard CT of industry. The non-reactive standard resistors and the calculable wide-ratio error CT are used to evaluate the linearity of the errors in the CT comparator. The shunt resistors are used to evaluate the CT burden of industry. The on-site calibration method using these travelling transfer standards has been successfully applied to calibration of CT testing systems in industries.

A new method for calibrating standard current transformers up to 20 kA

We have developed a method to obtain the ratio error and phase displacement of a single-stage current transformer (CT) by measuring four parameters in a CT equivalent circuit. The method was applied to CTs under test with a primary current in the range 10–20 000 A and a secondary current of 5 A. By determining the errors of the CTs under test using this method, the ratio error and phase displacement of the standard CTs used as a reference in a CT test system could be obtained. This implies the establishment of a traceability chain of a national standard for CTs. The ratio error and phase displacement of the standard CTs obtained in this study are consistent with those measured at the Physikalisch-Technische Bundesanstalt in Germany using the same standard CTs within an expanded uncertainty (k = 2) in the overall primary current range investigated.

CT Saturation Calculations: Are They Applicable in the Modern World?—Part I: The Question

Previously, ANSI/IEEE relay current transformer (CT) sizing criteria were based on traditional symmetrical calculations that are usually discussed by technical articles and manufacturers' guidelines. In 1996, IEEE Standard C37.110-1996 introduced (1+X/R) offset multiplying, current asymmetry, and current distortion factors, officially changing the CT sizing guideline. A critical concern is the performance of fast protective schemes (instantaneous or differential elements) during severe saturation of low-ratio CTs. Will the instantaneous element operate before the upstream breaker relay trips? Will the differential element misoperate for out-of-zone faults? The use of electromagnetic and analog relay technology does not assure selectivity. Modern microprocessor relays introduce additional uncertainty into the design/verification process with different sampling techniques and proprietary sensing/recognition/trip algorithms. This paper discusses the application of standard CT accuracy classes with modern ANSI/IEEE CT calculation methodology. This paper is the first of a two-part series; Part II provides analytical waveform analysis discussions to illustrate the concepts conveyed in Part I