Traditional Current Transformers Research Articles

Design of Laser-Powered Opto-Electrical Current Transformer and Its Application in Electrical Automation of Substations

The novel power equipment represented by the Opto-Electrical Current Transformer (OECT) possesses unparalleled advantages compared to traditional Current Transformers (CTs), rendering it with vast potential in power systems. In the design of OECT, this study employs laser power supply and focuses on the overall perspective, particularly the design of the laser power supply module and the optical fiber digital transmission module. The laser power supply module selects a high-power semiconductor laser to provide power to the circuit, incorporating line filters, thermistor resistors, and a laser overcurrent protection circuit. An automatic temperature control circuit based on PI regulation is utilized to stabilize the working temperature of the laser diode (LD). The photoelectric conversion device PPC-6E achieves a maximum photoelectric conversion efficiency of 40%, and the MAX639 DC–DC conversion chip ensures stable output voltage. The optical fiber digital transmission module adopts a circuit composed of multiple NOR gate logic for electro-optical conversion, a low-power driving circuit based on the 74LVC1G07 chip, and the HFBR-2412 photoelectric converter to simplify the circuit board design. To achieve a higher communication baud rate, a quartz multimode optical fiber with a diameter of 62.5 μm is chosen. In the experimental phase, a simulation of high-power AC power with equal turns is conducted. The linearity/temperature test results demonstrate that the designed OECT in this study meets expectations. In the electrical automation test of substations, a comparison between traditional CTs and the designed OECT is carried out using traditional protection algorithms and optical differential protection algorithms. The results indicate that, compared to traditional protection algorithms, the optical differential algorithm using the designed OECT achieves faster protection times.

A comprehensive design method of an active electronic current transformer for stable energy harvesting from power lines

AbstractThe real‐time monitoring technology of power line is an important guarantee for the digitization and intelligence of the modern power system. Traditional current transformers cannot provide continuous and stable energy when the power line current fluctuates in a large range. This paper proposes a comprehensive design method of an active electronic current transformer (AECT) based on a single‐phase full‐bridge voltage‐type PWM rectifier (VSR), which can acquire continuous and stable energy from the power line with a minimum magnetic core. The control of the AECT is divided into four different operation modes to adapt the fluctuation of primary current. Maximum power point tracking (MPPT) mode and minimum loss (ML) mode is designed to harvest the maximum power with lower iron core loss when the primary current is relatively low. Excitation mode and demagnetization mode are designed to harvest stable power when the primary current is higher than a certain current. The specific mode operation principles and control strategies are analysed in detail. Besides, the calculation method of the primary current is also presented for the primary current measurement. Finally, simulations and experiments verify that the iron core can obtain power steadily (9.91 W/kg) when the current fluctuation between 50 and 600 A.

Разработка и исследование катушки Роговского для работы в условиях эксплуатации открытого распределительного устройства

The interest to use Rogowski coils (RC) as current sensors to design secondary systems increases due to application of microprocessor relay protection and automation devices, including those that support the reception and processing of information in a form of a digital code in the format of protocols defined in IEC61850, as well as the disadvantages of traditional electromagnetic current transformers. Despite the popularity of RC, there are many unstudied issues of their functioning under operating conditions of open switchgears of power facilities, especially when operating with modern relay protection and automation devices. The aim of the study is to analyze the effect of ambient temperature on the error of current conversion by Rogowski coils. Studies of Rogowski coils have been carried out using the experimental and industrial samples. During the research modern testing equipment has been used. The equipment is listed on the Unified Register of Measuring Equipment of the Russian Federation and has valid verification certificates. The analysis of the factors influencing the error of metrological channels has been carried out. The analysis is conducted based on traditional current transformers (CT). The factors influencing the measurement error of RC are systematized. Experimental data obtained as a result of the study of the RC conversion error in a wide temperature range are presented. It has been established that due to design solutions, it is possible to neutralize the influence of such factors as the position of the primary conductor, external electromagnetic fields, uneven winding, the number of layers of the secondary winding, and thermal expansion. The use of materials with a low coefficient of linear expansion to manufacture the core of the Rogowski coil makes it possible to obtain the metrological characteristics of the transducer that meet the requirements of normative technical documentation in a wide temperature range. Application of RC as a primary current sensor for relay protection and automation systems is a promising area that requires further comprehensive research, and specifically in the process of industrial operation.

Nonintrusive overhead‐line current detection using a vertical magnetic‐field sensor array

AbstractWith the growing infrastructure for overhead lines, the demand for overhead‐line condition‐monitoring technologies has become increasingly urgent. Traditional current transformers used to measure three‐phase currents are bulky and difficult to install, which further increases the cost of monitoring. In order to meet the current state assessment needs of transmission and distribution networks, this study uses a novel magnetic‐field sensor array to realize nonintrusive overhead‐line current detection. Based on the magnetic fields measured by magnetic‐field sensors placed on the ground under the overhead line, the proposed algorithm can be used to calculate the three‐phase current values. The proposed method does not require on‐site calibration before the field measurements are conducted. With partially known overhead‐line geometry specifications from utilities, the algorithm can optimize the overhead‐line geometry and finally achieve accurate current detection. The simulation and experimental results verify that the proposed current‐detection technique can accurately calculate the three‐phase currents of the overhead line with only three three‐axis magnetic‐field sensors.

Fault location in overhead transmission line: A novel non-contact measurement approach for traveling wave-based scheme

Traveling wave fault location methods have gained considerable growth over the years, with the anticipation that power systems will be accurate and reliable everywhere. But the performance of classical traveling wave methods has focused only on traditional current transformer measurement in the literature till date. Therefore, in this paper, an innovative non-contact measurement of traveling wave formulation is proposed to achieve accurate fault location and time of arrival detection on a two-terminal overhead transmission line, since power cable naturally generates magnetic and electrical fields in the surrounding atmosphere. The proposed method solitary depends on the first two and successive time differences between the incident and reflective wave from the fault point of the transmission line. Thus, the proposed scheme does not depend on current transformer measurement that stimulates the accuracy and reliability of the conventional traveling wave fault location method. To ensure an accurate time of arrival waves detection, we exploit the magnetic field sensor array on each tower to measure the quality of current traveling waves generated on the transmission line and locate the transient disturbances as a result of a short circuit. By analyzing the line energization, a comprehensive fault analysis on a 500-kV line was simulated with PSCAD/MATLAB toolbox compared to the two-terminal classical method. The sensitivity analysis demonstrated that the proposed method is much accurate and reliable to be deployed and cost-effective, making it highly useful and efficient in comparison to the classical two terminals traveling wave method with an average error of 0.014%. The effectiveness of the proposed method was verified by laboratory experimental setup.

A Design Method of PCB Rogowski Coil in Limited Space and Modified Integral Circuit

Compared with the traditional current transformer with magnetic core, Rogowski coil has more advantages, such as non-saturation, high measurement accuracy, wide measurement range, simple fabrication technique and low cost. In recent years, Rogowski coil become an extremely popular method of measuring current in power grid. Rogowski coil is widely used to measure three - phase current in circuit breaker space. However, Rogowski coil brings the problems of low mutual inductance and phase angle difference. Integral circuit can address the issue of phase angle difference. But Rogowski coil with low mutual inductance has low signal-to-noise ratio and vulnerable to interference from external magnetic field. The design of Rogowski coil with large mutual inductance and integral circuit is required. This paper theoretically analyzes how to design a PCB Rogowski coil with many turns and large mutual inductance in the limited space. A modified integral circuit with excellent amplification and integration is proposed. Although the PCB Rogowski coil designed in this paper only occupies 1.435cm3 in volume, the value of mutual inductance is 89nH. The output voltage of PCB Rogowski coil is 0.023mV/A. In the range of 50Hz-1kHz, the integration and amplification of the circuit is reliable. The amplitude of the integral circuit is 62.2dB at power frequency and the crosstalk coefficient of the PCB Rogowski coil is no more than 0.26%. The temperature drift of the modified integral circuit is less than 0.16% and has no influence on the measurements.

Error Calculating and Compensating Method of All-Optical Fiber Current Transformer in ± 10 kV DC Distribution Network

Current transformer is one of the main equipment in ±10 kV DC distribution network. Traditional electromagnetic current transformer has poor anti-jamming capacity and poor insulation, so it cannot realize transient protection. The emergence of all-optical fiber current transformer brings solutions to these problems, which has attracted more and more attention. In this study, aiming at the problem of error and compensation of λ/4 waveform of all-fiber current transformer, the fabrication method of λ/4 waveform is studied, the mathematical model of the waveform is established, the influence of the waveform error on the scale factor is analyzed, and a calculating device of angle difference and ratio difference to compare the error is designed. The device adopts the principle of traceability of quantities to collect the state data of the wave plate. At the same time, in order to collect data for errors, database technology and network technology are used to realize remote transmission of monitoring data. The error of the λ/4 wave plate is compensated, the bidirectional principle generated by the birefringence in the fiber ring is analyzed, and the data logic description of the transformer sensitive ring is performed. The phase delay and the angle of the shaft are selected as the main factors causing the error. Compensation is performed by means of peak segmentation and variable cancellation. In the experimental process, compared with the error sampling of the traditional transformer, the error calculated by the current amplitude and phase angle parameters obtained by the all-fiber current transformer is more accurate, and the compensation scheme can suppress the size of the birefringence and improve sensing accuracy of fiber optic current transformers. This study provides a powerful reference for the error analysis of all-fiber current transformer λ/4 wave plates, which is beneficial to promote the better application of such current transformers.

PCB Rogowski Coils for 300 kA Current Measurement on a Multi-Split Conductor

In this paper, a measuring system based on printed circuit board (PCB) Rogowski coils is introduced for high current measurement with a multi-conductor structure. A high current up to 300 kA is split into 16 branches to fulfill engineering requirements. This high current and multi-split structure bring about many problems into the measurement. Traditional current transformers (TCTs) are not suitable for this situation since they are saturated by these high currents. Series of PCB-based Rogowski coils are used in the presented system, which can measure this high current easily with high accuracy (<0.5% at Hz). To ensure high accuracy, the measuring system needs to have high uniformity and strong anti-interference ability. Uniformity is analyzed and the relevant formula is proposed to calculate it. Anti-interference ability is mainly related to the adjacent coil’s mutual inductance (ACMI), which is analyzed carefully to find variables affect it. Uniformity analysis and anti-interference ability analysis provide a theoretical basis for parameter design. This paper elaborates on parameter design and gives the basis for selecting each parameter, which could be used as a reference for peers. Besides, supporting circuits and frequency analysis are also given to guarantee the integrity of the measuring system. Finally, experiments are carried out to prove high uniformity and anti-interference ability of the measuring system, thus ensuring high measurement accuracy. The tests show that the measurement error can be repressed under 0.5%.

A Stable Energy Acquisition System Based on Magnetic Circuit Coupled Current Transformer for Transmission Line

With the development of smart grids, high-voltage transmission line monitoring devices has become a new research hotspot, but the supply of stable energy for monitoring devices is a difficult point. By utilizing of current transformer (CT) to obtain stable energy from high-voltage transmission lines is one of promising approaches, but the current variation range of the high-voltage transmission line is often too large to be transformed linearly by the traditional electromagnetic CT. According to impedance matching principle, a method to regulate the load impedance of a CT is proposed in order to acquire energy effectively from a transmission line and provide stable output power. The method to regulate the load impedance of a CT can be implemented by controlling a single-phase PWM rectifier. It is proved by experiment that the PWM rectifier can be adjusted to supply a stable 20V output voltage and 9W output power for the DC load.

Measurement of inrush current in transformer based on optical current transducer

The accurate measurement of inrush current is a key problem in transformer protection. Traditional electromagnetic current transformer (CT) has nonlinear characteristics of its core. When the transformer generates inrush current, it will be saturated and lead to current distortion, which will lead to transformer differential protection misoperation. Optical current transducer (OCT) is not affected by saturation and can be used to measure and identify inrush current well. This paper analyses the mechanism of inrush current, the reason of CT saturation and the principle of OCT measurement. The simulation and experimental results show that OCT has better measurement quality than traditional CT and superiority to differential protection.

A Multi-Function Common Mode Choke Based on Active CM EMI Filters for AC/DC Power Converters

It is widely believed that hybrid electromagnetic interference (EMI) filter (HEF) is one of the most optimized solutions for EMI elimination. Nevertheless, the desire for a smaller and lighter filtering method has never stopped. To further reduce the size and weight of a hybrid EMI filter, this paper proposes a multi-function common mode choke (MCMC) used in HEF. It differs from the traditional common mode (CM) choke in that two functions of (1) CM signal sensing and (2) CM noise suppression are integrated into one choke. In such a proposed MCMC design, the traditional current transformer (CT) is eliminated in the hybrid filter topology. This technique could reduce the size, weight, and cost of the filter to a great extent. Based on the MCMC, a novel CM filter is proposed to achieve high-performance noise suppression. System model and stability feature of the new filter are developed and investigated in detail. A filter prototype based on the proposed MCMC is constructed. It is tested on a 100-W commercial ac/dc converter. The experimental results show that the proposed CM filter can provide better EMI noise suppression with a smaller size. The insertion loss could reach 50 dB at low frequencies and 40 dB at high frequencies.

Rectangular Magnetic Sensor Array for Current Measurement by the Quadrature Method

A novel current transducer based on rectangular array of magnetic sensors is proposed. It is a valid alternative to traditional current transformers in high power dc busbar systems for fusion applications. Since measured magnetic fields along rectangular closed integral path fluctuate violently, sum-average algorithm, which is most commonly used by circular arrangement sensor array, does not fulfill the accuracy requirement any more. Numerical quadrature algorithms of high orders are applied to improve rectangular array’s accuracy. The algorithm can be implemented by multilevel adder of analog circuit. Optimum dimension of rectangular array, error introduced by installing deviation and crosstalk interference are analyzed theoretically. The experimental results sufficiently verify the accuracy and reliability of the proposed transducer.

A new self-calibration method for electronic current transformers

Compared with traditional current transformers (CTs), electronic current transformers (ECT) have the following advantages: (1) no saturation problem, (2) wider measurement range, (3) smaller footprint, (4) lighter weight, and (5) easier for digitization. Hence, the ECT is one of the critical components in the development of intelligent substations. Since the majority of the sensing elements of ECTs are semiconductor materials, their accuracy and sensitivity are influenced by the ambient temperature. Moreover, the quiescent output voltage of the semiconductor-based ECT is also affected by the ambient temperature. This paper proposes a self-calibration method with the ability of quiescent-output-voltage compensation and sensitivity compensation. The proposed design can automatically adjust the quiescent output voltage and the sensitivity variation when the temperature changes. The experimental test results demonstrate that an ECT with the proposed self-calibration method can achieve Class 0.5 for measuring CTs and Class 5P20 for protective CTs.

Travelling wave pilot protection for LCC-HVDC transmission lines based on electronic transformers’ differential output characteristic

Considering travelling wave’s bandwidth which varies from several kilohertz to hundreds of kilohertz, traditional current and voltage transformers cannot transfer whole bandwidth of travelling wave. However, electronic transformers, including Rogowski coil based electronic current transformer and capacitance divided electronic voltage transformer, have much wider bandwidth (up to 500kHz) which could transfer almost whole bandwidth of travelling wave without distortion. Besides, secondary side’s output of electronic transformer is the differential signal of primary side. So, the integration circuit can be omitted when using differential travelling wave signal directly in protection principle. Traditional travelling protection for high voltage direct current (HVDC) lines is highly affected by grounding resistance. And the backup protection, such as current differential protection, has long operation time. So the paper proposes a novel travelling wave pilot protection based on the differential output signal of electronic transformers. Differential voltage and current travelling wave have axisymmetric relationship when the fault occurs at forward direction. Conversely, differential voltage and current travelling wave almost overlap when the fault occurs at reverse direction. Considering fault direction identification results of two ends, fault section can be determined. Besides, according to amplitude ratio of fault line and normal line, fault line can be selected correctly. The simulation results in PSCAD/EMTDC prove the correctness, sensitivity and reliability.

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 Redundancy Mechanism Design for Hall-Based Electronic Current Transformers

Traditional current transformers (CTs) suffer from DC and AC saturation and remanent magnetization in many industrial applications. Moreover, the drawbacks of traditional CTs, such as closed iron cores, bulky volume, and heavy weight, further limit the development of an intelligent power protection system. In order to compensate for these drawbacks, we proposed a novel current measurement method by using Hall sensors, which is called the Hall-effect current transformer (HCT). The existing commercial Hall sensors are electronic components, so the reliability of the HCT is normally worse than that of the traditional CT. Therefore, our study proposes a redundancy mechanism for the HCT to strengthen its reliability. With multiple sensor modules, the method has the ability to improve the accuracy of the HCT as well. Additionally, the proposed redundancy mechanism monitoring system provides a condition-based maintenance for the HCT. We verify our method with both simulations and an experimental test. The results demonstrate that the proposed HCT with a redundancy mechanism can almost achieve Class 0.2 for measuring CTs according to IEC Standard 60044-8.

Design and Implementation of the CT Analyzer on the Basis of the Low Pressure Test Principle

CT analyzer is a necessary instrument of on-site verification accuracy of current transformer. Traditional current transformer testing methods is mainly the differential method. Because of the limited by the principle, the current transformer of the on-site verification is very inconvenient. The CT analyzer designed and developed in this paper use the new method of indirect test - low pressure method, which gets current transformer contrast and angle difference by measuring the internal parameters of the CT. According to the design method of intelligent instrument, by combining with the digital sampling technology, taking DSP microprocessor as the core, and using the advanced digital signal processing method, use only one instrument current transformer can be realize the field test, which effectively resolve the problems of current transformer field test.

Design of a new Hall overcurrent protection relay with instantaneous characteristics

Traditional current transformers (CTs) detect fault currents and transfer signals to overcurrent protection relays for indicative and protective purposes as a part of the protection coordination system in a power network. By characterizing the transient response to a power failure, a reliable and accurate protection relay can be achieved. In this study, we propose a new Hall overcurrent protection relay (HCO) using Hall effect CTs based on the transient exponential signatures of fault current analysis. To minimize the distortion phenomena seen in fault current measurements because of the hysteresis caused by the CTs’ iron cores, and to obtain faster tripping signals, we adapt an HCO to accurately detect nonsinusoidal fault waves and to improve protection coordination accuracy. The experimental results indicate that the HCO significantly improves the protection system’s accuracy and response speed for various power systems applications.

Intelligent Current Sensors as Part of Smart Grid Network

The continuous development of the ITC systems, especially industrial Ethernet network using the IEC 61850 protocol allows to building a distributed sensor networks for control and monitor temporary states of the power infrastructure. In the paper the construction of the intelligent current sensors, Measurement Units (MU) for the Smart Grid network has been presented. In the paper the circuit diagram of sensor and synchronization method of the analog to digital converter based on the UTC timestamps has been described. The presented method allows to synchronize intelligent current sensors with an accuracy better than 1 μs. The presented innovative synchronization method is characterized by a high level of resistance to disappearance and attack of the reference time signal. The measuring device MU design is based on the embedded Linux-platform and as a sensing element used air core current transducers based on Rogowski coil. The connection between the current transducers and the central unit is made using a fiber optic link. Built-in IEC-61850 server provides a real-time measurement value of the True RMS, harmonics spectrum and samples value. The intelligent current transducers in addition to being used in the MUs create also new possibilities in the substation construction like the possibility to place the Autonomous Sensors in large distance from Intelligent Electronic Devices that are located in the substation. DOI: 10.12736/issn.2300-3022.2015205 Introduction Modern intelligent current sensors are gradually replacing traditional current transformers. The sensors, manufactured with the use of PCB technology, are composed of a current transducer based on the Rogowski coil principle and an electronic circuit integrated with the transducer. The current transducer is characterized by perfect linearity, high dynamic range, wide frequency bandwidth and very repeatable electrical characteristics. Equipped with signal processing circuits, autonomous power supply and communication modules, it allows the realization of the Smart Grid concept related to the monitoring of the temporary state of the power grid. Data transmitted by the intelligent sensor are collected by MU (Merging Unit) which, after time tagging make them available to IEDs (Intelligent Electronic Device), among them the Protection Relay, Power Quality Analyzer and PMU (Phasor Measurement Unit). Construction of the intelligent current sensor The block diagram of the intelligent current sensor is presented in Fig. 1. The main parts are current transducer, signal processing module, synchronization module, communication module and power supply module. The Merging Unit has also been shown because the intelligent current sensor communicates directly with it. The principle of operation of the current transducer has been described in [1]. This article will focus on the electronic parts of the sensor. Signal processing and communication module The signal processing module consists of an anti-aliasing filter, amplifier, A/D converter and additional processing circuits. The amplifier, together with A/D converter, must have a dynamic range high enough to cover the current measurement range of interest which may be in the range of a few amperes to several kA. Providing a high dynamic range is one of the more challenging issues when designing the current sensor. The A/D converter is followed by additional processing circuits responsible for packing the data into a format suitable for transmission. The transmission module changes the data from electrical into optical format. The block diagram of the APSP circuit described above is presented in Fig. 2. A. Nowakowski et al. | Acta Energetica 2/23 (2015) | 48–53

New Electronic Current Transformer With a Self-Contained Power Supply

Compared with traditional current transformers (CTs), electronic current transformers (ECTs) are characterized by small volume, light weight, good isolation, good linearity, and easy digitization. Hence, ECTs are one of the primary devices for signal digitization for supervising intelligent substations. We design a coreless ECT using Hall sensors as well as its mechanism and circuit configuration in order to replace traditional CTs and other CTs. The proposed ECT achieves Class 0.5 accuracy for measuring CTs and accuracy Class 5P21 for protective CTs according to IEC Standard 60044-8. An additional power supply is required to drive the ECT. Considering that there are no usable power supplies for special measurement environments, we use the surrounding magnetic fields induced by the cable currents as the power supply. A self-contained power supply is proposed and integrated with a backup battery power supply and a battery protection circuit to ensure that power is supplied for different operating conditions. In addition, we design a protective device to protect the back-end circuits of the self-contained power supply in case of a fault occurring in the power system.