Conventional Current Transformers Research Articles

Novel Bridge-Type FCL Based on Self-Turnoff Devices for Three-Phase Power Systems

In this paper, a novel bridge-type fault current limiter (FCL) based on self-turnoff devices for three-phase power systems is proposed. Compared with the SCR-based bridge type FCL, the new one is smaller in size, better in dynamic performance, and simpler in control method and control circuitry. The novel FCL is composed of a rectifier bridge-a dc-limiting reactor and one/three bypass reactor/reactors-for a single-phase/three-phase structure. The increasing speed of the fault current just after the occurrence of a fault is limited by a dc-limiting reactor, whose inductance is designed according to the response speed of the control circuit. The bypass reactors are designed in terms of the requirements of the coordination of protective relays. The source currents during limiting events are sinusoidal and do not have any harmful effects on the overall protection schemes, impedance relays, and conventional current and voltage transformers. The topologies of the proposed FCL for three-phase four-wire and three-phase three-wire power systems are investigated, respectively, in detail. Parameter design and optimization are presented. The fault and fault-mode judgment method is described briefly. Simulations and experimental results prove the feasibility of the new FCL.

Modified method of high alternating current measurement

The conventional current transformers (CTs) in the form of a bar primary and secondary winding on a highly permeable ring core suffer from ratio error and phase angle errors, which are primarily due to the no-load current of the transformer. A modified design of a CT has been described. In this design, a feedback technique has been used in order to reduce the no-load current to a negligibly small value so that the said error may be minimal. Using this technique, a CT has been designed and fabricated with the commercially available material. The performance of the transformer has been tested experimentally and the output and input AC waves have been observed in a storage cathode ray oscilloscope (CRO) and the experimental results are reported.

An LTS-SQUID Based Measurement Tool for Characterization of Superconductive RF Cavities

This paper presents a new system to measure very low currents in an accelerator environment, using a cryogenic current comparator (CCC). In principle a CCC is a conventional current transformer using the high performance SQUID technology to sense the magnetic fields caused by the beam current. Since the system is sensitive on a pA level, it is an optimum device to detect dark currents of superconducting cavities. The system presented here is designed for the test facilities of the superconducting accelerator modules for the European XFEL at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg. Measurements in a quiet environment showed that an intrinsic noise level of the CCC of 40/radicHz could be achieved.

APPLICATION OF POLYNOMIAL FITTING TECHNIQUES TO THE CURVE m 10-Z L OF THE CURRENT TRANSFORMER

Conventional ferromagnetic core current transformer (CT) is widely used to measure large alternating currents with lower rated instrumentations or ammeters. The improper choice of the load of secondary instrumentations (symbol in Z L , e.g., relay protection apparatus) will negatively influence the measurement accuracy. Testing Z L and multiple of current in the case of 10% error of current m 10 , and thus plotting the relationship between Z L and m 10 , is helpful to give valuable instructions for properly choosing the secondary instrumentations. The principle of calculating values of Z L and m 10, methods of operation of acquired data to determine key parameters, and technique to fit the curve m 10 -Z L resorting to least square estimator are presented. Hence the curve m 10 -Z L can be successfully plotted based on polynomial coefficients of fitting curve. Experimental results verify the validity of the proposed method, which removes most of the noise of the data, smoothes the curve, and hence increases the measurement accuracy.

Development of a current sensor based on active materials for high-voltage transmissionsystems

This paper presents the development of a new class of current sensor based on activematerials for high-voltage transmission systems. This current sensor is an innovative designwith respect to conventional current measurement transformers. The alternating currentsignal to be measured induces a magnetic field in an emitter which consists of amagnetostrictive material. The emitter transforms the current magnetic energy intomechanical energy in the form of mechanical waves due to the alternating nature of theinduced magnetic field. These waves are transmitted through a dielectric structure until apiezoelectric stack, the receiver, is reached which converts the mechanical energy backinto electrical energy. An electronic signal module processes this low electricalcurrent and estimates the primary current to be measured. A numerical modelhas been developed to evaluate the preliminary design. A small scale prototypehas been built and tested to demonstrate the feasibility of the current sensor.Experimental data have been used to fit the damping parameters of the model.

New Inductive Current Transformers Provided for to Work at a Frequency of up to 300 kHz

Phenomena occurring in electromagnetic current transformers operating in the areas of medium frequencies have been discussed, above all, with respect to the properties of particular constructional solutions. These constructions are characterized by appropriate solutions to windings and special magnetic materials (amorphous and nanocrystalline tapes). The degree of magnetic coupling that occurs between the primary winding and the secondary winding with a definite geometry of the magnetic core was assumed to be the criterion of a solution of the current transformer. Current transformers should be divided into three basic groups: 1) transformers with very small dispersion (i.e. of very good magnetic coupling between the windings); 2) transformers with medium dispersion; 3) transformers with very large dispersion. Current transformers can be assigned specified courses of the magnetic induction Bm as a function of pulsation ω. These courses are closely related to the construction of a given transformer. Thus, it can be stated that the specified variability of the magnetic induction Bm as a pulsation function which affects active losses and passive power has a direct effect on the core temperature and the transformer errors. Hence, the character of changes in Bm=f(ω) is the basic criterion determining the possibility of use of a given solution of the transformer in circuits of a specified elevated frequency. The possibility of application of a conventional current transformer to operate over the range of an elevated frequency is also determined by the value of the rated magnetic induction Bm in the magnetic core. Ill. 2, bibl. 4 (in English; summaries in Lithuanian, English and Russian).

Unconventional CT and VT Connections and How to Get Them Right

This paper describes current and voltage instrument transformer connections that are not physically conventionally connected as portrayed in ANSI Standards and Guides or relay manufacturers' literature. A three-line diagram of basic conventional current transformer connections for a two-winding-delta wye power transformer differential relaying is presented. The unconventional scenarios presented are: 1) one set of current transformers (CTs) being reversed or "rolled out"; 2) the other set reversed; and 3) both sets reversed from the conventional connection. Proper connection of these unconventional connected CTs is then explained. Also, the effect of reversed voltage transformer (VT) connections on directional overcurrent and directional power relay elements is presented. An example of how symmetrical component metering screens of a numeric relay were used to diagnose an incorrect VT connection is also presented. This paper is a primer on analyzing unconventional instrument transformer polarity connections and how to obtain the right connection to achieve the desired direction of operation or polarity as indicated in protective relay manufacturers' literature.

Optical Fiber Current Sensors for Electric Power Facilities

This paper describes development of optical fiber current sensors along with some of their applications. Optical fiber current sensors based on the Faraday effect have long been expected to replace conventional current transformers as compact and high performance current sensors. Yet because they require complex means for achieving stable characteristics, this type of sensor has never come into widespread practical use. To solve these problems we developed a special sensor fiber made from flint glass and created sensor systems whose sensing method ideally matches the characteristics of the fiber. These sensors we developed are immune to environmental effects such as vibration and temperature changes. They are also compact and flexible, and attach easily around conductors. This paper also introduces some typical applications of these sensors including cable current measurement using a portable type sensor, a fault section locating system for power transmission lines using these sensors, and sensor applications to protective relay systems as verified in field tests.

Research on optically powered ultra current transformer

In this paper an electro-optic current transformer (CT) is designed which is used for measuring the super-high-voltage station's line current. The measurement system utilizes a conventional CT as the sensing probe whose electronics is optically powered. The paper describes the principle of measurement system, including the system design, optically powered sensing probe, the data link measurement signal processing and results showing current measurement by means of CT.

Design and evaluation of an algorithm for detecting current transformer saturation

When the magnetic circuit of a conventional protection current transformer (CT) enters saturation, the wave-shape of the secondary current distorts due to the increase in the excitation current. The distortion can be detected by analysing the secondary current using an algorithm that evaluates the first, second and third difference functions. The first difference contains points of inflection, which correspond to the start and end of each saturation period. The second and third differences convert the discontinuities at the points of inflection into pulses that can be used to detect saturation. The design and evaluation of an algorithm for detecting CT saturation using the third difference function is described. A low-pass filter, required for anti-aliasing and noise rejection, softens the discontinuities and reduces the magnitude of the pulses seen in the second and third difference signals. The softening effect is pronounced when the cut-off frequency of the filter is reduced. The sampling rate of 64 samples/cycle is used and the currents are passed through a first-order low-pass RC anti-aliasing filter with a cut-off frequency of from 1920 to 480 Hz. Experimental test results clearly demonstrate that the algorithm successfully detects the start and end of each saturation period. Results are obtained when a prototype detector based on a digital signal processor was used on a high current test-rig. Results indicate that the detector can correctly detect when the secondary current signal is distorted and in all the test cases the detector operated correctly.

A novel electro-optic hybrid current measurement instrument for high-voltage power lines

Details of an electro-optic, hybrid current-sensing instrument, and its application to high-voltage power line current measurement, are presented. The current is first detected via a Rogowski coil, followed by conversion of the detected current into a frequency-modulated optical pulse signal in the high-voltage area. Subsequently, the optical pulse signal is transmitted to the ground through an optical fiber cable. Finally, the measured current is recovered from the optical pulse signal by a ground-located signal-processing module. Compared with conventional current transformers, widely used for high-voltage line current measurement, this instrument provides a significant reduction in size, weight, and cost, together with features of convenient installation, secure operation, and high accuracy. Furthermore, it can be configured to be compatible with conventional secondary meters widely used in today's power plants and substations. In 1996, a prototype of this instrument was demonstrated online at a power substation and has shown good accuracy, stability, and safety for more than two years.

Hybrid optical current transformer with optical and power-line energisation

The hybrid optical current transformer (HOCT) has previously been proposed as a suitable device to replace a conventional open-terminal post-type current transformer. The HOCT is much smaller than the conventional device leading to lower manufacturing costs. There is also a degree of electrical insulation inherent as optical fibre is used for connections between the line-mounted HOCT and ground level instrumentation; this reduces installation costs. A performance specification for a HOCT is presented; a HOCT unit satisfying this specification could, in principle, be a suitable replacement for a conventional device for power system protection purposes without loss of system quality or reliability. A prototype HOCT has been designed and constructed; initial tests show that it meets the specified accuracy requirements over the full operating temperature range. As far as can be determined, this is the first alternative to conventional current transformers that has been shown to meet these stringent requirements.

Electronic error reduction system for clamp-on probes and measuring current transformers

An original electronic device for compensating ratio and phase-displacement errors, of measuring current transformers and clamp-on probes, is proposed. It is intended for low-frequency applications (power frequency and its harmonic components), where the magnetizing current is the main cause of measuring errors. This system reduces the magnetic flux in the transformer core, reducing in this way the magnetizing current. The proposed system even reduces the influence of the error produced by the variation of the burden connected to the secondary winding. Large values of burden can be used without effects on the transformer measuring errors. This device can be directly applied to conventional current transformers, connecting the device to the secondary terminals. It is not necessary to use any auxiliary winding or auxiliary core, or any modification of those transformers.

A new electro-optic hybrid current-sensing scheme for current measurement at high voltage

A new electro-optic hybrid current sensor is presented here. The sensor consists of three parts: first is a Rogowski coil and integrator whose output voltage is proportional to the current flowing in the power line. Second is a combination voltage to frequency converter and electro-optic converter. The combination converter transforms the integrated Rogowski coil voltage into a FM optical signal. The optical signal can then be transmitted to a low voltage region via an optical fiber for subsequent demodulation processing. Third is an intrinsic power supply in which a small conventional current transformer is used to draw power from the power line. A current sensor based on this design was developed, having a diameter of 15 cm, length of 30 cm, and weight of 2 kg, and has been working well at a 110 kV substation for two years. This sensor features small size, light weight, high accuracy, low cost, and reduced hazard.

The interrogation of a conventional current transformer using an in-fibre Bragg grating

An alternative approach for measuring large currents at high potentials is presented. The output signal from a conventional current transformer is measured by transducing the voltage developed across the transformer secondary into a shift in the reflected wavelength from an in-fibre Bragg grating using a piezoelectric element. A pseudo-heterodyne detection scheme was used to determine this shift. We demonstrate the scheme's performance over a current range of 700 A. The current resolution was about over a frequency range of about 50 to .

A current sensor using the Faraday effect in optical fiber manufactured from flint glass

For application to current monitoring in electric power facilities, the authors have been developing a current sensor utilizing the Faraday effect of an optical fiber manufactured from flint glass having a very small stress-optic coefficient. This paper describes the result of study on constructing the sensing system, which is composed of a sensor head, a light source, fibers for light transmission and a signal processing circuit. First, results from investigation are reported with respect to causes affecting performance and durability of the sensor, and examinations of countermeasures also are described. As a result of the research, it was confirmed experimentally that several means are effective to improve characteristics of the sensor. Then, design and test results of a sensor model constructed with application of the means are described. From the test results it was proved that the model shows excellent characteristics satisfying basic requirements of the standard for conventional current transformers. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (3): 22–38, 1997

A novel transducer to replace current and voltage transformers in high-voltage measurements

A transducer which combines both current and voltage sensors in a single integrated unit is described. The principle of operation is based on Poynting's theorem which defines the rate of how of electromagnetic energy in terms of the electric and magnetic field intensities at a point in space. Current is measured by sensing the tangential component of the magnetic held, and voltage is measured by sensing the radial component of the electric field in a well-defined region around the high-voltage conductor. Signals from the two sensors are processed by simple electronic circuits to give an output suitable for direct connection to modern analog and digital meters, relays and computers for continuous on-line monitoring and protection. The new transducer overcomes the limitations of conventional current and voltage transformers. It is also ideally suitable for incorporation as a module in gas-insulated systems. Typical applications of the transducer in high-voltage power systems are also discussed.

Stabilization of temperature dependence of verdet constant of bi‐doped garnet and development of high sensitive optical fiber magnetic field sensor

AbstractBi‐doped garnet is a rare metal iron garnet made up of a ferrimagnetic substance, of which some portion of the rare metal element is replaced with bismuth. It illustrates a strong Faraday effect and has been applied to optical isolators. In magnetic field sensing applications, however, a shortcoming is apparent: Faraday rotation of Bi‐doped garnet reveals temperature dependencies when the garnet is not magnetically saturated. Close attention has been paid to the magnetization of each rare earth element's different temperature characteristics; garnet compositions have been investigated by calculations and experiments in order to find a garnet that shows flat temperature characteristics between −10 and 80°C. The liquid phase epitaxy (LPE) method was established to obtain a single crystal of the garnet [Bi13(Y, La)0.9Ho0.8Fe4.5Ga0.5O12]. A magnetic field sensor of fiber optic based on the Faraday effect of this single crystal is highly accurate in detecting conductor currents of power cables. The developed sensor is small and lightweight compared to conventional current transformers; additionally, optical fiber transmission of detected current signals is free from both electric and magnetic noise.

鉛ガラスから製造した光ファイバのファラデー効果を利用した電流センサ

For application to current monitoring in electric power facilities, the authors have been developing a current sensor utilizing the Faraday effect of an optical fiber manufactured from flint glass having a very small stress-optic coefficient. This paper describes the result of study on constructing the sensing system, which is composed of a sensor head, a light source, fibers for light transmission and a signal processing circuit. First, results from investigation are reported with respect to causes affecting performance and durability of the sensor, and examinations of countermeasures also are described. As a result of the research, it was confirmed experimentally that several means are effective to improve characteristics of the sensor. Then, design and test results of a sensor model constructed with application of the means are described. From the test results it was proved that the model shows excellent characteristics satisfying basic requirements of the standard for conventional current transformers. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (3): 22–38, 1997

Three‐phase and zero‐phase current sensors using hall ICs

AbstractThree‐phase current sensors are required in power‐line distribution systems to monitor currents and protect systems from possible failures. Zero‐phase current sensors also are required to detect leakage currents between power lines and the earth, thus preventing accidents such as a fire.This paper describes new three‐phase and zero‐phase current sensors which are based on a zero method. The sensors consist of Hall ICs, coils, and electronic circuits; no magnetic cores are used. The three‐phase current sensor provides measurements of currents from 0.8 A to 720 A with excellent accuracy, while the zero‐phase current sensor provides measurements from 90 mA to 20 A. The output signal levels are suitable for microprocessor‐based metering and control systems. The sensors are relatively small and lightweight compared to a conventional current transformer (CT). Moreover, they withstand excess currents and do not yield burning or deterioration. They do not require expensive components so they have an advantage in cost compared to an optical‐fiber current transformer.