A highly accurate, hand-held clamp-on current transformer

The design, operation, and performance of a 1000:5 A highly accurate, hand-held, clamp-on-type current transformer are presented. This precision current transformer uses a sensitive magnetic circuit to detect transformer ampere-turn unbalance between the primary and secondary circuits. The unbalance represents an error in the ratio and phase angle of the transformer. The largest of the errors is due to the core magnetization current and magnetic reluctance caused by the cutting of the core material. After sensing the errors, electronic feedback through a magnetic circuit is used to provide an error-correcting current. This reduces the overall errors dramatically. These types of devices are referred to as “active” current transformers because of the use of electronic amplifier and feedback circuits. The device described has a novel feature of an openable, split core. This “clamp-on” capability enables use of the transformer on a bus or cable without the complications or need to open the current-carrying circuit to be measured. Commonly used “clamp-on”-type current transformers generally have uncertainties from about 1 to 5% at full-scale rated current. This paper describes a commercialization of active current transformers having a ratio uncertainty of less than ±0.05% over a current range from full-scale rated to 1% of full-scale. Additionally, this product has a small phase angle which is an important consideration when measuring electric power, energy, and power factor. It is intended to be used by electric utilities, standards laboratories, testing laboratories, and in applications where high measurement accuracy and the split-core, clamp-on feature are attractive considerations     

Shielded electronic current transformer

A current transformer with nominal ratio 10 A to 10 mA, intended for low-frequency applications, was developed. It includes an electronic device to reduce the magnetizing current, and a continuous shield in the secondary winding (coaxial cable) in order to eliminate the effect of stray capacitances. No guard-source is connected to the shield. It is proposed in this paper to leave the cable-shield-potential floating. This leads to high-accuracy results (ratio errors and phase-displacements in the order of few parts in 10/sup 6/ from 50 Hz to 1 kHz).     

Soft-switching DC-DC converter with parallel-connected full-wave rectifiers

A soft-switching converter with parallel-connected full-wave rectifiers is presented. In the proposed converter, the primary windings of two transformers are connected in series. Two full-wave rectifiers with ripple current cancellation are connected in parallel at the output side to reduce the current stress of the secondary winding of the transformer. The clamp circuit, based on an auxiliary switch and a clamp capacitor, is connected in parallel with the primary side of the transformer to recycle the energy stored in the leakage inductance. The leakage inductance of transformers, the magnetising inductance and the clamp capacitance are resonant to achieve zero-voltage switching (ZVS) of the auxiliary switch. The resonance between the leakage inductance of the transformer and the output capacitance of the switch will achieve ZVS operation for the main switch in the proposed converter. The pulse-width modulation technique is adopted to regulate the output voltage. The operation principle and system analysis of the proposed converter are provided. Some experimental results for a 200 W (5V/40 A) prototype are given to demonstrate the effectiveness of the proposed converter.     

The Compensated Current Comparator; A New Reference Standard for Current-Transformer Calibrations in Industry

The compensated current comparator combines in one device the high ratio accuracy of the three winding ratio transformer or current comparator with the energy-transfer properties of a normal current transformer. Its ratio error is very small and so independent of the energy-transfer function that it can be operated at very high burden. Even negative burdens may be imposed, and, when operated in this mode, the device fulfils the functions of a high-current primary supply transformer. As such, it is highly suited to in situ calibrations in industry. Construction details of a multiratio compensated current comparator, covering all normally encountered ratios from 5/5 to 1200/5, are given. Its ratio accuracy, for burdens up to 750 va, is better than five parts per million. A test set, for use in calibrating current transformers up to errors of one per cent and 100 minutes, is also described. The set employs three conductance and three capacitance decades, with a three-position range switch, and is direct reading.     

Calculating the coupling factor in a multilayer coaxial transformer with air core

High-voltage transformers can be built with coaxial transmission lines. This paper describes a transformer design that uses the coaxial screen as primary winding and the inner conductor as secondary. An advantage of transmission line transformers is that the insulation problem is solved and the construction can be kept simple; the coupling between the primary and secondary coils is high even though the transformer uses an air core. The air core brings another advantage: the capacity to store large quantities of magnetic energy. The combination of a high coupling factor and large energy storage capacity makes this transformer ideal for charging high-voltage capacitors fast. The winding type for the transformer is alternating Archimedean spirals. Here, we present a magnetic field analysis of the transformer's primary, secondary, and mutual inductance using a finite-element solver. We compare the measured magnetic flux density versus the calculated value. The step-up winding ratio of the transformer influences the coupling factor marginally if the construction has an even number of spiral layers for each set of windings. However, the result from the finite-element solver predicts a drop in coupling factor if the step-up transformer construction has an odd number of spiral layers. The copper conductors used in the transformer resemble isotropic copper pipes.     

Analysis and implementation of an active snubber dc/dc converter with series?parallel connection in primary and secondary sides

An active snubber dc/dc converter to achieve zero voltage switching (ZVS) on power switch is presented. In the proposed converter, the primary windings of two transformers are connected in series so that the primary currents of the two transformers are equal. The secondary sides of the isolated zeta converters are connected in the parallel to share the load current and reduce the current stresses on the secondary windings of the two transformers. A boost type of active snubber is connected in parallel with the main switch to recycle the energy stored in transformer leakage and magnetizing inductors and to limit voltage stress of the main switch. During the transition interval between the active switch and the auxiliary switch, the resonance based on the resonant inductor and the output capacitor of the power switch will allow the switch to turn on at ZVS. The principle of operation, steady-state analysis and design consideration of the proposed converter are provided. Finally, experimental results for a 360 W (12 V/30 A) prototype circuit with 150 kHz switching frequency were given to demonstrate the circuit performance and verify the feasibility of the proposed converter.     

Design and operation of a protection system for transformers with superconducting windings

Power transformers with superconducting windings need a protection system to prevent damage to the low-loss superconducting winding by an abnormally high current. The generally accepted protection technique which uses auxiliary coils has been analysed using a network representation. The current distribution between main and auxiliary coil is expressed in terms of geometrical parameters. Experimental data on current transfer and main coil recovery in a test transformer are presented and a method of obtaining a very low auxiliary coil current is suggested.     

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.     

Pulse Shape Improvement in Core-Type High-Voltage Pulse Transformers With Auxiliary Windings

High-voltage pulsed power technologies are rapidly emerging as a key to efficient and flexible use of electrical power for many industrial applications. One of the most important elements in high-voltage pulse-generating circuit technology is the transformer, generally used to further increase the pulse output voltage level. However, its nonideal behavior has significant influence on the output pulse shape. The most attractive winding configuration for high-voltage, the core-type transformer with primary and secondary on different core legs, is seldom used in pulsed applications, because of its weak magnetic coupling between windings, which would result in a slow-rising output voltage pulse. This paper shows that auxiliary windings, suitably positioned and connected, provide a dramatic improvement in the pulse rise time in core-type high-voltage pulse transformers. The paper derives a mathematical model and uses it to describe the observed behavior of the transformer with auxiliary windings. It discusses experimental results, obtained from a high-voltage test transformer associated with a high-voltage pulse generating circuit, and the simulation results obtained from the numerical evaluation of the developed differential equations implemented in Matlab and taking into account the measured transformer parameters.      

The Design and Performance of Multirange Current Transformer Standards for Audio Frequencies

The design, construction, and performance of two nearly identical, multirange current transformers for operation 400 Hz to 10 kHz and with errors of only a few ppm are discussed. A general discussion of the parameters that influence the performance of single-stage transformers and their consideration in effecting minimum errors throughout a wide frequency band is included. Construction of the two standards having consecutive ratios 1 to 1 to 6 to 1 and operating at a rated secondary current of 5 amperes is described. Circuit diagrams are used to describe 1) the self-calibration circuit for measuring the errors of the 1-to-1 ratios and 2) the (N+1) circuit together with an auxiliary circuit for measuring the errors of ratios> 1. Balance equations are derived in the Appendix. Measured values are presented; included are the results up to 16 kHz obtained at the National Research Council, Canada, during an international comparison and described in a companion paper [6]. The effects of polarization at low frequencies are emphasized by utilizing the 400 Hz data together with information obtained in supplementary experiments; also the role of winding and interwinding capacitances and the necessity for a more explicit definition of current ratio at the higher frequencies is highlighted.     

Algorithm to identify the excitation inductance of power transformer with wye-delta connection

An algorithm to identify the excitation inductances of three-phase power transformer with wye-delta connection is proposed. Existing methods of determining the excitation inductances of three-phase transformers require that all winding currents to be known, making them impractical on some wye-delta transformers where the delta winding currents are not measured. Based on the transformer equivalent circuit, the proposed algorithm eliminates the influence of the delta circulating current, allowing the excitation inductances to be calculated using the line currents of the delta connection side directly. The algorithm?s accuracy has been verified by electromagnetic transients program including direct current (EMTDC) simulations, which show that the proposed algorithm is able to differentiate accurately and sensitively between transformer inrush and fault conditions. This lends itself to applications in the area of transformer protection.     

S级测量用电流互感器检定方法的讨论

该文讨论S级测量用电流互感器检定方法及其辅助检定设备的选用。由于大多数测量标;隹是不带S级的,提出一种方法,用二次电流为1A的标准电流互感器来检定带S级二次电流为5A的被测电流互感器,实验证明这种方法可以保证量值溯源的准确可靠。     

Analysis and implementation of a soft switching interleaved forward converter with current doubler rectifier

A soft switching interleaved forward converter with current doubler rectifier is presented. Active clamp circuit is used in the primary winding of transformers to recycle the energy stored in the leakage inductor and the magnetising inductor so that the voltage stresses of switches are reduced. The leakage inductance of transformers, the magnetising inductance and the clamp capacitance are resonant to achieve zero-voltage switching (ZVS) of clamp switches. The resonance between the leakage inductance of transformers and output capacitance of switch will achieve ZVS operation for the main switches in the proposed converter. The interleaved operation can reduce the current ripple on the output capacitor. Two current doubler rectifiers with ripple current cancellation are connected in parallel at the output side to reduce the current stress of the secondary winding of the transformer. All these features make the proposed converter suitable for the DC-DC converter with high output current. The operation principle and system analysis of the proposed converter are provided in detail. Finally, experimental results, taken from a laboratory prototype rated at 125 W, are presented to verify the feasibility of the proposed converter.     

电子式电流互感器校验方法研究

提出一种校验电子式电流互感器模拟量输出和数字量输出的方法,该方法基于采样测量技术,测量标准电流互感器和被校电子式电流互感器的二次输出,比较二者的差异得到被校电子式电流互感器的比值误差和相位误差。详细分析了电子式电流互感器模拟量输出及数字量输出校验方法的测量误差,论证了该方法的合理性和可行性。根据该方法研制的校验装置样机在国家高电压计量站进行了校准测试,测试结果表明,所研制的校验装置满足校验0.2及以下等级计量用电子式电流互感器的准确度要求。     

Turn-tester for windings with a magnetic core

A turn-tester for windings on a magnetic core is described. A temporary winding is wound on the core to form a current transformer. Based on the principle of current comparators for calibrating current transformers, the error of the current transformer thus formed is measured. The number of turns can be correctly determined from the characteristics of this current transformer. With an error of 2×10 ^-5 , the accuracy of this type of turn testing is very high, especially for cores made of iron-nickel alloy. An error on the order of 0.1% to 0.01% is attainable, which means that an error of less than 1 turn is possible when the number of turns is less than 1000 to 10000     

A wide-band active current transformer and shunt

An active current transformer and shunt with high accuracy and wide frequency range is described. The transformer, which uses two three-stage core transformers with two two-stage feedback amplifiers, can be used in the audio frequency band. It has a wide range of current ratio (0.01-1) and can dissipate 10 W in the external burden. If the external burden of the transformer is an AC standard resistor, it becomes an AC current to voltage transducer that can be used in many electrical measurements. The relative uncertainty (3σ) in the current ratio of the new transformer is between (1+j1) p.p.m. to (8+j8) p.p.m. over the frequency range from 40 Hz to 10 kHz     

A Simple Method for the Calibration of Traditional and Electronic Measurement Current and Voltage Transformers

The calibration of measurement transformers represents a classical task in the practice of electrical measurements. Most commercial instruments that are expressly designed for this purpose found their working principle on a scheme that is based on the idea of Kusters and Moore. Although they can assure very high accuracy, the need to employ a high-performance electromagnetic circuit makes them very expensive and usually not suitable for measurements at frequencies that are higher than 50 or 60 Hz. For this reason, these kinds of instruments cannot be employed for the calibration of the new generation of current and voltage transducers, such as electronic measurement transformers, whose employment is growing in all the applications where wide bandwidth is required. In this paper, a new method for the calibration of electromagnetic voltage and current measurement transformers (VTs and CTs) and electronic voltage and current measurement transformers (EVTs and ECTs) is discussed, and a deep metrological characterization is carried out. The novelty of the proposed method is represented by a completely different approach to the measurement of the ratio and phase errors of the measurement transformers. The method is based on the proper digital signal processing of the signals that are collected at the secondaries of the transformer under test and of a reference transformer when the same signal is applied to their primary. Since no auxiliary electromagnetic circuits are required, this solution can be easily implemented in a simple and cost-effective way. In spite of its simplicity, the tests that are developed on a prototype clearly point out that the proposed system is suitable for the calibration of measurement transformers with precision class up to 0.1 in the frequency range from 50 Hz to 1 kHz.     

Transformer design for reducing the inrush current by asymmetrical winding

Abstract

This study proposes a novel approach for designing transformers to reduce the inrush current using asymmetrical winding. Differently from traditional methods, the inrush current was reduced based on increasing the inrush equivalent inductance by changing the proportion of inner layer to outer layer in coil winding. To estimate the distributive ratio of winding, the formulae of the inrush equivalent inductance and the leakage inductance are calculated from the structural parameters of the transformer. By theoretical analyses, an optimum distributive ratio of coil winding is presented. Experimental results confirm that inrush current can be reduced concurrently with appropriate voltage regulation and short‐circuit current in transformer.     

Analysis on the Fusion Technology of a Commercial Transformer and an SFCL

In this study, a new concept is proposed on a combination device with functions of a commercial transformer and a superconducting fault current limiter (SFCL). This device serves as a transformer by stepping the voltage up or down in normal condition. When a transient phenomenon occurs in the power system, it serves as an SFCL to limit the fault current. The device quickly detects the line current using a current transformer (CT), and is based on the high-speed, silicon-controlled rectifier (SCR) interrupter operation. This is done by identifying the fault using an SCR switching control system. The test results showed that the fault current was limited by the impedance of the superconducting element a half cycle after a fault occurred. An SCR that initially had a normally open contact was turned on within a half cycle. However, an SCR with a normally close contact was turned off after a half cycle because the current dropped below the holding current after a half cycle. The voltage of the superconducting element was low in the step-down turn ratio condition of the transformer. This was because the secondary and tertiary windings were connected in series due to the SCR-1 turn-off condition, and the sum of voltages on each winding appeared on the superconducting element. By combining the existing power device technology and an SFCL technology, it is expected that the existing problems of an SFCL can be addressed to construct a smart power system.     

The Application of Current Comparators to the Calibration of Current Transformers at Ratios up to 36000/5 Amperes

The design and construction of a multiratio, 36 000-ampex e compensated current comparator and the technique by which it may be cascaded with a second multiratio comparator to calibrate, with a single balance, power frequency current transformers at ratios up to 7200/1 are described. The first comparator has a distributed, single-turn primary winding and a sectionalized 1200-turn secondary winding that may be series-paralleled to obtain eight ratios from 120/1 to 1200/1. A 300-kVA 1100-volt transformer is included in the single primary turn to provide the power required for the calibrations. In size the device is 40 inches in diameter and 20 inches in height, and its weight is 2700 lb. The two comparators are cascaded at nominal currents up to 30 amperes by connecting the secondary winding of the first comparator to the primary winding of the second and by making corresponding interconnections between the two compensation windings. At balance, the actual current ratio of the standard composed of the two comparators in cascade, or the first comparator alone, differs from the corresponding turns ratio by no more than a part per million (ppm).