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.     

A 100000-A high precision on-site measurement calibration devicefor heavy direct current

A 100000-A, high precision device of the magnetic modulation current comparator type, which can be used in industrial heavy direct current systems for online calibration and measurement, is presented. Testing of the comparator indicates that its current ratio accuracy is 5×10^-5, and that measurements of voltage with a standard resistor achieve an accuracy of 5×10^-4. These are slightly degraded to 3×10^-4 for on-site calibration and 1×10^-3 for on-site measurement. The device, which has a toroidal configuration, can be opened for each installation on a busbar, with a variation in accuracy of less than 2×10^-5. Its magnetic shielding renders it insensitive to magnetic fields up to 1×10^-2 Tea. Its accuracy is better by a factor of two than that of similar industrial measuring devices. The principle of operation and the characteristics of the comparator, the double shielded design, and an analysis of its errors are discussed     

The Application of the Compensated Current Comparator to the Calibration of Current Transformers at Ratios Less than Unity

A technique is described for extending the range of the compensated current comparator so that it may be used for calibrating current transformers at ratios both greater and less than 1/1. The concept of providing compensation windings for both the primary and secondary ratio windings is introduced and a new method of correcting for the capacitance error due to burden is presented. Details on how a compensated current comparator previously developed for ratios from 1/1 to 240/1 may be adapted for this purpose are discussed and results of measurements at ratios down to 0.01/1 with this comparator are given.     

Verification (Calibration) of Current Measuring Transformers in an Expanded Frequency Range

The method of verification (calibration) considered for current measuring transformers employs coaxial frequency-independent shunts with nonstandarized uncertainty and a current comparator as a standard measuring device. The method makes for a simpler design of verification (calibration) devices and allows current measuring transformers to be verified over a wider range of measurands and frequencies.__________Translated from Izmeritel’naya Tekhnika, No. 5, pp. 62–64, May, 2005.     

直流大电流比较仪谐波影响问题的研究

本提出了在谐波影响下直流大电流比较仪的等效电路模型及计算方法,通过仿真给出了该模型的频率特性,通过试验数据与仿真结果的比较证实了电路模型的可行性,对研究直流大电流比较仪和电流互感器的谐波影响具有指导意义。     

Power comparator based on-site calibration of isolating current transformers

A portable system for on-site calibrations of isolating current transformers in stationary energy meter test systems is described. The uncertainties of the calibration system are in the order of 0.005 % for the ratio error and 0.01 crad for the phase displacement of the current transformer at power frequencies. The power comparator based measurements can be done at test currents from 50 mA to 120 A.     

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).     

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.     

The Application of the Direct Current Comparator to a Seven-Decade Potentiometer

The design and construction of a self-balancing direct current comparator for use in a seven-decade potentiometer is described. The comparator generates an output current whose value, as a proportion of a constant input current, is determined to a very high accuracy by the ratio of the numbers of turns of two windings on a magnetic core. A linear, adjustable voltage scale is obtained by passing this output current through a resistor whose value does not vary with current. Since the voltage adjustment is made by varying turns on a magnetic core and not by means of a resistive divider, the usual problems of contact resistance and thermal electromotive forces associated with this adjustment in conventional potentiometers are avoided. The main sources of error in the comparator and the design techniques used to keep the errors less than the smallest step of the output current are discussed. A self-checking feature whereby the linearity of each step of the output current can be checked quickly and easily is described. The performance of the prototype model is given. The normal range of the potentiometer is from zero to 2 volts in steps of 0.1?V.     

测量用互感器及其测试仪器的发展和创新

详细分析了国内50年代以来,随着生产发展和市场需要,测量用互感器及其测试仪器在提高准确度和性能方面的发展与创新概况。其中主要有：互感器误差补偿的研究和创新,并据此研制各种规格的测量用互感器、带升流器或升压器的高准确度互感器、电流比较仪、双级互感器、比较仪式互感器校验仪及其整体检定装置和数显校验仪等。并展望今后在这方面的发展和创新,如二次为小电流和小电压的互感器以及计算机在互感器及其测试中的应用等。     

Electronic error reduction system for clamp-on probes and measuringcurrent 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 Direct-Reading Current-Comparator Bridge for Scaling Four-Terminal Impedances at Audio Frequencies

A direct-reading current-comparator bridge circuit, for scaling four-terminal impedances which do not deviate from nominal by more than a few thousand parts per million, is described. The scaling is performed at constant voltage and the impedances are treated as true four-terminal devices in that, at balance, no current is drawn from the potential terminals. The principal feature of the circuit is the use of a compensation winding to excite the magnetic shield of the current comparator and thus suppress the effect of lead unbalances. Possible applications of the bridge include the scaling of four-terminal resistance standards between 1 milliohm and 100 ohms at frequencies ranging from 50 Hz to 1600 Hz. Construction details and calibration of a current comparator for use in a 10-to-1 ratio bridge are given.     

A Self-Balancing Current Comparator

The development and construction of a self-balancing current comparator is presented. In principle, the instrument incorporates in one unit a current transformer and a current comparator. The transfer of power from the source to the burden is achieved with the current transformer. The current comparator detects the error current of the transformer and controls a feedback amplifier which supplies the error current to the burden. At the terminals, the overall instrument is equivalent to a very accurate nonreciprocal current transformer. At power frequencies, its current ratio differs from the turns ratio by less than half a part per million. A number of ratios from 10 to 1 to 1000 to 1 is available. Applications of the instrument are discussed.     

Cryogenic Voltage Comparator System for 2e/h Measurements

The design and operation of a cryogenic voltage comparator system for precision 2e/h measurements is described. Major improvements embodied in the new 2e/h system include the use of 1) a single microstripline-coupled Josephson tunnel junction to obtain usable step voltages up to 10 mV at 10.0 GHz, 2) a cryogenic voltage divider comprised of two resistors whose ratio is calibrated with a low-temperature dc current comparator, 3) a superconducting quantum interference device (SQUID) null detector, and 4) superconducting switching. The accuracy of the present 196:1 divider system is estimated to be about 2 parts in 108 on the basis of preliminary tests and is limited by resistor self-heating during calibration.     

A Wide-Range High-Voltage Capacitance Bridge with One PPM Accuracy

An impedance bridge for high-voltage capacitance and related measurements to an accuracy of 1 ppm is described and the background research and development which made it possible is documented. The bridge is of the transformer-ratioarm type, the principal components of which are a comparator and several two-stage transformers. The bridge can be used to measure capacitance ratios over a range from 1/1 to 107/1 with a resolution between 0.1 and 0.25 ppm. The highest accuracies are obtained at the principal power frequencies of 50-60 Hz, but the bridge is usable up to 400 Hz. The factors which limited the accuracy of previously developed bridges of this type were reexamined and their influence reduced. Two independent methods were developed for the calibration of the bridge.     

An International Comparison of Current-Ratio Standards at Audio Frequencies

The results and analysis of an intercomparison between the National Research Council (NRC), Canada, and the National Bureau of Standards (NBS), Washington, D. C., of two contrasting types of current ratio standards (current comparator and current transformer) are presented. The agreement achieved between the two laboratories when their respective designs and methods of measurement were quite different is emphasized. To this end, the basic theories underlying the design and operation of the current comparator and current transformer are contrasted; the origin and significance of their respective errors are summarized; the (N+1) method used at NBS to measure the errors of the multirange transformer standards up to 10 kHz is contrasted with the (N+1) method (one of several) used at NRC in calibrating the comparator standards. The comparison circuit used at NRC to compare the respective standards of the two laboratories is described. The errors of the transformers as measured at NBS and NRC are presented and contrasted in both tabular and graphical forms.     

一种在片薄膜铂电阻温度传感器的校准方法

在片薄膜铂电阻温度传感器以铂作为感温薄膜,采用半导体工艺制造,可以有效地监测晶圆片上的半导体器件温度。为了校准该类型温度传感器,根据其工作原理和结构特点,参考JJG 229-2010对校准装置的要求,提出了一种利用高低温探针台、八位半数字多用表以及直流探针组建校准装置的方法;通过组建校准装置,测量温度传感器在不同温度下的电阻值,得到电阻-温度特性的分度表;并对在片薄膜铂电阻温度传感器在25℃和125℃2个温度点进行校准。校准数据及校准结果验证表明,该方法切实可行,可有效解决无连接引线的在片铂薄膜电阻温度传感器的校准问题。该校准技术也可为其他类型感温元件的在片温度传感器校准提供参考依据。     

Current Comparator Using Ferromagnetic Cores and Its Application to Analog-to-Digital Converters

A current comparator using ferromagnetic cores with a rectangular hysteresis loop is described. Design criteria for this comparator are derived from the B-H characteristics of the core. To verify the performance of the current comparator, basic experiments and experimental A-D converters with these comparators have been made. By application of the current comparator, it would be possible to realize an A-D converter that could convert a current between a few milliamperes and a few amperes into 9- to 10-bit binary forms within an order of microseconds. The merits of an A-D converter of this type are 1) its very low input impedance, 2) its high impedance between the balanced input terminal pair and the ground, and 3) its ease of conversion of the sum/difference of two currents. Because of these features, the A-D converters can measure currents in circuits having high potential to ground without giving disturbance. The input impedance, measured at 500 kHz, of the experimental A-D converter is shown as a series connection of 0.3-ohm resistance and 0.24-?H inductance, with a stray capacitance of 7.5 pF between input terminals and ground.     

GPS频标比对系统的研制

应用GPS技术研制频标比对系统可对野外环境条件下的通信设备快速地进行时间频率校准.本文详细描述了系统中GPS频率标准源以及比相法、时差法频标比对单元的设计方案和软件设计思路,并给出了设计方案中关键技术解决的途径.该系统经校准完全能够满足要求,并在计量技术机构范围中得到应用.