Current Comparator Bridge Research Articles

A Current-Comparator-Based System for Calibrating Active/Reactive Power and Energy Meters

A system for calibrating active/reactive power and energy meters under sinusoidal conditions using a current comparator bridge is described. Measurement can be made at any power factor from zero lag through unity to zero lead, positive or negative power, at 100 to 120 V, 1 to 5 A, and 50 or 60 Hz. The system features a digital oscillator, a thermal rms ac/dc voltage comparator, an automatically compensated capacitor for producing an accurate and stable reactive component, and a microcomputer for control and data reduction. The systematic uncertainty of the calibration system is estimated to be not more than 15 parts per million (ppm).

A current comparator bridge for power measurement

A current comparator technique for the measurement of sinusoidal ac power at 60 Hz is described. The reference power is that generated by applying the voltage across a 10-kΩ resistor. The voltage itself is measured by comparing it directly to a dc reference with an ac/dc rms voltage comparator. The current is compared with the current comparator to the current in the resistor for the in-phase or active power component, and to the current in a capacitor for the quadrature or reactive power component. Six-digit resolution is provided for both components. The bridge can be operated over a voltage range from 50 to 150 V and a current range from 0.01 to 1 A at any power factor. An auxiliary current transformer provides extension of the current range to 10 A. Comparison measurements with a Drysdale wattmeter used as an ac/dc power transfer standard indicates agreement at unity power factor to within 25 ppm.

Recent advances in Josephson voltage standards

Recent advances of a new generation of Josephson EMF standards are described which are characterized by cryogenic operation of a resistive divider and a sensitive null detector. The advantage of using nonresonant in-line tunnel junctions and the results of resistance alloys at low temperatures are given. An AIMg5 alloy shows a temperature coefficient of resistivity of 1 part in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> /K between 1.5 and 5 K. The magnetoresistance ratio was measured to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> for a magnetic induction of 15 T. Three different methods for the calibration of cryogenic resistive dividers are discussed comprising the series-parallel ratio technique, the superconducting current comparator bridge, and the low frequency inductive voltage comparator. The long-term stability of a resistance ratio of 320:1 by using the AlMg5 alloy has been measured at 4.2 K which showed a small drift of less than 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> /h. Results of the recent intercomparison of Josephson EMF standards on the 1-V level are given. Close agreement was obtained to 6 parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> between the BIPM and the PTB-1 Josephson measuring systems which use conventional room-temperature techniques. The difference between the routine Josephson volt standard PTB-1 and a new developed cryogenic Josephson EMF standard PTB-2 is only 9 parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> . All results are within the range of the combined (σ) uncertainties of the two standards, respectively. The use of a cryogenic thin film divider in conjunction with a small in-line tunnel junction and SQUID null-detector lead to a significantly reduced size of the cryogenic component parts.

Measurement of short circuit load losses in large three phase power transformers using an alternating current comparator bridge

The application of a high-voltage alternating-current-comparator capacitance bridge to the measurement of the short circuit load losses of large three phase power transformers at rated current is described. Input of the transformer current to the bridge is provided through a 1000/1 ratio two-stage current transformer with a single turn primary winding Insulated for 50 kilovolts. Voltage input is provided through a loss-free gas dielectric capacitor which serves as the reference. Measurements on typical transformers ranging in size from 450 MVA to 750 MVA; with currents from 600 to 2100 amperes and short-clrcuit power factors from one percent to four percent, are discussed. Comparisons to appropriately corrected wattmeter measurements (aided by low-loss capacitors connected in parallel with the transformer) indicate an overall agreement of better than one percent.

Measurement of Shunt Reactor Loss at High Voltage with an Alternating Current Comparator Bridge

The application of a high voltage capacitance bridge based on the current comparator to the measurement of the loss angle of high voltage shunt reactors is described. Measurements on three core-less 55 MVA, 735 kV single phase shunt reactors are discussed and a comparison is made with the results of a calorimetric determination of loss which indicates agreement to better than 2 percent. Other bridge techniques for this type of measurement are reviewed.

A Direct Current Comparator Bridge for High Resistance Measurements

A dc comparator bridge, which is suitable for measuring resistors from a fraction of an ohm to several megohms with partper-million accuracy or better is described. An analysis of errors which may occur when measuring high-value resistors and methods used for reducing these errors to negligible proportions are included. The bridge can be connected to make either four-terminal or three-terminal measurements. The four-terminal connection is best for measuring low-value resistors. When it was used for measuring high-value resistors, a systematic bridge error was detected, which was found to be primarily due to leakage paths through the insulation, permitting current to bypass one of the measured resistors. This error is avoided by using the three-terminal connection for measuring high-value resistors. The leakage resistance is then in parallel with a low resistance winding rather than one of the measured resistors. The bridge has automatic reversal and a chart recorder output, which continuously indicates the deviation from the set point. The comparator is adjustable in steps of 10-' of full scale, and measurements can be made at ratios up to 11.111/1.

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.