National Research Council of Canada

Canadian Patents & Development Ltd.

Canadian Patents and Development Limited/societe Canadienne Des Brevets

Canadian Patents & Dev. Limited

Eddy So

William J Moore

Norbert L Kusters

Petar N Miljanic

Branislav Djokic

David A Bennett

A current comparator technique is applied to four-terminal resistance measurements for obtaining a highly accurate AC resistance bridge at power frequencies of 50 Hz to 60 Hz. Active circuits are used to establish equal voltage drops between the potential terminals of the two resistances being compared. The bridge is suitable for measuring resistances from 10 &mgr;&OHgr; to 100 k&mgr;&OHgr;. A cascading technique using two two-stage current transformers provides extension of the ratio range to 100,00,000 with a maximum applied current of 10,000 A. The bridge features measurement with a resolution of 0.1&times;10 . The total combined uncertainty (2&sgr;) of the bridge including the range extenders, at power frequencies, is estimated to be less than 5 &mgr;&OHgr;/&OHgr;.

Current-comparator-based four-terminal resistance bridge for power frequencies

A current ratio device for use in forming a hand-held openable-core clamp-on type of current transformer. The device employs electronically-aided three-stage circuitry for forming a current transformer capable of accurate low current measurements over a wide range of operation. Such a current transformer is specifically adapted for use with highly accurate active/reactive power and energy meters, and harmonic power analysers. For example a preferred form of clamp-on transformer can be used to measure currents in the range from 1 A to 200 A with ratio errors of less than 50.times.10.sup.-6 for both the in-phase and quadrature components.

Current ratio device for use in forming a current transformer

In a load loss standard for calibrating a power measurement system, a pair of reference voltage signals are generated, one in-phase and the other in quadrature with a test voltage ac source. A transconducting and summing amplifier generates an output current corresponding to the vector sum of these reference signals. By varying the magnitude of each of the reference signals independently of each other it becomes possible to vary the phase of the output current relative to the source, while keeping the magnitude of such current constant.

Load loss standard for calibrating power loss measurement systems

A transconductance amplifier circuit employs the known combination of a transconductance amplifier and a current comparator (with or without a ratio extender, preferably in the form of a two-stage current transformer) to compare the output current from the transconductance amplifier with that flowing through a reference resistor, both these currents having been derived from the same alternating input voltage. A current corresponding to the unbalanced ampere-turns in the current comparator is used to produce a feed-forward error signal that is used to modify the output of the transconductance amplifier. The invention is characterized by an output transformer that has coaxial inner and outer toroidal magnetic cores, and primary and correction windings outside the outer core. This primary winding receives the output current from the transconductance amplifier and the correction winding receives the error signal. A winding located between the cores detects the error current in the output transformer (its magnetizing current), and injects this error current into the current comparator. The secondary of the output transformer can be a single turn conductor extending through the hole formed by its toroidal cores. The result of this circuit arrangement is that the output current in the secondary of the output transformer follows the amplitude and phase of the input voltage with a very high level of accuracy.

Transconductance amplifier circuit

A current ratio device is used for forming a current transformer that can accurately scale down high currents to usable metering levels. The transformer has associated circuitry that renders it capable of functioning not only with ac and a mixture of ac and dc, but also when dc alone is present in the primary winding. The device employs a toroidal core and winding assembly that is cut along a plane containing the toroidal axis through a pair of diametrically opposite sections of the assembly that contain no windings, in order to separate the device into a pair of sub-assemblies that can be readily placed over a single turn primary, e.g. a busbar, without dismantling the same. Once so placed, the sub-assemblies can then be clamped together again to reform the device and the winding portions externally connected to the circuitry, the latter being such as to avoid the accuracy of measurement being significantly effected by the unavoidable increase in core reluctance that will have resulted from the separation of the core assembly into two sub-assemblies.

Current ratio device and toroidal core assembly therefor

A DC current-comparator-based circuit generates an adjustable output proportional to an input signal, i.e. an input voltage or current. One use of the circuit is in the formation of a DC resistance bridge that can be controlled automatically by a microprocessor. The ends of a pair of test resistors (the resistances of which are to be compared) are connected to respective ratio windings of the current comparator. The same potential is applied across these resistors by a master power supply. A microprocessor is alternately supplied with two voltage signals, a first being proportional to the current in a variable one of the ratio windings of the comparator, and the second being proportional to any inequality between the current in the other ratio winding and the test resistor to which it is connected. The microprocessor controls a slave power supply that receives both the first signal and a third signal that is indicative of any unbalance in the bridge. Balance is achieved by adjusting the variable ratio winding. Another use of the basic circuit is in the formation of an improved digital-to-analog converter, in which case the digital input controls the number of turns on the variable winding and the third signal provides the equivalent analog output.

DC Current comparator circuit for generating an adjustable output

A current comparator bridge for measuring the value of an unknown capacitor includes a current comparator having a first comparator winding with an adjustable tap which is connected to one side of a standard capacitor and a second comparator winding connected to one side of an unknown capacitor, a high voltage source being connected to the other side of the capacitors. The current comparator is provided with a detection winding for detecting when the ampere-turns in the first and second comparator windings are equal and opposed to each other. In addition to the current through the standard capacitor being applied to the first comparator winding, the bridge includes a circuit to add a current through a standard conductance to the first comparator winding as well as circuits to compensate for errors. The detection winding is connected to a current-to-voltage converter with an adjustable gain which provides a signal to a phase sensitive detector. A reduced replica of the high voltage source is also applied to the phase sensitive detector which provides output signals E.sub.o and E.sub.90 to indicate when the bridge is balanced for both in phase and quadrature components respectively. The values of the capacitance and dissipation factor for the unknown capacitor can then be determined from the balanced condition of the bridge which is designed so that it can be automatically controlled by a microprocessor and switch controller.

Microprocessor-controlled high-voltage capacitance bridge

A transimpedance circuit for generating an output voltage replica of an alternating current input has its accuracy improved by the use of a current comparator, the ratio windings of the comparator respectively carrying a current proportional to the input current and a current proportional to the output voltage. A further winding of the comparator detects any ampere-turns imbalance in the ratio windings, and, as a result, generates a correction signal proportional to such imbalance. This correction signal is then employed to correct the output voltage.

Transimpedance circuit

A current-comparator technique for obtaining an instrument for accurate high voltage power measurements at very low power factors is described. The quadrature component of the load current is automatically balanced in the current comparator against a quadrature current obtained from a slave supply, thereby reducing the stringent high accuracy requirement of the instrument when used for measuring power at a very low power factors. The current-comparator-based instrument according to the present invention has an estimated accuracy with respect to its readings of better than 3 percent at 0.001 power factor, 1 percent at 0.005 power factor and 0.5 percent at 0.01 power factor and higher. It features an internal high-voltage active-divider and displays to indicate source voltage, current, power factor and reactance of the load, in addition to the power.

Electrical measuring instrument for high voltage power measurements

A current-comparator technique for obtaining a load loss standard with adjustable loss tangent (or power factor) for in-situ calibration of transformer loss measuring system is described. The load loss standard, driven by the test voltage through a low-loss high-voltage compressed-gas-dielectric reference capacitor, simulates an inductive load which can be operated with load currents of up to 1000 A. For a 500 kV reference capacitor and a load current of 1000 A, the load loss standard simulates an equivalent inductive load with a rating of 500 MVA. Adjustment of the loss tangent (or power factor) is achieved by comparing the load current, using the current comparator, with in-phase and quadrature reference currents derived from the test voltage.

Load loss standard for testing and calibrating high voltage power

A converter for converting a series of digital input bits to an analog output employs a direct current comparator having a magnetic core and primary and secondary ratio windings coupled with the core. The primary winding is divided into two sections. A first group of the more significant input bits varies the number of turns of the first primary section which is traversed by a first direct current. A second group of less significant input bits varies the number of turns of the second primary section traversed by a second direct current. A direct current is also passed through the secondary winding, this secondary current being varied proportionally with the number of turns of the first primary section traversed by the first primary current. Modulation-detection windings arranged inside the core (which also acts as a magnetic shield) detect any net ampere-turns unbalance of the primary and secondary currents in the ratio windings. Detection of such an unbalance further varies the secondary current to tend to bring the unbalance to zero. The analog output is proportional to the secondary current. The splitting of the primary winding into two sections enables the converter to handle a larger number of bits, e.g. 20, than would be possible with a prior art type of primary winding, without the number of turns becoming unmanageably large.

Digital-to-analog converter

A circuit is provided for obtaining an accurate replica of an active electrical parameter, i.e. a voltage or a current. The circuit is an improvement over prior circuits in respect of accuracy, both in relation to magnitude and phase. When providing a voltage replica, the principal utility is to provide a scaled-down replica of a high voltage that can be used for accurate measurements of power. In its applicability to current, the principal utiity is to provide an accurate scaled-up current reference source, preferably in quadrature to the input voltage, for use in calibration of reactive volt-ampere or volt-ampere-hour meters. The objective of improved accuracy is achieved by combining known scaling-up and scaling down circuits with a current comparator.

Replica circuit

A method and apparatus for rendering a high-voltage current-comparator capacitance bridge insensitive to frequency fluctuations when used for measuring inductance. A compensating current, proportional to twice the change in the current through the reference capacitor due to frequency fluctuations, is driven into the first comparator winding, through which the current from the reference capacitor passes, resulting in a current in that winding having frequency characteristics similar to those of the current passing through the second winding, which is connected to the inductance. This compensating current is derived from a reduced replica of the applied high-voltage or from the current through the reference capacitor.

Frequency compensator for a current-comparator capacitance bridge

This reference source provides a current which is of relatively high magnitude and which leads the voltage by substantially 90.degree.. It includes a reference capacitor which is highly stable and pure but of low capacitance, and a power capacitor which has a high capacitance. The capacitors are connected together at one end for coupling to a voltage source. A current comparator compares the currents flowing through the reference capacitor and the power capacitor, and provides a difference signal. A circuit generates a current in response to the difference signal and adds the generated current to the current through the power capacitor resulting in a current having a waveform and phase substantially identical to the reference capacitor current which leads the voltage source voltage by substantially 90.degree..

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Gloucester, Canada

Eddy So