Current Comparator Bridge Research Articles

Development and implementation of an automated four-terminal-pair Josephson impedance bridge

The four-terminal-pair impedance bridge using pulse-driven Josephson voltage standards at PTB has been fully automated. The same bridge configuration was employed to determine R:R and C:C ratios over the frequency range between 53 Hz to 50 kHz. Only minor changes are needed to cover this large frequency range: amplifiers to increase the sensitivity of the current detections for low frequencies and signal generators with higher resolution at high frequencies to reach 50 kHz. Furthermore, the bridge can be operated for quadrature R:(1/ωC) measurements. The combined standard uncertainties (k = 1) for the new bridge were evaluated for all operating frequencies. They reach 2 nF F−1 and 4 nΩ Ω−1 at 1233.15 Hz. At this frequency, the 10 nF:10 nF ratio matched the ratio of PTB’s bridge employing inductive voltage dividers within 1 nF F−1 ± 3 nF F−1 (k = 1). Over 45 days, the 10 nF:10 nF ratio deviated less than −2 nF F−1 ± 3 nF F−1 (k = 1). The 12.9 kΩ:10 kΩ ratio at 53 Hz differed −2 nΩ Ω−1 ± 5 nΩ Ω−1 (k = 1) from the DC ratio measured by the PTB’s cryogenic current comparator bridge. Using a 12.9 kΩ resistance standard and a graphene AC quantum Hall resistance, the 10 nF:10 nF ratios derived from quadrature measurements agreed with the PTB’s inductive voltage divider bridge better than 9 nF F−1 ± 13 nF F−1 (k = 1).

On the calibration of DC resistance ratio bridges

Current comparator bridges are employed for the realization of the resistance scale from the quantum Hall effect in several National Metrology Institutes and calibration centers. Quantum resistance standards under development, based on novel materials and tabletop dry cryostats, make the more achievable DC current comparator bridges (DCCs) a viable alternative to the more accurate but more expensive cryogenic current comparator bridges (CCCs). A DCC ratios’ calibration against a reference CCC is a straightforward way to improve the DCC’s performances and the resistance scale overall accuracy.The paper reports the calibration results of two DCCs on the ratios employed in a 1Ω to 10kΩ resistance scale traceable to a 12.906kΩ quantized Hall resistance, showing a good reproducibility and stability of the DCC readings over the measurement period and supporting the possibility of a DCC errors’ correction and of a realization of the primary resistance scale at the 10-8 level.

Precise high resistance comparison between the NMIJ traveling dual source bridge and the NIST adapted Wheatstone bridge

A precise high resistance comparison was performed between the traveling dual source bridge developed by the National Metrology Institute of Japan (NMIJ) and the adapted Wheatstone bridge of the National Institute of Standards and Technology (NIST) from 10 MΩ to 100 TΩ at NIST. The NMIJ traveling bridge was shipped to NIST Gaithersburg and was installed right next to the NIST bridge and these bridges alternately measured the resistance ratio of the high resistance standards, without moving the location of the resistors inside the temperature controlled air-bath. Having the bridges and resistance standards in the same location for the comparison decreased the transportation and temperature coefficient effects on the resistance standards, contributing to the excellent agreement of the measured values. The NMIJ traveling bridge used an 8.5-digit digital multimeter and a relay switch box to determine a resistance ratio by measuring the ratio of the voltages applied to the resistors. The comparison was started from 10 MΩ based on the same 1 MΩ standard resistor calibrated using a NIST two-terminal cryogenic current comparator bridge, and standard resistors from 10 MΩ to 100 TΩ were calibrated by repeating 10:1 scaling measurements with both systems. Excellent agreement was obtained within the uncertainty of all resistance ranges and the difference between both systems was less than 1 μΩ Ω−1 up to 1 TΩ and the degrees of equivalence for 10 TΩ and 100 TΩ were less than 6 μΩ Ω−1.

Investigation on the Voltage Dependence of Compressed-Gas-Insulated Capacitors up to 400 kV by Improved Voltage-Doubling Method

This paper presents a recent improvement in the measurement of voltage dependence of compressed gas capacitors up to 400 kV using the voltage doubling method. Two identical 10nF high-voltage capacitors in series are employed to generate voltage-doubling operating points and this proved to be effective in providing consistent harmonic content for voltages. A commercial ground current comparator bridge is used to compare the capacitor under test with a reference capacitor at equal voltage and double voltage, respectively. In order to accurately determine the ratio of the voltage applied across the reference capacitor and the capacitor under test in the voltage doubling state, the other current comparator bridge with floating remote transmission function is developed in this paper to monitor the voltage ratio of the doubling voltage. The two current comparator bridges perform measurement process simultaneously without prior knowledge of the voltage coefficient of the reference capacitor. In this paper, the influence of the working state of the floating current comparator bridge on the measurement is determined by an inductive voltage divider. The experiment results show that the expanded uncertainty of the proposed method for measuring the voltage coefficient of capacitance and loss of 400 kV high-voltage capacitors is estimated to be about 5 ppm.

Precision measurement of single-electron current with quantized Hall array resistance and Josephson voltage

A single electron pump was incorporated with a quantum Hall resistance and a Josephson voltage for the current evaluation in the framework of Ohm’s law. The pump current of about 60 pA level was amplified by a stable amplifier with a gain of 103 to induce a Hall voltage of about 60 mV level across a 1 MΩ Hall resistance array, which was compared with the Josephson voltage. The gain of the current amplifier was calibrated with a cryogenic current comparator bridge. For two different drive frequencies and repeated thermal cycles, the comparisons demonstrated that the pump current averaged over the first plateau was equal to ef within the combined uncertainty level of 0.3 × 10−6 (k = 1).

Graphene Devices for Tabletop and High-Current Quantized Hall Resistance Standards.

We report the performance of a quantum Hall resistance standard based on epitaxial graphene maintained in a 5-T tabletop cryocooler system. This quantum resistance standard requires no liquid helium and can operate continuously, allowing year-round accessibility to quantized Hall resistance measurements. The ν = 2 plateau, with a value of R K/2, also seen as R H, is used to scale to 1 kΩ using a binary cryogenic current comparator (BCCC) bridge and a direct current comparator (DCC) bridge. The uncertainties achieved with the BCCC are such as those obtained in the state-of-the-art measurements using GaAs-based devices. BCCC scaling methods can achieve large resistance ratios of 100 or more, and while room temperature DCC bridges have smaller ratios and lower current sensitivity, they can still provide alternate resistance scaling paths without the need for cryogens and superconducting electronics. Estimates of the relative uncertainties of the possible scaling methods are provided in this report, along with a discussion of the advantages of several scaling paths. The tabletop system limits are addressed as are potential solutions for using graphene standards at higher currents.

A Synchronized Current-Comparator Bridge for the Calibration of Analog Merging Units

A synchronized current-comparator bridge for calibrating analog merging units at power frequencies of 60 and 50 Hz is described. Analog merging units take analog signals from instrument transformers, convert them into digital form, pack the sampled values into Ethernet frames conforming to the IEC 61850-9-2 LE specification, and stream the frames to other equipment in a digital substation for further processing. Since the sampled values are used for protection and metering, the merging units need to be calibrated. The expanded uncertainties ( $k = 2$ ) of the described calibration system are estimated to be better than $15~\mu \text{V}$ /V for voltage magnitude, $15~\mu \text{A}$ /A for current magnitude, and $60~\mu $ rad for phase of either voltage or current.

Direct comparison of fractional and integer quantized Hall resistance

We present precision measurements of the fractional quantized Hall effect, where the quantized resistance in the fractional quantum Hall state at filling factor 1/3 was compared with a quantized resistance , represented by an integer quantum Hall state at filling factor 2. A cryogenic current comparator bridge capable of currents down to the nanoampere range was used to directly compare two resistance values of two GaAs-based devices located in two cryostats. A value of 1–(5.3 ± 6.3) 10−8 (95% confidence level) was obtained for the ratio (). This constitutes the most precise comparison of integer resistance quantization (in terms of h/e2) in single-particle systems and of fractional quantization in fractionally charged quasi-particle systems. While not relevant for practical metrology, such a test of the validity of the underlying physics is of significance in the context of the upcoming revision of the SI.

Voltage Ratio Traceability of 10 kV Low-Voltage Excited Two-Stage Voltage Transformer

To establish a series of ppm-level high-voltage ratio standards up to 110 kV, we propose and have designed low-voltage-excitation two-stage voltage transformers (LVE-TSVTs) for high voltage. Ratio traceability for a 10 kV/100 V LVE-TSVT is vital in this scheme. This paper focuses on the voltage ratio traceability of the 10 kV LVE-TSVT by measuring the errors of the reference voltage and the voltage coefficient separately. A 1 kV double-TSVT with a 1 kV/100 V TSVT and a 100 V/10 V TSVT in cascade was developed and calibrated, and then was used to calibrate the error of the 10-kV LVE-TSVT at 10% rated voltage. The voltage coefficient was measured by comparing with a high-voltage capacitive divider through a current comparator bridge. The measurement was implemented in the range of 10%–120% rated voltage. The calibration results showed that the ratio errors of the 10-kV LVE-TSVT were less than $0.95 \times 10^{-6}$ in magnitude and $-1.26\,\,\mu $ rad in phase displacement at the 10% rated voltage. The expanded uncertainties are about $1.8 \times 10^{-6}$ in ratio and $2.2~\mu $ rad in phase, respectively, with coverage factor $k = 2$ .

A Three-Arm Current Comparator Digitally Assisted Bridge for the Comparison of Arbitrary Four Terminal-Pair Impedances

High accuracy impedance measurements are performed with coaxial transformer bridges, whose conventional design allows the comparison of like impedances—pure resistors or capacitors. Here, we present a current comparator bridge suited for the measurement of impedances of arbitrary magnitudes and phase angles. The bridge has three arms, to connect the impedance under measurement and two calibrated standards. The bridge is digitally assisted and its operation is based on a polyphase digital sine wave synthesizer. To allow the measurement of midimpedance to low-impedance magnitudes, the bridge network and the balancing procedure are designed to approximate the four terminal-pair definition of the three impedance standards. The bridge has been extensively tested with conventional impedance standards and custom designed phase standards. The relative base accuracy is in the 10−5 to 10−6 range at kilohertz frequency.

Stability and Performance of the Binary Compensation Unit for Cryogenic Current Comparator Bridges

As a key module of the redesigned cryogenic current comparator setup, the binary compensation unit is now in everyday use for resistance calibration at Physikalisch-Technische Bundesanstalt (PTB), as well as in a number of national metrology institutes. In a first step, its excellent performance was demonstrated in consistency checks involving the quantum Hall resistance and normal resistors with nominal values $100~\Omega $ and 12.9 $\text{k}\Omega $ . Meanwhile, the stability of this compensation unit could be evaluated, too. Our analysis is based on the results of calibrations at regular intervals of 20 weeks covering more than three years. Also the influence of temperature has been studied experimentally. From the data presented here, we conclude that the device under test can be considered to be stable within 1 part in 104 over ten years.

DC resistance comparison between a current comparator bridge and the quantum Hall system at Inmetro

This paper presents a comparison results between the Quantum Hall System (QHS) under development at the Quantum Electrical Metrology Laboratory (Lameq) and the current comparator calibration system, traceable to the Bureau International des Poids et Mesures (BIPM), at the Electrical Standardization Metrology Laboratory (Lampe), both part of the Electrical Metrology Division, at Inmetro. Comparisons were performed with 1 Ω, 10 Ω, 100 Ω, 1 kΩ and 10 kΩ resistors. The results obtained over two years of work are presented here, showing that the comparison contributed to improve the calibration systems of both Lampe and Lameq.

Self-Compensating Networks for Four-Terminal-Pair Impedance Definition in Current Comparator Bridges

The four terminal-pair definition of standards allows to reach ultimate accuracy in impedance metrology. The four terminal-pair definition requires dedicated circuitry to be included in the bridge network, and the reach of auxiliary bridge balances during the measurement. A careful choice of the network topology allows the bridge to be self-compensating: the four terminal-pair definition is in large part achieved by the behaviour of the network itself, without any adjustment. The additional circuitry required for impedance definition can therefore be simpler, the balancing procedure becomes easier, and a more robust four terminal-pair definition is achieved. This paper analyses a current comparator bridge network with a topology featuring self-compensation. A digitally-assisted test implementation was realized; test measurements on ac resistance comparison in the 25Ω to 100Ω range, at kHz frequency, are reported.

The Applications of Current Comparators in the Measurements on High Voltage Insulation

This paper describes the basic structure of the current comparator used for high voltage insulation measurements. Further applications for the current comparator in high voltage insulation are investigated and developed. A measuring system for the measurement of harmonics in the loss current of water tree aged insulation is described, as well as the principles to measure partial discharges with the current comparator bridge. A new system for the measurement of the DC component in the leakage current of insulation is de1veloped and presented. The results of experiments on XLPE cable insulation are also given.

Behavior of 1 Ω resistors at frequencies below 1 Hz and the problem of assigning a dc value

This paper presents a systematic study of the effects of reversals of current polarity on 1 Ω standard resistors. A selection of 1 Ω standards were measured on a dc cryogenic current comparator bridge and also on an ac current comparator bridge operating at 1 Hz. We find that the apparent 4 terminal resistance depends strongly on the dc reversal rate, in one case showing a variation of more than 0.1 μΩ/Ω for cycle times between 10 s and 1000 s. Even in the best cases, effects of at least 0.01 μΩ/Ω are present. Furthermore, the assumption that the apparent resistance value converges to a limiting value for long waiting times after reversal is not always upheld, making it difficult to quote a simple dc value. Existing models of Peltier heating at wire junctions do not completely explain our observations. The problems presented have been the limiting factor for 1 Ω measurements as part of a recent on-site comparison of quantum Hall effect resistance standards.

A Three-Arm Current Comparator Bridge for Impedance Comparisons Over the Complex Plane

We present here the concept of three-arm current comparator impedance bridge, which allows one to perform comparisons among three unlike impedances. Its purpose is the calibration of impedances having arbitrary phase angles against calibrated nearly pure impedances. An analysis of the bridge optimal setting and proper operation is presented. To test the concept, a two-terminal-pair digitally assisted bridge has been realized; preliminary measurements of an air-core inductor and of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$RC$ </tex-math></inline-formula> network versus decadic resistance and capacitance standards, at kilohertz frequency, have been performed. The bridge measurements are compatible with previous knowledge of the standard values with relative deviations of parts in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-5}$ </tex-math></inline-formula> .

Diagnosis of Water Tree Aging in XLPE Cable by the Loss Current Harmonic Component Under Variable Frequency Power

Water treeing is one of the main aging in XLPE cable insulation, which has the direct effect on the reliability of cable long-term operation. The diagnosis of water tree aging is a premise to ensure the safe operation of cables. The water treed XLPE insulation shows a nonlinear conductivity characteristic, which leads to a harmonic component in the corresponding lose current under the standard sinusoidal voltage. So the loss current harmonic component method can be an effective diagnosis of water tree aging. The loss current measurement system based on the high voltage current comparator bridge is used in this paper, which is designed by Harbin University of Science and Technology and Jiangsu Electric Power Company Research Institute. The feasibility of the application of variable frequency series resonant power source in the loss current measurement, which is aimed to diagnose the water treeing in XLPE insulation, is studied in the laboratory and field test. The results show that the output voltage waveform of the variable frequency series resonant power source is quite close to the standard sinusoidal voltage, although there are some high frequency sharp peaks. So the variable frequency series resonant power source can be used as the high voltage test power supply in the loss current harmonic component measurement system. The loss current harmonic component has power frequency interference during the field test, but this can be solved by differential frequency principle. The microscopic observation of water tree structures during the anatomy of the cable not only indicates the effectiveness of the results and diagnosis during the field test, but also proves the feasibility of the application of variable frequency series resonant power source in the loss current measurement. The existence and degree of the water tree can be estimated according to the test results, which can be a strong basis of the standardization of water tree aging diagnosis. Furthermore, the expensive standard sinusoidal wave generator and linear power generator is replaced by the variable frequency series resonant power source, which reduces the cost and weight of the testing system.

A Digitally Assisted Current Comparator Bridge for Impedance Scaling at Audio Frequencies

A current comparator impedance bridge, suitable for the comparison of four-terminal-pair impedance standards having similar phase angles (e.g., resistors or capacitors) in the audio frequency range at 1 : 1 and 10 : 1 nominal ratios, is here presented. The bridge is digitally assisted: Its accuracy is granted by an electromagnetic device, a high-permeability core current comparator, but the voltages and currents needed to achieve both principal and auxiliary equilibria are generated by programming a polyphase direct-digital-synthesis generator. The resulting implementation is neat and simple and does not include variable components such as decade dividers. The measurement is semiautomated: After an initial setting, the equilibrium can be achieved in a few minutes. Measurements performed on calculable resistors give a base accuracy of a few parts in $10^{7}$ at kilohertz frequency, sufficient for calibration purposes, with the potential for further improvement.

Precision comparison of the quantum Hall effect in graphene and gallium arsenide

The half-integer quantum Hall effect in epitaxial graphene is compared with high precision to the well-known integer effect in a GaAs/AlGaAs heterostructure. We find no difference between the quantized resistance values within the relative standard uncertainty of our measurement of 8.7 × 10−11. The result places new tighter limits on any possible correction terms to the simple relation RK = h/e2, and also demonstrates that epitaxial graphene samples are suitable for application as electrical resistance standards of the highest metrological quality. We discuss the characterization of the graphene sample used in this experiment and present the details of the cryogenic current comparator bridge and associated uncertainty budget.

Metrological management of the high dc resistance scale at INRIM

In the framework of the revision of the high dc resistance scale from 10kΩ to 1TΩ at National Institute of Metrological Research (INRIM) a measurement method for calibration of high value resistors based on the use of a digital multimeter (DMM) and of a dc voltage calibrator (DCVC) was projected, developed and characterized. A method based on the Hamon scaling technique was revised and extended to 1GΩ. Two other methods, based respectively on a modified Wheatstone bridge and on a current comparator bridge, were implemented. A new 100MΩ step Hamon standard, a humidity generator to characterize high value resistors vs. relative humidity and a scanner for high resistances were developed and characterized. The relative 2σ best measurement capabilities, obtained in the range 10kΩ÷1TΩ at INRIM with all these methods, span from 2.0×10−7 of the 10kΩ standard resistor at the measurement voltage of 10V to 1.0×10−3 of the 1TΩ standard resistor at a voltage of 100V. The relative standard uncertainties of the utilized methods at INRIM are summarized.