Leakage current detection in cryogenic current comparator bridges

Several tests have been developed to locate leakage currents in cryogenic current comparator (CCC) resistance ratio bridges used at NIST to measure ratios of 1000 Ω/100 Ω, 6453.2 Ω/100 Ω, and 10 kΩ/100 Ω. The major advantage of the tests is that they can be performed in situ using the sensitivity of the CCC bridge. These test procedures have been used to reduce the leakage error uncertainty of CCC ratio measurements, linking working standards to the quantized Hall resistance (QHR) and to the NIST calculable capacitor experiment. CCC bridges require that the current which passes through a standard resistor must equal the current through the appropriate CCC winding to very high precision. This can be difficult to verify at or below 1 pA because a large number of possible leakage paths exist. Errors due to six important leakage current paths are given, and the calculated changes in the resistance ratio are compared with measurements made with a controlled leakage resistance in a 100 Ω/1 Ω CCC bridge     

A current transfer method and its application to calibrating acurrent comparator

Current transfer by switching a comparison winding from a series to a parallel connection allows the error of an n:1 ratio comparator to be determined by self-calibration at a 1:1 ratio. The magnetic errors are shown to be the same for either comparison winding connection. The technique is limited to frequencies at which the magnetic error considerably exceeds the admittance error, i.e. typically up to several hundred hertz. The frequency range for which the method can be applied depends on the distribution of admittances to ground as well as the interwinding and shunt admittances, which cause the admittance error for a series section connection to be different from its value for a parallel section connection. The error characteristic also depends on resonances in the inductance circuits appearing in the comparator, on capacitances to ground, and on the interwinding and shunt capacitances     

The ratio error of the overlapped-tube cryogenic current comparator

Analytic expressions for the ratio uncertainty of the overlapped-tube cryogenic current comparator are derived for dimensioning the overlapped-tube comparator. Theoretical predictions agree reasonably well with experimental observations by H. Ramin (Arch. Elektrotech., vol.58, p.53, 1976). The construction of a very sensitive cryogenic current comparator and the possibility of minimizing the ratio uncertainty by reducing the size of the detection coil are also discussed     

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.     

Magnetic-field calculation for a high-temperature superconducting cryogenic current comparator

The problem considered in this paper arises in the design of a high-temperature superconducting cryogenic current comparator (CCC). The CCC consists of two currents flowing in opposite directions inside a toroidal superconducting shield. The shield has a radial cut, necessary for the measurement of the current ratio, but causing an error in the obtained ratio. The problem of interest is the dependence of the error on the geometric parameters of the device: the major and minor radii of the shield, the cut width, the material thickness, and the location of the currents. In the first part of the paper, a toroidal shield with an infinitesimal cut is considered and analytic expressions are derived for the magnetic field and the surface-current distribution. In the second part, a cut of finite width is introduced. Since all the perturbing currents are present in the narrow region around the cut, a shield of cylindrical shape is assumed. Expressions are derived for the flux through the cut and the magnetic field around the cut. Analytical results are in good agreement with the numerical results obtained by a finite-element method. In the final part, the expression for the ratio error is derived, which shows that in order to minimize the error, currents should be concentrated around the shield axis, the major radius of the shield should be maximized and the bore radius minimized. The error depends logarithmically on the cut width.     

Multiple exciton generation in films of electronically coupled PbSe quantum dots

We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. We demonstrate that the MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which has been shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in organic solvents at approximately 4 and 4.5 times the effective band gap. A slightly modified analysis is applied to extract the MEG efficiency and the absorption cross section of each sample at the pump wavelength. We compare the absorption cross sections of our samples to that of bulk PbSe. We find that both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in solution.     

直流电流比较仪磁调制器性能的仿真研究

为优化直流电流比较仪磁调制器的参数和结构,提升其性能,运用仿真软件研究其传输特性。通过分析磁调制器的两种绕组结构,证明激励绕组兼作检测绕组时,输出电压仍然是偶次谐波函数。偶次谐波的大小和相位可反映外加直流电流的大小和方向,因此提出一种以反双曲正弦函数加多项式的B-H曲线新数学模型,用拟合优度参数验证模型的可靠性。在Matlab中建立双铁芯磁调制器的仿真模型,研究激励电压源(正弦波、三角波、方波)、铁芯饱和深度对输出电压的影响,结果表明:采用方波电压作为激励源最优,且所施加的激励电压不应让铁芯工作在过度饱和状态,会造成检测绕组电压波形的畸变。     

A novel electro-optic hybrid current measurement instrument forhigh-voltage power lines

Details of an electro-optic, hybrid current-sensing instrument, and its application to high-voltage power line current measurement, are presented. The current is first detected via a Rogowski coil, followed by conversion of the detected current into a frequency-modulated optical pulse signal in the high-voltage area. Subsequently, the optical pulse signal is transmitted to the ground through an optical fiber cable. Finally, the measured current is recovered from the optical pulse signal by a ground-located signal-processing module. Compared with conventional current transformers, widely used for high-voltage line current measurement, this instrument provides a significant reduction in size, weight, and cost, together with features of convenient installation, secure operation, and high accuracy. Furthermore, it can be configured to be compatible with conventional secondary meters widely used in today's power plants and substations. In 1996, a prototype of this instrument was demonstrated online at a power substation and has shown good accuracy, stability, and safety for more than two years     

The potential transformer bridge with current comparator formeasuring the voltage dependence of compressed-gas capacitors

In this paper, an alternative method for measuring the voltage dependent capacitance variation (VDCV) of compressed-gas capacitors is described. In the proposed method, the VDCV of the capacitor under test is measured by comparing it with a capacitor standard supplied with the secondary (low) voltage of a potential transformer. This transformer has its primary parallel connected with the capacitor under test. The paper presents and criticizes the test results obtained in which an uncertainty of at least 0.5 ppm is achieved     

A bubble string comparator for information processing

A new magnetic bubble device is proposed: the bubble string comparator. It performs the sorting of couples of data without any logical control, depending only on the nature of the couples themselves. By means of an external control the bubble string comparator can also operate as a standard binary switch, either in bypass mode or in cross-over mode.