Research Of Key Techniques On Accurate Measurement For Time Constant Of Four-terminal Resistor

The establishment of time constant and phase angle standard of four-terminalresistors is the key technology to achieve the reunification of the ac impedance andac quality metrology, and also has become focus in establishment of ac current andac power standard. In the early twentieth century, several special structurecalculable samples have been designed. The equivalent inductance of thosesamples often involves complicated electromagnetic calculation, and accuratemeasurement of structure dimension will bring large uncertainties for thedetermination of inductance. With the development of the measurement techniqueand instruments, different methods to measure the quadrature part of the shuntshave been proposed at international metrology institutes, and the accuracy of theinstruments and the capacitive leakage errors become the main influence source.The conventional step-up method is widely used to realize the ranges extension forcurrent shunts by many steps comparisons, and will cause uncertaintiesaccumulation. Besides, float state in the series connection and linearity effectsneed to be considered in measurement procedure. Aiming at solving thoseproblems, the essence of the precise measurement named substitution method isflexibly applied to the establishment of phase angle standard of current shunts.According to the existed problem that the equivalent inductance of thefour-terminal resistor is unable to be precisely measured directly with largein-phase part, a structured alternative method has been developed. Based on themethod, a coaxial paralleled design resistor is built with high-resistivity wires andreplaced with low-resistivity wires at the same structure, to reduce the in-phasepart of the resistor. In order to reduce the influence of the measurement instrument,a high Q-value mutual inductor has been designed and analysized deeply its maineffect from structure parameters like parasitic capacitance, residual inductance andwire resistance. By meaning of current-voltage conversion method, the mutualinductance can be measured accurately and used as the impedence standard at highfrequency. Using alternative method, the equivalent inductance of the resistor canbe compared with known mutual inductor, and calibrated against the Ac voltageand current national standards. The time constant of the resistor can be determinedand provide traceability for phase angel errors measurement.With traditional in-series floating connection, the low-terminal potentialinconsistences between two resistors cause capacitive leakage for activemeasurement instruments, especially at high frequency. In order to reduce the leakage errors, a binary inductive current divider (BICD) has been developed togenerate two currents and to compare the two impedances with highly accuracy atcommon ground. The BICD has been studied deeply, and its influences from theexcitation windings errors, capacitive leakage and external loads are consideredand comprehensive evaluated. Basen on the BICD, a three-branch BICD has alsobeen designed. To calibrate the phase angle errors between two currents of theBICD, the method based on difference current transformer has been proposed.In the uncertainty budget of a step-up procedure, the accumulation of manysteps is very often the dominant part, especially at currents up to100A. To reducethis influence, a new range extension setup is built by comparing two differentresistors with only one step based on the voltage proportional technology. A binaryinductive voltage divider (BIVD) with ratio2:1and resistive voltage divider (RVD)with series and paralleled structure have been constructed, used as the phase anglestandard with ratio10:1and100:1. The structure model of the BIVD and RVD hasbeen analysized deeply and the phase angle errors have been also intensivelyconsidered and self-calibratedby. In combination with the BICD and voltageproportional technology, a new setup for range extension with highly measurementaccuracy is developed and meets traceability requirements for measuring timeconstant and the phase angle of the1A to100A four-terminal resistors.Based on the new setup, several lab-made cage like shunts from1to10m and an inductive current-to-voltage converter is measured against time constantstandard. In additional, a metal-foil structure shunt with10m resistance has alsobeen precisely calibrated and the relative uncertainty is estimated to be less than100μrad at200kHz. The phase angle errors of the high current shunts aremeasured at the current level of1A, instead of the normally rated currents of100A, so the level dependence in phase angle errors of the high current shunts is quitecrucial. A method of evaluating the level dependence in phase angle errors of thehigh current shunts at currents from1A to100A has been developed. The timeconstant standard of current shunts is applied to meet the traceability demand ofphase angle errors and also lay the foundation for establishment of high frequencynational power standard in future.