Study On Key Technology Of High Precision Wide-band AC Current Measurement

AC current national standard is one of the most important standard in the field of acquantities metrology. Nowadays fast development of electronic and electrical technologyneeds more and more calibration requirements for the high current at high frequencies. Thehighest current up to100A at the frequency of100kHz is widely used in the aircraft industry,removable military equipment, high frequency smelting equipment and medical instruments.In recent years, most of the national metrology institutes (NMIs) focus on the research forestablish national wideband ac current standard. AC shunts with small ac-dc difference aredeveloped to extend the ac current range. The step-up procedure is adopted for the currentextension based on the current level of10mA at thermal converter. Most of the calibrationrequirements are at the current level more than1A instead of some parts of mA, and manysteps for the build-up result in the accumulation in the uncertainties, especially at high currentand high frequencies. The heating effect may cause the small variation in the internalconstruction of high current shunts, and could have the possibility for the level dependence inthe ac-dc difference at high frequencies. There is no effective measurement method orverification way to investigate the dependence for NMIs. In addition, series connect betweentwo shunts under comparison introduces the ground floating errors for the reason of highpotential of the input current low terminal of the upper shunt and the leakage capacitance tothe ground. This is another importance uncertainty part compared with the level dependence.Considering the disadvantages in the traditional step-up method, this paper presents anew method for the traceability of AC current. The calculable shunt at1A ac current isdeveloped to be the reference instead of the thermal converter at10mA. We report on themeasurement techniques and results of the equivalent inductance of the calculable currentshunts at frequencies up to100kHz. With the known time constant of the special designedshunt, the ac-dc difference is then be calculated to be less than10-7at the wideband frequencyranges up to100kHz. The method on the comparison of two shunts by means of BinaryInductive Current Divider （BICD） is also descried. This method can make the comparisonwith the common ground and the shunts can be looked as in series connection with the knowncurrent. The coaxial high frequency Current Transformer (CT) is designed and used for thehigh current level extension refers to the1A calculable shunt. The current ratio of the highfrequency CT is100to1and10to1respectively. Compared with high curt shunts, the highfrequency CT may generate quite a little heat at high current and reduce the warm-up time dramatically. A new method for the measurement of level dependence of ac shunt is proposedin the paper, and the level dependence of ac shunt is first measured in the world. The resultsare quite small and negligible, and it also shows no remarkable frequency dependence up to100kHz. The absolute measurement results of level dependence of ac-dc difference for the acshunts solves the evaluation of the most dominate uncertainty part for the national ac current,and give the meaningful contribution for the international ac-dc society. The self-calibrationmethod for Binary Inductive Voltage Divider (BIVD) at100mV level is also discussed in thispaper. Three-stage BIVDs construct the8:1divider with known ratio error at frequency up to100kHz. The high precision ac voltmeters can be calibrated against the8:1divider at100mVwith very low uncertainty, then the ac current can be stepped up and down at10%level of thenominal current. Taking1A shunt as the reference, the speed for the step-up and step-downprocedure is improved several times during the extension of ac current from10mA to100Aat the frequency up to100kHz. The paper makes several breakthrough and innovative as thefollowing aspects.1. The design on the ac-dc difference calculable shunt at1A is proposed. The ac-dcdifference is taken as the basic reference root of the national ac current standard. The originalreference level is the10mA nominal current of thermal converter, and it is difficult toreproduce the calculation procedure because of the detailed unknown dimension size of theheater resistor and the thermal couples. The calculable shunt is designed as a coaxial structurewith the certain length of paralleled resistive wires. The calculation model is established tosimulate and estimate the value of the equivalent inductance and distributed capacitance.Anyway the final determination of the equivalent inductance is realized by substitutionmethod and calibrated against a known mutual inductance in nH level. Then with the knowntime constant of the resistive wire shunt, the ac-dc difference are calculated to be less than10-7at the frequency range up to100kHz.2. A coaxial design of high frequency current transformer is proposed. Compared withhigh current shunts, the power consumed by the current transformer is several ten times lowerthan that of the shunts, and it is more suitable for the current extension to the high currentrange in the current standard step-up. The100A or10A input currents of the current aredirectly transferred to1A, and if a coaxial1A shunt is taken as the load of the secondarywindings, the output voltage at1V of the shunt is proportional with the input high currentwhich becomes easy to be measured. The total power in the transition of the high current islower than1W, so in this method the warm-up time is shorted and the output of the currenttransformer becomes very stable during the longtime measurement interval. It is possible todo the fast measurement during the step-up procedure.3. A comparison method between two ac shunts with the common ground by means of BICD is proposed. The influence from the leakage capacitance at high frequency in thetraditional series connection method is eventually eliminated. The ground floating capacitanceleakage is unavoidable in the traditional setup. Even with the position exchange, the influencecan just be a certain degree depressed. The use of BICD can be looked as a device to supplytwo known current to the common grounded point, so the ground floating status is not crucialanymore.4. A method for the absolute measurement of the level dependence of ac shunts isproposed which is the first published method in the world. The level dependence is the mostdominate contribution in the uncertainty budget of ac current standard, and the measurementand evaluation method is lacked worldwide. The characteristic of BICD and high frequencyCT provides the possibility to measure it absolutely. The determination of the current leveldependence is a meaningful contribution for the uncertainty evaluation in ac-dc currenttransfer. The measurement results finally prove to be quite small and frequency independentwhich is agree with the assumption from some scientist of NMI.5. The extension method of ac current at10%nominal current level is proposed. Thismethod based on the development of BIVD and precision calibration of ac voltmeter. Thestep-up and step-down procedure becomes very quick with the aid of this kind of extension.The accumulation of uncertainty in the multi-step comparison is minimized greatly.Compared with the needs of12steps in the traditional step-up method, the new extensionmethod requires only4steps, and it also covers the current range from10mA to100A at thefrequency up to100kHz. The few steps results in the fast measurement speed and betteruncertainty specification of the new national current standard.The new national ac current standard based on the proposal methods in this paper has theimproved specifications in the uncertainty budget. The extension procedure is speeded updistinctly as well. The new national standard satisfied the calibration requirements in the accurrent in wideband frequency, and it has served for the development of domestic aircraft andaviation industry, the smelting industry and the manufacturer of high precisioninstrumentation companies. The ac current standard ensures the reliable traceability for thenational ac quantities standard, and paves the way for the future key international comparisonand establishment of national high frequency power standard.