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Measuring Variable-frequency big current has a great technical difficulty, you first need transfer Variable-frequency big current to small current is easy to measure, and then transfer variable frequency small current to a voltage. In this paper we uses a zero flux current transformer transfer high current into a small current, after then use a shunt transfer alternating current to a variable frequency voltage, realizing high accuracy variable-frequency big current calibration, measurement uncertainty up to 1×10-4,which can meet the needs of the current calibration,it has been used in many project, and achieved good results.

This paper describes a current transformer based ac four-terminal low value resistance measurement bridge. The design and performance of this bridge is presented, which compares resistors from 10 to 0.1 ohm over a frequency range of 100 Hz-10kHz. The reference ratio arm used in this bridge is a special designed high accurate zero-flux current transformer (ZFCT). The current ratio value of the ZFCT is self-calibrated by using the conventional build up method. An auxiliary current transformer combined with variable impedance is used to constitute the wagner circuit.

According to the problem of high cost of standard apparatuses and complex operating for the traditional AC current measurement, this paper introduces a method to measure the AC current with high performance which is based on using digitizing system and the AC-DC difference calculable resistance. Compared verification tests indicate a maximum deviation of 2.3 × 10−5 between the digital sampling measurement system and the traditional method in the typical frequency range of 40Hz to 10kHz, which achieves the equivalent technical specification. Meanwhile, it can provide a more reliable and simpler way for AC current calibration.

The design and calibration of high current with high-stability has been a problem in metrology, a current calibration system with high-stability is introduced, which includes a novel DC current comparator, an original sampling method for high-precision current source, an adaptive control method for air enclosure. The stabilities of major components are analyzed and measured individually. The measurement results show that when the current is 300 A, the stability in one hour can be up to 5.7×10−7, and the uncertainty of measurement is 1.8×10−6.

The calculable resistors can be used to calculate AC-DC difference dependence of frequency. Usually the resistance wires are chosen high resistivity material and as thin as possible to avoid skin effect and so on. However, this kind of wires often induce much temperature drift from DC base value, when current flows through the wires. The performance of the calculable resistors is limited by the loading capability. The loading capability limited the using of calculable resistors in some area, leading to low stability of the AC-DC calculable difference. To solve this problem, a kind of coaxial calculable resistors design was proposed here to improving the calculable resistors and acquires good results in experiments.