A Calibration System For Electrical Transducers With Digital Output

According to the standard IEC60044-8 the electric current transducers, the quality inspection and testing center for electric equipment of electric power need to compile a calibration criterion for electric transducers and design the corresponding calibration system to unify the calibration standards and test method. It is favorable for reaching market of the electric transducers and to ensure safety of power system.The paper discusses the development of the calibration system for electric transducers with digital output. The standard conventional transformers are needed for the calibration of the electrical transducers, the analog output of the electromagnetic transformer is converted to digital signals by the reference A/D convert, and then the signals are compared with the test signal. The calibration system is make up of the standard conventional transformers, the reference A/D convert, the tested electric transducers, the PC, the software, etc.Firstly, the digital output and communication of electrical instrument transducers is introduced. The serial unidirectional multidrop point to point communication between the merging unit and the secondary equipments is realized by Ethernet. The R&D of the merging unit offer digital signal for the calibration system.At the part of the development of the calibration system, the time delay introduced by the A/D convention and data processing is discussed, the time delay of the calibration system is analyzed, too. Then the arithmetic of the standard digital signal compared with the test digital signal is discussed.At the last of the paper, the source of uncertainty of the calibration system is analyzed, the expanded uncertainty is calculated. The calibration range is 20%~120%Un for 10kV~500 kV/electric voltage transducers, and 20%~120%In for 5A～5000A electric current transducers. The system is optimized for calibration with 50 Hz sinusoidal current or voltage and the measurement uncertainty is found to be 0,05 % uncertainty for ratio and 1.2 min for phase with a 99 % confidence level.