Research On Multi-Purpose Electronic Current Transformers

Compared with the traditional electromagnetic current transformer,electronic current transformer has some advantages of wide range of frequencyresponse, good insulation performance and strong anti-jamming capability so that itgradually becomes the hotspot in the research of current transformer. Electroniccurrent transformer is divided into two categories, measuring and protection. Theygenerally can’t use a current transformer to realize the two functions at the same timebecause their scope of work is not the same. But the measuring transformer andprotection of transformer can be assembled together, then we can realizemeasurement and protection functions respectively by different sensors.By analyzing the structure characteristics and the error characteristic of varioustypes of sensors applied in electronic current transformer, it uses low power iron corecoil and air core coil to realize measurement and protection functions respectively.Low power iron core coil has high accuracy of measurement in steady-state, butthere is iron core saturation problem. Air core coil has not occurrence of magneticsaturation by reason of no magnetic core，but its output voltage signal is very weakso it is diiffcult to measure accurately. And integrator needs to be designed to restorecurrent because the output voltage of air core coil and the measured current havedifferential relationship.The paper designed a mulit-purpose electronic current transformer based oncombined structure of low power iron core coil and air core coil. In order to improvethe overall performance of the current transformer, magnetic shunt compensationwas used to carry on error compensation for low power iron core coil，and wedesigned analog integrator and digital integrator to restore the measured current ofair core coil. At the same time, a hardware system of multi-purpose electronic currenttransformer based on STM32F103C8T6was designed. The system consists of signalconditioning, data acquisition and software integration of three parts. Firstly, twokinds of output signals of the current transformer need to be ifltered and ampliifedfor signal condition respectively. Then we used the analog-to-digital converterAD7656-1to complete synchronous signal sampling from two channels under the control of STM32F103C8T6and the sampling signal of air core coil needs toconduct software integration. The experimental data show that the designedmulit-purpose electronic current transformer reaches the accuracy requirements inIEC60044-8standard about measurement and protection, and the experimentalresults verify the validity of the designed hardware system.