Closed-loop Electronic Current Transformer

When the neutral-point of the transformer is connected with ground, it will often lead to theDC magnetic bias phenomenon of the transformer’s neutral-point. The phenomenon of DCmagnetic bias often lead the transformer’s abnormal working, local overheating and destroy theinsulation of the transformer, it also reduce the transformer’s life. Monitoring the current of ACtransformer’s grounded neutral point, you only need monitoring of AC and DC signal amplitudeand waveform data, then upload data to the online monitoring platform. When the groundedneutral working in abnormal situation, the operating company need to handle the workingtransformers, in order to ensure the normal working of the transformers and provide reliableprotection for the DC converter station.This article introduces a closed-loop current transformer which is used to monitor DCmagnetic bias online. It is an electronic compensated current transformer which is based on amagnetic reluctance chip sensing the current signal. The transformer consist of the measuringcircuit and the signal acquisition and analysis system. The measuring circuit consist of themagnetic collecting unit, the magnetic-electronic conversion unit and the current generating unit.It uses the closed-loop feedback sensing mechanism, and generates the feedback current which isproportional to the measured current. The signal acquisition and analysis system which is used tomeasure and analysis the feedback current, consist of the DSP signal processing unit and theLabview display storage unit. The article mainly to complete the following tasks:(1)The overall analysis of the system: According to the function of each module system, wemodel and analyze the system. Then we calculate the error of system, the response time in orderto provide the the theoretical basis for the design of system.(2)The analysis of the each module of the system: introduce the design requirement and thedesign method of the magnetic collecting unit, the magnetic-electronic conversion unit and thecurrent generating unit.(3)The seletion and design of the hardware system: According to the system’s functionalrequirements, we introduce the design of the magnetic calculate circuit, the current feedbackcurrent, the DSP signal acquisition circuit, the serial circuit and the power circuit. (4)The design of the software system: According to the system’s functional requirements,we program the DSP program and Labview program to achieve the current signal’s processing,display and storage.(5)The debug of system: Testing and analysing the system function of each module, thendebug the whole system.The frequency response range of the closed-loop electronic current transformer is wide,itcan measure both the DC and AC, and can measure current by non-contact. The results of thetesting show that the closed-loop current transformer has high anti-interference ability and goodstability. Its accuracy can reach1%, the angular difference is less than0.2°, measuring range canup to100A. By DSP analyze and handle the data and communicate with PC through Labview,the closed-loop current transformer can monitor the current in real-time.