Research On Key Technology Of High Voltage DC Optical Current Transformer

The high-voltage DC transmission system has improved the level of renewable energy consumption,and has made great contributions to our country's economy,society,environment coordinated development and sustainable development.At the same time,it has set a higher standard for the electrical equipment in the DC power system.As an important part of DC power system,DC current transformer is the key equipment of the entire DC transmission control protection and monitoring system.Therefore,designing a high-accuracy DC transformer is directly related to the safe and stable operation of the entire DC power system.DC Optical Current Transformer(OCT),due to its advantages of frequency bandwidth,good insulation,accurate measurement and digital interface,can fully meet the requirements of DC power systems for transformers.However,in practical applications,DC OCT also has key problems such as zero drift of the optical path,low frequency noise of the circuit,and temperature drift of the optical path.Therefore,this topic is to carry out research around these key issues,aiming to improve the measurement accuracy of DC OCT.The main research results of this paper are summarized as follows:First of all,the transmission principle of DC OCT is analyzed,and a mathematical model of the final output light intensity with both the magnetorotatory effect and linear birefringence is derived.The non-negligible interference DC component will be superimposed on the DC signal to be measured,reducing the measurement accuracy.Based on this,this paper proposes a new optical path model for bidirectional optical path transmission,which makes the original interference DC component in OCT can be canceled,and the problem of zero drift of optical path is solved based on the mathematical model and physical structure.Secondly,a new signal processing architecture of DC OCT is established,including the signal processing backbone link with photoelectric signal conversion,AD sampling,digital signal modulation and demodulation,digital filter,difference division sum module,and the signal processing closed-loop control link with FFT algorithm,criterion algorithm and balance controller.The backbone link adopts the digital modulation technology of the CORDIC.Through comparison and simulation,an optimal equal-ripple FIR digital filter is proposed to solve the problem of lowfrequency noise in the circuit from simulation results;the closed-loop control link is designed with a balanced control based on incremental digital PID to ensure the realtime balance of dual optical paths in theory,.Thirdly,the influence of ambient temperature change on the accuracy of DC OCT measurement system is analyzed.And the mathematical model of linear birefringence of magneto-optic glass about temperature change is derived.Through comparative analysis,a self-calibration DC OCT measurement system is proposed,which solves the problem of DC OCT optical path temperature drift from the simulation results.Finally,the software and hardware overall design of DC OCT is proposed.In terms of hardware,circuit schematics such as micro-control chip modules,AD conversion modules,front-end signal conditioning modules,various power supplies modules,and communication interfaces are designed;in terms of software,the software development process is introduced.Use Quartus II and Keil to carry on programming development,debugging and algorithm verification to FPGA and STM32 respectively.It provides feasible implementation scheme of hardware and software for DC OCT.