Signal Detection And Processing In Fiber Optical Current Transformer

Current transformer (referred to as CT) is widely used as the measuring equipment in the power system. It plays a very important part in the power system analysis, relay protection and current measurement. In recent years, with the rapid development of the electricity network, the level of the voltage is enhancing and the capacity of power network is increasing year by year, as a result, the conventional electromagnetic current transducers is gradually revealing its limitations duing to its working principle. A new type of current transducer-the fiber optic current transducer (FOCT) is becoming the future direction of development of the current transducer, because of a series of advantages such as a small size, light weight, large measurement range, wide frequency response, strong anti-electromagnetic interference.This paper first gives a theoretical analysis of the basic principle of the FOCT system, it’s work process and the overall design and then according to Jones Matrix, builds the output-signal expression of transducer which is used to make a detailed analysis of the characteristics of the output-signal. On this basis, the theory of signal detection and its implementation process are introduced in detail, especially the principle of a close-loop detection scheme based on square-wave modulation and ladder wave feedback. The transducer signal detection circuit is designed with a S3C2440microcontroller as the core.A general experimentation is made to test the sampling apparatus and several system problems are found out, for example, big phase difference, poor temperature characteristic. These problems seriously affect the performance of the transducer. Through analyzing the reasons for this situation, we found that there were too much remaining linear birefrigence in the sensor fiber and the other reason was the bad temperature characteristic of the fiber quarter-wave plate which induced the error of the interference results, and also, the bad detection algorithm increased the system delay. In order to improve the stability of the system temperature, reducing the system’s phase difference, many primary ways are brought forward, such as compensation in circuit, loop helix sensing coil. PID control and there times primary frequency are used to minimize the phase difference. The main contents of the paper are as follows:1. The defects of the traditional electromagnetic current transducer and the advantages of the fiber optic current transducer (FOCT) are introduced as a comparative analysis. It is pointed out that the fiber optic current transducer (FOCT) is the future direction of development of the current transducer because of its enormous potential.2. Magneto-optical effect is introduced in detail. The FOCT current transducer performs an indirect current detection through Magneto-optical effect. The composition of the the fiber optic current transducer, including both the optical route and electric route, is discussed. The math model of optical route is built through Jones Matrix and the output expression of optical route is deduced.3. The work principle and system scheme are analyzed at length. Correlation detection theory, phase modulation, the closed-loop feedback and digital demodulation which are applied in the detection process, especially the principle of a close-loop detection scheme based on square-wave modulation and ladder wave feedback.4. The composition of the electric circuit and analysis of the function of each circuit module. Estimation and simulation was carried out.5. The measurement system of phase difference of FOCT is set up. Sine function is adopted to calculate the phase difference, at the same time; lock-in amplifier is used to test it.Finally, the general experimentation is made to test the sampling apparatus and several system problems are found out, for example, big phase difference, poor temperature characteristic.On the basis of the causes of these problems, a series of methods are used to improve the performance of FOCT, such as the elimination of linear bi-refraction and wave plate error compensation technology.