| In recent years, the optical detecting techniques become more and more sophisticated, and people have done much research of the optical fiber current sensor. With the rapid development of the power industry, the traditional electromagnetic transformers can not meet the needs of the power system because of its structural features. By contract, the optical fiber current sensor with its good insulation, strong anti-electromagnetic interference ability, wide measurement range and other advantages gradually replaces the former in the power system, and is widely used. However, the domestic study of the optical fiber current sensor almost concentrates in the structure and principle of its optical path, as well as the design of the signal processing circuit, but rarely in developing the current monitoring software. As the advantages and development trends of the optical fiber current sensor, it is very urgent to develop the current monitoring system. This subject sets up a primary complete fiber optical current sensor system, and then designs a current monitoring software.This paper first analyses the needs of the whole system, and then provides the overall structure, and introduces the sensor scheme. Based on the analysis of optical fiber current sensor structure and sensing principle, a reflection-type of interferometric fiber optical current sensing scheme is established, combined with the experimental conditions. When the light through this type of sensor, it produces twice Faraday effect, and the phase difference is equal to four times of the Faraday phase shift. It makes the sensitivity of this type of sensor is four times that of other sensors under the same conditions, and twice that of the phase modulation optical fiber current sensor. Utilizing the Jones matrix to analyze the polarization state of the light, it is found that the Faraday rotation angle can be measured by the first harmonic amplitude of the output signal, and then the tested current value can be obtained. When extracting the first harmonic amplitude of the output signal, a method named signal correlation is used to remove the first harmonic to the DC component. And then the DC component is extracted through a low-pass filter. Thus the Faraday rotation angle and the tested current value can be calculated. Above is the model of measuring current through the optical fiber sensor. After the signal simulation, this subject builds a basic system including the optical path and the circuit, and makes it communicate with the host computer through the serial port.According to the demand of the system, an overall framework of the software is presented, so is the overall flow chart. Since the system use serial port to communicate, the problem of serial port occupation should be settled. It is solved by creating a Mutex to ensure that only one program is running in the same time. The main loop of the program is realized by means of a timer. Every one second the software does once data acquisition and conversion, matching the serial communication protocol. There are data collection, data conversion, data filtering, data storage, and data display functional modules in the main loop. The data filtering module is one of the most important modules among them, because it can effect the accurate of the tested current value. The limiting average algorithm, median average algorithm, and the first order lag filter algorithm are provided, which can basically meet the requirements of national standard of the accuracy of current sensors by class0.5. the software also provides the cross-border alarm, system setting, and user help module. The latter part of the software interface is added some displaying information to complete the whole system.By designing and debugging, the subject finally complete the current monitoring software based on reflection interferometer fiber optical current sensing scheme. In the experimental stage, the system is running well and can basically meet the current monitoring requirements. |
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