Research On Frequency Characteristic Of Fiber Optical Current Transformer

FOCT (Fiber Optical Current Transformer) is a key equipment for the realization of digitization and intelligence of smart substation. And it has been widely applied in various voltage levels of smart substations and converter station in our country. Wide working bandwith is the most prominent advantage of FOCT, which is of great significance to the increasingly severe harmonic environment of power grid. But so far, there has been little research on the harmonic measurement performance of FOCT. Devoted to harmonic measurement, this paper studies on the frequency characteristics of FOCT, which is important in engineering.The signal detct link of FOCT is a close loop system, including optical signal, voltage analog signal, digital signal, analog signal and optical signal. There are different kinks of devices, involving multiple parameters, when relate to system optimization, only simuliation can get to solve the problem. So the frequency characteristic of FOCT is studied by combining of simulation and experiment. Based on the principle of FOCT, a digital simulation system of the FOCT prototype is established, and the simulation results are compared with the measured results of the FOCT prototype, which proves the validity of the FOCT digital simulation model. The digital simulation system can be used to study the influence of the multi factors on the frequency characteristic of FOCT, such as the noise, the AD performance, the parameters of the optical fiber loop and the digital filter before merging unit. On the basis of this, the harmonic measurement ability of the mainstream FOCT is tested and analyzed. It is the total gain of the closed loop system and the digital filter before merging unit cause the change of bandwith of FOCT. The total gain of the closed loop system is too large or too small to increase working bandwidth of FOCT. The gain is too small, the measurement bandwidth is too small, while the gain is too large, the closed-loop system will not work properly. Total gain is made up of photodetector module transimpedance, amplifier multiple, bits of AD, bits of DA and the digital integral coefficients. When the hardware device is fixed, bandwidth of system can be adjusted through the adjustment of digital integral coefficient, in a certain precision range. Appropriatly, the bandwidth of FOCT can be gained by improving the digital filter output points. When requiring measurement error of FOCT less than 30%, the effect is obvious by improveing output points before merging unit. When less than 10%, close loop gain must be gain except for guaranteeing a little higher output points before merging unit. The research also finds a phenomenon that there occasionally exists of the waveform distortion measuring high harmonic current, and confirmed in the FOCT prototype. this exception is a major quality problems of FOCT, endangering the safety of power grid. Through digital simulation, found that the reason is found to be that AD sampling rate is not enough, Suggestion can be put forward to improve that of FOCT.In this paper, the influence factors and the law of FOCT measurement bandwidth and waveform are analyzed in detail. And methods to improve the performance of the optimization are put forward, which is a powerful guide to FOCT hardware selection, and an important theoretical basis on to solving the problem of harmonic measurement with FOCT. In addition, the discover, that a large hidden quality danger of FOCT can be eliminated by increasing AD sampling rate, to enhance the power grid security has important significance.