Research On Optical Current Transformer Based On Comparative Measurement

Optical current transformer (OCT) is regarded as a perfect substitute for traditional current transformer because of its inherent advantages such as high electrical insulation, broad frequency band, large dynamic range, small volume, light weight and so on. However, OCT is not popularized in electrical power system yet after several decades'development. The main reason is that OCT is sensitive to external environment factors such as temperature, vibration et al. A gradual sensitivity drift happens in its long-term outdoor running.A novel OCT based on comparative measurement is proposed to compensate the sensitivity drift of ordinary OCT. The OCT based on comparative measurement introduces a permanent magnetic field as the reference, an eventual result can be obtained which is irrelevant to environment factors but only proportional to the magnetic field intensity caused by the primary current. The comparative OCT compensates both the linear birefringence that exists in the sensor head and variety of Verdet constant of sensing material. In a word, the comparative OCT has more advantages than other OCT compensation schemes.The sensing principle of comparative OCT is expounded firstly. And based on the principle, the structure of comparative OCT's sensor head is designed. The light path design and the magnetic path design are the two most important hands. The eventual light path is structured by two independent, perpendicular strips of glass. And the magnetic path design make that the measured current magnetic field is vertical to the reference magnetic field, also the average magnetic intensity is as much as possible. At the same time, a pair of tongs is designed to fix the relative position between the primary conductor and the sensor head, thus the measuring result of the OCT made up of strips of glass is repeatable.Then, two key problems in signal demodulating are pointed out. One is the right demodulating of reference DC magnetic field. The other is the accurate detecting of weak modulated signal. A dual-input and dual-output demodulating scheme is applied to solve the former problem. This demodulating scheme makes up for the shortages of former schemes. It can demodulate both AC magnetic field and DC magnetic field accurately, and can eliminate the error caused by the unbalance of light path or circuit and the fluctuation of light intensity. In order to detect the weak modulated signal accurately, former analog demodulating system is replaced by an all-digital demodulating system, which is in fact a virtual instrument based on a high accurate data acquisition card and LabVIEW graphic language. The all-digital demodulating system benefits the accuracy and stability of the comparative OCT greatly.The comparative OCT is the integration of a DC system and an AC system. Both of them include four main parts: sensing, O/E transversion, sampling and demodulating. Every possible error source in these parts is respectively analyzed in each system. The conclusion that, the departure from 45°of included angle between the polarizer and the analyzer is one of the main system errors of the comparative OCT, is obtained. Some measures are taken for decreasing the error.At last, a series of tests including design tests and comparing tests are carried out. The design tests tend to provide helps for the sensor head structure design. The comparing tests including sensitivity, error and temperature comparing verify the superiority of the dual-input and dual-output demodulating scheme and the effectiveness of the compensation method based on comparative measurement. The performance relative to environment factors of the comparative OCT is much better than the ordinary one.