Research On Digital Signal Processing System Of Optical Current Transformer

Optical current transformer(OCT) is one of three kinds of new-style electronic current transformers. It has potential benefits over conventional current transformers (CTs) such as inherent high-voltage isolation, immunity to electromagnetic interference, enhancement in signal bandwidth and dynamic range, ease of integration into future digital control and protection systems and will be widely used in power electrical system.Firstly, the paper describes the work principles of OCT based on Faraday effect of crystal; several generally used types as well as the closed-loop structure of sensor head that has single-light-source and dual-light-path. Then several secondary signal-processing methods of OCT are discussed. An analysis is made why the metering output is fluctuated and the reasons are found out. In allusion to existent problems, soft synchronous sampling and IIR digital filter are adopted; the communication protocol between DSP and VB is modified; the flow of DSP algorithm is re-builded, thus the effective data length is increased. As a result, the measurement stability and accuracy is enhanced. The digital output error sources are indicated and their errors are quantified.Due to the difference between CT and OCT in measurement principle as well as output form, the conventional calibration instrument can't be used to verify the accuracy of OCT. For no appropriate calibration instrument, only the ratio error of OCT can be verified and no way can verify the phase error. In 2002, an automatic data acquisition and scaling system based on DSP was developed and accomplished. This system has no peculiar requirement on power stability and can be used to verify the current error and phase error of OCT synchronously. The measurement data is saved in database according to set command. This system had passed the identification by Hubei institute of Measurement and Testing Technology; its accuracy meets 0.05 class. So this system is suitable to verify 0.2 class OCT in laboratory.Finally, all type tests and EMC tests are done, and the results show that OCT achieves the anticipative design specifications.