DIGITAL OPTICS FIBRE CURRENT TRANSFORMER WITH C. T POWER SUPPLY
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
A digital optics fiber current transformer (CT) based on Rogowski coils and fiber transmission with excellent insulation to Electro-magnetic to replace a conventional open-terminal ferrous core one for use in high voltage systems is described in this paper. The Rogowski coil generates a low-level voltage signal in proportional to the differential of the primary current. A passive integrator is constructed from precision components to get the corresponding voltage signal of bus current. The technology of signal sampling, processing and communication for the current transformer is introduced. The large panel LCD screen displays the waveform, RMS value, frequency, periods and so on. At the same time the present supply power techniques at high voltage side are analyzed, and puts forward to a new closed-loop regulating method through tailor-made small CT to obtain stable supply power. The experiment results verified that the designed power supply can operate normally as the current of bus is above 8A. The accuracy of current measurement is 0.2% and the whole system can satisfy the demand of current measurement in power system.
With the rapid development of power system and the increase of voltage level of transmission and distribution, the traditional electromagnetic current transformer exposed a series of faults, such as the complex insulating construction, high manufacturing cost, combustible and explosive character and so on. Furthermore, not only the ratio error and phase error caused by the windings, but also nonlinear factors of the ferrous core, such as eddy current and hysteresis loss, contributed big errors that is hard anticipated to the current measurement. So it is no fit to high precision measurement and informative need of high voltage grid again. New type optics fiber current transformer is characteristic of good capability of counteraction of electromagnetic interface (EMI), wideband range, small size, light weight, low production cost, convenient setups, transmit the digital signal, conformability the tendency of modern power system. The paper proposes a novel optics fiber current transformer, which detected the current of grid bus by Rogowski coil, then integrated the output of Rogowski coil, fulfilled analog to digital conversion and followed electro-optic transformation of the detected current on high voltage side. Subsequently, the optical pulse signal is transmitted to the microprocessor on low high side through optical fiber cable. The corresponding digital signal of measured bus current is obtained by optic electric conversion under the DSP control. The large screen LCD displays the waveform, RMS value, frequency, period of current.
The line in Sub-Station operates at high voltage and carries current of thousands of amperes. The measuring instrument and protective devices are designed for low voltage (generally 110V) and current (about 5A). Therefore, they will not work satisfactory if mounted directly on the power lines. This difficulty is overcome by installing Instrument transformer, on the power lines. Instrument transformers are used for measuring voltage and current in electrical power systems, and for power system protection and control. Where a voltage or current is too large to be conveniently used by an instrument, it can be scaled down to a standardized, low value. Instrument transformers isolate measurement, protection and control circuitry from the high currents or voltages present on the circuits being measured or controlled. In electrical engineering, a current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are...