A Study On Optic-electric Current Transformer Applied To Direct And Harmonic Measurements

High Voltage Direct Current (HVDC) transmission network plays a critical role in our economy. We can measure some important quantities in HVDC transmission system by using new principles and new technologies. An Optic-electric Current Transformer (OCT) applied to direct and harmonic measurement is discussed in detail here. This work is in agreement with the requirement of the "Transmission of Electricity in Western Parts to Eastern Parts" in China.Based on analysis of the measurement methods for direct and harmonic current, the properties for the high voltage current transducer with air core coil and ohmic shunt is examined, then the transmission of signal and energy through optic fiber is proposed. The principle of this OCT and the main factors affecting the measurement are analyzed, and last the detailed measurement technologies are suggested.The main advantages of shunt are simple in the structure and no magnetic interference. The voltage drop across the shunt is digitized; digital signal is transmitted via an optic fiber link to ground potential. Therefore, the secondary circuits can be isolated from the primary part of the transducer. The measuring result can be changed by temperature and stress. However, the result shows that the affects produced by temperature could offset in part that result from the stress. By means of the available method and technology, high stability and accuracy (( 0.2%) of shunt can be reached.Air core coil as a sensor for harmonic current possesses an excellent dynamic performance. Because the output from the coil is proportional to the time derivative of the current, the obtained result is not connected to direct current. The linearity of the sensor is connected to well-distribute of the coil. The integrator designed specially can gain lower frequency and provide high sensitivity. Two methods designed as the "equal- ampere" and the scheme of complex coils that can be applied to the weak signal is proposed.Digital and analog signals are converted to light and transmitted by optic fiber respectively; they have different functions and distinguishing feature. The communication through signal optic fiber is explored for the first time. Test shows that power consumption of high voltage circuit is decreased obviously. Lower power consumption could improve reliability of OCT.The designs in the electromagnetic compatibility and insulation are also studied here. Because OCT belongs to electronic transformer and its secondary output is at low level, it is difficult to calibrate OCT by conventional calibrator. Therefore, new calibration methods are proposed. The calibration tests have been operated. A sample OCT for 120kV 0.5 classes is designed and made. The type tests are done in Xi'an High Voltage Apparatus Research Institute.