Optical Fiber Current Measurement Technique For HVDC Transmission System

In recent years, growing demand has arisen for high-voltage current measurement equipment localization with the vigorous development of high pressure direct current transmission technology in china. Conventional techniques for measuring dc current are mostly based on inductive principle adopting zero-flux or Magnetic modulation method, which has intrinsic high electric insulation requirement, complicated configuration, heavy and bulky. For large DC current superimposed on the harmonic currents, iron core current transformers hardly measure harmonic currents accurately because of CT saturation. Thus, A great demand has arisen for studying novel high-voltage current measurement technique.Based on general comparison and analysis of the measurement methods for direct and harmonic current, active current measuring system and noncontact passive current measuring system are studied as two distinct technique solutions, the former combines prevalent sensing head with optical fiber communication technology and the latter is the developing trend of direct current measurement in high voltage environment. These measuring methods are very promising because optical fibers are good isolators and immune to electromagnetic interference while used to link the high voltage module with low voltage module.DC shunt and Rogowski coil are utilized as sensing heads respectively for direct and harmonic current measurement in active current measuring system. Errors analysis and compensated solutions are presented, a novel Rogowski coil is designed based on PCB, electric energy supply techniques to high voltage side circuit in this special HVDC system are compared and analyzed in detail. Tested by Wuhan High Voltage Institute, the relative error for direct current measurement based on frequency modulation method is less than 0.15% from 300A to 3000A, high voltage side circuit power consumption is 17mW and the rise time of step response is 47mS. Aiming at the shortcoming of slow response speed, high voltage side circuit based on digital modulation method is proposed. Test results show that the relative error of circuit and signal process segment for direct current measurement system is less than 0.2%, the rise time of step response is 0.46mS, overshoot is less than 10% and high voltage side circuit power consumption is 46mW with sampling frequency of 8kHz. Test results show that the relative error of circuit and signal process segment for harmonic current measurement system is less than 2.3% from 100Hz to 2500Hz while input voltage varying from 0.107V to 1.06V, high voltage side circuit power consumption is 63mW with sampling frequency of 12.8kHz.The sensing principle and compensating principle of passive current measuring system are also derived in this paper, and the deficiency of the magnetic path design for AC measurement based on comparative method is analyzed. By taking the magnetic field strength on the axis line of reference arm reaching 8000A/m as design goal, factors affecting permanent magnet stability are deeply studied. Permanent magnet component and its shielding box are designed, and the suitable relative position between reference arm and measurement arm is determined by magnetic field simulation. The performance of passive current measuring system is tested on LabVIEW experimental platform. Test results show that the accuracy for DC measurement is less than 0.5% from 300A to 3000A. Even on the condition of 25.2% light power fluctuation and 34.2% asymmetric degree of the dual channel, the relative error varies from -0.4% to +0.42%. All data on the same test condition validates that passive strip-glass type current sensor with two incident light sources and dual output paths has better performance than traditional bulk-glass type current sensor. In addition, anti-electromagnetic interference ability of permanent magnet component is two order of magnitude larger than that of the original magnetic path design.Finally, the reliability of the two sensors mentioned above are studied. The system block diagram together with mathematical model is established, the parts count method and the parts stress method are adopted to estimate system failure rate so that quantificational reliability index is obtained. Various factors degrading and influencing the system reliability are pointed out. All kinds of safeguards are taken to enhance system inherent reliability.