Theoretical Modeling And Implementation Of Electronic Current Transformer Based On PCB Air-core Coils

Being of excellent insulation and no saturation, electronic current transformer (ECT) meets the needs of the electric power system development towards to large volume, high- level voltage and automation. However, such complicated factors as temperature, vibration and electromagnetic interference in working surroundings and the problem of the power supply on the high voltage side badly affect the measurement accuracy, long-term stability and reliability of active ECT, which baffled its extensive commercial utility in power system. Some major problems of active ECT are discussed from the points of view on theory and practice in this dissertation. The fundamental theory and design methods of high performance current sensor based on printed-circuit board (PCB) air-core coils and the power supply theory are presented. Based on the above research, the prototypes of PCB air-core coils ECT were developed. The TECT-10 prototype had successfully passed through type tests and is operating during the probative period in power system.The measurement accuracy of air-core coils ECT cannot meet the requirement of metering application in power system. To solve this problem, this dissertation stars with the fundamental theory study of the primary current sensor based on air-core coils, which directly affects the performances of active ECT, and investigates the design methods of high-performance air-core coils current sensor. By analyzing on the mutual influence relations among the structure characters, manufacturing technology and performances of applied air-core coils, it is discovered that the chief geometric factor affecting the geometric precision and farther the measurement performances of applied air-core coils is the small cross-section of coils'former. Thus, based on the geometric structure models and operating principles of applied air-core coils, the intrinsic influence relations between the large cross-section of applied air-core coils with its operating principles and measurement performances are investigated by theory modeling, simulations and experiments. The research results not only point out the incorrect opinion - the width of air-core coils is one cause of position errors but unfreeze the cross-section dimensions of coils'former, which can be applied to the design and manufacture of PCB air-core coils as instructive theory. It is for the first time that the precise mathematical models of air-core coils position errors and the mathematical models of the influence of current-carrying conductor effective length on measurement accuracy are built. A kind of PCB air-core coils current sensor is designed based on PCB air-core coils and T-integrator. The transfer function and the transient characteristics mathematical models at 50Hz of PCB air-core coils current sensor are educed. The dynamic simulation experiments show that PCB air-core coils current sensor is competent for the accurate reproduction of the fault current transient process in power system.The distinct structure characteristics of PCB air-core coils including the large rectangle cross-section, the regular discrete distribution of turns and the constrained coupling relation between wires and former are different from that of conventional air-core coils. And thus, the error characteristics of PCB air-core coils are also different from that of conventional air-core coils. Based on the above structure characteristics and the manufacture precision of PCB air-core coils, it is for the first time that the mathematical models including the position errors, the structure errors and the temperature error coefficient of PCB air-core coils are built. The structure factors causing the position errors are analyzed by simulation. The structure errors of two PCB air-core coils with different radii are tested by experiments. The temperature error coefficient model and steady-state theory error model of PCB air-core coils current sensor are also set up. The temperature-coefficient-match method for controlling the temperature error of PCB air-core coils current sensor is put forward. Tests show that the temperature error of PCB air-core coils current sensor is only±0.1 percent when the temperature of working surroundings varies from -40 centigrade to +70 centigrade by employing the above method.The steady-state temperature error and the dynamic characteristic of PCB air-core coils can be considered simultaneously by adopting the dynamic characteristics optimization method based on the optimal zero-pole point configuration and not the conventional optimization method of load resistor. For the first time, the low-frequency characteristic optimization model of T-integrator is set up. The conditionality relation between the upper cut-off frequency limit and the maximum peak instantaneous error of T-integrator is analyzed by simulation and experiment. Based on the analysis on the fundamental theory of PCB air-core coils, a set of more complete optimization principles and methods are put forward and the important mathematical model of the ratio (κ) of the outside diameter to the inside diameter of toroidal coils is set up. The full optimization designs of PCB air-core coils current sensor are shown in a flow chart. The hybrid power supplies scheme based on fast-saturation current transformer, magnetic-valve type saturable current transformer and laser-driving power supply is put forward to solve the problems including high-power supply, primary current dead range and wake-up time, which badly affect the long-term working reliability of the applied ECT products owing to the difficulty in acquiring high-power energy continuously of the power supply on the high voltage side. The study focus is on the on-line acquiring-energy using current transformer. The operating characteristics and the design method of fast-saturation current transformer are investigated. By employing a new method– the change of the iron core permeability self-adapts the change of the primary current to control the change of the iron core magnetic flux, it is for the fist time that a new-style magnetic-valve type saturable current transformer for the on-line acquiring-energy is developed, which greatly extents the upper limit of primary current dead range when applied for on-line acquiring 0.5 watt energy. The analysis on the tests data shows that the lower limit of primary current dead range can reach 1.5 ampere while the upper limit can reach 7000 ampere in practice (20000 ampere in theory) by employing fast-saturation current transformer and magnetic-valve type saturable current transformer simultaneously. The floating switch power supply is also developed.Based on the fundamental theory and design methods research of PCB air-core coils current sensor, two prototypes of PCB air-core coils ECT were developed, which can run respectively into 10KV and 110KV above power system. The type tests and technical evaluation of TECT-10 prototype (10KV) had been performed successfully. The type tests show that the accuracy of TECT-10 prototype reaches 0.2S in metering and 5P in protection. The applied products had already been developed and are operating during the probative period in power system. The major electromagnetic interference (EMI) sources and their characteristics in working surroundings of ECT are discussed. The corresponding measurements minimizing EMI were taken to ensure the electromagnetic compatibility of TECT-10 prototype.According to GB/T 20840.8-2007 standard, the accuracy test principle of ECT is described. The accuracy test system had been set up by employing virtual instrument technique for the accuracy test of ECT.