Study On Theory And Technics Of Electronic Current Transformer's Head

Being no saturation under fault current conditions and high level of insulation,Electronic Current Transformer (ECT) has been considered as the appropriate substituteof the traditional electromagnetic current transformer. However, the unsolved problemsare in the way to ECT's practicality up to the present time. The major reason for this is theinfluencing factors, such as ambient temperature, electromagnetic interference, thatdecreasing the measurement accuracy and stability of ECT and baffling its application inpower industry. This dissertation focuses researches on electronic current transformer'shead.Being a foundation of electronic current transformer, the principles of ECT currentsensor based on Faraday mageto-optical effect and Rogowski coil theory are investigatedand described. The components, composite construction and present status of differenttypes of ECT's development are summarized, which discovered the unsolved majorproblems existing in electronic current transformer's head.A digital ECT based on Rogowski coils is studied on its elements and construction.The current-sensing theories of the Rogowski coil having a circular cross section and thecoil having a rectangular cross section are systematically investigated by use of theknowledge within electromagnetic field theory. An analysis of the circuits of currentsensor is presented in time domain and frequency domain, which includes aself-integrating circuit, an additional integrating circuit and an integrator with a feed backresistor.The mathematical models of Rogowski coil's mutual inductance are systematicallybuilt, which is classified in terms of coil's cross section in circular shape, rectangularshape and racetrack shape. The calculating difference between the conventionalmathematical model and the presented mathematical model of mutual inductance isdescribed and verified by an experiment on the coil with a circular cross section. Aparticular mathematical model is expressed as the double-layer series for solution ofmagnetic flux density surrounding a current-currying conductor with the cross section inrectangular shape, which provides a method for numerical solution of the mutualinductance of the coil in plate shape for IPEM. The resultant data on program MATLABare verified by a simulation on ANSYS. In order to increase the sensitivity and keep thebandwidth necessary, the performance of the Rogowski coil by integrating several subcoils is discussed in this dissertation.Due to an analysis of thermal expansion effect on Rogowski coil, a novel factordescribing the ratio error of induced voltage per unit rise in temperature is defined. Thefactor is classified further in terms of the coil's component, wounding wire and former.Four equations are derived for calculation of novel factors describing the Rogowski coilswith a circular cross section and a rectangular cross section. A mathematical model is alsoproposed in order to evaluate the measurement error caused by thermal expansion effecton Rogowski coil;its numerical values from theoretical calculation are verified by anexperiment. The other influence factors on the coil are also discussed in this dissertation,such as the temperature variation of RC integrator, imperfect connect butt of winding,electromagnetic parameters related to coil's structure, electromagnetic interference and soon.The problem involved in power supply for ECT transformer's head is discussedfollowing description of three kinds of power, the power based on electromagneticinduction, the power based on laser and fiber and the power based on capacitor divider. Inorder to obtain the stability and an uninterruptible power supply, a new scheme of poweris proposed, which combines the application of Lithium-ion battery. The advantages ofthe Lithium-ion battery are discussed briefly according to its characters.The configurations are studied in this dissertation, which are mainly related tooptimization design of the electronic current transformer's head. An assemblage gapfactor is proposed for description of the coil's structure, technique, installation and design;the parameters describing the coil's configuration and relations between the parametersare systematically discussed. The configurations of the Rogowski coils with thesingle-layer windings and the multi-layer windings are studied. In order to improve theperformance of Rogowski coil, a new mathematical model is developed for optimizationdesign of the coil's configuration. The model is expressed as a computational processfinding the minimum of constrained nonlinear multivariable function. The coil'sreturn-loop for minimizing electromagnetic interference is discussed and the fulloptimization design is shown in a flow chart. All the calculation and simulation areprogrammed on MATLAB.Based on the previous analysis and experiment of the thermal expansion effect onRogowski coil, a new method called thermal expansion self-compensation is proposed tominimize the error caused thermal expansion, which is performed by using an embeddedcompensation ring in coil's former. An analysis of the thermal compensation ring is givenand the compensation effect is examined by an experiment.