Research On The Electromagnetic Coupling Mechanism Of Transient Electromagnetic Field To Secondary Cable In Substations

It is of great theoretical significance and practical value to research the electromagnetic coupling of the transient electromagnetic field to secondary cable in substations. In this thesis supported by the Scientific Funds for Outstanding Young Scientists of China (Grant No. 50325723), the wide-frequency measuring method of the parameters of the shielded cable (e.g. per-unit-length parameters, transfer impedance and admittance) and the electromagnetic coupling to secondary cable due to the electromagnetic transients in substations (e.g. short-circuit fault, lightning and switching operation) are emphatically researched. Compared with the calculated results in the published references and the measuring results, the proposed methods are proved to be correct. The main innovative achievements are as follows:1. Based on the transmission line theory, a simple method which is applicable for both the single-core and the multi-core shielded cables has been developed to measure the complex transfer impedance and transfer admittance of the shielded cable in magnitude as well as in phase. The test conditions are rather simple and the test system can be quickly set up and run. The measuring method is verified to be correct by comparing the results with those of the existing method.2. Based on the FDTD, a time domain method is presented for the electromagnetic transient analysis of the frequency dependent multi-conductor transmission lines (e.g. overhead line, underground cable and coaxial cable). Compared with the calculated results in the published references and the experimental results, this method is sufficiently proved to be correct.3. A finite element method is proposed to analyze the grounding performance of the substation grounding grids in frequency domain. The advantage of the proposed method is that the effect of the frequency is considered during the FEM modeling. In comparison with MoM and the measuring results, this method is tested to be correct and effective. Furthermore, combined with the method of nodal analysis in the circuit theory, the method can be conveniently used for modeling the grounding system with multiple exciting sources (e.g. voltage source and current source) and lumped elements (e.g. resistance, inductance and capacitance) connected.4. Based on the FDTD, a time domain model which is applicable for both the homogeneous and multi-layer soil is put forward to research the far-field coupling to the transmission line. As a hybrid method of MoM and transmission line method, a frequency domain model is also put forward to research the near-field coupling to the transmission line and used for the numerical prediction of EMI on the shielded cable due to the switching operation in substations. Compared with the results of existing method and EMTP, both models are demonstrated to be correct.5. Combined with the practical projects, the electromagnetic coupling impacts on secondary cable due to the short-circuit fault, lightning and switching operation in 500kV substations are respectively studied. By the on-site measurements in 500kV substation and the simulated experiments in the test grounding grids, the proposed methods are verified. This thesis establishes the foundation for the further research of the integrated effects from the multiple electromagnetic disturbances on the secondary equipments through different coupling paths.