Research On Wide-Band Transfer Characteristics Of Instrument Transformers And Transient Characteristics Of Grounding Grids In Substations

Supported by the National Natural Science Foundation of China (No. 50077006) and the Teaching and Research Award Program for Outstanding Young Teachers in Higher Eduacation Institutions of MoE, P.R.C, the thesis researches the following electromagnetic compatibility problems in substations:1. The pulse method and scattering parameter method for measuring the wide-band transfer characteristics of the instrument transformers are presented. The wide-band transfer characteristics and scattering parameters of the instrument transformers are measured, and the scattering parameters are converted into the open impedance parameters, the wide-band characteristics for the ports of the instrument transformers are acquired.2. The modeling methods for setting up the wide-band equivalent electric circuit models of the instrument transformers are put forward based on the rational approximation formulas of the voltage transfer characteristics, current transfer characteristics, impedance and inductance data, and the wide-band equivalent electric circuit models for the 500kV capacitive voltage transformer and 500kV current transformer are built.3. The paper researches the computation method for calculating the interference voltages and currents on the ports of the secondary equipments, which are caused by the transient voltages and currents on the high voltage bus via the instrument transformers. The interference voltages and currents are calculated with the transient voltages and currents on the high voltage bus as the source of the equivalent electric circuit models for the instrument transformers. The calculated results are consistent with the measured data at home and abroad.4. The paper researches the transient ground potentials caused by the lightning strokes in a 500kV substation grounding grids, and analyzes the effects of the different grounding modes for the grounding grid of the relaycell. The research indicates: (1)the asymmetrical transient ground potentials will be caused by the lightning strokes in the substation grounding grids; the relaycell should have the independent grounding grid connected with the substation grounding grid at its four corners, so that the transient ground potential diffidence in the relaycell will be lowest; (2)The grounding grid made of copper-bar will not reduce largely the ground potential difference in the relaycell than the one made of steel-bar; (3)the more the lightning rod is close to the relaycell, the more the amplitude, rise time and half amplitude time of the grounding potential difference in the relaycell are large and short, so the relaycell should be far away from the lightning rod.5. For calculating the electromagnetic interferences on the cables with the shields grounded at two ends caused by the transient ground potential differences in substations, a calculation method based on the multi-conductors transmission lines is presented, and its validity is verified with the experiment in the paper. The results indicate: when the lightning current of and 30kA is injected into the grounding grid of a 500kV substation, and the interference voltages and the shield current on the low voltage direct current cable with the shield grounded at two ends caused by the transient ground potential difference can't be neglected.