Fault Analysis And The Protection Design Of The Grounding Transformer

With rapid development of power systems in China, arc suppression coil grounding andresistance grounding are widely used in10kV distribution networks. Delta connection isgenerally used in the low voltage winding of main transformer (10kV) and the neutral point isnot grounded. For easy access of arc suppression coil, neutral point emerges in groundingtransformer. Special z-wiring is applied in grounding transformer, and the station transformercan be replaced by secondary coil. Since the grounding transformer directly connects to thebus as station power supply, its safe and reliable operation is of great importance. However,research on fault theory of grounding transformer and protective devices is limited indomestic and foreign country. Therefore, specialized analysis and in-depth study are made inthe thesis.Firstly, architectural characteristic, electromagnetic principle, the common external faultand internal fault of grounding transformer were studied. A MATLAB simulation platformbased grounding transformer model comprised with three single-phase five-windingtransformers was built to simulate various internal faults and external lead faults. Asymmetricoperation of grounding transformer with secondary load was analyzed. Then the faultsimulation model was used to do corresponding simulation analysis and dynamic experimentverification. Deficiencies in the existing grounding transformer protection technology werepointed out. An improved differential protection scheme used for the grounding transformerwas designed. The longitudinal differential protection with ratio and secondary harmonicrestraint was equipped as the main protection. Zero-sequence current differential protectionacted as backup protection. S function was used to realize differential protection criterion.Fault data generated was input into the MATLAB simulation program for analysis. Reliabilityand sensitivity of this protection scheme were verified.The fault theory of grounding transformer was analyzed in the thesis. The proposedgrounding transformer protection scheme will help ensure its safe and stable operation. It willprovide theoretical basis for further study on the grounding transformer protection equipment.Therefore, the reliability of substation power system and the whole distribution networks willbe improved. The scheme has great application foreground.