Study On Some Key Issues At The Protection Of Power Transformer

Electric power transformers have traditionally used differential protection as their main electromechanical and solid-state relays for protection. However, the characteristics of power transformers make this kind of protection based on Kirchhoff's current law less effective and reliability. So many algorithms were researched and proposed to solve the problem in the past. Those algorithms are reviewed, and their disadvantages are proposed.This thesis provides a new approach for protecting power transformers. The technique uses positive- and negative-sequence impedance and variation of magnetizing impedance, where phase voltages are used. The technique can distinguish external fault, internal fault and switching on slight fault, without information concerning parameters of the transformer and power system is required. The performance of the proposed technique was simulated on the electromagnetic transient program, EMTDC. The results of simulating show that the technique is effective and robust.Detection of the real-time fundamental negative- and positive-sequence components is the key of this technique. This thesis proposed a real-time detection method for fundamental negative- and positive-sequence based on space vector. Simulating shows, that the application of this new method can detect fundamental components effectively and quickly, and can eliminate the influence of frequency shift. Current transformer (CT) saturation is a important problem in all protection relay. There type of CT digital model used in the study of CT saturation characteristics is reviewed, and detail comparisons show that time-domain nonlinear equivalent circuit model is suitable for the purpose of the study in this thesis, which ensures the validity of the study. Wavelets are applied in the study of CT saturation. Simulating results show that dblO wavelet can examine the moment of CT being saturated accurately. What's more, a simple and effective method to distinguish between changes of the power system and CT saturation is proposed.