Study On Current Differential Protection Scheme For T Connection Transmission Lines

With the fast increasing of electric load and the density of power network, T connection transmission lines are playing a more and more important role in modern EHV and UHV electric power networks. As the transmission lines are always connected to large power plants and large systems, the protection must be tripped fast when faults occur. Current differential protection criterion is based on Kirchhoff Theory, so compared with the protection scheme used in two-terminal transmission lines, current differential protection can be better adapt to T connection transmission lines and multi-terminal transmission lines. This paper mainly studies on current differential protection scheme for T connection transmission lines, and the main research achievements are as follows:This paper firstly presents whole-current differential protection criterions for T connection transmission lines. Relative researches indicate that as the load current is used in these criterions, the sensitivity of the protection during inner faults is decreased, and the protection will reject to trip when large load flows out during high impedance faults. On this basis, the paper emphatically introduces three common current differential protection criterions based on fault component current. After analyzing the three criterions'working performance, the paper proposes a new current protection criterion for T connection transmission lines by improving the restraint coefficient which seriously influences performance of the criterion. The criterion can adjust constraint coefficient automatically, considering the fault is inner fault or outer fault. A large number of theoretical analysis and simulation results prove that the criterion has good phase separation action ability and the ability of withstanding CT saturation. In addition, the criterion has higher reliability during outer fault and higher sensitivity during inner faults, compared with the common criterions.As the CT near fault can easily get saturated during outer fault, if the saturation is severe, the protection may mal-operate. So, the main criterion of the new criterion will be changed in the situation. Through analysis and simulation on the reason of CT saturation and its influence, the paper proposes a new detection method for CT saturation based on wavelet analysis. Local modulus maxima and its number of wavelet transformation are used in the new method to judge whether the CT is saturated. Simulation results show that the proposed method can detect saturation of CT accurately and improve the reliability of the protection effectively.At the end of this dissertation, a systematical summary is given. Further work to be studied is pointed out.