Dual Protection Of Traction Transformer

With the rapid development of high-speed railway, traction transformers of220kV and above have been widely used in China. As the most important equipment in the traction power supply system, the protection reliability of traction transformers directly affects the security and stability of the system. Once the protection of transformer fails, it could cause the railway outage or large economic losses. In this case, it will lead to large damage to the transformer. To avoid the damage and ensure the reliability of the protection, dual protection was developed and has started to be used in the real traction system. However, there is no theoretical analysis about this issue. Therefore, this paper provides the theoretical analysis and the design for dual protection of traction transformers.In the paper, the background of the dual protection of power transformers and traction transformers is first introduced. Then the rest of this thesis is organized into two parts, namely the design for protection loop and the selection of the protection principles. In the first part, the primary and secondary equipment are designed according to the requirements of dual protection, and the equipment include the voltage transformers, current transformers, breakers, protection devices as well as the configuration scheme of DC power systems. Based on these components, the whole protection loop design of dual protection of traction transformers is proposed.The reliability of relay protection is largely determined by the discrimination of the excitation current. The second harmonic restraint is the most widely used principle in inrush restraint, however it is easy to regard the fault current as the inrush current in certain conditions, resulting in wrong protection. Therefore, it is necessary to choose an effective inrush restraint principle in dual protection to compensate for this defect. In this paper, taking V/x connection traction transformer as an example, the advantages and disadvantages of the second harmonic restraint principle, the waveform symmetry principle and the principle of wavelet are analyzed through MATLAB simulation of the traction transformer primary protection. And an appropriate traction transformer dual protection configuration scheme is selected, namely second harmonic restraint principle and waveform symmetry braking principle.The traction substation model is built in RTDS, and the accuracy of the model is verified by simulation. The reliability of the traction transformer dual protection is verified by simulating traction transformer no-load switching, the failure no-load switching and the district failure of normal operation by using the closed-loop test function of RTDS.