Simulation Of Mal-Operation Of Transformer Differential Protective Relay System And Study On New Method For Checking Inrush Current

Power transformers are key components in power system, security and stability of transformers are both important and necessary to power system operation. With the development of power system and the application of modern materials and techniques, the power capacity of transformers becomes bigger and bigger, and the requirement to improve the performance of transformer protection relay is getting stronger. At present, differential protection based on the second harmonic restraint principle is often used as the main protection for transformers, but because the second harmonic restraint principle has intrinsic disadvantage, it maybe fail to distinguish the internal fault current from inrush current on certain conditions, which will result in the mal-operation or delay operation of transformer differential relaying system, thus it is necessary to study new methods for checking inrush current.In this thesis, the mechanism of inrush current occurring in transformer is well researched, the advantages and disadvantages of several methods for checking inrush current are discussed, the theory of applying second harmonic current to lock the tripping operation of the circuit breaker during inrush current and its problem are analyzed. Taking the example of mal-protection of transformer's differential protective relay system in traction substation of QinShen passenger transport railway line, the reason is analyzed in the thesis and the system digital model is built with the simulation software SIMULINK based on MAT1AB. The simulation results are compared with our analysis with very good agreements, which both indicate that the differential protection system based on second harmonic restraint principle has certain possibilities of mis-operation. Due to the drawbacks of second harmonic restraint principle, a new method based on equivalent instantaneous inductance to discriminate between internal fault current and inrush current is put forward in the thesis. A numeric simulation model is built with the simulation software SIMULINK based on MATLAB, numerous simulations are carried out on an Y₀/â–³ connected transformer with...