Study On The Protection Of YNvd Connected Three-phase To Two-phase Balance Transformer

At present,China is vigorously developing electrified high-speed railway.The power of electrified high-speed railway is provided by the electric traction supply system.The traction transformer is the core equipment that connects the electric power system and the tractive power supply system and plays a role of voltage transformation and electric energy transmission.The electrified railways in China adopts single phase VAC tractive power supply system and the traction load is unstable single-phase load,which will lead to asymmetry of three-phase power systems and cause negative sequence and harmonic wave problems.How to suppress the negative-sequence current has always been a main research topic for the tractive power supply system.At present,one of widely-adopted methods is using three-phase---two phase balanced transformer,including frequently-used Scott balanced transformer and impedance-matching balance transformer.The balanced transformer can greatly reduce the influence of negative-sequence current caused by traction load on the electrical power system under the operation mode of secondary side two phase unsymmetrical load.YNvd is a new type of balanced transformer,which has a neutral point that can be directly connected to the ground,the two side of output circuit without electrical connection and easy to meet the impedance balance conditions,and other characteristics.With a wide development space in domestic China,it is likely to become the leading products for the tractive power supply system.With the development of heavy load and high speed electric railway,the traction transformer capacity is increasing constantly,which requires higher reliability and sensitivity of transformer protection.However,there are few researches on the protection scheme of the balanced transformer.In China,the balanced transformer mostly takes conventional differential current protection as main protection and its sensitivity needs to be improved when internal high impedance grounding and minor turn-to-turn short circuit occur.Besides,it is also easily misoperation which is caused by magnetizing rush current and external fault CT saturation.Therefore,the research on the fault analysis and protection principle of balanced transformer needs to be further studied.In this paper,take the YNvd connected balance transformer as the research object.The relationship of sequence components between three-phase system and two-phase system of YNvd balance transformer is demonstrated,and alpha-beta system voltage equation is derived with positive and negative sequence components for the first time.On this basis,negative sequence differential protection strategy of YNvd transformer is put forward to deal with asymmetry fault and theoretical analysis of this method is performed.Simulating test is conducted with software PSCAD.