Study On Relay Protection Of UHV Transformer

The electric power resources in China are concentrated in the western while the load center is mainly distributed in the eastern region of China.Large scale power transmission channels are needed.Therefore,with advantage of higher voltage level and larger transmission capacity,the UHV transmission mode has been developed rapidly in recent years.The UHV transmission system will play an increasingly greater role in China's power system.As one of the most important equipments in UHV transmission system,the safe and stable operation of UHV transformer has a pivotal position in the UHV transmission system,as well as the entire power system.Once the operation of UHV transformer relay protection goes wrong,the safe operation of the power system will be seriously threatened.At the same time,the special structure and special working condition of the UHV transformer brings some special problems to its relay protection.So it is necessary to carry out further study on the protection of UHV transformer.In order to improve the performance of protection scheme,this paper carries out research work on,combining with the special structure and working characteristics of UHV transformer,the traditional protection scheme's adaptability to the UHV transformer and the new protection principle.The research and analysis of the structure characteristics and operating characteristics of UHV transformer is a very important basis for the research of the relay protection scheme.According to the actual structure of UHV transformer,in this paper,the characteristics of excitation inrush current and the on load voltage regulation process of UHV variable transformer are analyzed.At the same time,this paper demonstrates the adaptability of traditional transformer protection scheme for UHV transformer protection,and lays a solid foundation for the next research work.Because of its big short-circuit impedance,the protection's sensitivity of the main transformer to turn to turn short circuit is not so ideal.Aiming at solving this problem,considering that the windings of UHV transformer have two branches,the author puts forward that the transverse differential protection could be introduced into the UHV transformer relay protection.Then through the dynamic simulation experiment,two kinds of transverse differential protection's CT configuration scheme and the performance of longitudinal differential protection are verified.The results show that the transverse differential protection has high sensitivity to turn to turn short circuit;besides,this method is affected little by external faults and running condition.The unbalance current of transformer differential protection is changing with the change of its working tap.The differential protection is usually improving the protection setting value to prevent the maloperation caused by the unbalance current.Due to the large voltage regulation range of UHV variable transformer,this method can hardly work.In this paper,an adaptive differential protection scheme is proposed,which can identify the current transformer tap and the process of on-load voltage regulation to improve the differential protection's performance of UHV variable transformer.And its feasibility is verified by simulation.