Study On Problem And Solutions Of Transformer Protection In Distribution Network

Power transformers are widely used in power system, and they are important connection equipments in the transportation of electric power from power plants to electric power users. The safe operation of power transformers is the necessary condition for the normal operation of the whole power system. According to the capacities and the importance degrees, power transformers are installed with different kinds of professional protections. In distribution network, the backup protections in the10kV side of110kV transformer include timing over-current protection. It works as both the backup protection for the110kV transformer in the10kV side and also the remote backup protection for the coterminous1OkV feeders. But in practical operation, the protection sensitivity of the backup protection is not enough to protect the full length of the feeders. To solve this problem, to analysis the reasons of the problem and put forward countermeasures is full of practical significance and application value.This paper gives an introduction to the transformer protections, including main protection, backup protection and the setting of the related protective values. According to the actual situation in Yantai, factors that influence the sensitivity of the backup protection are analyzed, including capacity of the transformer, system impedance and line parameters.This paper gives fault analysis for distribution network (small current neutral grounding system), and summarizes the characteristics of the voltage and current in different fault types such as single-phase earth fault, two-phase short circuit fault and three-phase short circuit fault. It also gives the electrical sequence map of the system with different fault types using symmetrical component method. The characteristic of negative sequence current is given particularly.Thirdly, this paper puts forward a new backup protection scheme for the110kV transformer in distribution network, combined the original backup protection scheme with negative-sequence current protection theory. This paper also gives simulation verification in PSCAD that proves the feasibility of the new protection scheme. The protection sensitivity is showed to be improved using the new backup protection scheme.Finally, the research contents are summarized, and future research directions are also prospected.