Measures to Improve the Short Circuit Resistance of Power Transformers

Power transformer is the hub of power transmission and distribution, and the core component of power network. Its reliable operation is not only related to the power quality of users, but also to the security of the whole system. The reliability of power transformer is determined by its health condition. It depends not only on the design, manufacture and structural materials, but also on the maintenance. The improvement of short circuit resistance of transformer in power system is discussed.

Measures to Improve the Short Circuit Resistance of Power Transformers

(1) Standardize design and attach importance to the axial compression process of coil manufacturing. In the design of the transformer, the manufacturer should consider not only reducing the loss of the transformer and improving the insulation level, but also improving the mechanical strength and short-circuit fault resistance of the transformer. In terms of manufacturing technology, because many transformers adopt insulating pressure plate, and the high and low voltage coils share a pressure plate, this structure requires a high level of manufacturing technology, gaskets should be densified, after the coil is processed, a single coil should be dried under constant pressure, and the height of the coil after compression should be measured. The coils of a pressure plate are treated by the above process, then adjusted to the same height, and the oil pressure device is used to exert the prescribed pressure on the coils in the final assembly, so as to reach the height required by the design and process. In general assembly, besides paying attention to the compression of high-voltage coils, special attention should be paid to the control of the compression of low-voltage coils.
(2) Short-circuit tests of transformers are carried out in order to prevent accidents in the future. The operation reliability of large transformer depends first on its structure and manufacturing technology level, and secondly on the various tests of equipment during operation, so as to grasp the working conditions of equipment in time. To understand the mechanical stability of transformers, we can improve the weak links by withstanding short-circuit tests to ensure that the structural strength of transformers is designed with certainty.

(3) Use reliable relay protection and automatic reclosure system. Short-circuit accident in the system is an accident that people try hard to avoid but can not absolutely avoid. Especially, the possibility of short-circuit accident caused by wrong operation of 10KV line, small animals entering, external force and user responsibility is great. Therefore, for transformers that have been put into operation, first of all, reliable DC power supply for protection system should be equipped, and the correctness of protection action should be guaranteed. Considering the poor short-circuit strength of the transformer outside Hangzhou in current operation, the disadvantageous factors should be noticed for the automatic reclosure or forced operation of the system after short-circuit tripping. Otherwise, the damage degree of the transformer will be aggravated sometimes, and even the possibility of repairing will be lost. At present, according to the probability of instantaneous automatic elimination of short-circuit faults, some operating departments cancel the use of reclosure for near-zone overhead lines (e.g. within 2km) or cable lines, or prolong the reclosure interval to reduce the harm caused by reclosure failure, and should try their best to check the transformer with short-circuit tripping.
(4) Actively carry out transformer winding deformation testing and diagnosis. Usually, after the transformer is impacted by short-circuit fault current, the winding will be deformed locally, even if it is not damaged immediately, it may leave serious hidden trouble. Firstly, the insulation distance will change and the solid insulation will be damaged, which will lead to partial discharge. When lightning overvoltage occurs, it may occur turn-to-turn and cake-to-cake breakdown, resulting in sudden insulation accidents. Even under normal operating voltage, the long-term effect of partial discharge may also lead to insulation breakdown accidents.

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