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.
Overview of Power Transformers
Electronic power transformer is mainly realized by power electronics technology. Its basic principle is to convert power frequency signal into high frequency signal through power electronics circuit in the original side, that is, to raise frequency, then to couple to the vice-side through intermediate high frequency isolation transformer, and then to restore the power frequency signal, that is, to reduce frequency. By adopting appropriate control scheme to control the work of power electronic devices, the power of one frequency, voltage and waveform can be transformed into another frequency, voltage and waveform. Because the volume of the intermediate isolation transformer depends on the saturated flux density of the core material and the maximum allowable temperature rise of the core and winding, and the saturated flux density is inversely proportional to the working frequency, thus increasing the working frequency can improve the utilization of the core, thereby reducing the volume of the transformer and improving its overall efficiency.
Measures to Improve the Short Circuit Resistance of Power Transformers
The safe, economical and reliable operation and output of transformer depend on its manufacturing quality, operation environment and maintenance quality. This chapter tries to answer the effective measures to prevent transformer sudden faults in the process of transformer operation and maintenance.
The short-circuit is often caused by lightning strike, misoperation of relay protection or refusal of operation in power grid. The strong impact of short-circuit current may damage transformers. Therefore, efforts should be made to improve the short-circuit endurance of transformers from all aspects. The statistical results of transformer short-circuit impact accidents show that about 80% are caused by manufacturing reasons, while only about 10% are caused by operation and maintenance reasons. The design and manufacturing measures are discussed in Chapter 2. This chapter focuses on the measures to be taken in the process of operation and maintenance. In the process of operation and maintenance, on the one hand, the short-circuit fault should be minimized, so as to reduce the number of transformer shocks; on the other hand, the transformer winding deformation should be tested in time to prevent accidents in the future.
(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 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 can be measured; each coil of the same pressure plate has been processed by the above technology. After that, the coil is adjusted to the same height, and the required pressure is applied to the coil by the oil pressure device 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 test and inspect transformers 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.
Therefore, active diagnosis of transformer winding deformation, timely detection of faulty transformers, and planned verification and maintenance of hoods can not only save a lot of manpower and material resources, but also prevent transformers from changing.
