Analyze And Study Of Control Strategy Of Transformer Inrush Current

Power transformers are the major electric equipments in power plants and substations. Whether they are safely operated or not is directly responsible for the continuous stability of the power system. Now, along with the increasing of their power capacity and voltage level, the manufacturing cost of power transformers is growing also. In case a transformer is broken down, the repair process can be very complicated and time-consuming. In addition, the economic loss can be enormous. Therefore, a lot of worldwide research has focused on the issue of seeking a better scheme to ensure the transformers be protected more safely, reliably and sensitively.Practically, misoperations do exist in the transformer protection. Under most circumstances, it is due to the exciting surge current, which is one of the major reasons that can cause the misoperations. Aiming to aviod this phenomenon, a lot of research has been done to demonstrate a clear mechanism of how the exciting surge current is generated. There are two main methods. One is to avoid the misoperations of the protection when the exciting surge current occurs; the other one is to restrain the exciting surge current itself. Practical applications prove that the function of these two methods are limited and can not resolve the misoperations thorouly.Based on the systematically and thoroughly researching work and the analysing work of profound literatures and materials at home and abroard, this paper proposes a new theory on inrush exciting current limitation and more significantly, a current limitation device is correspongdingly designed according to the real practice in China’s present power grids. Experiments and practical applications have strongly proved the effectiveness of the research idea and the current restraining strategy which are proposed in this paper.Utilizing hysteresis loop theory, the paper analyzes the magnetization process of magnetic materials by AC sources. Based on this analysis and considering the construction of the transformer, further research is done on the core flux in the single phase transformer. Thus, the mechanism of the generation of the inrush exciting current is obtained. This paper also proposes another theory on inrush exciting current control based on the switching angle. Furthermore, simulation experiments on single phase transformer switching are done to prove the effectiveness of the theory. The results show that, under the switching angle control method, the inrush exciting current can be restrained if the transformer is switched on the appropriate switching angle. At last, based on the further research on the inrush current in three-phase transformer, a method on restraining the inrush exciting current in three-phase transformer is also proposed.The new method this paper proposed is to restrain the inrush current based on the thoughts that magnetic bias and remanence will offset each other while transformer being put into or cut out of the system. In this method, the switching angle is calculated with the established magnetic hysteresis model of the transformer based on the Preisach, and the estimated remanence based on the last cutting time. This paper sets up the IEA inrush exciting current limitation model based on H_TCPN. The simulation results in MATLAB show that:with the control of switching angle, the IEA can effectively restrain the inrush current which will happen when the transformer is connected in the system.In order to satisfy the requirements of the real engineering practice, and to enhance the robustness of the method-control switching time which restrains the transformer inrush exciting current, this paper proposed a switching time optimization method based on fuzzy reasoning with the remanence calculation method. This fuzzy reasoning derives from human thinking pattern, sufficiently considering the fuzziness and logic when human are in the process of cognition. Therefore, the method is strongly robust and adaptive. What’s more, the author simulates the proposed optimization method with the established practical model. The comparing analysis of experiments under different conditions proves that the fuzzy reasoning based switching time optimization method can appropriately determine the switching time. The inrush current restraining method is promoted to be more robust and reliable.Based on the theoretical derivation and analysis of transformer inrush exciting current restraint principle and algorithm, and bear the aim to apply this method to the power grid, this paper proposes a design method of the transformer inrush exciting current suppressor, including both hardware and software design. A detailed analysis is also conducted on the structure, operating principle and the performance of the suppressor.In order to prove the correctness of the design method of transformer inrush exciting current suppressor proposed in this paper and the effectiveness of the device designed in this paper, the field experiment is conducted at the 5# transformer S10-40000/35 and 2# transformer S9-6300/30 in Jiangdong Fulida Themal Power Plant in Hongzhou City. According to the analysis of the results and the wave forms recorded in the experiment, it is concluded that this transformer inrush exciting current suppressor can effectively restrain the inrush exciting current generated when the transformer is switched on without load.This paper conducts a systematic and profound research on the transformer ferrite core magnetism characteristics, ferrite core residual magnetism and the original reason of the inrush exciting current generation. It proposes a restraint scheme of transformer inrush exciting current and an optimization computation of the switching time of the transformer under actual operating circumstances. Based on all these above, this paper demonstrates the designing and the manufacturing details of the transformer inrush exciting current suppressor. This device has been highly favored by the customer in the field testing and the application. It is regarded to be an innovation achievement, which is valuable to be referenced by other research on restraint of transformer inrush exciting current. And also it is considered to be of great value in the real power engineering.