Investigations On Controlled Switching Of Power Transformer

Power transformer is one of the most important facilities in the Power System. Its safe operation is of vital importance in the Power System. It will generate overvoltage of Current Chopping during the de-energization operation, which is due to its intrinsic inductive load character. The hysteresis effect of the iron core material will lead to tremendous amplitude inrush current when closing unload transformer. These electro-magnetic transitions effect will induce great potential safety hazard to the safe operation of the Power System, such as the disoperation of the differential protective system, the insulation failure of the transformer winding, the decrease of the work life, the increase of the maintenance costs, ect. Although the method that the system is using now restrains these electro-magnetic transition effect to some degree, but it increases the cost of the investment and the complexity of the operation. The introduction of the Controlled Switching can fundamentally solve the problems that mentioned above. Since the proposal of the controlled switching technique, the Controlled Switching of unload transformer has been widely used in the electrical engineering.The intrinsic character of the Controlled Switching of unload transformer is the energization of unload transformer when the residual magnetism equals to the pre-induction flux in the three phase iron core, which can avoid the saturation of the iron core and the generation of the Inrush Current. Based on the two key factors during the design of the controlled system, the residual flux and the operation time dispersity of the breaker, this thesis gives the different Controlled Switching strategies and the compensation algorithm of the operation time. And it also establishes the unload transformer's simulation model using the EMTP/ATP for the conditions such as:overvoltage of Current Chopping, Sympathetic Inrush Current, Residual Flux, and Inrush current. Simulation method is used to verify the application of the Controlled Switching technique of unload transformer. Meanwhile, detailed simulation analysis of the three strategies during different Residual Flux is carried out. The result of the simulation reveals that Controlled Switching of unload transformer can restrain the Inrush Current when it is energization, and the rate of the inrush current is 81.8 times less than the random switch.The developing of the digital signal processing offered the great support to Controlled Switching technique, especially the introduction of the DSP2812 processor of the Texas Instrument. The Controlled Switching based on the DSP2812 realized Controlled Switching techniques during three different residual magnetism circumstances. This system can collect the three phase voltages and currents of the Power System, promptly extract the fundamental wave signal of the reference phase through the FIR filter, and obtain the zero-voltage point of high accuracy through the differential value algorithm. The relevant experiments reveal that the precision of the three phase operation signal sent by the system is within±0.4ms, which achieves the requirements of the CIGRE that regard to the precision of Controlled Switching.