Research On Controlled Switching Of No-load Power Transformers

As a result of the iron-core saturation and the nonlinear response of the iron core material, the amplitude of the exciting current generated by the energization of unloaded transformer is large. The exciting current, which is called inrush current, has unfavorable effects such as harmonic amplification, power system voltage sag, protective relay misoperation, damage to the power electronic devices, imposed sever mechanical stresses on the transformer windings, induced sympathetic inrush, and vibration aging of the motor. In order to eliminate inrush current, people have devoted to identify the characteristic of the inrush current with the physical or mathematical method for a long time, or have used the complex expensive auxiliary equipment, or have optimized the design of the transformer. However the overall effect is barely satisfactory. And controlled switching is a new way to eliminate inrush current.Controlled switching is a terminology applied to the principle of coordinating the instant of opening or closing of a circuit with specific target point on an associated voltage or current waveform. Controlled switching has become an economical solution and commonly used to reduce switching surges. It is one of the advanced subjects of intelligent apparatus. The main contents of this paper are composed of the mechanism of inrush current occurring in transformer, transformer modeling, controlled switching strategies of unloaded power transformer, and the software and hardware design of the synchronous control system. The obtained results in this thesis supply references to future research on the controller of synchronous switch.The single-coil permanent magnetic actuator (PMA) has small mechanical scatter due to its fewer moving parts. PMA has been used more and more widely. Controlled vacuum switch is supplied with independent-pole operated PMA, and capacitor discharge supply can be chosen in the PMA exciting circuit. Thus the operation is controlled by high-current power electronic devices. Test results demonstrate that PMA can satisfy with the request of the controlled switching, and it is the ideal choice of synchronous switching.Here, the design of software and hardware for synchronous control system of permanent magnetic actuator, in which the digital signal processor TMS320LF2407A is used as the kernel, is presented. In this system the real-time acquisition of the electrical parameters is adopted, and the crossover point of reference signal is detected by FIR digital filter. The system controls the large-capacity capacitor discharge to actuate the PMA through IGBT, and makes the circuit breaker open or close in an ideal phase in order to reduce the transition impact.The experiment results indicate that this system already achieves the anticipated effect, and provides some useful experience and the reference for the future research. The system is useful to optimize the synchronous control system.