Research On Main Protection Device Of Transformer

Power transformer is a very important equipment in power system,the security operation affect directly whether the whole power system can operate steadily,so real-time protection is necessary countermeasure to the transformer. Based on the protection function ,transformer guard is divided three classes,such as main protection,back up protection and body protection. The main protection device of transformer is designed in this paperr based on the microcomputer.At first, the author discuss the current situation and developing tendencies of digital protection at home and abroad,and analysis the protection principle of main protection and execution ways by using a microcomputer. Based on the concept, a digital protection device of main protection of transformer is designed. The system hardware includes signal sampling circuit, A/D conversion circuit, inputing and outputing circuit of switching value, LCD displaying circuit, keyboard circuit, CAN bus communications circuit. Using the flexibility of software, the device includes differential fast-trip protection function, restraint ratio-restraint differential protections of second harmonic braking function. The system software includes interrupting and sampling program, serial communication program, parameter account program, fault handle program, LCD displaying program, key response program, CAN bus communication program.The static test is worked by sample accuracy, calculation accuracy and protection function, the results show that the model device reaches the desired target.Aiming at discriminating the inrush current and fault current which is a difficult point in theory of transformer protection, a method based on the wavelet theory is presented. Wavelet analysis can detect the abrupt change of the signal, the B-spline wavelet is choosed to analyze the sample data attained from the Matlab. The wavelet coefficient of the local maxima can be get by wavelet transforms, and then the Lipschitz which meaning the abrupt level of the signal can be attained. The inrush current can be separate from the fault current by Lipschitz. The results of computer simulation show that the method is workable.