Transformer Monitoring And Protection And Fault Diagnosis

Power transformers as very important elements in power system require very reliable protection. Differential protection has been adopted as main protection against inner fault of transformer for a long time, since it has high reliability and high sensitivity, which are most important for transformer protection. But there are some problems troubling differential protection: identification of inrush current from short circuit current; unbalance current in differential circuit when outside short circuit happens; make sure differential protection device won't mal-operate in process of voltage recovering after cutting off outside short circuit; make sure that the saturation of CT won't influence the operation of differential protection. Identification of inrush current has been a key problem to be resolved. Many researchers have done a lot of work on this aspect. Because the inrush current of transformer is influenced by many factors such as residual flux of iron core, saturation flux density, system impedance, mode of connection, iron core structure, closing angle and so on, but all of the proposed methods can't make sure 100% reliability. Identification of inrush current is the most critical factor that limits differential protection operation.This paper proposes a new criterion for differential protection. Compared with the existed criterions, this criterion has great improvement on reliability and sensitivity. To solve the problem that differential protection may mal-operate when a faulted transformer closing onto system or after cutting off out side fault, an impedance relay is added in this criterion. This principle has great sensitivity to inner fault and great brake ability to outside fault. It can reflect to fault when closing onto system.The method based on the vibration signal of the transformer is a new method studying in civil and abroad in recently years. The author hopes to find an effective method to diagnose the status of the core and winding of the transformer by analyzing the vibration signal. The key technology is to find the feature of the vibration signal and gives acriterion for condition diagnosis. Taking advantage of the powerful function for signal processing of the wavelet analysis method, the vibration signal is analyzed both in time and frequency domain. At last, a diagnosis method based on frequency band-energy is proposed in this paper. The analysis result indicates that using wavelet method, the feature of the signal can be found effectively and the diagnosis of mechanical status of the core and winding based on the frequency band-energy method is applicable.