Internal Asymmetric Analysis of AC Machines

Abstract

Internal faults in ac machine stator winding will cause severe damages to the machine. In order to protect electric machine and lighten the damage owing to internal fault, the speediness and reliability of relay protection are very important which demand understanding the currents in electric machine windings under internal faults clearly.

Under the situation of internal fault of electric machine winding, there are stronger space harmonics in air-gap magnetic field and strtonger time harmonics in winding currents, which make symmetric component method be not suitable for analyzing it. At the same time the ideal electric machine model is no longer suitable, accordingly, adopting d, q, 0 coordinates system based on the corresponding model will cause very large errors. Therefore it is needed to adopt Multi-loop Theory and Method based on single coil as stated in chapter 1 for analyzing it. In this chapter using the presented method the rotor failures of bar broking and end-ring broking of induction machine, the internal faults of stator windings of salient pole synchronous machine and non-salient pole synchronous machine, etc. are discussed. The short circuit current of internal fault for stator winding is large, so it is necessary to pay special attention to the protection. In addition, the transient ac component is a little more than the steady-state current not just as the short circuit of external terminals. By the way, the short circuit current depends upon not only the short circuit sorts but also the short circuit turn ratio and the space position of short circuit turn, etc. The calculation results are supported by corresponding experiments.