Comparative Analysis Between Offline and Online Methods for Turn-to-Turn Fault Detection in Rotor Winding of Large Turbogenerators

Rotor winding is generally reliable part of the turbine generators. However, the rotor turn insulation in these machines is susceptible to degradation or/and punctures over time, primarily due to thermal aging, load cycling and contamination. Although shorted turns do not usually cause immediate machine failure, they can lead to higher bearing vibrations, increased strain on excitation systems, flux integrity degradation, decreased generator capacity and reduce overall output. Offline tests can detect shorted turns in rotor windings, but they are sometimes not enough reliable. These tests are conducted while the rotor is at standstill, without thermal and mechanical stresses due centrifugal forces, which sometimes results in undetected shorted turns. Online testing of rotor flux includes instalation of magnetic flux sensor in the generator air gap. This sensor allows monitoring of the rotor flux at regular service condition with rated machine parameters. As a result, real-time insight into the condition of the inter-turn insulation of the rotor windings can be continuously obtained during the generator’s operation.