Simplified mathematical model of inter-turn short circuit of field windings in hydro-generators and its application

Abstract

Inter-turn short circuit of field windings is a common electrical fault of generators. Simulation is an important method of investigating the fault and providing data support for fault monitoring. However, huge numbers of pole pairs and damper loops in large hydro-generators would lead to lengthy calculation time, hindering scientific research and engineering application. To deal with this problem, we analyze a theoretical basis for a damper winding simplified model and then propose an equivalent treatment method. Through the analysis of steady-state current harmonic characteristics of generators with different stator winding configurations during the fault, the simplified models suitable for steady-state calculation are derived from two aspects, namely, additional rotor harmonic current frequency characteristics and the relationship of the amplitude as well as the phase of each branch current of the stator. The calculation and experimental results of the two simplified models are then compared to verify the models’ correctness. A calculation example of the Three Gorges left bank VGS generator shows few deviations between the calculation results of the simplified model and the original model. Moreover, the calculation time using the simplified model is 1/1500 that using the original model, which provides a more effective tool for on-line fault monitoring. Finally, the sensitivity-verification application of the fault-monitoring scheme based on the stator steady-state unbalanced current RMS is depicted. The result shows that the scheme can monitor two-turn short circuits of field windings in the Three Gorges generator and provide high sensitivity.