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| Fault Detection of Stator Non-Roundness in Hydro-Generator Based on Branch Circulation |
| Wu Yucai1, Zhang Guangxiang1, Yue Jianfeng2, Yang Guangyong2, Dao Yajuan2 |
1. Hebei Key Laboratory of Green and Efficient New Electrical Materials and Equipment Department of Electrical Engineering North China Electric Power University Baoding 071003 China;
2. Huaneng Lancang River Hydropower Co. Ltd Maintenance Branch Kunming 650051 China |
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Abstract Problems such as manufacturing tolerances in the stator of large hydro-generators and uneven seams during lamination assembly are complex to avoid, making it impossible to guarantee an ideal roundness of the stator bore. During operation, mechanical and electromagnetic vibrations, as well as thermal expansion due to temperature variations, can readily cause loosening and deformation of the stator core. The induced unbalanced magnetic pull will continuously worsen the stator non-roundness, ultimately causing severe stator core deformation. Accurate early warning of stator non-roundness faults can effectively enhance the safe operation level of hydro-generator units. Therefore, this paper proposes a circulating-current comparison detection method for stator branches, which can be employed for online detection of stator non-roundness faults.
First, using the mesh current method and Kirchhoff's second law, the paper derives an expression for the circulating currents in multiple in-phase branches of the generator. Considering the irregular distribution of the air-gap magnetic field caused by stator non-roundness, it is deduced that differences in the magnetic permeability harmonics in the regions of each in-phase branch due to stator non-roundness result in discrepancies in the amplitude and phase of the branch electromotive forces (EMFs). Thus, a correlation between the characteristic harmonics of the circulating currents and the severity of the stator non-roundness fault is established.
Subsequently, a finite-element electromagnetic simulation model was constructed, using a 334 MV·A hydro-generator as an example. The stator non-roundness was simulated at the spatial locations of single-, dual-, and multi-branch windings. Odd-order characteristic harmonic circulating currents consistent with the predicted frequencies were extracted. The relationships between the time/frequency domain characteristics of circulating currents and the stator deformation range, radial displacement, and concave-convex points were analyzed, revealing a connection between the circulating currents and stator core deformation in the spatial regions where the branches are located.
Finally, an experimental platform was used for a salient-pole synchronous generator, and an outwardly convex stator non-roundness fault was simulated. The circulating-current comparison method for stator branches was used to compare circulating currents across branches of each phase, determine the relative deformation magnitude in branch regions, and detect the overall non-roundness of the stator. The results indicate that after a stator non-roundness fault occurs, odd-order harmonic circulating currents appear between the in-phase branches of the stator. The greater the differences in the deformation range and magnitude of stator non-roundness in each branch region, the higher the harmonic circulating currents between the branches.
Therefore, by real-time online monitoring, comparison, and analysis of the relative magnitudes and trends of these circulating current signals, the relative severity of non-roundness at different spatial locations along the stator circumference can be determined. Online assessment and monitoring of the overall roundness condition of the generator stator can be achieved by integrating the relative non-roundness information from all compared branch circulating currents.
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Received: 11 April 2025
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2026, Vol. 41 
