Inter-Turn Short Circuit Detection of Pumped Storage Unit Based on Optical Fiber Weak Magnetic Sensing

The stray flux (SF) at the back of the stator core of pumped storage unit (PSU) can provide an effective response to the operating condition of the rotor winding. This study investigates the SF characteristics of the rotor winding before and after an inter-turn short circuit (ITSC), with a view to achieving online monitoring of the inter-turn insulation health level. Firstly, the expressions for the SF in the healthy state and ITSC state of the rotor winding are derived, obtaining the characteristic harmonics of SF corresponding to the different states. Secondly, the sensing mechanism of reflective magneto-optical crystal (RMOC) is analysed, on the basis of which an optical fiber weak magnetic (OFWM) detection system is built and a conversion expression between the Faraday magneto-optical deflection angle and the flux density is obtained. Thirdly, a two-dimensional finite element simulation model is built based on the actual dimensions of the operating PSU, according to which the evolution of the time/frequency domain characteristics of the radial SF before and after the ITSC fault is analyzed, obtaining the peak radial SFes of no-load/load conditions are 111 μT and 115 μT respectively, which can provide a reference for the design of OFWM sensor. Finally, the OFWM probe is calibrated with a sensitivity of 0.244 mrad/μT and a lower detection limit of 0.1 μT _rms . The probe is fixed vertically to the stator housing of the synchronous generator for real-time monitoring, and the radial SF information is collected at different levels of ITSC, with the peak values are 2.57 μT and 2.81 μT for no-load/load conditions respectively. The monitoring results demonstrate that the changes in the time domain characteristic waveform and spectral characteristic harmonics of the SF can effectively characterise the rotor winding inter-turn insulation health level, which expands a new technical path for rotor winding ITSC condition monitoring.