Investigation of Permanent Magnet Demagnetization in Synchronous Machines during Multiple Short-Circuit Fault Conditions

Abstract

If a PMSM is subject to a short-circuit, only slightly damaging the magnets and nothing else, it might be kept in operation. Since a variable speed synchronous generator is allowed to vary in voltage and is connected to a frequency converter, a decrease in voltage is acceptable. An increase in torque could give problems with increased vibrations, especially at lower frequencies when the damping from the generator and shaft dynamics is lower. [...]low vibrational frequencies are more easily transferred through the generator and shaft to the turbine [30]. Since the comparison study only consists of a short high current pulse (see Section 4.2), this will have a great impact on the results. If the load had been adjusted between the different short-circuit events, to increase the current and thereby maintain the output power, the short-circuit currents would also have been increased for the later cases, leading to higher demagnetization. Since the load was kept constant, the current was reduced due to the decrease in induced voltage as a result of the demagnetization of the permanent magnets.