Partial Discharge Characteristics of Electric Machine Insulation Under Square Wave Voltage for Flywheel Energy Storage

As a critical method of physical energy storage, flywheel energy storage offers high specific power, high energy conversion efficiency, and excellent reliability. To minimize wind friction loss, the flywheel energy storage system operates in a vacuum state. Consequently, the motors in flywheel energy storage systems typically function under a medium vacuum range from tens to hundreds of Pa air pressure, which imposes new demands on the reliability of the motors' insulation. According to Paschen's law, the partial discharge inception voltage (PDIV) decreases and then increases as air pressure decreases, leading to poorer insulation performance at low vacuum compared to high vacuum and standard atmospheric pressure. An experimental platform with the ability to generate square wave voltage for partial discharges (PD) under vast vacuum levels has been established, employing a detection method that combines a high-frequency current transformer (HFCT) with an ultra-high-frequency (UHF) antenna. A finite element simulation model is constructed to simulate the discharge of different electrodes under vacuum, to verify the experiment method that fixes the discharge distance. To guide subsequent insulation design, the PDIV under different square voltage rise times is tested under different vacuum conditions. Differences in PD characteristics, along with the PD evolution, under low-medium vacuum conditions are analyzed and further studied with a high-speed camera.