Piezoelectrically-actuated fast mechanical switches provide a low-loss conduction path in hybrid circuit breakers for medium-voltage, direct-current system protection. With the desired actuation performance being pushed towards the driving limit of the piezoelectric actuator, excessive vibration starts to dominate the underdamped travel curves of contact movements, which will lead to insulation failures and delayed operations in the fast mechanical switch. To improve underdamped responses into critically damped actuations, several switching motion control strategies have been proposed with active damping filters such as notch, lead and lag compensators in the closed-loop system. The switching motion controllers are built upon a vibrational dynamics model of a prestressed piezoelectric stack actuator with experimentally identified parameters. The controller tuning principles are derived to achieve optimized step responses with a minimized rising time down to 250 μs, a reduced undershoot around 10%, and a closed-loop control bandwidth up to 1760 Hz. The closed-loop simulation is performed to verify the performance of proposed switching motion controllers on both low-frequency external disturbance elimination and high-frequency internal vibration attenuation. According to the hardware implementation tests, the proposed control strategies have optimized the switching motions of a heavily loaded piezoelectric actuator with a 60% reduction in undershoot and a 45% reduction in settling time. At the same time, the sub-millisecond switching time has been preserved in the actively damped travel curves of this piezoelectric actuator. With optimized switching operations of the piezoelectric actuator, the overall fast mechanical switch can better serve the advanced hybrid circuit breakers to achieve reduced fault current and fault clearance time during circuit interruptions. Consequently, the overall medium-voltage direct-current systems can get better protected by the piezoelectrically-actuated fast mechanical switch.Ph.D
