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Piezoelectrically actuated fast mechanical switches provide a low-loss conduction path in hybrid circuit breakers for protecting medium-voltage direct-current systems. As the desired actuation speed approaches the driving limit of a piezoelectric actuator, excessive vibration starts to dominate its underdamped travel curves, resulting in insulation failures and delayed switching in the fast mechanical switch. To improve the underdamped travel curves into critically damped travel curves, several switching motion control schemes with active damping filters are proposed in this paper. The switching motion controllers are based on an electromechanical model of a fast mechanical switch with experimentally identified parameters. According to hardware implementation tests, the proposed closed-loop control schemes optimized the switch travel curves with a 56% reduction in the undershoot and a 75% reduction in the settling time. Equipped with the critically damped travel curves, the piezoelectrically actuated fast mechanical switch is better suited for sequential submodule operations to reduce the interruption time, the peak current and the fault energy to be absorbed in hybrid circuit breakers. [ABSTRACT FROM AUTHOR]

High-voltage direct current (HVDC) circuit breaker development and deployment strongly depend on the testing process, which ensures that the HVDC circuit breakers will satisfy design requirements. This article presents an HVDC circuit breaker test bench circuit configuration that can provide controllable large output currents to simulate different fault conditions for the current breaking test and high output voltage for the dielectric withstand test. The current breaking test circuit is based on multiple cascaded power converters connected in parallel to provide the necessary output current capability. Each cascaded power converter is composed of multiple cells that are operated by a phase-shifted pulsewidth-modulated signal for greater controllability and higher quality of the output waveform. The dielectric withstand test circuit is a simple high-voltage source with a low power rating that can also be used to charge the test bench and the internal circuitry of the circuit breaker that is to be tested. The proposed test bench ensures that fault conditions can be replicated accurately and offers greater flexibility by being able to test mechanical, semiconductor-based, or hybrid HVDC circuit breakers with different current and voltage ratings on the same hardware without any changes. The idea and the operating principle of the proposed test bench are verified experimentally on a downscaled system that consists of three cascaded power converters connected in parallel with three cells per cascaded power converter and with a total equivalent switching frequency of 92.5 kHz. [ABSTRACT FROM AUTHOR]

A new series-type hybrid circuit breaker (S-HCB) concept for dc fault protection is reported that is fundamentally different from other prior-art hybrid circuit breakers (HCB) or solid-state circuit breakers (SSCB) in the literature. The S-HCB conducts its load current through metal wires instead of power semiconductor switches and curtails its fault current to near zero throughout the entire opening process of a series mechanical switch. It offers the low on-resistance of the conventional mechanical contacts for normal operation and µs-scale fault response even faster than the fast-acting SSCBs. A proof-of-concept S-HCB prototype experimentally demonstrates the interruption of a fault current of 30 A within 10 μs at a dc voltage of 600 V. The operating principle, simulation, and experimental results are discussed in this letter. [ABSTRACT FROM AUTHOR]

Medium voltage direct current (MVDC) power systems are in their early phase of development, but the lack of efficient and reliable DC circuit breakers is currently one of the most significant bottlenecks for widespread adoption of MVDC applications. Compared to protection devices in high voltage direct current (HVDC) systems, MVDC circuit breakers are expected to interrupt fault currents with higher rates of rise in more power-dense and more controllable structures. A hybrid approach with a piezoelectrically actuated fast mechanical switch in parallel to a solid-state circuit breaker is expected to meet the above requirements. A prototype of a 15 kV-class piezoelectrically actuated fast mechanical switch is presented in this paper. The prototype demonstrates an actuator opening time of 1 ms, a breakdown voltage of over 20 kV, and a continuous current rating of 500 A. After integrating the fast mechanical switch into a hybrid circuit breaker testbed, the fast mechanical switch was able to open within 1.5 ms and the fault was cleared within 5 ms. Therefore, piezoelectrically actuated fast mechanical switches can enable fast, reliable and controllable protection for various MVDC systems like electric ships, electric aircraft and microgrids that integrate distributed energy resources. [ABSTRACT FROM AUTHOR]

A mechanical switch can be used instead of a power electronic load commutation switch in orderto reduce the losses during normal power flow in standard hybrid DC circuit breakers. The success of current commutation in such hybrid breakers depends on the arc voltage formed across the contacts in the mechanical switch. The behaviour of rapidly elongated, wall constricted arcsgenerated during the opening of an ultra-fast mechanical switch in the air is studied here. The voltage-current characteristics of the generated arcs are obtained for steady state contact opening velocities of 6, 11 and 21 m/s. It is found that the arc voltage at a given current and contact separation increases with the contact opening velocity. It is shown that stationary,zero-contact-velocity characteristics can not be used to accurately quantify the voltage build-up in fast elongating arcs in hybrid breakers. A best-fitting black-box equation for the dynamic arc characteristics is presented for the tested velocities. The obtained voltage-current characteristics are used as input to simulate the current commutation process in a 12 kV hybridDC circuit breaker case-study. Different scenarios of current commutation failure related to the arc voltage build up are identified. It is shown that these failures scenarios can be avoided by increasing the contact opening velocity. [ABSTRACT FROM AUTHOR]