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The development of turboelectric distributed propulsion (TeDP) systems requires the identification of operating voltage standards particular to this revolutionary airborne form of microgrid. This paper introduces a holistic approach for nominal operating voltage and voltage limit definition. Preferred nominal design voltage ranges were identified for the NASA N3-X DC superconducting TeDP system. This was accomplished by decomposing the architecture concept to the subcomponent and device level and integrating technology sensitivities into a system level parametric model. The effect of voltage on electrical system mass and efficiency are presented considering the potential effects of device improvements and protection system variations. Nomenclature AC= Alternating Current BSCCO = Bismuth Strontium Calcium Copper Oxide DC = Direct Current FBC = Force Balanced Coil LN2 = Liquid Nitrogen SFCL = Superconducting Fault Current Limiter SMES = Superconducting Magnetic Energy Storage TeDP = Turboelectric Distributed Propulsion TRL = Technology Readiness Level SSCB = Solid State Circuit Breaker UPS = Uninterrupted Power Supply YBCO = Yttrium Barium Copper Oxide

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The Turboelectric Distributed Propulsion (TeDP) concept uses gas turbine engines as prime movers for generators whose electrical power is used to drive motors and propulsors. For this NASA N3-X study, the motors, generators, and DC transmission lines are superconducting, and the power electronics and circuit breakers are cryogenic to maximize efficiency and increase power density of all associated components. Some of the protection challenges of a superconducting DC network are discussed such as low natural damping, superconducting and quenched states, and fast fault response time. For a given TeDP electrical system architecture with fixed power ratings, solid-state circuit breakers combined with superconducting fault-current limiters are examined with current-source control to limit and interrupt the fault current. To estimate the protection system weight and losses, scalable models of cryogenic bidirectional currentsource converters, cryogenic bidirectional IGBT solid-state circuit breakers (CBs), and resistive-type superconducting fault current limiters (SFCLs) are developed to assess how the weight and losses of these components vary as a function of nominal voltage and current and fault current ratings. The scalable models are used to assess the protection system weight for several trade-offs. System studies include the tradeoff in fault-current limiting capability of SFCLs on CB mass, alongside the fault-current limiting capability of the converter and its impact on CB fault-current interruption ratings and weight.

Purpose -The purpose of this paper is to highlight and discuss the unique safety and protection requirements for the electrical microgrid system in a turboelectric distributed propulsion aircraft. Design/methodology/approach -The NASA N3-X concept aircraft requirements were considered. The TeDP system was decomposed into three subsystems: turbogenerator, distribution system and propulsors. Unique considerations for each of these subsystems were identified. Findings -The fail-safe requirements for a TeDP system require a divergence from the standard safety case used for conventional propulsion systems. Advantages in flight control and single-engine-out scenarios can be realized using TeDP. Additionally, a targeted use of energy storage and reconfigurability may enable seamless response to propulsion systems failures. Practical implications -The concepts discussed in this paper will assist to guide the early conceptual and preliminary design and evaluation of TeDP architectures. Originality/value -The safety case for TeDP architectures is currently immature. The work presented here acts to frame some of the major issues when designing, evaluating and verifying TeDP conceptual architectures.