Turboelectric Aircraft DC Network Fault Analysis and Protection Solution using Superconducting Fault Current Limiters

Abstract

Excluding the strong impact of COVID-19 on the aviation sector, the annual increase in aircraft passengers and freight traffic were estimated to be around 6.4% and 4.2%, respectively, according to the International Civil Aviation Organization (ICAO). According to the United States Environmental Protection Agency (EPA), the transportation sector is the US’s largest source of carbon dioxide (CO2) emissions at 28%, and will only dramatically increase with current aviation technologies.
The National Aeronautics and Space Administration (NASA) in the US and the Advisory Council for Aviation Research and Innovation in Europe (ACARE) have set goals to limit atmospheric pollution and reduce greenhouse gases. The environmental goals of NASA and ACARE are the reductions of CO2 by 75%, NOx by 90% and external noise by 65% relative to their levels in 2000; targets which are unfeasible using traditional aircraft design due to the relatively low efficiencies. To achieve these goals, aircraft, including the propulsion system, must work with superior efficiency.
The turboelectric distributed propulsion (TeDP) system is considered one of the best approaches for future large-scale aircraft to achieve the desired environmental goals. The main idea of the TeDP system is to replace the traditional turbofan/combustion engine with a turboshaft that runs electric generators connected to electric motors via a DC microgrid, which includes converters to control motors/propellers to produce the thrust required by the aircraft.
The merging of electrical components in turboelectric aircraft (TeA) and the installation of onboard electrical power systems offer several design opportunities for optimization and system improvement. However, there are several design challenges for the TeA system, including protecting the DC distribution system against faults. It is a significant challenge due to the high magnitude of fault currents, the absence of zero-crossing points, low line impedance and high bus voltage (6 kVDC). As the expected fault currents have a high magnitude and a short rise time in the airborne DC microgrid, and DC circuit breakers (CBs) have long operation times of up to 4 msec, the need for a device to limit fault currents and ensure safe operation of the CBs is inevitable.
In this thesis, one set of the TeA is modelled in MATLAB®/Simulink environment to conduct the DC fault analysis, with results indicating that fault currents are extremely high in pole-to-pole and pole-to-ground with low grounding impedance faults. Based on the DC fault analysis results, a multilayer thermoelectric resistive superconducting fault current limiter (r-SFCL) is modelled to reduce the fault currents and support the protection system. The multilayer thermoelectric r-SFCL is tested with different copper stabilizers and shunt resistors. The best candidate model, in terms of fault current limiting capability and recovery time, is integrated into the power protection system to reduce the fault current in the time gap between the fault occurrence and the fault clearance. The nominated r-SFCL model was successfully able to reduce the prospective fault currents by up to 80% for up to 64 msec without reaching a permanently damaging temperature (400 K). Finally, a controlled superconducting magnetic energy storage (SMES) is integrated into the TeA power system architecture to supply the propulsion system during a short period of temporary power loss.
This thesis provides an effective and embeddable r-SFCL model which considers the electrical and thermal behaviour of each layer in the superconductor tape as well as the heat transfer between the r-SFCL and the coolant, liquid nitrogen (LN2). The robust model presented in this thesis can help with the protection planning for power-dense electrical networks such as those in TeA.

Date of Award	17 Feb 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Xiaoze Pei (Supervisor) & Vincent Zeng (Supervisor)