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Electrical Insulation Design and Qualification of a SiC-based Generator-Rectifier Unit (GRU) for High-Altitude Operation

Hardware prototypes
Fig. 1. (a) GRU components: dc bus (left), dc CM choke (middle), gate driver (right); (b) Full system as-sembly.
  The aerospace industry is in the process of moving towards a more-electric aircraft by replacing the traditional hydraulic and mechanical systems with high efficiency electrical systems to reduce system weight, cost and fuel consumption. The main generator rectifier unit (GRU) forms the heart of the power processing system in the aircraft feeding all downstream loads. The low air pressure environment associated with the cruising altitudes (up to 15,200 m) reduces the break-down electric field (E-field) strength of air thereby increasing the risk of surface discharges within the GRU which can have consequences ranging from insulation lifetime degradation to complete insulation breakdown and catastrophic system failures.
  Printed circuit boards (PCBs) are ideal for integration of electronic components in low profile, high power density assemblies where precise E-field control techniques can be easily imple-mented to achieve sufficient PD performance even under the specified altitude conditions. With the knowledge of the Paschen’s curves for air, it is possible to deduce the breakdown E-field of air for varying conductor clearances at different altitude conditions to directly guide the GRU design.
  In this work, the GRU in question is designed and qualified for PD free operation at 15,200 m altitude (11.6 kPa). Finite element analysis simulations are used to optimize the design of the GRU PCBs and their assemblies to constrain the surface E-field (Eair in air) of the system under a predetermined limit (Eair-limit ⩽ 300 V/mm) chosen based on the Paschen curves to eliminate PD occurrences under nominal conditions. The three main GRU component assemblies, the dc bus, the CM choke, and the gate driver, are shown in Fig. 1. A high altitude PD test bed was designed for GRU insulation qualification (Fig. 2 (a)) and PD experimental results of individual GRU com-ponents and their assemblies are presented in Fig. 2(b) showing satisfactory PDIV values (> 1.4 times the working voltage) at the specified air pressure conditions.
PD tests
Fig. 2. (a) High altitude PD test bed; (b) GRU components PDIV summary for different altitude settings.

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