The Doubly Fed Induction Machine as an Aero Generator

Modern aircraft require a robust and reliable supply of electrical power to drive a growing number of high-power electrical loads. Generators are driven by a mechanical offtake from the variable speed gas turbine (GT), while a constant frequency ac network is preferred. Here, doubly fed induction machines are advantageous since they can be controlled, through a fractionally rated converter, to decouple electrical frequency from the mechanical drive speed, making control of the network frequency possible. However, the converter must be suitably rated, according to drive speed range, electrical voltage and frequency regulation, and power requirements. This paper develops and validates a simulation model of the doubly fed induction generator (DFIG) system, which is applied to find the power flow through the machine's stator and rotor connections over a wide mechanical speed range in order to size the converter. A field-orientated control scheme is implemented, to provide stand-alone voltage and frequency regulation across a drive range of +/- 40% synchronous speed, on a purpose-built 6.6-kW hardware test platform. Based on the mechanical speed range of an aero GT and the identified converter sizing, the suitability of a DFIG for aero applications is appraised. It is shown that a converter rated at 18% of full system rating can be used to meet the aircraft electrical specifications and offers a potential improvement in aircraft performance, with no additional mechanical components.