The doubly-fed induction machine as an aero generator

Abstract

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, 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 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 standalone voltage and frequency regulation across a drive range of ±40% synchronous speed, on a purpose-built 6.6kW hardware test platform. Based on the mechanical speed range of an aero gas turbine and the identified converter sizing, the suitability of a doubly-fed induction generator 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 offer a potential improvement in aircraft performance, with no additional mechanical components.

Fractional kVA Rating PWM Converter Doubly Fed Variable Speed Electric Generator Systems: An Overview in 2020

“…For vehicular or on-ground applications, standalone dc output variable speed DFIGs have been proposed since 2001 [23] and relaunched recently [24] due to the control simplicity, lower weight at acceptable efficiency and, finally, lower cost via a single dc-ac fractional (20%) kVA rating PWM converter, as shown in Fig. 4, for a speed range of 2.5/1 and a frequency range of 2/1.…”

Analysis of electro-mechanical interaction in aircraft generator systems

“…The test platform includes a doubly-fed induction generator (DFIG), complete with rotor-side converter, controlled to achieve voltage and frequency regulation across a speed range of 600rpm to 1400rpm on the generator side, the parallel axis gearbox has a drive ratio of 1.5:1. A DFIG is chosen for the generator because it provides a way of decoupling electrical frequency from mechanical drive speed, this is discussed in [19], along with further details of the generator control scheme. Connections to the DFIG rotor give the machine fast dynamics, allowing control schemes to be developed to mitigate interaction.…”

“…The machine rotor-side is fed via a commercial inverter, the stator side terminals supply a standalone electrical network, loading is provided by a resistive load bank. A fieldorientated control scheme is implemented to provide standalone voltage (215V rms ) and frequency (50Hz) regulation, with cascaded current and voltage control loops, this is described in more detail in [19]. The second, identical, electrical generator is represented by a flywheel wheel with matched inertia and coupling stiffness.…”

“…The electro-mechanical model, with reduced order drivetrain, has been validated against the test platform, and is now used to simulate more challenging electrical loads, which may damage the hardware. The generator is represented by a standard fourth-order model, the DFIG on the test platform has been characterised to provide the machine parameters and is discussed in [19]. A field-orientated controller has been implemented to provide standalone voltage (215V rms ) and frequency (50Hz) regulation.…”

“…Weight constraints result in low stiffness shafts and very low levels of damping in a complex mechanical drivetrain and the electrical network is relatively weak with highly dynamic loads. The prime mover speed may vary over a range of greater than 2:1 and in some aircraft, the generator frequency also varies over a similar range [19].…”