IEEE Electrification Magazine - December 2017 - 27

generator while the dc power demand is shared by both
the HP and LP generators. An open-end winding induction
starter/generator (OEWIS/G) is used to develop an APU for
power generation and a management system for supplying/absorbing the highly dynamic power demand/regeneration from the EHA/EMAs. The OEWIS/G used in this
system creates a separate dc bus for the actuators without
adding significant hardware to the system. During the
entire flight mission, the regenerative power is recovered
by one side of the OEWIG terminals; meanwhile, the
power delivery to the main dc network of the aircraft EPS
can be independently controlled by using the same generator through the other side of the terminals.

Induction Machine-Based ac/dc
Hybrid Power-Generation Architecture
In current MEA systems, WFSG-based ac primary generation systems are widely used for the main-engine electrical power generation, the system configuration of which is
shown in Figure 1. In this system, a wound-field synchronous starter/generator is connected to the HP spool of the
gas turbine engine through a mechanical gearbox. The
field current of the synchronous generator is controlled by
a smaller PM machine with a diode bridge rectifier
installed on the generator shaft. By adjusting the excitation of the field winding, the ac source voltage can be
regulated with variable shaft speed. In this way, the
WFSG-based ac primary generation system can feed the
frequency-insensitive loads directly from the synchronous

Gas Turbine
HP Spool

generator terminals. However, the complex rotor structure
makes the torque-to-inertia ratio of WFSG lower than
other types of electric machines. Moreover, the rotating
diode bridge structure limits the top speed of the generator shaft.
In aircraft systems, the effect of electrical power offtake can sometimes have a significant impact on the
dynamics and control of the aircraft engine. For instance,
during the transition from cruise to descent phase, the
aircraft engine power is transiently reduced while maintaining high electrical power demand. This transition
may create engine instability and require substantial
electric load shedding. Furthermore, with the increasing
electric power consumption in MEA, the potential instability will be more severe if the electric power is solely
extracted from the HP spool of the gas turbine engine.
This issue can be resolved by installing an extra generator on the LP spool of the engine and sharing the power
extraction between the HP and LP spools. In this way, the
power generated from the LP spool could compensate for
the decreased power from the HP spool, so the electrical
power demand is not compromised.
The idea of adding a fan-shaft generator on the LP
spool has been widely accepted with the concept of MEE.
In a twin-spool aircraft engine, the generators on the HP
and LP spools operate at different frequencies. To parallel
the two generators with enhanced efficiency and
reduced size and weight, a dc primary generation system
with power electronic converters is preferred as an

Gearbox

Synchronous
Starter/
Generator

Three-Phase VF (360-800 Hz) 230-Vac Distribution

Auto Transformer
Rectifier Unit

Transformer
Rectifier Unit

Auto Transformer
Unit

±270-Vdc Distribution

28-Vdc Bus

115-Vac Distribution

Figure 1. The system configuration of the synchronous generator-based ac primary generation system.

IEEE Elec trific ation Magazine / D EC EM BE R 2 0 1 7

27



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