IEEE Electrification Magazine - December 2017 - 32

energy to the power source, such as a main engine or APU
generator. Using the energy-restoring method with ESEs, a
separate electric actuator bus can be created to protect the
main electrical power network from disturbance and risks.
However, the ESEs, jointly with multiple bidirectional
power converters, require considerable additional hardware. Returning the regenerative energy to the power
source has minimal hardware requirements, but the
returning regenerative energy needs to pass through the
main electric power network. This could expose the entire
aircraft electrical power grid to risks such as instability of
the bus voltage regulation.
The sources that can absorb the regenerative power in
the aircraft include the turbine shafts of the main engines
and APU engine. Returning the regenerative energy to the
main-engine shaft is presently prohibited by the U.S. Federal
Aviation Administration due to engine operation reliability
concerns. As a result, the APU engine turbine shaft becomes
a favorable choice as the power source for regenerative
power absorption. A power generation and management
system architecture that allows the regenerative power from
the actuators to be absorbed by the turbine shaft of the APU
is shown in Figure 8. In this architecture, the open-end winding topology is adopted to provide a direct power-flow path
from the EHA/EMAs to the power source and create a separate electric actuation bus without a significant additional
hardware requirement. Since induction generators present

higher power density compared to WFSGs and better robustness and fault-tolerant capability compared to PM generators in MEA systems, an OEWIS/G is used as the APU starter/
generator. The configuration of the APU system with regenerative power management capability is shown in Figure 9.
The generator terminals are connected to two IRUs. The
main dc bus is connected to the left side of the generator terminals through IRU 1, while the electric actuation dc bus is
located at the right side of the generator terminals through
IRU 2. In this way, the operation of the aircraft main dc power
network is not affected by the perturbation and disturbance
caused by the actuators.
In MEA systems, the integrated APU can also act as an
ECS and thermal management system (TMS) during the
flight mission (cooling mode). The four major operating
modes of the integrated APU for power generation and the
management system are explained in the following section.

Self-Start Mode of Operation
As shown in Figure 10(a), in self-start mode, the electric
actuation dc bus is disconnected from the system. The
right side terminals of the induction machine are shorted
through IRU 2. The open-end winding induction generator
is thereby transformed into a wye-connected induction
motor. IRU 1 is operated as an inverter, driving the induction machine to accelerate the APU turbine shaft using the
electrical power from the aircraft battery or ground power

Inverter/
Converter
Unit

Electric Drives

EHA /EMA

Inverter/
Converter Unit

APU

Engine

ES/G

Power
Distribution
Network

ES/G

Loads

Inverter/
Converter Unit
Figure 8. The power generation and management system architecture with regenerative power absorption capability. ES/G: electrical
starter/generator.

32

I E E E E l e c t r i f i cati o n M a gaz ine / DECEMBER 2017



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