IEEE Electrification Magazine - December 2014 - 28

1.6
1.4
1.2

Speed

1
0.8
TEM: Turbine Engine Model
MTE: Motor-Emulating TEM

0.6

W_ref = 1.0 MTE
W_ref = 1.0 TEM
W_ref = 1.5 MTE
W_ref = 1.5 TEM

0.4
0.2
0

0.5

1
Torque

1.5

Figure 5. Speed versus torque characteristics of gas turbine model
(open loop) and motor drive emulation in closed loop.

load-frequency control is the main control loop during normal
operating conditions. The temperature control reduces the
output power of the gas turbine if the temperature of the
exhaust gases exceeds the limit value. The output of the two
control loops are input to a minimum-value gate so the loop
that takes control is the one with the lower output of the two.
In the simulation, a 48-MW turbine base
and the associated parameter values
were used. To check the performance of
the two control loops, time-domain
simulations of several disturbances
have been conducted. For various given
torque loads, a speed controller modulates the fuel input to maintain the rotational speed constant.
Next, how to achieve the speed-
torque characteristic of the gas turbine model at different speeds is
described. For the fixed rotational
speed, the speed is kept constant as
the load is increased. When the torque
reaches the maximum value, the
speed begins to decrease as the fuel
supply reaches its limit just after the
engine has reached its maximum
power output. Figure 5 shows the simulation results of using two different
speeds, and the two curves present a
similar tendency. This analytical model will be called a turbine engine observer in the following discussion.

shaft, which simulates a coupled turbine engine-generator.
The load torque from the electrical loading of the generator
goes to the turbine engine observer, and it generates the
speed reference that goes to the motor speed controller.
Hence, this model enables an emulation of the analytical
performance of the speed-torque characteristic in the
closed-loop control.
Figure 6 shows a complete two-motor drive system that
consists of an ac motor drive model from the simulator
library and a brushless dc motor drive model from the
library, coupled by a mechanical shaft model. The ac motor
drive model is a three-phase synchronous-motor-based
drive system that produces sinusoidal output, while the
brushless dc motor drive model is a three-phase motor
drive system that produces trapezoidal output. For step
changes in commanded torque, the trapezoidal output
provided a simulation with less numerical noise. This coupled model can emulate the speed-regulated turbine
engine and the torque-regulated generator. Note that the
motor type represented can be easily changed by changing
the commutation method. The signs of the ac electric
torque and speed should be the same for the motor that
mimics the turbine engine when it is driving. The torque
and speed should be of opposite signs for the motor that
simulates the generator. The tracking performance of the
developed speed controller and torque controller is
analyzed following step changes in the electrical loading
and unloading of the generator.
Figure 7 shows the simulation
results of the ac motor drive startup at
reference load followed by the application of speed ramp and load disturbance torques. The ac motor speed
reaches the reference value of 400 r/
min at t = 1.0 s. At that moment, the
ac electric torque drops down to 10
N∙m. Then, at t = 1.3 s, a reference
torque of 0 N∙m is applied to the
brushless dc motor drive; the ac
motor drive electric torque immediately drops down to zero to maintain
the regulated speed. At t = 1.8 s, a reference torque of +10 N∙m is applied to
the brushless dc motor drive, forcing
the ac motor drive to operate as a generator and the brushless dc motor
drive to operate as a motor. Finally, a
negative reference speed ramp of −400
r/min is applied to the ac drive at t =
2.4 s. A new steady state is reached at t = 2.85 s, and the ac
motor drive electric torque stabilizes at −10 N∙m. Notably,
the ac motor drive responded well to a variety of the speed
ramps and load-disturbance torques.
Now, we will discuss the implementation of a gas turbine
observer into the motor-generator drive. We describe a
method by which the ac motor and drive can be controlled

An advantage of the
turboelectric drive is
that it allows the
adaptation of highspeed turboshaft
engines to the slowly
rotating propellers or
wheels without the
need for a heavy and
complex gearbox.

Simulink modeling of Turbine
engine-Generator coupled by Shaft
The goal of this section is to describe an analytical model
for a two-motor drive system coupled by a mechanical

I E E E E l e c t r i f i c ati o n M agaz ine / december 2014

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2014