02-07-2013, 01:08 PM
A MATLAB/SIMULINK Model to study the performance of the VFT for the interconnection of Weak and Strong AC Grids
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Abstract:
This paper represents a new model of the
Variable Frequency Transformer (VFT) using
MATLAB/SIMULINK. The VFT is used to connect
two power systems. The simulations shown in the
paper accurately represents the VFT’s dynamic
characteristics. Based on this model, some further
simulations are conducted to study VFT’s
characteristics under fault conditions and its roles in
preventing the spread of faults into the other area. The
simulation results show that the VFT effectively
suppresses the power oscillations between the two
interconnected power systems and thus prevents the
faults from spreading.
INTRODUCTION
The variable frequency transformer (VFT) is a
controllable, bi-directional transmission device that can
transfer power between asynchronous networks.
Functionally, the VFT is similar to a back-to-back
HVDC converter. The technology is based on a rotary
transformer (continuously variable phase-shifting
transformer) with three-phase windings on both rotor and
stator. A drive system adjusts the VFT rotor position in
order to control the phase shift between the two networks
through the action of a fast power controller. The VFT
controls power transfer up to 100 MW in both directions.
network with and without the VFT was discussed by D.
Nadeau [10]. A comparison between the performance of
a back-to-back HVDC system with series compensation
external to the converter transformers, and a variable
frequency transformer for power transfer power between
asynchronous AC systems and flow control feeding or
supplying a weak AC network was introduced by B.
Bagen, D. Jacobson, G. Lane, and H. M. Turanli in
reference [11].
EXPERIMENTAL SETUP
In the above model of the VFT we used a wound rotor
induction machine with its rotor ends connected to
another source to simulate the VFT and test its operation
under different conditions for power flow control.
We then implemented a simple setup to test the
capability of the doubly fed induction machine to
transfer power between two asynchronous systems
without any control topologies to verify the idea of
power transfer.
The experimental procedure goes as follows:
1. We connected the system according to the
connection diagram shown in figure (17) and
the pictures of the equipments are as shown in
figures (18-21).
2. The speed of the dc motor driving the
synchronous generator was used to adjust the
frequency of the synchronous generator, to test
the power flow under different
frequencies.(49Hz, 50Hz and 51Hz)
3. At each frequency we adjusted the value of the
variac voltage that represents the grid at a
constant frequency of 50Hz. Hence the wound
rotor induction machine is connecting two
asynchronous systems to transfer the power
between them.
CONCLUSIONS:
This paper presents a complete and comprehensive
model of the VFT system using MATLAB/SIMULINK.
The model shows the dynamic performance of the VFT
system under faults and frequency disturbances. It shows
the VFT’s outstanding capability in improving power
stabilit. Also a simple test system was implemented
showing the change in power flow through the single
rotor machine with the change in voltage and frequency.