Proceedings of the 14th International Middle East Power Systems Conference (MEPCON’10), Cairo University, Egypt, December 19-21, 2010, Paper ID 106.
A New Simulink Model to study the VFT
performance when transferring power between
Weak and Strong AC Grids
Prof. Dr.Ahmed Hossam El Din
Dr.Mohamed Abdullah Ashraf
Department of Electrical Engineering
University of Alexandria
Egypt
Department of electrical engineering
University of Alexandria
Egypt
hossamudn@hotmail.com
mohamed_abdulla@ieee.org
Abstract :This paper represents a new model of the
Variable Frequency Transformer (VFT) using
MATLAB/SIMULINK .The model is then used to
connect two power systems .The simulations show that
the model 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 of the VFT show that the VFT
effectively suppresses the power oscillations between
the two interconnected power systems and thus
prevents the faults from spreading.
I.
INTRODUCTION
The VFT system is based on a combination of hydro
generator and transformer technologies .The variable
frequency transformer (VFT) is essentially a continuously
variable phase shifting transformer that can operate at an
adjustable phase angle. The core technology of the VFT is a
rotary transformer with three phase windings on both rotor
and stator. A drive motor and a variable speed drive system
is used to apply torque to the rotor of the transformer and
adjust the rotational position of the rotor relative to the
stator, thereby controlling the magnitude and direction of
the power flow through the VFT. The concept, theory and
structures of the VFTs, and then a description of the
characteristics and control strategies are presented by
Robert Gauthier, E .Larsen et al, [1-7]. They also use
control systems and strategies to show the dynamic
performance of the VFTs. Various applications where VFT
technology can be used as a tie between power system
control areas were discussed by J .J .Marczewski [8]. Its use
to facilitate connections between synchronous and
asynchronous systems is discussed along with its general
operational characteristics in these applications. The
stability characteristics of the Laredo area power system and
Eng .Mona Ibrahim
Department of Electrical Engineering
University of Alexandria
Egypt
eng_mona_ibrahim@ieee.org
the improvements made with the addition of the VFT was
introduced by P .Hassink, P .E .Marken, R .O'Keefe, and G .
R .Trevino in reference [9]. The VFT is found to provide
stability benefits similar to that of a local generator .In
particular, the VFT improves the system’s post-fault voltage
recovery characteristics and thereby extends the stability
margin even with zero power scheduled. The VFT is also
shown to enhance stability both prior to and following the
introduction of 345kV transmission into the Laredo area .
The VFT reaction to grid disturbances by showing curves of
real events like single phase to ground faults, three phase
faults, islanding, how its natural reaction helps weak
networks by comparing the results of a simulation of the
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].
The steady state and dynamic simulations show that both
technologies are able to control power flow accurately .The
variable frequency transformer consumes less reactive
power than a back-to-back HVDC system, provides faster
initial transient recovery and better natural damping
capability. A digital simulation model of VFT and its
control system are implemented by G. Chen, and X. Zhou et
al. [12, 13]. Studies on VFT had been accomplished with
these models, including VFT energization, speed regulation
of rotor, synchronizing, ramp power control, step power
control, dependent power limit control power supply for an
isolated passive system reactive power balance and fault
analysis. Hence it is important to implement a model for the
VFT to test its performance before installing one. Reference
[14] implements a model of the variable frequency
transformer using PSCAD/EMTDC. The model verification
is conducted and the characteristics are analyzed afterward.
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II.
The variable frequency transformer is used to control the
power flow between the two systems by means of
changing the rotor position with respect to the stator
(angle δ). To control the rotor position a DC motor is used
coupled with the rotor of the asynchronous machine, thus
controlling the rotor position according to the ordered
power. Figure (3) shows the controller of the DC motor.
VFT MODELING
Figure (1) illustrates a conceptual system diagram of the
VFT.
Figure1-System diagram of the variable frequency
transformer
The VFT model is constructed using the
MATLAB/SIMULINK software to study the dynamic
performance of the VFT when tie a weak AC grid to a
strong AC grid. We used the MATLAB software package
because other research papers used PSCAD/EMTDC to
build the model and hence we decided to use a new
software package which is equally reliable and accurate.
Figure 2 shows the proposed model.In the proposed
model, the VFT is modeled as a doubly-fed induction
machine, where the stator is connected to system 1 and
the rotor is connected to system 2.
Figure 3-The Controller
Upon starting the simulation the following sequence takes
place:
•
During starting the rotor will be open-circuit, and
the speed controller is applied on the dc motor
which is mechanically coupled with the doubly
fed IM to control the speed of the shaft to be
equal to the reference speed (the difference
between stator and rotor rotating fields)
•
After 5s from starting a check for the phase angle
differences across the circuit breaker is
performed, and when the difference is zero the
breaker is closed.
•
At t=10s the speed controller is replaced by
torque controller to control the shaft torque +ve
value or –ve value or zero (to control the power
flow) from stator to rotor or from rotor to stator
or zero power transfer (respectively).
Figure 2 -VFT Proposed Model
System 1:
Voltage: 220V (line to line RMS voltage)
Frequency: 60Hz
System 2:
Voltage: 220V (line to line RMS voltage)
Frequency: 50Hz
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Below are the output waveforms of the VFT model :
Applying a single line to ground fault:
Figure7 -Actual and reference speed upon
changing the power order
Figure 4-The actual and reference power
A single line to ground fault is to be applied to the system
at t=20s to t=22s .The waveforms are shown in figure 8.
Figure5-The actual and reference speed
Below we will discuss the effect of changing the power
order:
Figure8 -Actual and Reference power upon applying a
single line to ground fault
Change in power order:
We will study the effect of changing the power order to
test the system dynamic response, the power order is
changed from 5000W to 10000W at t=25secs.
Figure6-The actual and reference power upon
changing the power order
Figure9-The actual and reference speed upon
applying the single line to ground fault
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The simulations above show clearly that the vft can
significantly improve the power system stability, restrain
power oscillations, and thus prevent faults from spreading
into the neighboring power systems.
The vft response to frequency disturbances:
We will investigate here the behavior of the vft in case of
change in frequency in the ac grid to which the vft
connected.
Figure12-The AC current at the stator side of the
VFT.
The procedure would be as follows:
•
a change in frequency would take place at AC system
1 where the frequency changes from 60Hz to 66Hz at
t=30secs
•
the disturbance lasts till t=60secs and it returns back to
60Hz
Below are the waveforms of the VFT under frequency
disturbance :
Figure13-The AC current at the rotor side of the
VFT
From the above waveforms we can conclude that the VFT
system has a very good performance under frequency
disturbances. That is although the VFT may be
connecting weak AC grids however power could still flow
from the sending to the receiving end.
II. 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
stability, suppressing power oscillations and preventing
faults from spreading into the neighboring systems.
Figure10-The actual and reference power upon
frequency disturbance
III.
REFERENCES:
[1] Robert Gauthier, “A World-First VFT Installation in
Quebec,” Transmission and Distribution World, Nov 2004.
Available:
http://tdworld.com/mag/power_worldfirst_vft_installation/
[2] E. Larsen, “A Classical Approach to constructing a
power Flow Controller”, IEEE Power Engineering Society
Summer Meeting, 1999, Volume :2, pp 1192 – 1195, 18-22
July 1999
Figure11-the reference and actual speed upon
applying the frequency disturbance
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