Simulation of VFT For Power Transfer Between Two Networks

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CHAPTER 1
INTRODUCTION
1.1 GENERAL
The ever growing demand of electricity needs the adequate increase in generating
capacity as well as transmission & distribution infrastructure. The capital funding needed
for the improvement of high voltage transmission and distribution (T&D) infrastructure.
T&D lines in the power division assume a key part in meeting the energy demands. The
delay in the development of new transmission lines decreases the generation potential.
Developing the new transmission lines with least investment is of great concern for
power engineering. [1]. Table 1.1 Shows the electricity statics of few highest generating
nations.
Table 1.1 Statistics of electricity in top generating nations [2-3]
Country Power
Generation
(TWh/year)
(2015)
Power Access
(% of occupants)
(2014)
Consumption
per Capita (per
kWh) (2014)
Line Losses
(% of
output
power)
(2014)
China 5,682 100 3927 5
USA 4,324 100 12973 6
India 1,368 79.2 805 19
Russia 1,062 100 6603 10
Japan 995 100 7829 4
Germany 638 100 7035 4
Canada 632 100 15542 9
Brazil 586 99.7 2578 16
France 569 100 6944 6
South Korea 546 100 10564 3
United Kingdom 338 100 5130 8
Saudi Arabia 336 100 9411 7

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1.2 TRANSMISSION LINES
Transmission system is a link between generating stations and distribution
system. It receives power from generating station & delivers to the distribution system.
Fig. 1.1 indicates a general structure for power system.
Fig. 1.1 General Structure of power system
The transmission structure is a spine of the coordinated power structure. Fig. 1.2
Indicates the investment made & planned in power sectors of developing nations during
2000–2030.
Fig. 1.2 Investments by developing countries in power sector for the period 2000–2030

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1.3 DIFFERENT OPTIONS FOR TRANSMISSION OF ELECTRICAL ENERGY
Generally, there are two methods for transmission of electrical energy, which are
given as:
1. High voltage DC transmission system (HVDC).
2. High voltage AC transmission system (HVAC).
HVAC transmission offers more versatility in operation as compared to HVDC
and hence it is most popular. High voltage direct current (HVDC) structures are
discovered for long length transmission and to interface two asynchronous AC networks.
Table 1.2 demonstrates the transmission line voltage levels accepted in different
countries.
Table 1.2 Transmission voltage levels in various countries [1]
Country HV (kV) EHV(kV) UHV(kV) HVDC (kV)
China 220 500 1000 800
USA 115, 138, 230 345, 500, 765 1200 800
India 132,220 400,765 1200 800
Russia 110, 154, 220 330, 400, 750 1200 750
Japan 110, 132, 154,
187, 220,
500 1000 650
Canada 115,230 345,500,735 1200, 1500 1200
Brazil 138,230 345,440,500,750 1000, 1500 1000
UK 110, 132, 275 400 – 500
After development of power electronics based switching, high voltage direct
current (HVDC) technology is gathering acceptance, however it is economical only for
long distance (600-800 km) power transmission. Some areas, like rural communities in
Alaska have a vast amount of resources in both renewable and non-renewable and spread
in a remote location. Due to integration of the large amount of small power plant, it does

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suffer from the synchronization problem so that the large AC grid does not work, and
HVDC could be a better choice to avoid the problem of synchronization.
However, HVDC have the following constraints:
 The converter stations are costly and have restricted overloading capacity.
 Toward short transmission line, those losses in the converter stations might be higher
over On an AC transmission line to that same length.
 The HVDC causes degradation in the quality of power by introducing harmonics the
reactive power cannot be transmitted.
To overcome above mention problems, an alternative technology has been
recently developed and being used in commercial operation, for exchanging the power
between two asynchronous grids. This technology is Variable Frequency Transformer
(VFT).
1.4 VARIABLE FREQUENCY TRANSFORMER (VFT)
The General Electric was effectively commissioned and tested after installation of
first VFT of world in 2004 at Hydro-Quebec's near Langlois substation for
transformation of large amount of power between an asynchronous system, where the
transformation capacity of that VFT plant is 100 MW, and installed in between the two
asynchronous networks of Canada (Quebec) and USA (New York).[5]
In spite of the fact that those VFT ideas may be new, VFT supplies incorporate
well-established hydro-generator, dc motor. The VFT is basically a similar operating
function to a phase shifting transformer in with adjustable phase angle.
1.4.1 VFT OVERVIEW
VFT, a transformer which one side is rotating (another name as ―Runkle
Machine‖ inside GE) having mainly two parts stator and the rotor. Both sides are laced
with three phase windings. For connection with three phase rotor winding, a collector
system also having three phase winding is available there. Those two separate electrical
networks would associate with the both sides of Runkle machine individually. Electrical

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power may be transferred in between the two electrical systems alongside VFT via an air
gap. A drive motor having a variable speed is utilized for applying a torque on the rotor
side of VFT. Rotor regulates its rotational position with respect to stator power flow
through VFT. Fig. 1.3 illustrates a schematic of the VFT. The control mechanism is
always proportional to the difference between stator and rotor phase angle. [4].
Fig. 1.3 Connection diagram of the VFT [6]
VFT can transfer power in between or within both networks like synchronous and
asynchronous system. In the synchronous system, both the networks, which are to be
connected through a VFT, have both voltage and frequency is same. Therefore, the speed
of the field of both the sides of the Runkle machine is almost the same. Therefore, the
speed of the VFT rotor is zero (standstill position). Then transfer of power is also zero.
For exchanging of power from one side to another side, the VFT rotor has to be taken
into the dynamic position. For this, the external drive motor which is connected with VFT
rotor has to vary the value of the torque. When positive torque is applied on the rotor
shaft, the flows of power from one system to the other system and when negative torque
is applied on the rotor shaft, the flows of power in the opposite direction (from second
system to first system). Therefore, flow of power always depends on the direction and
magnitude of applied torque on the rotor shaft of VFT.
If a synchronous network leaves the state of its synchronism due to any
disturbance or behaves like an asynchronous network, then the VFT rotor rotates to

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maintain the continuous power flow, and this rotation is equal to the frequency difference
of both networks.
1.4.2 VFT COMPONENTS
A VFT is made by three parts – (a) drive motor (b) collector and (c) rotating
transformer. The different parts are indicated in Fig. 1.4 and Fig. 1.5.
Fig. 1.4 Rotating parts of VFT [7] Fig. 1.5 Assembly of three phase
VFT [6]
Stator of VFT has laminations for stacked steel bits inside the body of stator. The
bars of winding are arranged under a four pole & three phase system. The rotor needs
also three phases, four pole winding.
A drive motor is essential for the rotor shaft of the VFT, used to adjust the relative
motion of both stator and rotor fields. It’s basically a dc motor having low self-cooling
arrangement due to low rotational speed, hence additional cooling is required. The inertia

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of the whole rotating structure is relatively high. At the time of grid disturbances the high
inertia helps to keep balance of stability. [4]
The three phase collector is manufactured by traditional carbon-brush imposed on
slip rings which is made of copper. Those rings of collector are associated with the
windings of rotor by means of bus duct. Fig. 1.5 indicates a three phase collector
arrangement.
1.4.3 ADVANTAGE OF VFT TECHNOLOGY
Following are the advantages of VFT:
1. It provides easy and fully control path between two synchronous or asynchronous
grids, so that electricity can be transferred in both directions.
2. The VFT occupies a smaller space; a 100 MW VFT substation should occupy 30 m x
80 m space, while the HVDC substation with its same capacity takes 2 to 3 times
more space compared to the VFT.
3. The problem of harmonics in VFT is very low. This is present in the HVDC system
due to electronic equipment. VFT cannot cause undesirable communication with any
neighboring device such as generator or other device on the VFT connected grid.
4. When compared with its parallel technology HVDC, VFT is an inexpensive and have
a good coordination with traditional machines.
1.4.4 VFT SYSTEM APPLICATIONS
Some applications of VFT are given below.
1.4.4.1 CONTROL OF POWER FLOW WITHIN SYNCHRONOUS SYSTEMS
The most basic application of the VFT is to transfer and control the power within
the synchronous systems, most of the parts of the world have synchronous embedded
buses. There is also one grid one frequency system in India, in this application (Fig. 1.6)
VFT works exactly like a traditional phase angle regulating (PAR) transformer with
much greater accuracy & control than that. For controlling of real power flow via a
conventional PAR, the variable voltage and angle flows in windings are required, which

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are in the discrete step. This is in a fix proportion of movement and angle. Whenever the
loading power demand is changed, the real power flow and current also changes, so that
its angle automatically changes based on the fixed setting. PAR is a more useful tool for
regulation of long term, where power flow does not require the fine control level.
VFT System
Fig. 1.6 VFT plan within a power system.
Generally, an interconnection having VFT is used for the fast response and to
adjust the power flow at the faults occurring condition. For a controlling purpose, the
VFT can be run back, changing the direction of power flow, controlling telemetry-
dependent flow with a remote bus, and increasing the transfer capability as required.
Whatever line or structure the VFT is connected, it can be helpful in transmitting
an economic power flow. For example, assume that there is such a synchronous network
area having two independent zones, there is a big difference in per unit electricity charge,
and the distance between the both zones is less. At that place, the VFT can be installed to
reduce the price difference. However, in order to interconnect the both areas, the cost of
the installed VFT can be more, which can be compared with the power losses in the lines.
1.4.4.2 CONTROL OF POWER FLOW BETWEEN SYNCHRONOUS SYSTEMS:
For a few situations it might a chance to manage to control flow in existing tie
lines the middle of two independently dispatched energy networks (i.e. Differentiate
control areas) as demonstrated on Fig. 1.7, alternately on make a controlled tie line the
place none at present exists.

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Fig. 1.7 VFT system between control areas
VFT is a fully controllable and bidirectional technique. It can be installed between
two networks, which is connected to the uncontrolled tie line. Thus it will be helpful in
reducing the load from the previously installed tie line and fulfilling the desired power
requirement. Due to these qualities of VFT, makes it better in the field of power market
than other parallel techniques like PAR-regulated system and free-floating arrangement.
Tie-lines are used for load balancing in between two networks, while VFT can be
controlled and flow the power in both directions as per requirement. That is why the tie-
lines are seen as a source in so many the power markets.
For example, A VFT project of 300 MW was installed in between New Jersey and
New York City (NYC) of United States (US), which is a synchronous system. These two
zones (New York City and New Jersey) are part of Eastern Interconnections. Therefore,
both operate in the synchronisms state and there are so many tie-lines present between both
of them (mostly PAR-controlled). The energy charge in Northern New Jersey zone is lower
than New York City (NYC). So this connection has helped the people of New York City to
get electricity on low price.
1.4.4.3 CONTROL OF POWER FLOW BETWEEN ASYNCHRONOUS SYSTEMS:
The major advantage of VFT over traditional PAR is that it can also flow the
power between the synchronous and asynchronous both networks. As in Fig. 1.8 it’s
Existing Ties
or
Area B
VFT System

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shown a systematic interconnection diagram. Many electrical systems such as
conventional transformers, PAR and transmission lines, etc. all require synchronism state,
and they face many problems in asynchronous state and also stop working. While as,
VFT does not require synchronism state.
This kind of interconnection maintains the reliability of the respective systems in
that zone, which can get help from the neighboring network in the emergency fault
condition and can also help the neighboring network. It can also be an economical in
some situation, which was not possible previously by having an asynchronous system.
Fig. 1.8 Asynchronous power network connection through a VFT
For example, the VFT becomes tie-link between an asynchronous system, which
is the world's first VFT project of a 100-MW link, which is located in Langlois, and
installed between TransEnergie and Grid of Ontario. Furthermore in future Lots of
potential are possible between North America and Canada. There is already a lot of
HVDC links operating here. At The time of upgrade in capacities the VFT will prove to
be a useful technique in the future. And large coal resources have been found in the same
zone, which may be possible that electricity generation would be start in future by using
thermal power plant. The VFT will also be very useful for interconnection in that
condition. [8]
1.5 OBJECTIVE OF DISSERTATION WORK
The following objectives are set for this dissertation work:

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 To develop a simulation model for VFT.
 To use developed simulation model for power transfer within synchronous, in-
between synchronous & asynchronous grid.
 To check the behavior of VFT by varying the torque using dc drive motor.

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CHAPTER 2
LITERATURE REVIEW
2.1 LITERATURE REVIEW
VFT is a fast growing technology in field of power transmission with a controlled
direction and magnitude. The controlling is done by help of a dc motor, which is known
as drive motor. A lot of work has been done in this field in the last decade. Various
schemes and related concepts are proposed by different authors in this field of VFT.
Some papers with work relating to this field are given below.
Merkhouf et al. [4] proposed the basic concept of the VFT, a device by which a bi-
directional power can be transfer between an asynchronous power networks. Also, they
presented summary of the electromagnetic and mechanical design of a 4-pole unit and
rated at 100 MW, 17 kV, 60 Hz. Apart from this, they also mentioned the drawbacks of
VFT in two different conditions (with and without conjunction of power electronic
devices) when it is used as a phase-shifting transformer. They also focused on
application, concept, and mechanical design overview, theory of operation and
electromagnetic design evaluation of VFT.
Marczewski [8] described the applications of VFT technology, in which it can be used as
a link between the two power networks. It has been reported that till now, the HVDC link
is used to connect the asynchronous network, which is a very useful technique, but the
installation cost of HVDC is very high so that it cannot be used in short transmissions
lines. It is used primarily in medium and long transmission lines. VFT is described as an
alternative to HVDC equivalent technology, which can be used in asynchronous power
transmission as well as in synchronous power transmission. Apart from this, it can be also
used as a tool to meet other objectives like to connect the load areas with lower cost
generation areas, load sharing of two power grid, operating smoothly, and increasing grid
stabilization.
Bakhsh and Khatod [5] discussed the applications of VFT through digital simulation
result. Matlab-Simulink software has been used to represent the result and a Simulink

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model has been introduced. In this simple model, 2 power sources have been taken,
which reflects 2 independent power networks. According to this, power can be
transferred from one source to another source by changing the value and direction of the
rotor speed. In order to change direction and speed of rotor, an external variable torque-
drive system has been used which has been manually operated. Automated torque control
systems can also be used in the future. Power has been transferred with the help of
variable torque system and the results are represented by tables and wave form graphs.
Bakhsh and Khatod [9] investigated another application of the VFT, in which a stand-
alone wind energy conversion system (SWECS) based synchronous generator (SG) fed a
separate load through the help of VFT. Initially, it has been described by the conventional
method to convert wind energy into electrical energy, in which power electronic based
interface such as rectifiers, inventor, and low-pass filter have been used. But there is a
problem of harmonic distortion in such interfacing systems, which deteriorate the quality
of the power. So to avoid total harmonic distortion (THD) and to meet the standard power
demand, a low-pass filter (LPF) is used in the side of the inverter, which again increases
the system's price and complexity.
Therefore, to reduce all these problems, another model containing the VFT has
been shown to reduce the cost of the interface system. It has been also represented a
Simulink model for transmission the power from wind mill to load. The simulation model
were constructed with the help of Matlab-Simulink software and operated at different
loading condition, the result is shown with the help of tables and graphs and compared
the performance of both system (conventional and by using VFT).
Bakhsh and Khatod [10] proposed the investigation of the wind energy conversion
system, with grid through VFT as a control system. A step ahead in the same direction,
using the VFT, wind turbine generator (WTG) system on the basis of grid interconnection
of permanent-magnet synchronous generator (PMSG) is being seen as a useful way.
Conventional power electronic control system has not been used due to reduce the cost
and simplicity of circuits, and once again, it has been proposed to a new scheme using
VFT. For validating the proposed scheme, a simulation model has been made with the
help of Matlab-Simulink software.

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With the help of proposed model, flow of power from PMSG to grid has been
studied on its different PMSG speed. Along with this, power flow transfer plots have
been achieved on various torque positions. The received results such as efficiency and
THD of the output voltage are compared with the conventional method and finally the
negligible THD has been seen.
Ambati et al. [11] explained the working of VFT has been explained in detail, and
described some new applications such as to establish of a new hierarchical control
strategies for bidirectional power transmission system The middle of those two
asynchronous power framework networks and also to keep the grid error propagation
starting with particular case side on another side of VFT. The basic control is mainly
divided into three parts, which is given as below:
1. Frequency Matching
2. Phase angle matching
3. Power transfer control
To prevent the error propagation starting with particular case side on another side
of VFT a fault ride-through (FRT) scheme has been presented on the basis of a series
dynamic braking resistor (SDBR). Performance of VFT has been presented with the help
of actual time hardware in Loop (HIL) system at various conditions like synchronization,
grid error, steady state and dynamic. The plant has also been simulated with the help of a
real time simulator.
Bakhsh et al. [12] introduced a new model of VFT for bidirectional power transmission,
which is useful for transmitting the power between two asynchronous links and operating
function is same as back-to-back HVDC technology. NMVFT, which is a new
technology, has been primarily used to control v/f of induction motors. A model is
proposed to fulfill the NMVFT concept and as well as a digital simulation model for
validation has also been made, which is developed by using Matlab-Simulink software.
And the simulation results of the output voltage has been represented at different
conditions like when speed of the DC shunt motor is zero, when the direction of the rotor
is similar to the direction of the field, At the rotation of the rotor will be equivalent to that

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synchronous speed of stator field and with zero relative speed (in the same direction),
The point when that direction of the rotor is the inverse of the direction of the stator field,
When those rotations of the rotor may be equivalent to the synchronous speed of the field
and in the opposite direction (double relative speed) etc. In addition to this, the voltage-
frequency graph and voltage-torque characteristic have been represented and results are
shown on various speeds in a table.
Chen and Zhou [13] showed that a new technology VFT, can remove many problems in
power system interconnection. So that, using this VFT technology, they attempted to
overcome the problems that arise in interconnection systems in China. Interconnection
technology is being adopted in many countries such as China, North America, European
etc. to meet the power demand now days. There are two ways of interconnection. First
synchronous interconnection; in this method, both power systems need to be
synchronized which is connected to an AC line. For example power networks of Center
China, North China and Northeast China have synchronous interconnection and
connected to the 500kv AC transmission line. This is a simple way. But increment of
complexity in power system operation and decrement of system stability during the
severe fault condition etc. is generated. To overcome all these problems another way,
asynchronous power transmission technology can be used with the help of back-to-back
HVDC system. It has a capability of transmission of the large amount of power. But in
small lines and low power transmission it proves to be expensive due to its conversion
systems. Hence VFT is being seen as a very useful technology in this field and parallel to
HVDC technology.
Therefore a digital simulation model of VFT has been shown including with
Electromagnetic Transient Program (ETMP) control system. this VFT model has been
studied on some topics like energization of VFT, self-synchronization of VFT, ramp
power regulation, step power regulation, power supply for an remote passive system,
control of voltage through a shunt capacitors, voltage dependent power limit, fault
simulation.
El Din et al. [14] presented a model of the VFT to achieve the power transfer control. In
which power is being transferred with the help of VFT in between a strong and a weak

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AC grids. And this model has been represented with the help of Matlab-Simulink
software. In their model they considered two, three-phase sources as a two-grid area, both
are operating at different frequency (one is on 50Hz & another is on 60Hz) and same
voltage (220 volts). And the performance of the VFT has also been checked on the
condition of the faults, in which it has been found that VFT is a technology that avoids
the fault to spread into the neighboring system and enhances stability of the system. And
the evident can be seen in the simulation result.
Aravindhraj and Radha [15] presented an advance model of VFT for solving to the
problem of grid interconnection. In which the model is being used as a link to power
exchange, in the middle of two asynchronous networks, and true power is, no doubt
controlled. A variable torque system is used to control the true power. Also a detailed
study has been done on VFT technology and compared it with HFDC technology, and
highlighting the specialty of the VFT. It is said that the problem of harmonics is not
generated in the VFT system. Some points like VFT model, VFT analysis and reactive
power relationships have been described in particular. And the result is shown with the
help of simulation, for which Matlab-Simulink software has been used. The result
element such as the current waveform and waveform of three phase supply voltage is
shown with the help of software.
Yuan et al. [16] presented a simulation model of VFT technology for power transfer in
this paper, which is created with the help of power system computer aided design
(PSCAD) software. The development and verification of model is on one-machine
Infinite-Bus system and presented by a clean and accurate dynamic characteristic. Some
control systems have been used such as, phase angle control, start-up control, and
frequency control. With the help of this simulation model, some more simulation models
have been developed which proved to be very useful in the faults condition. And it
prevents the fault spreading from one place to another. Through the simulation study
VFT is compared with a conventional transformer and it has been explained, how it can
be remove the problems of harmonics between two interconnected networks and
improving the stability of power system networks.

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Pratico et al. [17] described the Laredo Project, commissioned by the company, which is
also the world's second VFT project. In the first project 2003 Hydro-Quebec Langlois
was commissioned to transfer the power between two asynchronous power networks. The
Laredo project is also used to exchange that power in the middle of two asynchronous
power system ARCOT (Texas) and CFE (Mexico), capacity 100 MW. This project works
similar to the back-to-back HVDC system. This paper describes main parts of VFT like
rotor & stator with 3-phase windings and a rotary transformer. A drive system has been
used to match the position of the rotor. With the help of this drive system the direction
and magnitude of real power is controlled and transferred. Also, the single line diagram
of the Laredo Project has been represented which contain, one DC motor, one variable
speed drive system, rotary transformer and two traditional step-up transformers. Along
with this, an auxiliary power feeder has also been used, which can be used at the time of
VFT repairing and maintenance to fulfill the continuous power demand. These paper are
also mentioned the applications of VFT and in the case of severe fault it is considered
very useful in the rescue of other equipment. Eventually, Laredo VFT response has been
shown in various conditions with the help of Real Time Simulator.
Marken et al. [18] discussed the suitability of the VFT with power transmission and
distribution related equipment such as existing electromechanical electronic
interconnection techniques, PARs and generators facts devices such as HVDC and
STATCOMs. Using VFT, all these actions like asynchronous or synchronous power
transfer, system-stability and grid shock-absorbers can be performed. In addition, in the
near future, it can be connected with a smart grid to flow the power smoothly, whose
proven compatibility is stated by the authors in this paper. For the authenticity of its
logic, the author has shown the result with the help of simulation, as well as some
operating results of General Electric's (GE) project which is first VFT of the world
(Langlois project) on the ON-load (running) condition are shown. In which the tripping
case of the generator is displayed when the frequency of the system gets down, but the
same time VFT balanced its frequency through rotor movement, and the output power
frequency remains the same. In this paper, the compatibility of the VFT has been seen
with the near parts of grid like- generation, traditional voltage support device, power
electronic device, phase shifter, and parallel with back -to-back HVDC.

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Bakhsh and Khatod [7] presented the state of the art review of VFT, in which VFT is
described as a bidirectional and fully controllable device which can transfer the power
between the both synchronous and asynchronous networks. VFT construction is similar
to the three-phase induction machine, in which two electrical networks are connected
respectively with three-phase stator winding and rotor winding. One power framework
associate of the VFT's stator side and another is associated with the VFT’s rotor side with
the help of the duct. There is an air gap between the stator and the rotor, and both are not
directly connected to any electrical connection to each other.
Power transfer is done by magnetic coupling from one network to another. In this
paper author has presented the single line diagram for the Langlois Project (World first
VFT). A wave form has been shown for the frequency balancing operation on the running
condition of the project, and the applications of VFT have been described in detail.
Khalik et al. [19] presented another machine to fulfillment of VFT applications, which
can complete all the tasks of VFT. Brushless doubly fed induction machine (BDFIM) can
be used as a bridge-connection between two different frequency networks. In which DC
motor are used for torque control. This is connected to the rotor shaft. This BDFIM can
be used to control those streams of power from claiming the middle of the two networks,
similar to the VFT. The basic difference between VFT and BDFIM is simply that in the
VFT one network Is associated with a stator winding and the another associated with the
rotor winding, whereas in this BDFIM both networks are associated with the two ends of
stator winding. Proposed Model of this technology is currently under construction which
has been given in the paper and a simulated result is shown to validate the validity of the
proposed system, which proves that these technologies can also use for bulk power
transmission.
2.2 GAPS IDENTIFIED
Some gaps are identified on the basis of literature review.
 VFT can be used to enable communication in the smart grids. Power system networks
can connect to SCADA system to facilitate unmanned control.

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 Fuzzy Logic Rules can be used with fault conditions for construction of a proposed
system.

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CHAPTER 3
VFT MODELLING
3.1 VFT MODELING
In this work, doubly-fed wound rotor induction machine (WRIM) has been used
to simulate VFT. A three-phase network is connected to the both sides of the Runkle
machine. Both power networks (#1 and #2) are connected to a three-phase connection of
stator and rotor winding, such as shown in Fig. 3.1. Stator winding is excited by
connected power network#1with Vs voltage and θs phase angle. Rotor winding is
excited by connected power network#2 with Vr voltage and θr phase angle.
A DC motor is connected with shaft of WRIM. This DC motor is operated by the
command of the torque control system, which is adjusted to the rotor position of the
VFT as per the requirement, and the direction and amplitude of the active power are
controlled.
VFT
Mechanical link
PD
Fig. 3.1 VFT model Representation [5]
The power flow may be in both directions as per requirement. The power transfer
direction for both systems may be negative or positive. For power network#1, It shows
the negative power flow when the transmission of power toward power network#1 and it
shows positive power flow when the transmission of power toward the power

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network#2. And similarly for power network#2, It shows the negative power flow when
the transmission of power toward power network#2 and it shows positive power flow
when the transmission of power toward the power network#1. The direction of power
flow depends on the value of torque applied on shaft connected rotor. [5]
3.2 BASIC THEORY OF MODELLING OF VFT
The basic theory of modeling is based on the ―power control theory‖. The power
control of VFT depends on the power regulator system (PRS). PRS of VFT is adjusts the
power by regulating the torque value. The flow of power through the VFT from
network#1 to network#2 can be approximated as: [4]
{ }
(3.1)
Where,
= VFT power from network#1 to network#2,
= Stator voltage or terminal voltage on network#1,
= Rotor voltage or terminal voltage on network#2,
= Reactance of Runkle machine,
= Stator voltage’s phase-angle,
r = Rotor voltage’s phase-angle,
= Runkle machine’s phase-angle,
= { } net angle or phasor relationships, indicated in Fig. 3.2 and
PXMAX = , maximum possible value of power transfer at ( radians)

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Fig. 3.2 Phasor diagram of VFT
For a steady state operation for VFT, angle ought to bring an outright worth
that is altogether under radians, which implies that the power exchange will make
restricted to a portion of the maximum theoretical level provided for toward (3.1). Inside
this range, the power exchange takes after a monotonic what's more about straight
association of the net point what's more it might be approximated by:
(3.2)
The power stream equations the following are shows a perfect Runkle machine,
at minimal leakage reactance furthermore magnetizing current. Further, the real power
and reactive power flow are represented by graph.
Fig. 3.3 represents to a VFT structure joined the middle of two power networks,
with a third power network giving work to a power source or sink related to states for the
torque-control drive arrangement.

23
Fig. 3.3 Flow diagram of VFT real power
Those directions of power flow demonstrated on Fig. 3.3 are similar to the sign
convention of generator like, for positive sign demonstrating power going outside from
the windings of machine and similarly for negative sign demonstrating power coming
inside the windings of machine. Yet the direction of real power might be positive and
also negative related to the functioning scenario.
On the basis of power flow direction from different source like network#1 or
network#2 and sometime torque control power system also need a power relation
between all. Then according to power balancing equation it can be represented as: (acc.
to Fig. 3.3):
– (3.3)
Where,
Ps = Stator side power (Electrical),
Pr = Rotor side power (Electrical),
PD = Drive motor power (Mechanical) to the torque-control drive system,

24
Since, there is a lot of similarity in between Runkle machine and transformer. So
that Runkle machine must fallow the ampere-turns relationship in middle of stator and
rotor:
(3.4)
Where,
= Stator winding turns,
= Rotor winding turns,
= Stator winding current,
= Rotor winding current,
Since both the winding of Runkle machine is connected with a common magnetic
field. But whenever, the frequency varies along these lines the voltage will also vary
toward that same ratio.
(3.5)
and (3.6)
or, (3.7)
Where,
= Rotor voltage or terminal voltage on network#2,
= Stator voltage or terminal voltage on network#1,
= Frequency of stator voltage (Hz),
= Frequency of rotor voltage (Hz) and
= Flux of air-gap medium.

25
At steady state condition there is no rotation in rotor of VFT but at a time of fault
occurring condition or presence of an asynchronous network VFT rotor rotates with the
speed of frequency difference of both systems i.e. the frequency difference between stator
and rotor ends.
– (3.8)
And (3.9)
Where,
= Frequency or Runkle machine electrical speed (Hz)
= Number of poles in VFT
= Angular velocity or Runkle machine mechanical speed (rpm).
From the above equations we get a new relationship and given as below:
–
(3.10)
therefore torque developed by the drive motor (TD) is:
( )

26
Or, (3.11)
From the above relation it is clear that drive motor torque depends on the
Stator winding turns ( ), frequency of stator voltage ( ) and air gap flux ( ). And it is
not depend on VFT rotational speed. Since at a normal operating condition it act as an
stationary machine i.e. no movement in VFT rotor have. [4]
3.3 SIMULATION MODEL OF VFT
The main aim of the work (Simulation of VFT for power transfer between two
networks) has been successfully done. The VFT working principle is similar to the
induction motor as a transformer. So recently in this thesis a Simulink model has been
designed to fulfill the power transfer task between and within two synchronous or
asynchronous networks. In this model two elements (asynchronous machine and 3-phase
source) are playing a vital role. Besides these two elements there are some other
important elements also used like RMS block, Three-phase V-I measurement, Three
phase instantaneous power measurement, Gain block, Bus selector, Scope and Display.
Both 3-phase sources are using like two individual sources. Simulation model is shown
in Fig.3.4.

27
Fig 3.4 Developed model of VFT.
3.4 MODEL DATA
3.4.1 ASYNCHRONOUS MACHINE
Implements a three-phase asynchronous machine (wound rotor, squirrel cage or
double Squirrel cage) modeled in a selectable dq reference frame (rotor, stator, or
synchronous). Stator and rotor windings are connected in wye to an internal neutral point.
Some rated parameters of the asynchronous machine are given as:
1. Nominal Power: 4 KW
2. Line to Line voltage: 400 Volts
3. Speed: 1430 RPM
4. Frequency: 50 Hz
5. Pole Pairs: 2

28
3.4.2 THREE PHASE SOURCE
Three-phase voltage source in series with RL branch, total two source boxes are
used to fulfill the requirement of two synchronous and asynchronous power system
zones. Parameters (Rated) of the three phase source are:
1. Phase to phase voltage: 415 Volts
2. Frequency: 50 Hz
3. Phase angle: 0°
3.4.3 THREE PHASE V-I MEASUREMENTS
Ideal three-phase voltage and current measurements, total two boxes are used for
measurement of both source data (voltage and current). The block can output the voltages
and currents in per unit values or in volts and amperes. But here output data (voltage and
current) has been represented in volts and amperes unit.
3.4.4 THREE PHASE POWER MEASUREMENTS
Compute the three-phase instantaneous active and reactive powers associated with
a periodic set of three-phase voltages and currents. Instantaneous value of active power &
reactive power is accurate only for balanced and harmonic-free three-phase voltages and
currents. Total two boxes are used for measurement of both source data (active power &
reactive power).
3.4.5 BUS SELECTOR
This block accepts a bus as input which can be created from a Bus Creator, Bus
Selector or a block that defines its output using a bus object. There are two list boxes, in
the left list box shows the signals in the input bus. By use of ―Select‖ button two output
signals are selected here. The right list box shows the selected signals. Use the ―Up‖,
―Down‖ or ―Remove‖ button to reorder the selections. The both selected signals are
given as:
1. Mechanical rotor angle thetam (rad)
2. Mechanical rotor speed (wm)

29
3.4.6 GAIN
Gain box is used to convert the unit of signals. There are two gain boxes are used,
first box (Gain 1) is used to convert the unit of rotor speed from radian/second to
cycle/second and second box (Gain 2) is to convert the unit of rotor angle from radian to
degree. The gain value for both boxes is given as:
1. 30/
2. 180/
3.4.7 RMS BLOCK
Measure the true root mean square (RMS) value of the input signal at the
specified fundamental frequency. Here two different frequencies (50 Hz and 60 Hz) are
selected in third condition for exchanging the sources power. Total four boxes are used
here. When True RMS value parameter is unchecked, the block outputs the RMS value of
the fundamental component of the input signal.
3.4.8 DISPLAY AND SCOPE
These are used to show the input and output data. In the model total six scopes are
used for measurement of wave form of voltage and current, active power and reactive
power, torque and rotor speed. Here Total 10 Displays are used to show the result in
numeric form for same parameters.

30
CHAPTER 4
RESULTS AND DISCUSSIONS
The simulated results have been taken for three different cases and each case is
divided in to two different conditions. So that total six results has been represented by an
individual table and with simulated graphs at different torque condition.
4.1 POWER FLOW CONTROL WITHIN SYNCHRONOUS POWER NETWORK:
For the analysis of power flow control within synchronous power systems
networks in a new version software of Matlab-16a, the both power network#1 and power
network#2 are kept as a same line voltage and frequency power system. i.e. 415V, 50 Hz
supply. So that the model, represented in Fig. 3.4, have been used to transfer and control
to the flow of the both power networks. On the varying of torque value (from 0-30 Nm)
in positive and negative cycle the current, voltage, reactive power and active power in
power network#1 and power network#2 are simulated. The simulated waveforms of
torque, rotor speed, stator voltage, stator current, stator active power, stator reactive
power, rotor voltage, rotor current, rotor active and rotor reactive power are shown in
Figs. 4.1- 4.7.
4.1.1 POWER TRANSFER FROM POWER NETWORK#1 TO POWER NETWORK#2:
From the plotted graphs result it is clear that, the power transfer through the
power network#1 and power network#2 is either positive or negative not zero, under
different torque condition which is provided by a DC motor. The supply of both power
network#1 and power network#2 are kept same for all torque conditions and the power
transfer is shown in Table 4.1 and Figs. 4.1- 4.4.
Table 4.1 Simulation result for power transfer within synchronous power network from
Power network#1 to Power network#2 (positive torque):
S.No.
Td Power network#1 (Stator side) Power network#2 (Rotor side)
(Nm) Vs Is Ps Qs Vr Ir Pr Qr
(Volt) (Amp) (W) (VAR) (Volt) (Amp) (W) (VAR)
1 0 239.6 2.19 9.5 1575.48 239.6 2.15 30.97 1546.1

31
2 5 238.8 1.4 786.41 615.265 240.5 3.66 -721.78 2538.26
3 10 237.6 2.28 1584.62 -308.2 241.4 5.3 -1445.16 3554.71
4 15 236.5 3.81 2410.85 -1202.43 242.2 7 -2144.7 4608.05
5 20 235.5 5.47 3264.76 -2066.1 243 8.72 -2819.96 5697.86
6 25 234.3 7.19 4143 -2893.8 243.7 10.46 -3467.05 6816
7 30 233.1 8.95 5048.5 -3686.65 244.4 12.22 -4087.35 7967.46
From the Table 4.1, It can be seen that in starting (Td=0) there is no (zero) power
is being transferred from any of power network#1 or power network#2 through VFT,
While the both networks have positive value of power. Therefore VFT is behaving like a
power consumer for both the power networks i.e. the algebraic summation of power at
VFT is zero. For the positive torque conditions, the power transmission takes place from
power network#1 to power network#2. The negative sign of Pr (active power in rotor
side) shows the power transfer toward power network#2 i.e. the Table 4.1 represent
power network#1 as a sending end and power network#2 as a receiving end.

32
For Td= 0 Nm, below Fig. 4.1 shows the simulated result waveforms.
Fig. 4.1 Waveforms showing torque, rotor speed, current, voltage, reactive power and
active power transfer with Td= 0 Nm.

33

34

35

36
Table 4.2 shows the plotted graphs result. The power network#1 and power
network#2 is either positive or negative not zero, under different torque condition which
is provided by a DC motor. The supply of both power network#1 and power network#2
are kept same for all torque conditions and the power transfer is shown in Table 4.2 and
in Fig. 4.5- 4.7.
Table 4.2 Simulation result for power transfer within synchronous power networks from
Power network#2 to Power network#1 (negative torque):
S.No.
1 0 239.6 2.19 9.5 1575.48 239.6 2.15 30.97 1546.1
2 -5 240.5 3.7 -739.15 2563.27 238.6 1.38 805.62 591.75
3 -10 241.3 5.34 -1465.15 3588.67 237.6 2.29 1610.07 -337.3
4 -15 242.1 7.03 -2160.88 4638.64 236.5 3.82 2433.43 -1225.62
5 -20 242.9 8.75 -2833.47 5723.25 235.4 5.5 3285.51 -2083.88
6 -25 243.7 10.48 -3479.33 6844.34 234.3 7.21 4165.45 -2912.04
7 -30 244.4 12.24 -4095.81 7994.31 233.1 8.96 5067.96 -3701.69
In a Table 4.2, it shows that the direction of power transmission can be change by
changing of the torque direction i.e. for negative torque condition the power transmission
from power network#2 to power network#1 takes place. The negative sign of Ps (active
power in stator side) shows the power transfer towards the power network#1 i.e. the
Table 4.2 represent power network#2 as a sending end and power system#1 as a
receiving end.

37
active power transfer with Td= .

38

39

40
4.2 POWER FLOW CONTROL IN-BETWEEN SYNCHRONOUS POWER
NETWORK:
For the analysis of power flow control in-between synchronous power networks,
the both power network#1 and power network#2 are different power system having
different line voltages and same frequencies. The power network#1 is kept at 415V, 50
Hz of supply while power network#2 is kept at 400V, 50 Hz. So that the model,
represented in Fig. 3.4, have been used to transfer and control to the flow of the both
power networks. On the varying of torque value (from 0-30 Nm) in positive and negative
cycle the current, voltage, reactive power and active power in power network#1 and
power network#2 are simulated. The simulated waveforms of torque, rotor speed, stator
voltage, stator current, stator active power, stator reactive power, rotor voltage, rotor
current, rotor active and rotor reactive power are shown in Figs. 4.8- 4.14.
Table 4.3 Simulation result for Power transfer between synchronous power networks
from Power network#1 to Power network#2 (positive torque):
S.No.
1 0 239.5 4.44 73.87 3194.61 230.9 0.18 11.01 -127.2
2 5 238.5 3.29 824.62 2202.23 231.9 1.635 -767.13 832.2
3 10 237.5 2.86 1597.17 1248.67 232.8 3.4 -1513.94 1815.95
4 15 236.6 3.42 2401.54 321.94 233.8 5.16 -2240.52 2843.12
5 20 235.5 4.65 3228.94 -567.3 234.6 6.93 -2936.77 3896.47
6 25 234.4 6.16 4084.73 -1424.1 235.4 8.71 -3607.18 4986.49
7 30 233.2 7.8 4969.02 -2248.2 236.2 10.52 -4250.3 6114.7

41
power network#1 as a sending end and power network#2 as a receiving end.

42
Fig.4.8 Waveforms showing torque, rotor speed, current, voltage, reactive power and
active power transfer with Td = 0 Nm.

43

44

45

46
are kept same for all torque conditions and the power transfer is shown in Table 4.4 and
in Fig. 4.11- 4.14.
Table 4.4 Simulation result for Power transfer between synchronous power network from
Power network#2 to Power network#1 (negative torque):
S.No.
1 0 239.5 4.44 73.87 3194.61 230.9 0.18 11.01 -127.2
2 -5 240.3 5.91 -649.23 4218.17 229.9 1.91 814.07 -1050.58
3 -10 241.1 7.51 -1344.66 5273.14 228.8 3.68 1642.1 -1938.43
4 -15 241.9 9.18 -2012.95 6362.74 227.7 5.47 2497.4 -2793.22
5 -20 242.6 10.9 -2654.61 7487.97 226.5 7.26 3381.11 -3614.42
6 -25 243.3 12.65 -3264.45 8644.13 225.2 9.07 4287.6 -4396.43
7 -30 244 14.43 -3849.54 9843.08 223.9 10.91 5230.9 -5146.3
Table 4.4 represent power network#2 as a sending end and power network#1 as a
receiving end.

47
Fig.4.12 Waveforms showing torque, rotor speed, current, voltage, reactive power and

48

49

50
4.3 POWER FLOW CONTROL IN-BETWEEN ASYNCHRONOUS POWER
NETWORK:
For the analysis of power flow control in-between synchronous power system
networks, the both power network#1 and power network#2 are different power system
having different line voltages and different frequencies. The power network#1 is kept at
415V, 60 Hz of supply while power network#2 is kept at 300V, 50 Hz. So that the
model, represented in Fig. 3.4, have been used to transfer and control to the flow of the
both power networks. On the varying of torque value (from 0-30 Nm) in positive and
negative cycle the current, voltage, reactive power and active power in power network#1
and power network#2 are simulated. The simulated waveforms of torque, rotor speed,
stator voltage, stator current, stator active power, stator reactive power, rotor voltage,
rotor current, rotor active and rotor reactive power are shown in Figs. 4.15- 4.21.
4.3.1 POWER FLOW FROM POWER NETWORK#1 TO POWER NETWORK#2:
Table 4.5 Simulation result for Power transfer between asynchronous power network
from Power network#1 to Power network#2 (positive torque):
S.No.
1 0 233.6 9.06 340.91 6308.3 172.9 5.43 113.25 -2819.03
2 5 232.7 7.66 1184.81 5107 174.4 4.08 -727.64 -2007.04
3 10 231.9 6.67 2068.8 3973.4 175.8 3.64 -1529.1 -1150.9
4 15 230.9 6.23 2991.67 2904.3 177.1 4.35 -2292.55 -250.6
5 20 229.8 6.44 3953.24 1900 178.2 5.79 -3018.14 691.12
6 25 228.6 7.24 4952.52 959.4 179.3 7.56 -3706.3 1675.03

51
power network#1 as a sending end and power system#2 as a receiving end.

52

53

54

55

56
4.3.2 POWER FLOW FROM POWER NETWORK#2 TO POWER NETWORK#1:
are kept same for all torque conditions and the power transfer is shown in Table. 4.6 And
in Fig. 4.19- 4.21.
Table 4.6 Simulation result for Power transfer between asynchronous power networks
from Power network#2 to Power network#1 (negative torque):
S.No.
1 0 233.6 9.06 340.91 6308.3 172.9 5.43 113.25 -2819.03
2 -5 234.3 10.72 -460.8 7580.44 171.4 7.2 995.47 -3584.88
3 -10 235 12.57 -1217.98 8927.25 169.7 9.24 1921.45 -4301.34
4 -15 233.8 39.08 -5315.38 -21687.9 146.9 40.07 7566.8 14503.6
5 -20 235.4 38.86 -10985.9 -20263.4 147.7 39.3 8625.02 13690.5
6 -25 237.6 37.43 23790.9 -3607.2 151.8 38.52 11144.1 14540.5
7 -30 234.5 38.57 -2962.38 28905.9 148.3 40.45 9921.43 13902.5
Table 4.6 represent power network#2 as a sending end and power network#1 as a
receiving end.

57
active power transfer with = 10 Nm.

58

59

60
CHAPTER 5
CONCLUSION & FUTURE SCOPE
5.1 CONCLUSION
This report gives an idea about efficient and fully controlled operation of power
transmission in between or within two synchronous or asynchronous networks by using
VFT. Several advantage and application with their challenges and remedies have been
implemented by VFT. The proposed Simulink model also provides efficient management
and fully controlled between grids by using adjustable torque drive system.
Matlab/Simulink version 2016a software has been used to prepare the model of
VFT. Based on the simulation result following conclusions are drawn:
1. VFT is a very simple technology to exchange and control the power between
synchronous or asynchronous networks.
2. The Simulink model shown in Fig. 3.4 has been tested on various synchronous and
asynchronous network conditions, in which the power direction and magnitude are
controlled with the help of variable torque drive system. Also the rotor and star power
transfer plot, torque, rotor speed, voltage and current, are also shown.
3. It is quite clear from the simulated results that the power control is directly related to
the external torque drive system and it is its proportionate. Therefore, both the
direction and magnitude of power is controlled with the help of torque.
5.2 FUTURE SCOPE OF WORK
The VFT might be of a great potential use in future. The future scope to further
enhance the attractiveness of VFT is given below.
(i) This could be used as a bulk power transmission system as a power system
stabilizer (PSS).
(ii) There could be recognized a particular interconnection of power system in India
and can be find a specific potential point for VFT installation, then study about
working environment of that point and check that how a VFT can solve those
problem like power flow, frequency oscillation and asynchronous interconnection.

61
(iii) By using VFT, pumps or hydro turbines could be operated near about their
maximum efficiency condition. It could also be used to stabilize the networks and
for smooth operation. [4]

62
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review‖ Renewable and Sustainable Energy Reviews 39 (2014)949–974
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[accessed on 12.01.17].
[3] Power infrastructure indicators. Available from:
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[accessed on 19.02.17].
[4] Arezki Merkhouf, Sanjoy Upadhyay, and Pierre Doyon, ―Variable Frequency Transformer –
An Overview‖, Power Engineering Society General Meeting, 2006.
[5] Farhad Ilahi Bakhsh and Dheeraj Kumar Khatod, ―Digital Simulation of VFT Applications
between Power System Networks‖ Fourth International Joint Conference on Advances in
Engineering and Technology, (2013), India
[6] Arezki Merkhouf, Pierre Doyon, and Sanjoy Upadhyay, ―Variable Frequency Transformer—
Concept and Electromagnetic Design Evaluation‖ IEEE Transactions on Energy Conversion,
23 (2008) 989-996.
[7] Farhad Ilahi Bakhsh and Dheeraj Kumar Khatod, ―Variable Frequency Transformer - State of
the Art Review‖ Int. Con. on Energy Efficient Technologies for Sustainability (ICEETS),
(2013) 1012-1017
[8] John Marczewski ―Variable Frequency Transformer: Applications between Grid Control
Areas‖, Power Engineering Society General Meeting, 2007.
[9] Farhad Ilahi Bakhsh and Dheeraj Kumar Khatod. ―A new synchronous generator based wind
energy conversion system feeding an isolated load through variable frequency transformer‖
Renewable Energy 86 (2016) 106-116.
[10] Farhad Ilahi Bakhsh and Dheeraj Kumar Khatod, ―Application of variable frequency
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[11] Bharath Babu Ambati, Parag Kanjiya, Vinod Khadkikar, Mohamed Shawky El Moursi, and
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VOL. 30, NO. 1, MARCH 2015

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[12] Farhad Ilahi Bakhsh, Mohammad Irshad, and Shirazul Islam, ―New Model Variable
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ACEEE Int. J. on Electrical and Power Engineering, 02, (feb 2011)
[13] Gesong Chen, and Xiaoxin Zhou, ―Digital Simulation of Variable Frequency Transformers for
Asynchronous Interconnection in Power System‖ Transmission and Distribution Conference
& Exhibition: Asia and Pacific IEEE/PES, Dalian, China 2005
[14] Ahmed hossam El Din, Mohamed Ashraf Abdullah and Mona Ibrahim, ―A Novel Model to
Study the VFT Performance When Controlling Power Transfer between Weak and Strong
AC Grids Using MATLAB/SIMULINK‖ IEEE International Energy Conference, 2010.
[15] Aravindhraj.V. and Radha P., ―Power Transfer In-Between Two Power System Networks
Using Variabl Frequency Transformer‖ Int. J. of Advanced Information Science and
Technology (IJAIST) Vol.23, March 2014
[16] Rongxiang Yuan, Ying Chen, Gesong Chen, and Yong Sheng, ―Simulation Model and
Characteristics of Variable Frequency Transformers Used for Grid Interconnection‖ IEEE
Power & Energy Society (PES) General Meeting, 1 (2009). 1-5
[17] Elizabeth R. Pratico, Carl Wegner, Einar V. Larsen, Richard J. Piwko, Daniel R. Wallace,
and David Kidd, ―VFT Operational Overview –The Laredo Project‖ IEEE Power & Energy
Society (PES) General Meeting, Tampa.1 (2007)
[18] Paul.E. Marken, John J. Marczewski, Robert D’Aquila, Paul Hassink, Jim H. Roedel and
Richard L. Bodo, ―VFT – A Smart Transmission Technology That Is Compatible With the
Existing and Future Grid‖ Power Systems Conference and Exposition (PSCE) IEEE/PES 1
(2009) 1-7.
[19] A.S. Abdel-Khalik, A. elserougi, S. Ahmed, and A. Massoud, ―Brushless Doubly Fed
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Drives (PEMD 2012), 6th IET International Conference on 27-29 March 2012.
[20] Srinivasa Kishore T ―Cost Optimization of Power Transmission Lines in Hilly Areas‖, PhD
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