Operational VFT instalation in Laredo. It shows how it works the VFT conecting two grids

1. 1
Abstract-- A variable frequency transformer (VFT) is being
installed at AEP’s Laredo substation, where it will be used to
exchange up to 100 MW of power between the asynchronous
power grids of ERCOT (Texas) and CFE (Mexico).
Functionally, the VFT is similar to a back-to-back HVDC
converter. The core technology of the VFT is a rotary
transformer with three-phase windings on both rotor and stator.
A motor and drive system are used to adjust the rotational
position of the rotor relative to the stator, thereby controlling the
magnitude and direction of the power flowing through the VFT.
The world’s first VFT was installed at Hydro-Quebec’s Langlois
substation. Some additional control features have been included
in the Laredo VFT design, related to tie flow regulation, power
runback, reactive power control, and black start capability. This
paper describes the VFT application at Laredo and includes
simulation tests of performance during severe grid disturbances.
Index Terms—Asynchronous, interconnection, VFT, back-to-
back HVDC, power transmission, tie flow regulation, reactive
power control, black start.
I. INTRODUCTION
he first variable frequency transformer (VFT) was
commissioned at Hydro-Quebec’s Langlois substation in
2003 and has been reliably transferring power between two
asynchronous networks [1]-[3]. The second VFT is being
installed at AEP’s Laredo substation near the Texas-Mexico
border and is the subject of this paper.
A single channel can transfer up to 100 MW in either
direction. Power can be expanded with multiple channels.
The VFT has operating capabilities similar to a back-to-back
HVDC converter station. It also has some special control
characteristics and is able to ride through severe grid
disturbances [3]. AEP selected VFT over conventional back-
to-back HVDC and voltage-source converter back-to-back
devices as the “best technical fit,” due in part to VFT’s
superior stability performance [4].
E. R. Pratico, C. Wegner, E. V. Larsen, and R. J. Piwko are with GE Energy
in Schenectady, NY 12345 USA (email: elizabeth.pratico@ge.com).
D. R. Wallace is with GE Energy in Syracuse, NY 13088 USA.
D. Kidd is with AEP in Tulsa, OK 74119 USA (email: dekidd@aep.com).
This paper was prepared and submitted to IEEE for an invited presentation at
the panel session on "Rethinking Interconnections: Design Concepts and
Operational Experience of the Variable Frequency Transformer (VFT)" at the
2007 IEEE PES General Meeting, Tampa.
II. GENERAL VFT CONCEPT AND CORE COMPONENTS
The variable frequency transformer (VFT) is essentially a
continuously variable phase shifting transformer. The core
technology of the VFT is a rotary transformer (RT) with
three-phase windings on both rotor and stator (see Figure 1).
The collector system conducts current between the three-
phase rotor winding and its stationary buswork. One power
grid is connected to the rotor side of the VFT and another
power grid is connected to the stator side of the VFT.
Power flow is proportional to the angle of the rotary
transformer, as with any other ac power circuit. The
impedance of the rotary transformer and ac grid determine the
magnitude of phase shift required for a given power transfer.
Figure 1. Core components of the VFT
Power transfer through the rotary transformer is a function
of the torque applied to the rotor. If torque is applied in one
direction, then power flows from the stator winding to the
rotor winding. If torque is applied in the opposite direction,
then power flows from the rotor winding to the stator winding.
If no torque is applied, then no power flows through the
VFT Operational Overview –
The Laredo Project
Elizabeth R. Pratico, Member, IEEE, Carl Wegner, Einar V. Larsen, Fellow, IEEE,
Richard J. Piwko, Fellow, IEEE, Daniel R. Wallace, and David Kidd, Member, IEEE
T
1-4244-1298-6/07/$25.00 ©2007 IEEE.

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rotary transformer. Regardless of power flow, the rotor
inherently orients itself to follow the phase angle difference
imposed by the two asynchronous systems, and will rotate
continuously if the grids are at different frequencies.
Torque is applied to the rotor by a drive motor, which is
controlled by the variable speed drive system. When a VFT is
used to interconnect two power grids of the same frequency,
its normal operating speed is zero. Therefore, the motor and
drive system is designed to continuously produce torque while
at zero speed (standstill). However, if the power grid on one
side experiences a disturbance that causes a frequency
excursion, the VFT will rotate at a speed proportional to the
difference in frequency between the two power grids. During
this operation the load flow is maintained. The VFT is
designed to continuously regulate power flow with drifting
frequencies on both grids.
A closed loop power regulator maintains power transfer
equal to an operator setpoint. The regulator compares
measured power with the setpoint, and adjusts motor torque
as a function of power error. The power regulator is fast
enough to respond to network disturbances and maintain
stable power transfer.
Reactive power flow through the VFT follows
conventional ac-circuit rules. It is determined by the series
impedance of the rotary transformer and the difference in
magnitude of voltages on the two sides.
Unlike power-electronic alternatives, the VFT produces no
harmonics and cannot cause undesirable interactions with
neighboring generators or other equipment on the grid.
III. LAREDO 100 MW VFT STATION
AEP’s Laredo substation is located in southwest Texas, at
the electrical interface between the USA Electric Reliability
Council of Texas (ERCOT) power grid and the Mexico
Comision Federal de Electricidad (CFE) power grid. A
100 MW VFT is being installed at Laredo to enable power
transfer between the two asynchronous power grids.
Figure 2 shows a simplified one-line diagram of the Laredo
VFT, which is comprised of the following:
• One 100 MW, 17 kV rotary transformer
• One 3750 HP dc motor and variable speed drive system
• Four 25 MVAR switched shunt capacitor banks
• Two 142/17.5 kV conventional generator step-up
transformers
• Two auxiliary power transformers
On the ERCOT side, there are several 138 kV transmission
lines, generators, switched capacitor banks at 138 kV and
69 kV, and a ±150 MVAR STATCOM that regulates 138 kV
bus voltage. On the CFE side, the VFT connects to the CFE
system through 138 kV and 230 kV transmission lines with a
230/138 kV autotransformer, and has a switched capacitor
bank at 138 kV.
IV. LAREDO VFT OPERATION AND CONTROL FEATURES
The Laredo VFT operation and control features provide
automatic sequences, power transfer control, reactive power
control, and black start capability.
Figure 2. One-line diagram of Laredo VFT.

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A. Automatic Sequences
After the operator closes the 138 kV breakers to energize
the two VFT step-up (VSU) transformers, the operator can
initiate automatic sequences to energize the rotary
transformer, start and stop, and de-energize the RT.
1) Energize and Start Sequences
The normal startup from the off-line state requires two
operator steps: energize and start. In the energize sequence,
the VFT Control System (VCS) verifies breaker positions and
voltages, starts the cooling system, and energizes the RT by
closing the isolation breaker. This puts the VFT in the
energized/stopped state.
In the start sequence, the VCS starts the drive system,
engages the automatic phase angle control to align voltages
across the synchronizing breaker, closes the synchronizing
breaker, and engages the power regulator at zero power. The
VFT remains in the running state at zero power until the
operator enters a new power order.
2) Stop and Off-Line Sequences
The normal shutdown from the running state requires two
operator steps: stop and off-line. In the stop sequence, the
VCS checks for zero power order, opens the synchronizing
breaker, and stops the drive system. This puts the VFT in the
energized/stopped state.
In the off-line sequence, the VCS de-energizes the RT by
opening the isolation breaker and shuts down the cooling
system. When the VFT is in the off-line state, the operator
may open the 138 kV breakers to de-energize the VSU
transformers.
B. Power Transfer Control
After completing the energize and start sequences, the
operator may enter a desired power order (MW), power flow
direction, and ramp rate (MW/minute). Power regulation is
the normal mode of operation. The VFT uses a closed-loop
power regulator to maintain constant power transfer at a level
equal to the operator order. The power order may be
modified by other control functions, including automatic
governor, isochronous governor, tie flow regulator, power
runbacks, and power-swing damping control.
1) Automatic Governor
The automatic governor adjusts VFT power flow on a
droop characteristic when frequency on either side exceeds a
deadband. This function is designed to assist one of the
interconnected power grids during a major disturbance
involving significant generation/load imbalance.
The VFT is designed to operate with one side isolated. If
the local grid on one side of the VFT becomes isolated from
the rest of the network, the VFT will continue to operate
regardless of whether the isolated system has local generation.
If there is no local generation, the VFT will automatically feed
all the necessary power up to its full rating. If there is local
generation, the VFT will make up the difference between
local generation and local load, and share frequency
governing with the local generator.
2) Isochronous Governor
The VFT also has an isochronous governor that will
regulate the frequency of the isolated network to 60 Hz, when
engaged by the operator. The Laredo VFT has the option of
regulating to 60.05 Hz, which may help re-synchronization
during block load transfer or during black start.
3) Tie Flow Regulator
Normally, power transfer is regulated at the rotary
transformer 17 kV bus. The Laredo VFT has a tie flow
regulator that will regulate power transfer at an alternate
location, when engaged by the operator. In this case, the total
power transfer of the 230 kV and 138 kV CFE lines is
measured by customer metering at the CFE tie. This regulator
can change the VFT power order by up to ±2 MW.
4) Power Runbacks
The runback function quickly steps VFT power to a preset
level. It is externally triggered following major network
events (e.g., loss of a critical line or generator or for
undervoltage conditions). The VFT control system is
designed to accommodate up to four runbacks with separate
triggers and runback levels.
At Laredo, runbacks are triggered by undervoltage (UV)
conditions, detected by relays. There are two UV contacts on
each side, and runback levels can be set by SCADA to be
coordinated with VFT dispatch. The first UV contact on an
exporting grid would trigger a power reduction, and a second
UV contact on the same side would trigger a power reversal,
to try to help relieve the undervoltage condition.
5) Power-Swing Damping Control
The power-swing damping control function adds damping
to inter-area electromechanical oscillations, normally in the
range of 0.2 Hz to 1 Hz. This function is available but
disengaged at Laredo, as system conditions do not require it at
this time.
C. Reactive Power Control
The standard VFT Reactive Power Control (RPC) function
switches 17 kV capacitor banks (VFT zone). At Laredo, there
are two additional zones for reactive power and voltage
control: the AEP Laredo zone, and the CFE zone.
1) VFT Zone RPC
Like any other transformer, the VFT has leakage reactance
that consumes reactive power as a function of current passing
through it. The four 17 kV shunt capacitor banks are
switched on and off to compensate for the reactive power
consumption of the VFT and the adjacent transmission
network. The VFT reactive power controller has three modes:
Power Schedule Mode – the capacitor banks are switched
as a function of VFT power transfer, with appropriate
hysteresis to prevent hunting. This mode includes a voltage
supervision function that takes precedence if the 17 kV bus
voltage falls outside of an acceptable range.
Voltage Mode – the capacitor banks are switched to
maintain the 17 kV bus voltage within an operator-settable
range. At Laredo, the voltage deadband is widened when the
Laredo STATCOM is out of service.

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Manual Mode – the capacitor banks are switched on and
off by the operator.
2) AEP Laredo Zone RPC
At Laredo, a ±150 MVAR STATCOM regulates voltage on
the 138 kV bus. When it is in service, five capacitor banks
(two at 138 kV bus and three at 69 kV bus) are switched by
the RPC to keep the STATCOM operation inductive. When
the STATCOM is out of service, the 138 kV capacitor banks
are also out of service, and the 69 kV capacitor banks are
switched by the RPC to regulate the VFT 138 kV bus voltage.
3) CFE Zone RPC
At Laredo, there are two tie lines on the CFE side: a
138 kV line and a 230 kV line. A 138 kV capacitor bank is
switched by the RPC, based on total MVAR flow to CFE, to
try to keep a net positive MVAR flow to the CFE system.
There is also a 230/138 kV autotransformer on the 230 kV
line, and it has a load tap changer (LTC) that is switched by
the RPC to balance the MVAR flow on the 138 kV and
230 kV tie lines to CFE.
D. Black Start Capability
The Laredo VFT includes special sequences to permit the
operator to “black start” either side, if one of the grids is dead.
The VFT can be used to energize the dead side and pick up
load up to the rating of the VFT, by transferring voltage and
then power from the live side, and to enable system
restoration.
Once initiated by the operator, the black start sequence
enables the isochronous governor to regulate the frequency on
the side being restored to 60 Hz. With this function, the
power order is automatically adjusted when load is added
while regulating frequency. As discussed earlier, the operator
also has the option of regulating the frequency to 60.05 Hz.
After generation is brought on-line or tie lines are closed, the
operator disables the isochronous governor and can then
change the VFT power order normally.
V. PROTECTION CHARACTERISTICS
The VFT unit is protected by redundant protection
systems, using standard protective relays, plus some special
protections in the Unit VFT Control (UVC) and drive system.
The Laredo VFT is designed with three major protective
zones: ERCOT 138 kV zone, CFE 138 kV zone, and RT zone.
The four 17 kV shunt capacitor banks and two 17kV/480V
auxiliary transformers are protected individually.
A. ERCOT 138 kV Zone Protection
The ERCOT 138 kV zone includes the 138 kV tie circuit
connecting the ERCOT 138 kV substation with the VFT, the
stator-side 142/17.5 kV VSU, and stator-side 17 kV buses.
Protective trips occur due to faults within the zone or stator-
side breaker failure.
B. CFE 138 kV Zone Protection
The CFE 138 kV zone includes the 138 kV tie circuit
connecting the CFE 138 kV substation with the VFT, the
rotor-side 142/17.5 kV VSU, and rotor-side 17 kV buses.
Protective trips occur due to faults within the zone or rotor-
side breaker failure.
C. RT Zone Protection
The RT zone includes the rotary transformer and the drive
system. Protective trips occur based on RT currents and
voltages, protective functions in the UVC and drive system,
and external trips of 138 kV breakers of the above 138 kV
zones. The UVC performs a unit trip sequence to put the
VFT into a normal state and trips the 17 kV capacitor banks.
VI. RESPONSE TO SYSTEM DISTURBANCES
The performance of the VFT for grid transients was
validated with extensive real-time simulator testing prior to
commissioning, and subsequently confirmed by field
observations at Langlois [1, 2, 3]. Response to power order
steps is fast and stable. The VFT rides through faults and
damps subsequent power swings quickly. Furthermore, the
VFT is inherently capable of supplying power and voltage
support to a suddenly islanded load.
For the Laredo project, extensive tests have been
performed on a real-time simulator with the actual control
system. One example is shown in Figure 3. This event
involved a programmed sequence of voltage steps on the
ERCOT side designed to validate ability of the VFT to ride
through a fault and subsequent severe undervoltage, while
simultaneously providing voltage support to help the system
recover. The voltage sequence includes a 3-ph fault cleared in
6 cycles, followed by recovery to 25% voltage for 3.5 sec,
then recovering to 75% voltage, and then to 90% after 10
seconds. This scenario represents an extreme situation where
motors stall during the fault and require reactive support for a
long time before reaccelerating to normal speed. The VFT is
initially importing 100 MW to the ERCOT side from CFE.
During the very low voltage period following fault clearing,
the VFT delivers reactive current to the ERCOT side. When
the voltage recovers to 75%, the real power is restored. This
is the desired behavior for such an event.
VII. CONCLUSIONS
The new VFT technology for power transfer between
asynchronous power grids is being installed at AEP’s Laredo
substation in Texas. Several control features have been added
for the Laredo application, including AEP and CFE system-
dependent reactive power and voltage control functions,
special power runback logic based on undervoltage contact
inputs, an alternate CFE tie flow power regulator, and black
start sequences to enable system restoration. Real-time
simulator tests verified the Laredo VFT dynamic
performance, including a severe undervoltage event.

5. 5
0 5 10 15 20
−200
−100
0
100
200
P(MW)
0 5 10 15 20
−1
0
1
2
3
ReactiveCurrent(pu/100MVA)
Time (sec)
0 5 10 15 20
0
0.5
1
1.5
ERCOT138kVV(pu)
0 5 10 15 20
0
0.5
1
1.5
CFE138kVV(pu)
Time (sec)
Figure 3. Laredo VFT Response to Undervoltage Event on ERCOT 138 kV (Real-Time Simulator Test).
VIII. ACKNOWLEDGMENT
Application of the VFT at Laredo has been an outstanding
team effort, including many GE individuals and business units
working in excellent cooperation with AEP.
IX. REFERENCES
[1] R. Piwko, E. Larsen, “Variable Frequency Transformer – FACTS
Technology for Asynchronous Power Transfer”, Presented at the 2005
IEEE PES T&D Conference and Exposition in Dallas, Texas USA on May
21-24, 2005.
[2] D. McNabb, D. Nadeau, A. Nantel, E. Pratico, E. Larsen, G.Sybille, Van
Que Do, D. Paré, “Transient and Dynamic Modeling of the New Langlois
VFT Asynchronous Tie and Validation with Commissioning Tests”,
Presented at the International Conference on Power Systems Transients
(IPST’05) in Montreal, Canada on June 19-23, 2005.
[3] J.-M. Gagnon, D. Galibois, D. McNabb, D. Nadeau, E. Larsen, D.
McLaren, R. Piwko, C. Wegner, H. Mongeau, “A 100 MW Variable
Frequency Transformer (VFT) on the Hydro-Quebec Network: A New
Technology for Connecting Asynchronous Networks”, Presented at the
2006 CIGRE General Session (Paper A2-208) in Paris, France in August
2006.
[4] M. Spurlock, R. O’Keefe, D. Kidd, E. Larsen, J. Roedel, R. Bodo, P.
Marken, “AEP’s Selection of GE Energy’s Variable Frequency
Transformer (VFT) for their Grid Interconnection Project between the
United States and Mexico”, Presented at the North American Transmission
& Distribution Conference & Expo in Montreal, Canada on June 13-15,
2006.
X. BIOGRAPHIES
David Kidd (M) received his BSEE from New Mexico State University in
1987. He spent 10 years at El Paso Electric Company in system protection. In
1997 he joined AEP in the Texas Transmission Planning group where he has
been working with FACTS Technology. He is a member of IEEE and works
with various groups within the Electric Reliability Council of Texas (ERCOT).
Einar Larsen (M’74, F’91) is currently a Director with GE’s Energy
Consulting group in Schenectady, NY, where he has worked since graduation in
1974. His experience is in system engineering for applying new equipment to
power grids. He is a Fellow of IEEE for contributions to High Voltage DC
systems, and recipient of the 2001 IEEE award for “FACTS” - application of
power electronics to ac transmission systems.
Richard Piwko (M’76, F’96) is a Director with GE’s Energy Consulting
group in Schenectady, NY. His responsibilities include management of large-
scale system studies, power plant performance testing, control system design,

6. 6
and analysis of interactions between turbine-generators and the power grid. He
has led numerous system design projects involving high-power electronics,
including HVDC, static var systems, and thyristor controlled series capacitors.
He recently contributed to GE’s development of the Variable Frequency
Transformer (VFT), a new technology for transferring power between
asynchronous power grids. Mr. Piwko is a Fellow of the IEEE. He has served
as chairman of the IEEE Transmission and Distribution Committee and the
HVDC and Flexible AC Transmission Subcommittee.
Elizabeth Pratico (M’86) joined GE in 1991 and is currently a Principal
Consultant with GE’s Energy Consulting group in Schenectady, NY. She
received a BSEE degree from Florida Institute of Technology in 1986 and a
Masters degree in Electric Power Engineering from RPI in 1995. Her
background includes various aspects of power system modeling and simulation,
electromagnetic transient analysis, and control system development. Ms. Pratico
was a key contributor to GE’s development of the Variable Frequency
Transformer (VFT), which is a new technology for power transfer between
asynchronous networks. Before joining GE, she tested space shuttle payload
experiments for NASA at the Kennedy Space Center.
Daniel Wallace received his BSEE from Clarkson University and has been
with GE for more than 20 years. He is currently a Systems Engineer in GE
Energy’s Network Reliability Products & Services organization, focusing in the
area of FACTS technology. Mr. Wallace recently contributed to GE’s Laredo
VFT project.
Carl Wegner is an engineer with GE Energy in Schenectady. He led the
VFT and TCSC control system implementations, and leads the design of GE’s
generator torsional protection and monitoring products. He has contributed to
battery energy storage and other high power electronic systems, and worked for
six years on controls at the HVDC and static var product operation in
Philadelphia. He has a BSEE and MSEE (1983) from the University of Illinois
in Urbana.