controls&Automation
United States and Mexico
Cross-Border
by Rob
O’Keefe and David Kidd, American Electric Power
Variable frequency
transformer to
reinforce power
transfer between
countries.
A new asynchronous transmission link will
interconnect the U.S. and Mexico grids. The link will
be between American Electric Power (AEP) and Mexico’s Comisión
Federal de Electricidad (CFE) and will be located in Laredo, Texas,
U.S. This project is one of several reliability-must-run (RMR) exit-strategy projects for the Laredo Power Station and is endorsed by the Electric Reliability Council of Texas (ERCOT) board of directors for the
purpose of maintaining reliable power supply to the Laredo area.
AEP selected the 100-MW variable frequency transformer (VFT)
from GE Energy for this project. The VFT is a controllable, bidirectional transmission device that allows power transfer between two networks that might not be synchronized. Functionally, the VFT is similar
to conventional high-voltage direct current (HVDC) converters and
voltage source converters (VSC) that are arranged back-to-back (BTB)
to provide power-transfer capability across asynchronous grids.
Project Description
In the past, local generation supported the Laredo area, but that
changed in 2002 when the generation owner notified ERCOT of its
intention to shut down the Laredo Power Station due to market conditions. ERCOT responded by entering into an RMR agreement with
the generation owner to keep the units operating until power-delivery
system improvements could be constructed. AEP then began studying
short-term solutions to reduce the unit run times until a new 345-kV
transmission line could be brought on-line in 2010. Part of the shortterm plan required that a fifth 138-kV source flow into the Laredo area.
Given the presence of a strong 138-kV CFE network just across the border, and cross-border 138-kV transmission infrastructure already in
place, an asynchronous tie to CFE made sense as an expedient shortterm solution to provide this fifth source.
After careful analysis of alternative technologies, AEP selected the
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Connection
August 2006
21
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Fig. 1. VFT stator build-up in progress.
VFT, which will add the capability for import of up to 100
MW of real power to ERCOT from CFE, thus allowing more
flexibility to serve the Laredo area load while meeting AEP,
ERCOT and North American Electric Reliability Council
(NERC) stability and reliability requirements.
The VFT project initial design and system study began in
early 2005 and is scheduled for commercial operation in early
2007. A one-line system diagram of the VFT project is provided in Fig. 2.
VFT Overview
The VFT is essentially a continuously variable phase-shifting transformer that can operate at any adjustable phase angle. The core technology of the VFT is a rotary transformer
Fig. 2. AEP Laredo VFT one-line diagram.
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with three-phase windings on both the rotor and
stator sides (Fig. 3).
The collector system conducts current between the three-phase rotor winding and its stationary bus work. In the case of the Laredo VFT
system, the rotor side of the VFT is connected to
the CFE grid and the stator side of the VFT is
connected to the ERCOT grid. This arrangement
is arbitrary and could have been configured the
other way just as easily.
Power flow is proportional to the magnitude
and direction of the torque applied to the rotor.
This torque is applied to the rotor by a drive motor, which is controlled by a variable-speed drive
system. If torque is applied in one direction, then
power flows from the stator windings to the rotor windings. If torque is applied in the opposite direction, then power flows from the rotor
windings to the stator windings. If no torque is
applied, then no real power flows through the rotary transformer.
A closed-loop power regulator maintains power transfer according to the operator setpoint.
The regulator compares measured power with the setpoint,
and adjusts motor torque as a function of power error. The
power regulator will respond quickly to network disturbances
and maintain stable power transfer.
Regardless of power flow, the rotor inherently orients itself
to follow the phase angle imposed by the two asynchronous
systems, and will rotate continuously if the grids are at different frequencies. The motor and drive system are designed to
continuously produce torque while at a standstill. 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 such a disturbance, if the VFT is transferring power, it
will continue without interruption and at full-expected pow-
controls&Automation
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 voltage magnitude on the two sides. And, unlike power-electronic alternatives, the VFT produces no harmonics and cannot cause undesirable interactions with neighboring generators or other
equipment on the grid.
Fig. 3. VFT core components.
er. The VFT is designed to continuously regulate power flow
with drifting frequencies on both grids.
Stability Studies
Because the Laredo area is vulnerable to dynamic voltage
collapse, particularly during summer peak-load conditions,
the studies performed to evaluate the prospective asynchronous devices consisted of power-flow analysis and dynamicstability analysis. In the dynamic studies, detailed modeling of
the ERCOT transmission network and connected generation
was included in its entirety. Existing flexible alternating current transmission systems (FACTS) devices—including the
Laredo and Military Highway 150-MVAR STATCOM and
the Eagle Pass 36-MW VSC BTB tie—were modeled. Alternate representations of the CFE system also were considered
but did not affect results.
Special consideration was given to the modeling of the
South Texas area load. A primary objective was to simulate
the dynamic behavior of a heavy concentration of air-conditioning load that would be typical of a summer demand peak.
To accomplish this, an aggregate load model was derived
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HVDC BTB
Condition
VSC BTB
VFT
Stability performance beyond N-1
150 MW only adequate through 2008
150 MW adequate through 2010
100 MW adequate beyond 2010
Blackstart capability
Requires a synchronous condenser
Untested
Demonstrated the ability
SSTI* potential with adjacent generators
Known to be capable of SSTI
Known to be capable of SSTI
Inherently avoids SSTI
Harmonics
Requires a large amount of filtering
(changes with power level)
Requires less constant filtering
No harmonics generated so no filters
required
Coordination with future plans
Conventional but not adequate
Adequate
Adequate
STATCOM support 2007-2009
Required in 2009
Not required
Not required
*
Subsynchronous torsional interaction (SSTI).
Summary of overall reliability comparisons.
from estimated load class percentages, typical summer peak
composition data, and typical load device modeling data. The
resulting model consisted of approximately two-thirds of the
load represented as dynamic induction machines and onethird as a static polynomial-type load model. This special load
model is applicable to transient voltage-collapse studies and
was applied at each South Texas area load bus.
Evaluation of the asynchronous interconnection centered
on the following three different asynchronous tie device types:
Conventional HVDC BTB, VSC BTB and VFT. Dynamic models from individual vendors were integrated separately into
the base study case, thereby providing a basis for comparative
performance evaluations among the various asynchronous
devices.
A set of N minus one (N  1) contingency events involving
three-phase faults and nonfault-initiated line tripping on the
138-kV network supplying Laredo and the vicinity was simulated. Due to induction machine stall tendencies, the fault cases
proved to be the limiting contingencies for power import. In
every case, the system post-fault response exhibited an acute
transient voltage-recovery problem, even with the added reactive injection supplied by the Laredo STATCOM. If the initial
power import into Laredo was high enough, voltages did not
recover. Thus, the power import limitation into the Laredo
area is defined by transient voltage collapse. (Figures 4 and 5
illustrate a typical fault case simulation with the VFT in service. Note the delay in post-fault voltage recovery in Fig. 5.)
Successive load year cases spanning 2007 to 2010 were established based on forecasted load growth with the objective
being to determine how each of the asynchronous devices
would support that growth. The study time horizon of 2010
corresponds to a maximum Laredo area load of 539 MW. The
stability study results are summarized in Fig. 6. The best performing of the conventional HVDC BTB device is exhibited
here. This indicates the magnitude of load that may be stabilized by injection of power from CFE through each asynchronous interconnection technology. The area to the right and
below each plot is the unstable region. Increasing area load
requires increasing steady-state real power imports to maintain stable operation.
Also in Fig. 6, the VFT and VSC BTB devices show an increasingly substantial benefit as area load is increased compared to the conventional HVDC BTB. The VFT at 100 MW
is observed to have a small but consistent advantage over the
150-MW VSC BTB. While the 150-MW VSC BTB transient
injection of reactive power helps to stabilize system voltage,
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Fig. 4. VFT real and reactive net power to ERCOT from CFE.
Fig. 5. VFT voltage at ERCOT and CFE sides.
the 100-MW VFT combined transient flow of real and reactive
power is more beneficial even though the overall reactive injection is significantly less than that of the 150-MW VSC BTB.
The benefit of supplying an immediate real power boost into
controls&Automation
horizon. Therefore, the steady-state system requirements were not a key factor in the selection process.
Fig. 6. AEP Laredo stability study final results.
the faulted system is that it helps to keep the Laredo area voltage angle from lagging further behind the ERCOT system.
This real injection occurs just after the fault is cleared (Fig. 4).
A smaller ERCOT-Laredo angular separation translates into
a smaller voltage dip during the post-fault swing as the induction machines reaccelerate.
Another noteworthy result in Fig. 6 is that the VFT can supply a stability benefit with zero-scheduled steady-state power
flow at the 2007 peak load (469 MW). This is a benefit of the
inherent characteristic of the VFT that makes it look more
like a phase-shifting transformer than does a conventional
HVDC BTB or a VSC BTB. As a result, a transient real and
reactive power boost through the VFT is attainable at zero
steady-state flow. The VSC BTB also can remain in service and
supply reactive power at zero-scheduled steady-state power
flow. However, a conventional HVDC BTB must be turned off
completely.
The ability of the asynchronous device to ride through severe voltage depressions is essential for this application. An
interruption of the device transmitting capability during a
post-fault voltage-recovery period could result in a system collapse. The AEP specification included several scenarios to address this requirement. This also had been a requirement for
the Langlois VFT. GE Energy demonstrated this capability via
extensive simulator testing, and there have been field events
that substantiate the expected behavior from the initial VFT
installation.
Steady-State Requirements
Steady-state system requirements are related to the thermal capacity of the 138-kV transmission system connecting
Laredo with the rest of ERCOT. The asynchronous device import capability offsets the need to import power from ERCOT,
relieving the 138-kV network loading. And so the steady-state
power-import requirements are a simple function of the asynchronous device capacity.
The conventional HVDC BTB and VSC BTB devices, rated
at 150 MW, were obviously better in this regard in comparison
to a single 100-MW channel VFT. However, further power-flow
studies showed that a 100-MW injection was still adequate to
achieve the required power import relief within the study time
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RMR offset of the Laredo Plant
Continued operation of the Laredo Power Station,
while offering much toward stabilizing system voltagecollapse tendencies, is dependent on favorable market
conditions and economic factors that cannot be assured. While both the VFT and the VSC BTB were able
to achieve necessary stabilization without running the
Laredo Plant throughout the study period, the VFT was
selected because it offered better overall performance
with regard to the stability requirements. This is indicated in Fig. 6 as the VFT and VSC BTB graphs exceed
the 539-MW Laredo area load level at steady-state power
injections still lower than their respective rated values. The
best-performing conventional HVDC BTB device was unable
to achieve system stabilization beyond 500 MW of total area
load, thus running the Laredo Plant would be necessary in the
latter years of the study period.
Transmission Solution
ERCOT determined the cost of the RMR contract to the
market and requested AEP find a transmission solution to
eliminate the RMR contract. A new 345-kV transmission line
is required for the long-term support of the Laredo area, but
cannot be placed in service before 2010. The solution for the
interim period from 2007 to 2010 includes the asynchronous
tie to CFE for which the VFT option was chosen. The VFT will
operate continuously at 0 MW for reliability. This operation
will substantially decrease the need for the RMR unit, which
will still be retained and utilized if needed for reliability.
This approach will allow AEP to increase the transfer limit
into the Laredo area up to 450 MW. The VFT also will allow
even higher transfer levels into the area without the two Laredo 35-MW units required to operate and with reduced run
time on the 110-MW unit throughout the year. If the 110-MW
unit is down for maintenance or trips, then ERCOT has the
flexibility to schedule energy across the tie from CFE. At high
load periods prior to completion of the 345-kV transmission
line, ERCOT has the ability to preemptively schedule energy
from CFE in preparation of the worst single contingency for
the Laredo area.
Rob O’Keefe received bachelor’s and master’s degrees in
electric power engineering from Purdue University. Between
1983 and 1990 he was employed with General Electric’s Power
Systems Engineering department. Since 1990, he has been with
American Electric Power Service Corp.’s Transmission
Planning Group in Gahanna, Ohio, U.S. rjo’keefe@aep.com
David Kidd received his BSEE degree from New Mexico State
University in 1987. After working at El Paso Electric for 10 years
in system protection, he joined the Texas Transmission Planning
group at American Electric Power in Tulsa, Oklahoma, U.S.
dekidd@aep.com