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.

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 short-term 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 short-term solution to provide this fifth source.

After careful analysis of alternative technologies, AEP selected the 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.

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 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 power. The VFT is designed to continuously regulate power flow with drifting frequencies on both grids.

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.

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 dynamic-stability 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 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 one-third 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.