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, 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 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 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 horizon. Therefore, the steady-state system requirements were not a key factor in the selection process.

Continued operation of the Laredo Power Station, while offering much toward stabilizing system voltage-collapse 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.

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