Second & Future Applications of Stability Enhancement in ERCOT with Asynchronous Interconnections

In the summer of 2001, AEP Texas placed in- service its first asynchronous interconnection between Electric Reliability Council of Texas (ERCOT) and the Comision Federal de Electricidad (CFE) in Eagle Pass, Texas. A 36 MW Voltage Source Converter (VSC) was selected for this application in order to supply reactive power support in addition to real power injection. It was also the first such ERCOT interconnection to be justified on the basis of reliability, and was selected over a transmission line project with a lesser benefit/cost. In Laredo, Texas, reliability was also the basis for expanding interconnections between ERCOT and CFE, when the second asynchronous interconnection was endorsed by the ERCOT Board. In order to allow time for the construction of a new 345 kV line from the San Antonio area to Laredo, a 100 MW Variable Frequency Transformer (VFT) was selected as the most appropriate device for this asynchronous interconnection. The VFT is to be placed in-service by the summer of 2007, and is the device most capable of supporting the Laredo load growth through 2010; its passive response to transient declines in frequency with power injection is ideally suited for this remote city with a rapidly increasing air conditioning load. Future applications of asynchronous interconnections may include ties in west Texas, where the transmission system is sparse, and the proliferation of wind generation, with its dependency on the grid for real power absorption and reactive power support, inherently deteriorates stability.