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Load growth in Laredo, Texas has stressed the area power system to the point of requiring Reliability Must Run (RMR) generation in order to maintain an adequate stability margin. AEP has recently improved the stability of the Laredo area by adding an asynchronous interconnection with the Comision Federal de Electricidad (CFE) using the Variable Frequency Transformer (VFT). The VFT is based on a combination of hydro generator and transformer technologies. It consists of a rotary transformer, drive motor and control system. The phase shift angle and speed of the rotary transformer are continuously adjusted to regulate the power flow through the VFT, resulting in an ability to interconnect and schedule constant power between two asynchronous AC systems. This paper discusses the stability characteristics of the Laredo area power system and the improvements made with the addition of the VFT. The VFT is found to provide stability benefits similar to that of a local generator. In particular, the VFT improves the system's post-fault voltage recovery characteristics and thereby extends the stability margin even with zero power scheduled. The VFT is also shown to enhance stability both prior to and following the introduction of 345 kV transmission into the Laredo area.
Date of Conference: 21-24 April 2008