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A transmission expansion project, Competitive Renewable Energy Zone (CREZ), in the ERCOT region will be implemented to deliver wind generation from wind resource areas to load centers. Several extra high voltage transmission lines will be equipped with series capacitors to increase the transfer capability. A SubSynchronous resonance (SSR) phenomenon that can be induced by having a conventional generator radially connected to the series compensated line has been well studied and documented. However, a similar phenomenon, Sub-Synchronous Control Interaction (SSCI) can also be induced between a wind generation resource and series compensated lines. It is important to identify proposed points of interconnection at risk of sub-synchronous control interaction. Identifying and sharing this information can prevent damage to transmission and generation equipment. ERCOT anticipates that control settings can be adjusted to mitigate or avoid SSCI or alternate turbine designs selected for a particular location.

Generators connecting to a series compensated transmission system can be exposed to subsynchronous resonance (SSR) and require a comprehensive vulnerability assessment. This study presents an SSR assessment framework and tools to more efficiently assess all types of generation in a large system with multiple generators and meshed series capacitors. The developed framework covers torsional interaction, torque amplification, and induction generator effect/control interaction and includes three assessment stages: topology screening, frequency scan, and detailed electromagnetic transient study. Topology screening based on max-flow min-cut theorem is applied to identify the set of transmission outages forming radial connections between generators and series capacitors. Frequency scan analysis is then conducted to further analyse the SSR vulnerability with all the possible combinations of transmission outages identified in the topology screening results. Nearby generators are modelled by frequency-dependent impedance tables, which consider the equivalent impedance looking into the generators over the range of 1-120 Hz. An electromagnetic transient simulation will be required to confirm the SSR vulnerability identified in the frequency scan analysis. The developed framework and tools have successfully been applied to a large power system with multiple series compensated circuits to ensure reliable generation interconnection and operation.

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