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Project: T173700 #3135

This report provides an introduction to sub-synchronous interaction (SSI) phenomena, including sub-synchronous resonance (SSR), sub-synchronous torsional interactions (SSTI), and sub-synchronous control instability (SSCI). It includes a discussion of:

  • SSTI phenomena between high voltage direct current (HVDC) installations and other power system equipment, including conventional turbine generators;
  • SSCI phenomena between series-compensated line and windfarms, especially with type 3 doubly-fed wound induction generators (DFIG) and FACTS devices; and
  • SSCI phenomena between grid-connected inverters for renewable energy sources integration.

This report will also act as a guide on how to identify the potential where SSTI and/or SSCI may be an issue; how to study the phenomena; how to control, mitigate, and/or protect against any adverse effects associated with these interactions; and potential types of system changes or additions that would require equipment owners and/or operators to re-examine their system for the possibility of SSTI and/or SSCI.

The commonly-used tools that will be discussed include a unit interaction factor (UIF) screening study and an electrical damping torque analysis for SSTI; a harmonic impedance scan both for the linear portion of a system and for the non-linear and active electronic devices for SSCI; and a full-time domain electromagnetic transient analysis.


SSR, SSI, SSTI, SSCI, power electronic devices, turbine generator, wind turbine generation, HVDC, renewable energy integration