Sub-synchronous interactions (SSI) are a family of physical interactions that involve exchange of energy between a generator and a transmission system at ac frequencies below the system nominal frequency. They include sub-synchronous resonance (SSR), sub-synchronous torsional interaction (SSTI), and sub-synchronous control instability (SSCI). SSR is a phenomenon that can cause increased fatigue or critical failure of generator turbine shaft systems due to an energy exchange between the generator and a series-compensated transmission system, either through sustained or poorly damped oscillations, or transient effects. SSTI occur when an interaction happens between an HVDC link, FACTS device, or other power electronic controller and the mechanical mass system of a generator. The power electronic controller can exhibit negative damping at sub-synchronous frequencies, which can cause un-damped or growing oscillations in the known mechanical torsional modes of oscillation in the generator shaft system. SSCI phenomenon is a control interaction that can occur between any power electronic devices, such as wind turbine, and a series-compensated system. The oscillations resulting from SSCI may grow very quickly. Another SSCI phenomenon is the interaction between power electronic controllers such as grid-connected inverters for renewable energy sources integration. Such resonances can be both sub-synchronous and super-synchronous and may lead to inverter control instability and other dynamic problems when the connected grid is becoming weak.
The objective of this Seminar is to provide an introduction to SSI phenomena including SSR, SSTI and SSCI. To give a guide on how to identify potentiality 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.