Sub-synchronous torsional interaction (SSTI) analysis

Turbine-generators can be subject to electrical environments that react with turbine-generator shafts to produce resonance (torsional vibrations) at shaft natural frequencies. These vibrations cause cumulative fatigue damage when they exceed material fatigue limits and result in reduced component life of parts such as shafts, buckets (blades), retaining rings, and rotors. In some cases, adverse interactions have led to growing oscillations and shaft damage, including twisted couplings and broken shafts. Known electrical environments that cause concern include:

• SSR – Subsynchronous Resonance due to series capacitors in transmission lines
• HVDC converters
• Large industrial motor drives
• SVC – Static Var Compensators
• Faults and high speed reclosing
• Steel mills with arc furnaces
• Cycloconverter frequency changers
• Sustained off-nominal frequency excursions

GE Energy Consulting has pioneered the analysis and mitigation of torsional problems, providing protective relays and monitoring equipment, since the early 1970’s. Our latest generation of torsional protection and monitoring products build on GE’s extensive experience, and feature digital processors, built-in monitoring, and higher reliability.

Torsional Stress Relay

The Torsional Stress Relay is a digital protective relay that continuously monitors the turbine generator shaft for torsional oscillations, and provides trip output contacts when shaft fatigue reaches predetermined levels. Torsional event data capture is also provided.

The TSR is an updated digital version of the proven GE SMF relay. GE technology has been used since 1976 to protect turbine-generators exposed to risk of harmful torsional interaction.

Application screening

Subsynchronous Resonance caused by series capacitors is the most common reason that torsional protection may be needed. Accordingly, SSR studies are prudent when series capacitors are added to transmission lines, the amount of series compensation is changed, or if other system (or shaft) changes are expected that affect a turbine-generator's exposure to SSR. Our studies evaluate the risk of damage due to SSR for normal and contingency system conditions. Depending on the level of risk, it may be prudent to add torsional protection (TSR). In some cases the risk is high enough to justify redundant protection.

Torsional protection may also be required for high rated power electronic converters that are near to turbine-generators. HVDC is especially of concern, due to typically high power ratings. Large motor drives and SVC can also be a problem, typically for smaller turbine-generators nearby. Energy Consulting can perform a simple Subsynchronous Torsional Interaction (SSTI) screening study in these cases.

Subsynchronus Resonance or Subsynchronous Torsional Interaction may, in rare cases, require active damping or resonance blocking filters to protect high value capital equipment.

GE Energy Consulting can determine the amount of risk posed by your unique circumstances, from high risk caused by series capacitors to low risk caused by local high speed reclosing or steel mill arc furnaces.