Subsynchronous Torsional Interaction

Subsynchronous Torsional Interaction

We have a number of recognized transmission and distribution experts serving on our board of "Grid Masters." Several times each month we’ll post what we judge to be the toughest questions that also have high interest to our readers. At least one of our experts will respond. Want to challenge our Grid Masters for a chance to win?

Q: Please explain how the subsynchronous torsional interaction (between VSC*-HVDC and a nearby generator) is affected by:

  • Active and reactive power level of the generator
  • Active and reactive power level of the converter in case it operates as a rectifier as well as an inverter

* Voltage Source Converter



  • Mechanical damping of the turbine generator (T-G) is higher at higher power level.
  • Subsynchronous torsional interaction (SSTI) depends on the degree of coupling between the HVDC converter and the T-G, i.e. unit interaction factor (UIF), the type of converter technology and its control response in the torsional frequency range.

The torsional modes could be destabilized if there is negative electrical damping from the controls which exceeds the positive mechanical damping of the T-G. With VSC-HVDC positive electrical damping in the torsional frequency range increases with higher UIF while operating in active power and reactive power control modes, regardless of power direction or level.  Reactive power level has little effect on the damping if operating in power control.  If operating in dc voltage control, however, damping can vary with reactive power level.

These are general conclusions from a number of specific projects. Results could vary from project to project depending on control modes and frequency response in the torsional range. There is potential for negative electrical damping in the torsional range from the controls if not designed properly.  SSTI should be investigated for each project as part of the design process.

Michael Bahrman
HVDC Business Development

See previous questions and answers and join the discussion. Add your comments below:


Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.