Case Study

Mitigation of Sub-Synchronous Oscillations using TGSSR

Client locations:

South America
Services: Power Systems Analysis
Client/Project Profile:

Series compensation is largely used to improve the power transfer capabilities of long AC transmission lines. Apart from the benefits provided by series capacitors, series capacitors can also negatively interact with other dynamic devices in the system. Sub-synchronous interactions with generator-turbine units (SSR & SSTI) and wind power plants (SSCI) are one of the main concerns. TGS is well equipped to analyse such interactions using our own frequency domain analysis tool: TGSSR and using time domain (EMT) analysis.


One of our recent projects on the subject of sub-synchronous interactions was to evaluate the risk of sub-synchronous interactions of a system that includes more than 10 series compensated lines. There were more than five large generating stations at risk. The scope of work included the screening and analysis of sub-synchronous interactions, and the proposal of short-term and long-term mitigation measures.


Using TGSSR, we evaluated the SSR and SSTI risk and identified the generating stations at risk and the locations of the series capacitors contributing to the risk. The worst-case operating conditions of the system were also identified using the features available in TGSSR. The findings were validated using the time domain simulations (PSCAD).

As a mitigation measure, we proposed two solutions: bypassing several of the series capacitors as a short-term solution, and the use of Thyristor Controlled Series Capacitors (TCSCs) as a permanent solution. Using frequency domain analysis and time domain simulations, it was shown that the risk of sub-synchronous interactions can be fully eliminated by converting segments of two of the series capacitors into TCSCs. In addition to the inherent damping provided by the TCSCs, sub-synchronous damping controllers were also designed for the TCSCs using the control design tools available in TGSSR.


Having our specialized in-house dynamic phasor based small signal stability analysis tool (TGSSR) puts us in a unique position to be able to quickly and accurately study large areas of a power system for sub-synchronous oscillations and likewise determine optimized mitigation measures. 

Time domain simulations alone may not be able to find the issue, since the issue may occur under very specific circumstances, which may not be easily observed in the time domain simulation unless it is first known.

TGSSR can quickly “find the needle in the haystack” and directly pinpoint the equipment that is at risk to experience sub-synchronous oscillation issues and the mitigation measures that can eliminate these issues. Once the specific issue is known, it can easily be modeled in the time domain to verify that the issue exists and that the mitigation measure proposed is effective.

With the increased use of dynamic devices in the systems around the world, interactions are becoming more probable. Our small signal stability experts use TGSSR as an excellent tool for analyzing these issues.

For more information on TGSSR, please contact us.