Analysis of frequently over‐threshold subsynchronous oscillation and its suppression by subsynchronous oscillation dynamic suppressor

To mitigate the frequently over-threshold subsynchronous oscillation (FOSSO) phenomenon which occurred in Hulunbuir power plant in the northeast of China, a novel equipment based on voltage source converter (VSC), named subsynchronous oscillation dynamic suppressor (SSODS) is developed and installed. In this study, the root cause of FOSSO is revealed by analysing the response characteristics of the system. Then the control strategy of the developed SSODS is presented. Damping characteristics of SSODS are analysed by the complex torque coefficient method. To verify the effectiveness of the control strategy and the physical controller of VSC, as well as provide initially optimised controller parameters for the field tests, real-time closed-loop tests are carried out. A novel method of torsional modal frequency measure for onsite commissioning test is proposed. The operation and field verification test of SSODS is introduced, and test results demonstrate that SSODS can mitigate the new SSO phenomenon dramatically, proving the validity of SSODS.