Suppressing Intermittent Subsynchronous Oscillation via Subsynchronous Modulation of Reactive Current

Recently, a new oscillation phenomenon - intermittent subsynchronous oscillation (ISSO) - has been observed in a large interconnected power grid. Rather than leading to oscillatory instability, ISSO causes cumulative fatigue damage and reduces the shaft life of the generator. The existing phase-compensation-based damping technologies are found to be limited in mitigating these types of oscillations. To mitigate ISSO, a subsynchronous modulation of reactive current (SMRC) approach is established and implemented in the voltage-source converters (VSCs) at the generator terminals. The contribution of SMRC to system damping has been analyzed in order to select control parameters for the VSC. The SMRC controller has been prototyped and extensively tested on a real-time closed-loop simulation platform equipped with a newly developed speed sensor module for precisely interfacing the real-time digital simulator with the rotating speed-acquisition board. Extensive experiments on the input and output characteristics of the VSC have been carried out to test and validate the damping current generating module. The effective damping areas are obtained from test observations to fine tune the parameters of the VSC controller for adapting to various system operating conditions, resulting in the common optimal damping region. Real-time simulations, onsite commissioning, and statistics have demonstrated the ability of the SMRC to mitigate ISSO and subsynchronous oscillation in power plants.