• Energy Research

The impact of Type 4 wind turbine generator (WTG)-based 10 million megawatt clusters (TMMC) on small-signal dynamics of power systems was investigated using the second-generation generic models (GM) of Western Electricity Coordinating Council (WECC). A WTG participation index (WTG PI) was defined to investigate the impact of Type 4 WTGs on the traditional interarea electromechanical modes. To identify the new electromechanical modes dominated by Type 4 WTGs, an identification factor (IF) was also defined using participation factors. Given the increasing penetration of Type 4 WTGs replacing synchronous generators, the changed law of damping and frequencies of the traditional interarea modes was also investigated using the WTG PI. One new type of electromechanical mode dominated by Type 4 WTGs was identified by using the defined IF. These new modes can be divided into two categories: strong-interaction modes and weak-interaction modes, depending on the number of participating WTGs. The strong-interaction modes dominated by Type 4 WTGs can result in widely spread power oscillations in power systems. The results of small-signal analysis were validated by time domain simulation and mode detection.

The subsynchronous oscillations (SSO) caused by the dense integration of direct-drive permanent magnet synchronous generator (DPMSG) based wind farms have threatened the secure and stable operation of the local Hami power grid in Xinjiang Region, China. The DPMSG-involved SSO events show three new characteristics: random occurrence, time-varying frequency and amplitude, and wide involvement of all types of power electronics devices. To reveal the mechanism of the three new characteristics shown in the real SSO events, this paper develops the forced SSO (FSSO) theory and its method of use. Firstly, the generation and the time-varying performance of subsynchronous interharmonics (SSIH) from the DPMSG converters are analyzed based on the modulation theory. It is discovered that the SSIHs from the DPMSGs can be the disturbance source that excites the positive-damping subsynchronous mode to cause the FSSO. Further, the three occurrence conditions of the FSSO are proposed, and the three characteristics of the FSSO are revealed. It is concluded that the characteristics of the FSSO coincide with those of the real SSO events in the local Hami power grid. Finally, the theoretical analyses are validated by simulations in case studies.