DC Impedance Modeling and Stability Analysis of Wind Farm Integrated MMC-HVDC System with Coordinated Frequency Support Control Strategy

Due to the high proportion of renewables and power electronics, power systems exhibit weak inertia. The coordinated control between wind farms and the modular multilevel converter (MMC) based high-voltage direct current (HVDC) system is recognized to provide frequency support for AC grid. To investigate the stability of the above MMC-HVDC system, this paper designs a coordinated frequency support control in a non-communication mode and develops the detailed DC impedance models using the multiharmonic linearization method. The internal harmonic dynamics and frequency coupling are considered. The effect of the frequency support coordinated controller on system DC impedances is revealed. The results indicate that the additional controllers mainly affect the DC impedance of the wind farm-side MMC in the sub-synchronous frequency band (below 10 Hz), while their influence on the grid-side MMC is negligible. The stability under different frequency support control parameters is further conducted. Simulation in PSCAD/EMTDC validates the correctness of both control and DC impedance modeling.