Mixed Grid-Forming and Grid-Following Inverters with Secondary Control Providing Fast Voltage and Frequency Support

As the penetration of renewable energy generation increases, grid-forming (GFM) inverters are deemed to be a promising solution for future power systems. However, restricted by the power rating of the inverter, the limited power output ability makes GFM inverters behave not exactly the same as synchronous generators. Namely, GFM inverters may have difficulties in meeting all the Grid Codes simultaneously. When the active power requirement is prioritized to be satisfied by GFM inverters, the reactive power requirement may be difficult to meet at the same time. To address this issue, a mixed GFM and grid-following inverter scheme is proposed, where the GFM inverter is prioritized to provide active power to support the grid frequency while the GFL inverter is prioritized to provide reactive power to support the grid voltage. It is found that the proposed mixed scheme can provide fast voltage and frequency support in weak grid case.