With the fast development of renewable energy, system operators intend to require renewable energy generation to have more grid support capabilities to ensure the safe operation of AC grid. This paper studies the grid-connected control strategy of a battery-free medium-voltage DC (MVDC) system with distributed photovoltaic generation units (DPVGUs), aiming to realize the rapid grid support function to AC grid frequency. Firstly, a master-slave coordinated control (MSCC) strategy of DPVGUs is proposed. Under this strategy, the master DPVGU with the maximum dynamic output characteristics adopts an improved DC constant voltage control (DCCVC) strategy to maintain the MVDC bus voltage, and the remaining DPVGUs adopt an improved droop control strategy to quickly provide corresponding power and can keep some reserve capacity for the MVDC system. Secondly, when the frequency deviation of AC grid exceeds the deadband of primary frequency control, DPVGUs with the MSCC strategy in the MVDC system can deliver responsive power to AC grid through MMC adopted fast frequency response control (FFRC) strategy, which can quickly provide frequency support to AC grid and help the grid bringing the frequency back quickly. Finally, simulations are carried out in RTDS to verify that the proposed control strategies are effective.