With promotion and application of ultra high voltage direct current(UHVDC) and continuous increase of renewable energy power generation proportion, Chinese energy transformation strategy is strongly supported. However, due to the poor network-related protection performance of renewable energy units in the DC sending power grid where renewable energy is centrally connected, AC and DC faults can easily result in large-scale disconnection of renewable energy, which brings great challenge for power system transient frequency security. The failure process of large-scale disconnection of renewable energy from the UHVDC receiving grid is firstly analyzed. Then, the existing measures to deal with disconnection of renewable energy are discussed. The emergency control technical scheme using the information of DC commutation failure is designed according to the characteristics of fault process. Finally, a simulation example of the northwest power grid is built to verify that the proposed frequency emergency control strategy can effectively deal with the problem of large-scale renewable energy off-grid caused by AC and DC faults, and greatly improve the UHVDC transmission capacity and renewable energy generation of the power grid.