Abstract: A control strategy of off-grid start-up of energy storage system（ESS）was proposed，which can

make all power converter systems（PCSs）entered off-grid mode smoothly and quickly in MW level battery energy

storage power station，and make off-grid voltage set up. The unified off-grid phase angle and enable signal were

provided by off-grid control box to realize that multiple PCSs can be entered to off-grid mode quickly. During offgrid start-up，there are two ways to meet the different application needs of all users，one is raising voltage from

zero，the other is seamless switch.

      Key words: power converter system（PCS）；off-grid control box；raising voltage from zero；seamless switch