Black startup with large amount of load has enormous impact on system voltage. An approach based on superposition is used to decompose sudden three-phase symmetrical load resumption in black startup into two simpler independent problems. Synchronous generator model including damping winding is then used to derive the relationship between voltage magnitude and machine， circuit parameters， and load types. After further analyzing the relationship derived， a general load resumption principle is proposed. It increases the power factor and composite impedance of the load in a certain range. Simulation results using RTDS and filed test measurement verify the correctness and effectiveness of the proposed methods.