Abstract The S-shaped characteristics of the pump-turbine may cause instability and thus leads to difficulties in grid synchronization. This paper develops a complete model for a pumped storage power plant and studies the start-up and grid synchronization procedure of two 300 MW variable speed units at no load in turbine mode. Based on the grid-voltage-oriented vector control method, the stator voltage of the doubly-fed induction machine is controlled to meet the grid connection requirements. Compared with the fixed speed units, the simulation results show that for a pump-turbine with typical S-shaped characteristics, the fixed speed unit cannot meet the grid connection requirements due to the unstable unit speed. Whereas, the variable speed unit can quickly reach synchronization with the grid voltage. For a pump-turbine without typical S-shaped characteristics, the synchronous unit can meet the grid connection requirements, but it takes a longer time and the stator voltage is not stable enough, while the variable speed unit is 11 times faster and more stable. The effects of PI parameters on the synchronization process of a variable speed unit are also studied. The results show that with suitable PI control parameters, the synchronization process can be accelerated without large overshoot of the rotor power.