The three-phase voltage source converters (VSCs) usually adopt a dual-loop control to achieve the bi-directional power flow in grid-connected applications. The VSCs may suffer from serious transient instability when the grid fault occurs. In this paper, the post-fault transient current, which is critical for the protection design, is calculated according to various types of grid faults. The physical origin of the expanded transient current is analyzed and the calculated methods are proposed for typical balanced/unbalanced short circuit and open circuit faults. The analysis results are verified by detailed simulations.