  Two-dimensional Josephson junction arrays at zero temperature are
investigated numerically within the resistively shunted junction (RSJ) model
and the time-dependent Ginzburg-Landau (TDGL) model with global conservation of
current implemented through the fluctuating twist boundary condition (FTBC).
Fractional giant Shapiro steps are found for {\em both} the RSJ and TDGL cases.
This implies that the local current conservation, on which the RSJ model is
based, can be relaxed to the TDGL dynamics with only global current
conservation, without changing the sequence of Shapiro steps. However, when the
maximum widths of the steps are compared for the two models some qualitative
differences are found at higher frequencies. The critical current is also
calculated and comparisons with earlier results are made. It is found that the
FTBC is a more adequate boundary condition than the conventional uniform
current injection method because it minimizes the influence of the boundary.
