3W transformer equivalent can have zero impedance

[WinfriedL]

For three-winding transformers with specific parameters it is possible to get equivalent transformers with zero impedance. While this is probably due to bad data, currently pandapower does not check this and the powerflow does not convergence with the warning

RuntimeWarning: divide by zero encountered in true_divide Ysf = stat / (branch[:, BR_R] + 1j * branch[:, BR_X]) ## series admittance

While this is a clear hint at the problem, finding the faulty element is not very straightforward.

I suggest at least a check in _trafo_df_from_trafo3w() like the following:

if any(trafo_df.vsc_percent==0):
    raise UserWarning("Equivalent Trafo with zero impedance!")

One could also add a small Impedance to let the powerflow continue...

[WinfriedL]

The transformer in my case has the following parameters:

std_type                                  None
hv_bus                                      24
mv_bus                                      25
lv_bus                                      23
sn_hv_kva                               140000
sn_mv_kva                                70000
sn_lv_kva                                70000
vn_hv_kv                                   150
vn_mv_kv                                    22
vn_lv_kv                                    22
vsc_hv_percent                              12
vsc_mv_percent                              24
vsc_lv_percent                              12
vscr_hv_percent                              0
vscr_mv_percent                              0
vscr_lv_percent                              0
pfe_kw                                       0
i0_percent                                   0
in_service                                True