Three-phase transformers - Combining star and delta connections

[alexblancoeroots]

Describe the bug
When a Dy or Yd power transformer is implemented in VeraGrid, numerical issues can appear. The power flow does not converge or provide incorrect results because the bus admittance matrix Ybus is singular.

To Reproduce
To implement in VeraGrid either a Dy (Delta-star) or Yd (Star-delta) transformer:

trafo= gce.Transformer2W(name='trafo',
bus_from=bus_1,
bus_to=bus_2,
HV=4.16,
LV=0.48,
nominal_power=0.5,
rate=0.5,
r=1.1,
x=2)
trafo.conn_f = WindingType.Delta
trafo.conn_t = WindingType.GroundedStar
grid.add_transformer2w(trafo)

Expected behavior
VeraGrid should give the correct power flow results, validating them with other three-phase solvers like OpenDSS.

Desktop:

• OS: Windows 11