[FEATURE] generic_branch3

[scud-soptim]

Describe the feature request

Feature request: generic_branch3 — 3-winding generic branch (convenience component)

Motivation

generic_branch covers arbitrary two-winding (2WT) branch models via r1, x1, g1, b1, k, theta, sn.
For three-winding transformers imported from external formats (e.g. CGMES star equivalents), users currently have to build the star topology manually:
create an auxiliary node and three separate generic_branch objects, including the
per-winding impedance conversion. A convenience component would encapsulate this.

Proposed solution

Add a composite component generic_branch3 connecting three nodes (HV/MV/LV) via an
internal auxiliary (star-point) bus:

• Internally expanded into 3 × generic_branch + 1 hidden auxiliary node.
• Impedance handling via the MVA method (star configuration):
  • Pair impedances Z12, Z13, Z23 given at their respective winding ratings
    are first converted to a common power base (system or max. winding MVA),
  • then transformed to star impedances:
    Z1 = ½(Z12 + Z13 − Z23), Z2 = ½(Z12 + Z23 − Z13), Z3 = ½(Z13 + Z23 − Z12).
• Per-winding inputs: r, x (pair values or precomputed star values — to be discussed),
  g, b (magnetizing branch, assigned to one side or the aux bus), k, theta, sn per winding.
• Output: results per winding (side 1/2/3), analogous to three_winding_transformer.

Alternatives

• Keep building the star manually with 3 × generic_branch (status quo, error-prone).
• Use three_winding_transformer — not applicable when only generic per-unit
  branch parameters are available (e.g. from CGMES or external tools).

Additional context

This mirrors the relationship transformer ↔ three_winding_transformer,
but on the generic-branch level.

Community Meeting

• I wish to discuss this feature in the bi-monthly PGM community meeting. Please join the PGM community meeting.