[FEATURE] Feature request: GenericBranch3 (3-winding generic branch with internal AUX bus)

[scud-soptim]

Description:

Introduce a new component GenericBranch3 representing a generic three-winding branch, analogous to the existing 2-winding generic branch.

Key idea:
The component internally creates and manages the required AUX bus, so users and importers/exporters (e.g. CGMES-Converters) do not need to manually decompose a 3-winding device into multiple GenericBranch instances plus an extra AUX bus.

Proposed interface:

• For each winding i (i = 1…3), provide:

  • Series parameters: r_i, x_i
  • Shunt parameters: g_i, b_i
  • Rated voltage: Vn_i
  • Rated apparent power: Sn_i
  • ratio, shift etc.

• Parameters are conceptually identical to the 2-winding case, just extended per winding.

Behavior:

• The function/component automatically:

  • Creates a single internal AUX bus
  • Connects each winding to this AUX bus via its own branch representation

• Electrically equivalent to the current MVA-based modeling using:

  • Three GenericBranch elements
  • One explicit AUX bus
    but fully encapsulated in a single, reusable component.

Initial scope / limitations:

• Like the existing GenericBranch, only the symmetrical case is supported in the first implementation.
• Asymmetrical windings or phase-specific modeling are explicitly out of scope for this initial approach.

Motivation / Benefits:

• Simplifies network construction for 3-winding equipment
• Reduces boilerplate and modeling errors
• Greatly eases import of CGMES (and similar) data, where 3-winding components are common
• Improves consistency with the existing 2-winding generic branch abstraction

Scope (non-goals):

• No change to solver behavior required
• Purely a modeling / component-level enhancement