Changes

requires PyPSA/linopy#568
closes #592
closes #1540
supersedes #1540

This refactor consolidates ramp limit constraint definitions into a unified formulation that handles all operational types (fixed, extendable, committable) with a single constraint. It also introduces p_init to enable ramp constraints at the first snapshot.

New Attribute: p_init added to generators and links (pypsa/data/component_attrs/):

• Specifies the initial active power for ramp limit constraints at the first snapshot
• When set, ramp constraints are applied at sns[0] using p_init as the previous dispatch
• For committable**: Initial status is derived from up_time_before > 0

Default Value Changes (Breaking)

ramp_limit_start_up and ramp_limit_shut_down:

• Before: Default 1 (full ramp allowed)
• After: Default NaN (no limit / ignored)
  (like in Fix: ramp_limit_start_up attribute is ignored in binary UC #1540)

Consolidated Constraint Formulation

The constraint code (pypsa/optimization/constraints.py) was simplified from ~230 lines with separate handling for fix/ext/com to ~75 lines with a unified formulation:

Ramp Up:

p[t] - p[t-1] <= ru * p_nom * u[t-1] + rusu * p_nom * (u[t] - u[t-1])

Ramp Down:

p[t] - p[t-1] >= -rd * p_nom * u[t] - rdsd * p_nom * (u[t-1] - u[t])

Key insight: For non-committable components, status = 1 for all t, so start-up/shut-down terms vanish naturally. For extendable components, p_nom is replaced by the capacity variable.

Constraint Naming (Breaking)

• Before: {c}-fix-p-ramp_limit_*, {c}-ext-p-ramp_limit_*, {c}-com-p-ramp_limit_*
• After: {c}-p-ramp_limit_up, {c}-p-ramp_limit_down

First Snapshot Handling

Merge Order

This should go in before #1007 because there the ramp limits not yet fully sorted out.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• Unit tests for new features were added (if applicable).
• A note for the release notes docs/release-notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.