Apparent power limit at both ends of an AC branch

[erikfilias]

Problem

Under the branch flow model, the rating limits the current at the lowest voltage of the sending bus, l <= (TTC / Vmin)^2.
The apparent power this admits is TTC * V / Vmin: 2% above the rating at 1.0 p.u. on a 0.98 lower limit, 5% on 0.95, and
16% at 1.10 p.u. on 0.95. The DC model limits the active power to the rating itself, so an AC and DC comparison gives the AC
model up to 5% more transfer capacity on most branches.

Solution

IndACApparentPowerLimit = 1 in oT_Data_Option adds P^2 + Q^2 <= TTC^2 at both ends of each branch.

• SOCP and piecewise linear: a 24-sided polygon inside the circle, linear and at most 0.9% below the rating. The circle as
  a quadratic constraint gave Gurobi numerical trouble on a 695-bus case.
• Exact NLP: the circle, scaled by the rating.
• AC recovery step: the circle replaces the polygon.
• The current limit is kept. Off by default, which leaves the model unchanged.

Results

9n_AC, 8736 hours:

	Current limit only	With the option
SOCP: branch-hours above the rating (largest)	618 of 52416 (105.8%)	0 (99.9%)
SOCP: generation cost	149.019 MEUR	149.249 MEUR (+0.15%)
Piecewise linear: branch-hours above the rating (largest)	614 (105.2%)	0 (99.9%)
Piecewise linear: generation cost	149.059 MEUR	149.266 MEUR (+0.14%)

695-bus Nordic case, one hour, generation fixed at a reference power flow solution: with the case's ratings, the option
sheds 22.5 MW of load, because the reference flows exceed six ratings. With those ratings raised to the reference loading,
the option changes nothing; the solve takes 90 s.

Tests

• The option is off by default, and 2 is refused.
• 9n_AC: largest loading above 101% without the option, at most 100% with it.
• First 12 hours of 9n_AC with IndACRestore = 1: the circle replaces the polygon, largest loading 99.6% against 105.0%.
  Ipopt does not converge on the full year, with or without the option.
• AC tests without the solve marker: 80 passed; AC recovery step tests: 4 passed.

Documentation

doc/md/InputData.md: the option in the oT_Data_Option table and a section "The thermal limit of an AC branch". The
comment listing the AC options in openTEPES_InputData.py is shortened.