Notes on PowerFlowData constructors

[luke-kiernan]

We have large chunks of code copy-pasted between the different constructors of PowerFlowData, which makes keeping track of what's what tricky. What follows are notes on the differences and similarities between the constructors for the different types of power flows: written with an eye towards refactoring things, but also generally useful for high level understanding.

Things in common across the 4 constructors (copy-pasted 4x):

• timestep_names
• converged

Things in common between make_dc_powerflowdata and the AC power flow constructor (copy-pasted 2x):

• timestep_map
• branch_types (named as branch_type) this is removed in PR #225, temporarily. The field only exists for PF-PSI compatibility reasons; for now, I'm focusing on PNM-PF compatibility, for which that field is problematic.

Things different across the 4 power flows, but in a regular fashion:

	ACPowerFlowData	ABAPowerFlowData	[v]PTDFPowerFlowData
power_network_matrix (pnm)	Ybus	ABA	[v]PTDF
aux_network_matrix (anm)	ABA or nothing	BA	ABA
bus_lookup	pnm.lookup[2]	anm.lookup[1]	pnm.lookup[2]
arc_lookup	pnm.branch_admittance_from_to.lookup[1]	anm.lookup[2]	pnm.lookup[1]
ref bus positions from...	pnm	anm	pnm
matrix for calculating arc flows	pnm.branch_admittance_{from/to}_{to/from}	anm.data, transposed	pnm.data, transposed

Updated for PR #225

The swap on lookups for PTDF is due to the fact that we're storing the transpose, yet the axes and lookups are in the order of the non-transposed matrix. [A slicker approach would be to make it a subtype of an Adjoint type.] One pattern that emerges: for DC power flows, we use the non-ABA matrix for things.

The ref bus positions, in 3 cases, are retrieved via get_ref_bus_position: looking in PNM, the function call turns into [get_bus_lookup(M)[x] for x in keys(M.subnetwork_axes)]

Then, we have several things that only make sense for AC power flows:

• extra functionality: [calculate_]loss_factors, generator_slack_participation_factors, and [calculate_]voltage_stability_factors
• neighbors (empty for DC)