Transmission networks

A balanced network is the positive sequence transmission model. All of the balanced formats in the format table parse to it, so one reader and one writer per format cover all the conversions between them.

using PowerIO
module_ = parse("case118.m")       # PioModule{BalancedNetwork}
net = module_.value
length(net.buses)                  # 118
net.branches[1]                    # buses, branches, generators, loads, and the other tables

The network keeps what the source says: the element inventory, terminal connections, impedances, ratings, generator capability bounds and cost curves, and the source’s operating assignment. Ratings and costs stay on the network as reusable data, and which bounds a calculation enforces is decided when you construct an instance, so one parsed case serves power flow, DC OPF, and AC OPF without reparsing (see Calculation instances and solutions).

A few conventions hold across the accessors. Bus identifiers are the source’s own; dense zero based indices exist only in matrix results, which include the mapping. Powers are MW and MVAr as the source gives them, and angles are degrees; to_normalized derives a per unit, radian, in service only copy when a solver wants one. A branch tap of 0 means 1, and a rate_a of 0 means unrated.

Sources that describe more than the balanced calculation view, such as XIIDM, CGMES, and PSS/E RAW 35, also fill in the detailed connectivity: substations, voltage levels, connectivity nodes, terminals, switches, operational limit groups, and tap changer controls. A writer whose format can represent those writes them, and one whose format cannot reports what it left out.

emit(module_, "matpower", "copy.m")            # the source bytes, unchanged
result = emit(module_, "psse")                 # fresh PSS/E text
for finding in result.diagnostics
    println(finding.code, ": ", finding.message)
end

From a library you call parse, keep the module, and call emit; on the command line, powerio convert does both in one call.

Merging buses

A closed switch or a zero impedance branch joins two buses into one electrical node, which no finite admittance describes. merge_buses resolves them explicitly: each set of joined buses becomes one bus, every element moves onto it, and the joining elements are removed. The rule states what joins buses, and no threshold applies unless the rule names it.

Every rule considers in-service branches only. The surviving bus of a set is its reference bus, else a bus with an in-service generator, else a bus a generator regulates, with the smallest id breaking each tie. In Rust, BusMergeRule::with_survivor keeps the smallest id or each merged element’s from bus instead, the survivors of merge_zero_impedance_buses and reduce_zero_impedance. The survivor takes the strongest bus kind of its set. Loads, shunts, generators and their regulated buses, storage, branch, switch, and HVDC ends, three winding transformer windings, control buses, and area swing buses all follow the survivor. The result lists every removed element with its source row, identity, and ratings, and maps each source branch and switch row to its merged row. A removed in-service branch becomes a fixed shunt with its full equivalent admittance Yff + Yft + Ytf + Ytt at the survivor. This includes the series contribution of a bypassed transformer with an off-nominal tap or phase shift, and terminal charging with the from side divided by the tap squared. charging="drop" discards only terminal charging; the series equivalent remains. charging_shunts lists all these generated shunt rows. A branch the merge shorts, such as a line in parallel with a jumper, is removed and reported. Bypassing an ideal transformer with a nonunity tap or phase shift returns an error: no finite shunt can preserve that voltage constraint. A jumper whose merge would join two windings of one three winding transformer stays, also reported. Merging the result again changes nothing.

The removed elements’ flows are not variables of the merged network. calc_removed_flows recovers their active power from a solution of the merged network. Kirchhoff’s current law fixes the flows where the removed elements form a tree, the elements’ reactances split a loop, and a loop of zero reactance elements, such as a ring of closed switches, gets the minimum norm split and a diagnostic.

merge = network.merge_buses(zero_impedance="psse")
merged = merge.network                 # solve this network
flows = merge.calc_removed_flows(p_from, p_to)

powerio summary case.raw --merge-buses psse
powerio convert case.raw --merge-buses psse=0.0001 --to matpower -o merged.m

Islands and reference buses

A large case can hold several AC islands: systems joined only by HVDC, pockets behind an open breaker, and stranded equipment. calc_islands partitions the energized buses into islands joined by in-service branches, closed switches, and in-service three winding transformers. Buses typed isolated belong to no island, and HVDC lines do not join islands. Each island lists its buses, reference buses, in-service generators, and the in-service HVDC lines that tie it to another island, largest island first.

subset_buses(&buses) returns a BusSubset: the cut network and a map for all eleven tables (buses, branches, switches, three winding transformers, generators, loads, shunts, SVCs, storage, HVDC lines, and areas). Each map has one entry per source row: Some(row) points into the result table and None means that row was excluded. IDs are preserved. For example, cut.branch_rows[source_row] locates a statement’s branch in cut.network without reproducing the cut’s filtering rules.

A dead winding can bring its bus along as an isolated stub: its bus_rows entry is present, while injections excluded from that stub map to None. Detailed connectivity follows the cut, including calculated buses, topology connections, equipment metadata and remote regulation references. Unassigned source topology remains when no calculated bus identifies it as excluded. Empty voltage levels are pruned and nodes stranded by a cut retain explicit calculated-bus records so fresh XIIDM emission preserves the selected buses. The older selector-based subset shares this filtering but still returns a network directly.

To keep row indices fixed instead, keep_island(&buses) works in place: it de-energizes every other bus not already typed isolated and returns those buses, so the equipment outside the island stays in its table, out of service. de_energize_buses(&buses) is the step it builds on, the same one assign_island_references applies to an island with no source; it also takes the generators at those buses out of service.

A power flow needs one reference bus per island, and an island with no source cannot be solved. assign_island_references with the per island policy gives each island exactly one, on a bus that hosts an in-service generator:

• a stated reference that hosts no in-service generator cannot balance its island and is demoted to PQ first; the rules below count only the references that remain;
• an island with one reference keeps it;
• an island with none takes the bus of its largest pmax in-service generator, the smallest bus id breaking ties;
• an island with several keeps the one hosting the most generation and demotes the others to PV;
• an island with no in-service generator is de-energized: its buses are typed isolated and the equipment on or touching them is taken out of service. Three winding transformers lose only windings on those buses; the whole transformer goes out when fewer than two live windings remain. Supply counts in-service generators only, so an island that only an HVDC line feeds is de-energized too, and the line with it. It gets its own code, CANONICALIZE.ISLAND.HVDC_FED_DE_ENERGIZED, which names the lines and the power they schedule into the island; model that infeed as a generator at the converter bus to keep the island.

Every change is reported under CANONICALIZE.ISLAND. Normalization applies the same rule when asked, picks the same reference buses, and leaves the unsupplied islands out of the normalized network; by default it keeps the case’s references and designates one only when none survives. powerio summary lists the islands under topology.islands.

normalized = network.to_normalized(island_references="per_island")

Building matrices from a balanced network has its own chapter, Matrices and graphs.

PSS/E impedance correction before topology reduction

BalancedNetwork::apply_impedance_correction() applies retained PSS/E transformer correction tables at the present tap or phase shift. Call it before merge_buses or closed-switch normalization: a bypassed transformer becomes a shunt, whose admittance must include the correction. Reapply after editing tap or shift values while the transformer still exists; the nominal cache prevents repeated scaling.

The original table range selects ratio or angle. Ratio tables use bus-base per-unit winding ratios regardless of the RAW CW setting. A branch table is mapped to its effective t1/t2 tap coordinates. Three-winding ZCOD=0 scales star impedances; ZCOD=1 scales Z12/Z23/Z31 at the one nonfixed control winding, even when automatic regulation is disabled. Ambiguous RAW ZCOD=1 input is refused; an edited in-memory transformer with no unique controlling winding is skipped by correction application.

Fresh RAW35 output states nominal impedances beside the tables and preserves their interpolation and angle/ratio classification. When winding 2 cannot be set to one without changing a ratio table’s meaning, the writer chooses an equivalent nonunit ratio. RAW33/34 cannot state ZCOD=1; output freezes its present corrected pairwise impedances and reports the lost dependence on tap position. RAWX reads and writes the same tables as its impcor table, one itable, tap, refact, imfact row per point, with the RAW 35 rules.