Buses, loads, shunts, and branches

The passive network model includes bus types, composite loads, fixed shunts, and the branch pi equivalent.

Bus types

The IDE field assigns each bus one of four roles. Type 1 buses carry fixed injections. Type 2 buses host voltage-regulating machines; the regulation contract is defined on the generator page. Type 3 buses are swing buses: the voltage magnitude and angle are held, and the connected machines absorb the system power balance. Type 4 buses are out of service: they are excluded from the solve, together with everything connected to them, and each exclusion is reported (ISOLATED_BUS).

Every electrical island must contain a swing bus; a case with a swingless island is rejected, with the island's buses reported. A case may carry several swing buses: each is held at its setpoint, and they jointly absorb the balance. The entered VM and VA seed the iteration and do not constrain the solution, except at swing buses, where VA sets the held angle.

Loads

A load consists of constant-power, constant-current, and constant-admittance components. With \(V\) the bus voltage magnitude in per unit, the consumed power is

\[ P(V) = P_L + I_P V + Y_P V^2, \qquad Q(V) = Q_L + I_Q V + Y_Q V^2 . \]

Each coefficient is entered as the power consumed at \(V = 1.0\) per unit, in MW and Mvar, and its sign is preserved as entered. Loads with STAT = 0 and loads at out-of-service buses are excluded. In revision 34 cases, distributed generation on a load (DGENP, DGENQ) is netted against the consumption when DGENF is non-zero; DGENF = 0 leaves PL/QL unchanged (v33 default). Only the constant-power terms are netted; ZIP coefficients are passed through as entered.

An optional solve setting (low_voltage_load) tapers the constant-power component below a chosen voltage to bound the current it draws at depressed voltages. It is disabled by default, and the equations above then hold as written.

Fixed shunts

A fixed shunt is an admittance to ground. GL is the active power consumed and BL the reactive power injected at \(V = 1.0\) per unit; both scale with \(V^2\). Positive BL is capacitive (injection); negative BL is a reactor (consumption).

Branches

A branch is a pi equivalent: series impedance \(R + jX\) in per unit on the system base, total line charging B split equally between the two ends, and optional per-end admittances (GI, BI, GJ, BJ). Out-of-service branches (STAT = 0) are excluded. Flows are reported at both terminals, so the difference between the two ends is the branch loss. The branch rating fields do not constrain the solution; they scale the loading percentages in reports. The bus voltage limit fields (NVHI, NVLO, EVHI, EVLO) have no effect on the solve.

Zero-impedance ties

The network preparation can consolidate buses joined by negligible impedance. The case threshold controls which connections qualify. Consolidated buses share one reported voltage, and the result identifies each affected bus and connection. Use the case doctor to review these changes before a study.

System switching devices

In-service system switching devices (revision 34 and later) solve as network branches. RAW and RAWX exports do not write these records. Review the export report before using an exported case.