Power flow

NumaSim can solve the steady-state power flow of your electrical circuit alongside the time-domain simulation. Use it to size a network, check bus voltages under load, or validate a PSS®E case before streaming it into a transient run.

A power-flow case is a set of buses joined by branches, with generators and loads attached. Each bus is one of three types — a slack bus that sets the reference voltage and angle, a PV bus that holds a scheduled voltage, or a PQ bus that draws fixed real and reactive power:

A small power-flow case. The slack bus sets the reference voltage and angle, the PV (generator) bus holds its scheduled voltage, and the PQ (load) bus draws fixed real and reactive power. The branches carry the resulting flows.

When to use it

A time-domain simulation answers "what is the voltage at this node right now?". A power-flow solve answers "what is the steady-state voltage at every bus, given the loads and generation specified?". They complement each other:

• Run power flow first to verify your network is feasible.
• Then run a transient simulation to see how it responds to a disturbance.

Solving a power flow

1. Build (or import) an electrical circuit with buses, branches, generators, and loads.
2. Open the Power Flow dialog from the toolbar.
3. Pick the solver (Newton-Raphson is the default; Gauss-Seidel and fast-decoupled are available).
4. Click Solve. The dialog reports per-bus voltage magnitude and angle, slack bus power, and any convergence issues.

Under the hood, the default Newton-Raphson solver is iterative: it starts from an initial guess, computes how far each bus is from its scheduled power (the mismatch), solves a linearized update from the Jacobian, applies it, and repeats until the mismatch is within tolerance.

The iterative power-flow solve. Each pass computes the bus power mismatch, solves a linear update $\mathbf{J}\mathrm{\Delta }x=\mathrm{\Delta }S$ from the Jacobian, and applies it; the loop ends when the mismatch is within tolerance.

Visualizing the result on the canvas

When a power-flow solution is loaded, every bus and branch on the canvas can display an overlay (voltage magnitude / angle, line MW / MVAR flow). Toggle the overlay from the dialog's footer.

Importing PSS®E cases

If you already have a network in PSS®E format, use File > Import PSS/E. That opens the PSS/E Import Wizard, where you choose the files, pick how the records should be modelled, and review everything before anything lands on the canvas.

Files

A power-flow .raw file is required. A dynamics .dyr file is optional; see Dynamic models below.

Modeling options

Two global choices, applied to every record that does not override them in the review table:

• Transmission line model — how each BRANCH record is built. Bergeron travelling-wave and Nominal PI both produce a Transmission Line Model box holding the line data plus a Transmission Line Terminal at each end; Lumped Pi Line cell produces a single Pi Line component instead. Bergeron is the default. A Bergeron line whose travel time is shorter than your time step automatically falls back to a nominal-pi section at run time.
• Generator model — whether each GENERATOR record becomes an ideal three-phase Voltage source (the default) or a Synchronous machine. A raw file carries no dynamic data, so machines built from a .raw alone get the ratings and power-flow schedule from the file with placeholder reactances and inertia.

Reviewing the records

Below the options is one collapsible section per record type — buses, generators, loads, fixed shunts, branches and transformers — showing what the file contains. Each row can be:

• excluded with its checkbox, so it is left out of the import. Dropping a bus drops everything attached to it, and those rows grey out immediately so you can see the knock-on effect before importing.
• edited, for the values that carry physical meaning (generator PG / QG / MBASE, branch R / X / B / rating, load P / Q, transformer R / X / taps). Import stays disabled while any edited cell is not a number.
• overridden, on generator and branch rows, to use a different model from the global choice.

Dynamic models

Attach a .dyr and the wizard gains a Dynamic models section listing each machine, the controllers attached to it, and whether it will reach the canvas. Supported models:

Kind	PSS®E models	NumaSim component
Machine	GENROU, GENROE, GENSAL, GENSAE	Synchronous Machine
Exciter	IEEET1, ESDC1A	Excitation System (DC1A)
Exciter	ESAC4A, EXAC4, SEXS	Excitation System (AC4A)
Governor	TGOV1	Turbine-Governor (TGOV1)
Governor	GGOV1	Turbine-Governor (GGOV1)
Stabilizer	IEEEST	Power System Stabilizer (PSS1A)
Stabilizer	STAB1	Power System Stabilizer (STAB1)

Anything else is listed as unsupported and skipped — the rest of the file still imports. A generator with a machine record is always built as a synchronous machine regardless of the global choice, because dynamics cannot attach to an ideal source.

Controllers are placed next to their machine and wired up for you: terminal voltage and field current into the exciter, field voltage back out, shaft speed into the governor and stabilizer, mechanical torque back out, and the stabilizer's output into the exciter's PSS input. Because the pairing is a real wire, the controllers initialize themselves from the load-flow operating point the same way they would if you had drawn them by hand.

After the import

Because a Transmission Line Model box has no pins of its own, the importer parks all of them in a grid to the right of the network rather than leaving them scattered through the diagram. The two ends of a line are paired by name rather than by a wire, but the auto-layout still treats that pairing as a connection, so a line's two terminals land next to each other instead of at opposite corners of the canvas. The same applies when you run Auto layout on a circuit you drew yourself.

The raw file only carries positive-sequence R, X and B, so the imported model gets equal positive- and zero-sequence values — the same as saying the phases have no mutual coupling. Open the model box and enter real zero-sequence data if you plan to study unbalanced faults.