Run a load flow
What it tells you. A load flow finds the steady state of your network. It gives the voltage at every busbar and the power in every line and transformer. Use it to check that voltages stay in their band, that nothing is overloaded and how much power the network loses.
Before you start
Check that the network has the data below. Select an element to see its data in the inspector on the right. The inspector groups data under Basic data, Load flow, Short circuit, Stability and Diagram.
| Data | Where to enter it |
|---|---|
| Busbar Nominal voltage | Inspector, Basic data |
| Busbar Lower voltage limit and Upper voltage limit, in per unit | Inspector, Load flow |
| Line Length, Resistance R′, Reactance X′, Susceptance B′ and Rated current | Inspector, Basic data |
| Transformer Rated power, Rated voltage HV, Rated voltage LV, Short-circuit voltage and Copper losses (resistive part) | Inspector, Basic data |
| Transformer No-load current, Iron losses and tap changer settings | Inspector, Load flow |
| Machine Control mode, Active power, Voltage setpoint, Reactive power minimum and Reactive power maximum | Inspector, Load flow |
| External grid Voltage setpoint and Voltage angle | Inspector, Load flow |
| Load Active power and Reactive power | Inspector, Load flow |
| Shunt Reactive power per section and Sections in service | Inspector, Load flow |
Each connected part of the network needs a source. That is an external grid, or a machine whose Control mode is Reference.
A machine's Control mode sets what it holds. PV holds active power and voltage. PQ holds active and reactive power. Reference sets the angle and balances the system.
Set up the study
- Select the Calculate tab.
- Under Steady state, switch on the options you want: Recalculate on edit, Respect Q limits and DC start.
- To change other settings, press Ctrl+, to open Study case.
- Scroll to the Load flow heading and change the settings you need.
- Select Apply.
The settings you will use most are in this table.
| Setting | What it does |
|---|---|
| Power mismatch tolerance | Sets how small the power imbalance must be before the solution is accepted. The default is 0.001 MVA. |
| Maximum iterations | Sets how many steps the solver may take. The default is 30. |
| Respect reactive power limits | Holds a machine at its reactive limit, instead of its voltage setpoint, when it reaches that limit. Off by default. |
| Start from a DC load flow | Takes the starting angles from a DC load flow. It helps meshed networks with phase-shifting transformers to converge. On by default. |
| Load scaling | Scales every load, as a percentage. The default is 100 %. |
| Tap changers regulate voltage | Lets transformer tap changers move to hold a busbar voltage. Off by default. |
| Phase shifters regulate flow | Lets phase-shifting transformers move to hold a power flow. Off by default. |
| Switched shunts regulate voltage | Lets switched shunts add or remove sections to hold a voltage. Off by default. |
| Share each island's imbalance among | Chooses which machines take up the difference between generation and load. |
The ribbon switch Respect Q limits is the setting Respect reactive power limits. DC start is Start from a DC load flow. For how the solver works, see the engine description.
Run it
- On the Calculate tab, select Load flow. You can also press Alt+L or Ctrl+Enter.
- Wait for the progress bar in the status bar to finish.
The Load flow tab opens in the results panel, and the diagram shows the results. The Output tab logs a summary, such as the iterations and the highest loading.
To stop a calculation that takes too long, press Ctrl+. or select Cancel calculation on the Calculate tab.
With Recalculate on edit on, the load flow runs again after each change you make. It starts only after you have run it once.
Read the results
The top of the Load flow tab shows a summary:
- A pill that says Converged in 2 iterations, or the reason the solver did not converge.
- Generation, Load and Losses, in MW.
- Shared imbalance, if machines shared the imbalance of an island.
- Mismatch, the largest power imbalance left, in MVA.
- Time, how long the calculation took.
Use the buttons on the right of the tab to switch between its tables.
| Table | What it lists |
|---|---|
| Busbars | Busbar, Type (Ref, PV or PQ), U in kV, u in p.u., Angle, P injected and Q injected |
| Branches | Branch, Type, Loading in %, P from, Q from, P to, Q to, I max and Losses |
| Machines and loads | Element, Type, P, Q and a Note such as At upper Q limit |
| Controls | Each tap changer, phase shifter or switched shunt that moved: Before, After, Moved and Range |
| Areas | Each area's Net export against its Target |
| Warnings | Warnings and De-energised busbar rows |
The tables Controls, Areas and Warnings appear only when they have something to show. The first 3 short warnings show as pills instead of a table.
On the diagram, each busbar's result box shows its voltage and angle. Each branch shows the power at both ends and its loading. Colours show loading from 0 % to 100 % or more. Busbars outside their band have their own colours.
To show only the rows that need attention, use the filters above the table. See Work with results.
Check your results
Use these checks before you rely on a result.
- Power balance. Generation should equal Load plus Losses. If it does not, look for Shared imbalance or a warning.
- Voltages. Most busbars should be close to 1.00 p.u. A busbar far from 1.00 p.u. is a finding. Check the data around it before you trust it.
- Reference. The reference machine or external grid takes up the imbalance. If its P looks unrealistic, check your loads and Load scaling.
- Reactive limits. With Respect reactive power limits on, a machine marked At upper Q limit cannot hold its voltage setpoint.
- A change you know. Change one load, then check that the losses and the flows move the way you expect. Press Ctrl+Z to undo the change.
To check your results against another tool, see Check PowerStudio against your own tool.
If something goes wrong
| What you see | What it means | What to do |
|---|---|---|
| A red pill, such as "No convergence after 30 iterations", that names a busbar | The solver stopped with a large mismatch near that busbar | Check the loads and data around that busbar. Try a lower Load scaling. Switch Start from a DC load flow on or off. |
| A Load flow cannot run message about a line that joins busbars with different nominal voltages | A line connects 2 voltage levels | Replace the line with a transformer. |
| A Nothing to calculate message | The network has no busbars | Draw a network or open a sample. |
| De-energised busbars in the Output tab | A part of the network has no external grid or reference machine | Connect it to a source, or set a machine in it to Reference. |
| A machine says At upper Q limit or At lower Q limit | It cannot hold its voltage with the reactive power it has | Change the setpoint, the limits or the network. |
| The pill Calculated before the last edit, and old on the tab | You changed the network after the calculation | Run the load flow again. |
A load flow starts from the last one that converged. If it does not converge from that start, PowerStudio tries again from the usual start before it reports a failure.
Try it on the IEEE 14-bus sample
The IEEE 14-bus sample is a 132/33/11 kV transmission network with 5 machines.
- Select the Help tab.
- Under Samples, select IEEE 14-bus system.
- Select the Calculate tab, then select Load flow.
- Read the summary. The pill says Converged in 2 iterations. Generation is 272.39 MW, Load is 259.00 MW and Losses are 13.393 MW.
- Check the balance. 259.00 MW plus 13.39 MW is 272.39 MW.
- In the Busbars table, select Outside band. It lists 3 busbars: Bus 6 at 1.0700 p.u., Bus 7 at 1.0615 p.u. and Bus 8 at 1.0900 p.u.
- Select Bus 8 to see its data. Its Upper voltage limit is 1.06 p.u., and the machine there holds 1.09 p.u.
- In the Branches table, look at Loading. The highest is Transformer 5-6 at 76.4 %, then Line 1-2 at 72.5 %. No branch is above 90 %.
The Riverside sample is a 110/20/0.4 kV distribution network. Under Samples, select Riverside distribution. Its load flow also converges in 2 iterations. Generation is 26.61 MW, Load is 26.32 MW and Losses are 0.293 MW.