In short. A load-flow study calculates steady-state voltages, currents and power flows in the system for a given set of sources and loads. It shows whether cables and transformers are overloaded, whether voltages are within limits, and how power divides between utility and generators.
What it tells you
- Voltage at every bus, compared with allowed limits.
- Loading of cables, transformers and generators as a percentage of rating.
- Active and reactive power flows, and power factor at key points.
- System losses.
- Effect of tap changers, capacitors and reactive support.
Inputs
| Input | Detail |
|---|---|
| Network | Single-line diagram, cable data, transformer impedance and taps |
| Sources | Utility voltage and fault level; generator limits for kW, kVAr, voltage setpoint |
| Loads | kW and power factor, by bus, for each case |
| Control settings | Tap positions, AVR modes, capacitor steps |
Cases to run
- Normal: maximum and minimum load on utility.
- Generator supply: loads on the generator alone, including the critical subset.
- Contingencies: a transformer or feeder out of service, one generator lost in a parallel plant.
- Future: including planned expansion.
- Parallel operation: with utility and generator, checking reactive power sharing.
Interpreting the results
- Voltage low at a remote bus: check cable size, transformer tap, source voltage. See cable sizing and voltage drop.
- Overloaded transformer or cable: reduce load, add capacity or reconfigure.
- Generator near its kVAr limit: improve power factor locally. See reactive power studies.
- High losses: examine cable lengths, power factor and loading.
Limits of the method
Load flow is steady-state. It does not show motor starting dips, fault currents or transient behaviour, which need other studies. It is only as good as its model: wrong impedances or loads give wrong voltages.
Illustrative example
A site fed by a long cable from a generator room shows a voltage of 94 percent at the farthest board at full load in the load-flow study, below the allowed minimum. Options are a larger cable, a local transformer tap change or a generator voltage setpoint adjustment. The study compares each and finds the tap change enough for normal load, but not for the motor-starting case, which is addressed separately.
What load flow solves
Given the network, the loads and the sources, load flow finds the steady-state voltage magnitude and angle at every bus, and the real and reactive power in every branch. From these come currents, losses and loading percentages.
Inputs and outputs
| Input | Output |
|---|---|
| Source voltage and impedance | Bus voltages |
| Transformer data and tap positions | Transformer loading and tap suitability |
| Cable lengths and sizes | Voltage drop and losses |
| Load kW and power factor | Reactive flow, power factor at incomer |
| Generator output and voltage setpoint | Generator loading and reactive share |
What to check in results
- Bus voltages within the permitted band, commonly plus or minus five percent in service.
- No transformer or cable above its rating.
- Generator kVA and power factor within its capability.
- Losses at the expected level.
Cases to run
- Normal mains supply at maximum and minimum load.
- Generator supply at the critical load.
- Parallel operation, with and without a transformer out of service.
Generator-specific point
Under generator supply the lagging power factor seen by the alternator may be lower than on the mains, because capacitor banks and power factor correction can be disconnected or unsuitable. Load flow shows whether the alternator operates inside its reactive capability.
See reactive power studies and cable and voltage drop.
Common mistakes
- Running only the utility case.
- Using nominal rather than actual transformer taps.
- Ignoring reactive power limits of the generator.
What to send us
Send the single-line diagram, load list and cable data. Send your application details. See also voltage drop basics.
Questions engineers ask
Is load flow needed for small sites?
Hand calculations may be enough for simple radial systems. Larger or multi-source systems benefit from a model.
What voltage limits apply?
They depend on standards, equipment and the utility agreement. Define them before the study.
Does load flow include harmonics?
No. Harmonics need a separate study.
Can it size a generator?
It checks loading and voltage. Generator sizing also needs starting and transient studies.
This guide gives general principles. Results depend on the accuracy of the model.