In short. A sudden kW step is met first by stored rotating energy, so speed falls, then by the engine as fuel and air catch up. The size of the dip and the time to recover depend on the step, engine type, inertia and governor. Staging loads in smaller steps is often the cheapest fix.
Physics
Rotating inertia supplies the shortfall until the governor increases fuel. Frequency change is proportional to the power imbalance and inversely to inertia, so a heavier rotating mass and smaller step both give a gentler dip.
df/dt ∝ (Pmech − Pelec) ÷ (2H × S)
Engine limits
- Naturally aspirated engines accept large steps more readily.
- Turbocharged engines lag at first as air supply builds, so first-step capability is lower.
- Manufacturers publish block-load capability, often as a percent of rating per step with recovery times.
What the study checks
| Item | Question |
|---|---|
| Step size | Is each step within the block-load capability? |
| Frequency dip and recovery | Within limits of UPS, drives and process? |
| Voltage effect | Does under-frequency roll-off reduce voltage as designed? |
| Protection | Do under-frequency or overload relays stay clear? |
| Sequence | Does the restart order respect recovery time between steps? |
Staging a restart
- Rank loads by criticality. See load prioritisation.
- Energise the base group first, usually lighting and control.
- Add motors and transformers one at a time with recovery delays.
- Leave the largest step for when more sets are online.
Parallel plants
Sets share the step according to governor droop and settings. Misaligned governors push one set harder. See frequency dip and load-sharing control.
Illustrative example
A 1,000 kVA set at 800 kW base load receives a 250 kW chiller start in one step. The study finds the frequency dip near the limit of the UPS input. Splitting the chiller start with a soft starter and a 10 second delay after the last motor keeps the dip well inside the limit.
What a load step does to a generator
When load is added suddenly, the engine slows until fuel and air catch up, so frequency dips. The alternator voltage dips and the AVR restores it. Both recover in seconds, and their depth and speed define transient performance.
Factors that set the frequency dip
| Factor | Effect |
|---|---|
| Step size as a fraction of rating | Larger step, deeper dip |
| Turbocharger lag | Slower air response, deeper dip |
| Rotating inertia | More inertia, shallower dip, slower recovery |
| Governor type and tuning | Electronic governors recover faster |
| Load type | Motor loads also draw reactive power |
Staged loading (illustrative)
A set that can accept about 60 percent step load in one go from cold may accept 100 percent in two or three stages with a few seconds between steps. A 1,000 kW load can then be connected as 400, 300 and 300 kW steps. Confirm acceptable step sizes with the engine maker’s data.
UFC and load shedding
Under-frequency can trip protection or lose synchronism with a parallel source. Frequency-based load shedding gives the engine relief before it stalls. See load shedding.
Study output
- Frequency and voltage versus time plots for each step.
- Recovery times.
- Recommended step sequence.
See frequency dip and transient response.
Common mistakes
- Using steady rating as step capability.
- Restarting everything at once on return of the generator bus.
- Untuned governor on parallel sets.
What to send us
Send engine block-load data and your restart list. Send your application details.
Questions engineers ask
What frequency dip is acceptable?
It depends on the most sensitive load. Define it first.
Does a flywheel help?
More inertia reduces the initial dip, but not engine response time.
Can batteries help?
Yes, a BESS can supply the step fast while engines ramp. See hybrid system studies.
Is isochronous mode better?
It restores nominal frequency, but sharing in parallel needs proper controls.
This guide gives general principles. Use manufacturer data for design.