In short. When a block of load switches on at once, a generator’s voltage and frequency dip and then recover. The size of the step, the engine and alternator characteristics and the loads’ tolerance decide whether that is acceptable. Screen the step against the manufacturer’s data and stage loads when it is too large.
What happens in a step
- The engine slows, because it must supply more torque than the governor can instantly deliver, so frequency dips.
- The alternator voltage dips, as armature reaction and the load’s reactive demand increase.
- The governor and the voltage regulator then restore both, over a recovery time of a second to several seconds.
The dip depends on the step as a share of the generator rating, the power factor of the step and the engine type. Turbocharged diesels typically accept a smaller first step than naturally aspirated ones, and gas engines often accept smaller steps than diesels.
Screening inputs
| Input | Why |
|---|---|
| Step size in kW and kVA | Defines the disturbance |
| Power factor of the step | Reactive part drives the voltage dip |
| Preload before the step | Loaded engines respond differently |
| Allowed voltage and frequency dip | From the sensitive loads |
| Recovery time allowed | Before the next step or for loads to remain on |
| Engine type and rating | Step acceptance differs |
Typical screening logic
- Compare the step as a percentage of generator rating with the manufacturer’s step acceptance table, which usually gives the maximum first step for a given frequency dip.
- Estimate the voltage dip from the alternator’s transient reactance and the step’s kVA.
- Check that dips stay within the loads’ tolerance.
- If not, split the step into smaller blocks with delays, or add generator capacity or a soft starter.
The generator’s compliance classes, such as the performance classes in ISO 8528-5, describe permissible dips and recovery. Use the class required by the loads.
Worked example (illustrative)
A 1,000 kVA diesel set with 400 kW running must accept a 300 kW block. The step is 30 per cent of rating. The manufacturer’s table might allow a first step of about 50 to 60 per cent for a frequency dip within the limit, so the step is acceptable on frequency. The reactive part may cause a voltage dip of, say, 12 per cent; if the loads tolerate 15 per cent, the check passes. If they tolerate only 10 per cent, the block is split in two with a 5-second delay. Numbers here are examples; the manufacturer’s table is the authority.
Staging as the cure
Staging is often the cheapest way to fix a step problem. It costs a delay, not capacity. See restoration and restart sequencing.
When to ask for a study
- Large motor starts and sequences.
- Many non-linear loads, such as UPS and drives.
- Strict voltage or frequency limits.
- Parallel generators or hybrid systems with BESS.
See load-step and frequency response engineering and transient response.
Screening a step load
A load step is the change in load when equipment connects at once, such as an ATS transferring a whole board. The generator must accept it with acceptable voltage and frequency excursions.
Step ratio = added load (kW) รท generator rating (kW)
| Step ratio | Likely outcome (indicative, confirm with maker data) |
|---|---|
| Up to about 40 percent | Most diesel sets accept in one step with moderate dip |
| 40 to 70 percent | Larger dip; check maker acceptance table or stage |
| Above 70 percent | Stage the load, or choose a unit with faster response |
Worked example (illustrative)
A 750 kW set must take a 600 kW board at transfer: step ratio 80 percent. Stage as 300 kW, 200 kW and 100 kW with 3 to 5 seconds between steps, so each step is 40, 27 and 13 percent. Alternatively, add a battery to buffer the step.
Performance class
ISO 8528-5 defines performance classes for voltage and frequency deviation and recovery. Choose the class by the load sensitivity and request the maker’s declared step-load performance, confirming the current edition of the standard.
Where screening stops
It does not predict a specific dip. Run a transient study when the loads are sensitive. See transient response and load-step and frequency response.
Common mistakes
- Using steady-state sizing and ignoring steps.
- Assuming acceptance of 100 per cent step.
- Not allowing for the reactive part of the step.
What to send us
Send your load blocks and sensitivity limits. Send your application details.
Questions engineers ask
What is the largest step a generator can take?
It depends on the engine, alternator and allowed dip. The manufacturer’s step-acceptance data gives the answer.
Why do gas engines handle steps worse?
They typically have less torque reserve, so the allowed first step is often smaller.
How does a BESS help?
It can take the step quickly while the engine responds, cutting the dip.
What is the performance class?
A classification in standards such as ISO 8528-5 for acceptable voltage and frequency deviation and recovery.
Figures are illustrative. Use manufacturer data for design.