In short. In a parallel plant you rarely need every unit running. A dispatch scheme starts and stops units as load rises and falls, keeping the running units in a healthy loading band and always keeping enough reserve to take the next step or lose a unit. Good rules save fuel and wear without risking a blackout.
Goals of dispatch
- Keep each running unit within a target loading band.
- Maintain spinning reserve for a step or a unit trip.
- Balance running hours so wear is shared.
- Avoid frequent start-stop cycling.
- Keep redundancy for the critical load.
Typical rules
| Rule | Example (illustrative) |
|---|---|
| Start next unit | When running load exceeds 80 per cent of online capacity for 10 seconds |
| Stop a unit | When load falls below 50 per cent of online capacity after removing it, for 5 minutes |
| Start on failure | Immediately if a unit trips or fails to take load |
| Minimum run time | Each unit runs at least 10 to 15 minutes after start |
| Priority order | Rotated by running hours or schedule |
Choose thresholds from engine guidance, load profile and step needs.
Operating combinations
For a plant of four equal units, the combinations are one, two, three or four running. Each has a capacity band, and a hysteresis gap between the start and stop thresholds prevents cycling. Example with 1,000 kW units, start at 80 per cent and stop at 50 per cent after removal:
- One unit runs to 800 kW, then a second starts.
- Two units (2,000 kW) run until load falls below 500 kW on one unit’s worth, that is, below about 1,000 × 0.5 = 500 kW, so the second stops.
- The thresholds repeat for three and four units.
Reserve and redundancy
Decide whether the critical load must be carried with one unit out, at all times. If so, the plant keeps one extra unit running or on fast standby, not only the number needed for the load. This costs fuel, so some sites run the spare only when the load is high or the risk is elevated.
Step capability
Online capacity must handle the largest expected step. If the biggest motor start would stall two units but not three, then three run when that start is possible. See load-step screening.
Controls
The plant controller or power management system implements these rules. See automatic sequencing and dispatch. It should have a manual override and a fallback when communications fail.
Worked example (illustrative)
A site’s load swings between 600 kW and 2,600 kW across the day, with four 1,000 kW units. At night, one unit carries 600 kW (60 per cent) and a second is on fast standby. In the morning ramp, units start at 800 kW, 1,600 kW and 2,400 kW of load. At the 2,600 kW peak, all four run with one as reserve. Fuel use falls compared with running all four all the time, and each unit stays in a good loading band.
Choosing which sets run
With several sets available, the plant runs only as many as the load needs, plus the reserve required. The operating combination is chosen by load level, wear and redundancy rules.
| Load (of 4 × 500 kW, N+1 requirement) | Sets running | Loading each |
|---|---|---|
| Up to 400 kW | 2 (one carrying, one hot standby) or 1 with the other ready to start | 80 percent or 40 percent |
| 400 to 800 kW | 2 | 40 to 80 percent |
| 800 to 1,200 kW | 3 | 53 to 80 percent |
| 1,200 to 1,500 kW | 4 | 60 to 75 percent |
Dispatch rules (illustrative)
- Start the next set above 80 percent loading for 10 seconds.
- Stop one when remaining sets would be under 70 percent after it leaves, after a minimum run time.
- Rotate lead set by run hours.
- Always keep the N+1 reserve startable within the time the load can bear.
Hysteresis
A gap between start and stop thresholds avoids cycling. Without it a load hovering at the boundary causes repeated starts and stops.
Constraints
Minimum run time, cool-down, maximum starts per hour and the engine maker’s guidance on light running.
See multi-DG sequencing and dispatch.
Common mistakes
- Thresholds with no hysteresis, causing cycling.
- No reserve for a trip.
- Same unit always first, so it wears out first.
What to send us
Send your load profile and generator data. Send your application details. See also number of generators.
Questions engineers ask
How much spinning reserve should I keep?
Enough to cover the largest credible step or the loss of the biggest running unit, depending on your resilience target.
Why add hysteresis?
So units do not start and stop repeatedly around one threshold.
Should I run units in rotation?
Yes, to balance wear, subject to maintenance schedules.
Can dispatch run without a controller?
Manually, yes, but automatic dispatch is faster and more consistent.
Figures are illustrative. Set thresholds from engine guidance and your load profile.