In short. Load shedding is needed when available generation can be less than the load, for example after a unit trips or when only a standby set is available. Find the shortfall in each case, decide which loads can go, in which order, and what triggers each stage.
Step 1: Find the cases
| Case | Available capacity | Load to carry |
|---|---|---|
| Normal, all units | Full | Full |
| One unit failed or in maintenance | Total minus the largest unit | Critical plus essential |
| Single generator on standby | One unit | Priority loads only |
| Temporary or rental source | Rental capacity | Limited set |
| Fuel or derating limit | Reduced capacity | Reduced |
Step 2: Compute the shortfall
Example (illustrative): load 2,200 kW, three 1,000 kW units. With one tripped, available is 2,000 kW at 100 per cent, but with a target loading of 90 per cent it is 1,800 kW. Shortfall is 400 kW, so at least 400 kW of load must be shed, with margin.
Step 3: Choose what to shed
Use the classification. Shed the lowest tier first, and check the total. See critical-load classification.
Step 4: Plan the stages
- Stage 1: comfort and non-essential loads.
- Stage 2: deferrable process loads.
- Stage 3: large loads that can be paused.
- Never shed Tier 1.
The scheme should shed enough in the first action for each case, with later stages for frequency or overload triggers. See load shedding and priority control.
Step 5: Triggers and speed
- Event-based triggers for unit trips act within a few hundred milliseconds.
- Capacity-based triggers act when the online capacity falls below the load.
- Frequency or overload triggers act as a last defence.
Step 6: Restoration
Plan staged restoration once capacity returns. See restoration sequencing.
Checks
- Do the shed loads sum to more than the shortfall, in each case?
- Are shed breakers actually controllable and wired to the scheme?
- Are any shed loads needed by Tier 1, for example chillers for the IT room?
Worked illustration
In the case above, the 400 kW shortfall is met by shedding 300 kW of comfort cooling (Tier 4) and 200 kW of a deferrable process line (Tier 3), total 500 kW, giving margin. The plan lists those feeders, the triggers and the order, and is tested by simulating a unit trip.
Do you need load shedding?
| Situation | Need |
|---|---|
| One generator carrying nearly all load | Simple overload shedding of non-essential loads |
| N+1 generators, load fits N | Shedding for double failure or abnormal events |
| Loads exceed generator capacity by design | Mandatory, as a normal operating feature |
| Island or hybrid operation | Mandatory, with battery state-of-charge triggers |
Shedding amount (illustrative)
Sets online 3 × 500 kW. One trips. Available = 2 × 500 × 0.9 (safe loading) = 900 kW. Load before the event 1,300 kW. Shed at least 400 kW. Choose shedding blocks: comfort HVAC 250 kW, process auxiliary 200 kW, giving 450 kW. The shed is done in under a second by a fast controller, ahead of frequency decay.
Design requirements
- A priority list approved by the operator.
- Triggers: source loss, overload, frequency.
- Shedding speed suited to the generator’s response.
- Restoration rules to avoid re-tripping.
- Test with real breakers.
What to avoid
- Shedding loads that are needed for safe shutdown of others.
- Restoring all at once.
- Untested shed lists that no longer match the plant.
See load shedding automation and load shedding engineering.
Common mistakes
- Only planning for the normal case.
- A list of loads with no breakers wired to shed them.
- Shedding something a critical load depends on.
What to send us
Send the load list, classification and generator arrangement. Send your application details. See also load-shedding automation.
Questions engineers ask
When is load shedding not needed?
When any credible failure still leaves enough capacity for the entire load. That usually needs larger N+1 capacity.
How much load should I shed?
At least the shortfall, with margin, for each case.
Is manual shedding acceptable?
For slow events perhaps, but generator trips need fast automatic action.
How do I test the scheme?
Simulate each case, including a unit trip, and record the response.
Figures are illustrative. Shed lists must be validated with operations.