In short. A PMS keeps generation matched to demand. It decides how many sources run, shares load, sheds load when capacity is short and restores it in order. It sits above device controllers and below SCADA, and it must degrade gracefully when it fails.
Core functions
- Generator dispatch: start and stop sets by load and reserve. See sequencing and dispatch.
- Load sharing: kW and kVAr. See kW sharing.
- Load shedding: fast and slow. See load shedding.
- Restoration: staged load return. See restoration.
- Blackout prevention and recovery.
- Reporting: run hours, fuel, events.
Inputs and outputs
| Inputs | Outputs |
|---|---|
| Generator status, kW, kVAr, availability | Start and stop commands |
| Breaker positions | Breaker commands (within permissives) |
| Bus frequency and voltage | Speed and voltage set points |
| Load priorities and feeders | Shed and restore commands |
| Utility status and limits | Mode and alarm messages |
Spinning reserve
Dispatch should keep enough online capacity to absorb the loss of the largest running generator, or the largest step, without a blackout. Reserve is a design decision tied to the redundancy level. See multi-DG capacity.
Fast load shedding
When a generator trips, frequency can fall within a fraction of a second. A PMS can shed pre-selected loads based on the event, before frequency falls, which is faster than waiting for under-frequency relays. Pair with relays as backup.
Where it fits
The PMS must not remove protection from device controllers or relays. It should have defined behaviour on loss, for example generators continue sharing load by their own controllers. See control hierarchy.
Illustrative example
A plant with four generators and a 3 MW load runs two sets at light load and three at higher load. When one running set trips, the PMS sheds the lowest-priority feeders in under 100 ms, starts the standby set and restores the feeders in stages after it synchronises. Frequency stays within limits and no breaker trips on under-frequency.
What a power management system does
A PMS coordinates several generators, the utility and loads. It decides how many sets run, how load is shared and what to shed when capacity falls short.
| Function | Purpose |
|---|---|
| Load-dependent start/stop | Start another set when demand rises, stop one when it falls |
| Active and reactive load sharing | Equal percentage loading |
| Frequency and voltage control | Hold bus within limits |
| Load shedding | Protect the running sets in a shortage |
| Blackout recovery | Restart in an orderly sequence |
| Standby selection and run-hour balancing | Even wear, ready spare |
Start/stop thresholds (illustrative)
Start the next set when loading exceeds 80 percent for 10 seconds. Stop a set when the remaining sets would be below 65 percent after it leaves, with a minimum run time of 10 to 15 minutes to avoid cycling. The gap between thresholds (hysteresis) stops hunting.
Architecture
- Distributed: each generator controller holds the logic and communicates by a dedicated bus. No single point of failure.
- Centralised: a PLC or dedicated controller coordinates. Easier to modify, needs redundancy.
Design checks
- Response time of load shedding versus the engine’s tolerance.
- What each set does if the PMS communication fails: it should keep sharing by fallback methods such as droop.
See multi-DG sequencing and load shedding automation.
The master engineering question
At any moment, what is the largest single event the plant can absorb without losing priority loads, and is the PMS keeping that margin?
What to send us
Send generator data, load priority list and one-line diagram. Send your application details.
Questions engineers ask
Is PMS the same as EMS?
No. PMS is real-time plant control. EMS optimises energy and cost over longer periods. See EMS.
Do small plants need a PMS?
Two or three sets with simple needs may manage with generator controllers.
What happens if the PMS fails?
Generators should keep sharing load and protection should remain active.
Can it be added later?
Often yes, if device controllers and breakers have the required interfaces.
This guide gives general principles. PMS design must be project-specific.