In short. In a DG and BESS hybrid, the generator supplies bulk energy at an efficient load while the battery handles fast changes, peaks and short gaps. The control must keep enough reserve, avoid generator light-load running and keep the plant stable if either source drops out.
Roles
| Source | Best at | Limited by |
|---|---|---|
| Generator | Sustained energy, long outages | Step response, minimum efficient load, start time |
| Battery | Instant response, peaks, ramps | Stored energy, power rating |
Dispatch strategies
- Load following: generator follows load, battery smooths steps.
- Peak shaving: generator runs at a set point, battery covers peaks above it.
- Charge on generator: run the generator in its efficient band and use surplus to charge the battery.
- Generator off: battery supplies low load alone; generator starts when state of charge or load requires.
- Spinning reserve substitute: battery holds reserve in place of an extra running set.
Control rules
- Keep the generator in a loading band that avoids long light-load running.
- Define the minimum battery state of charge before the generator must start.
- Limit battery discharge so the generator never faces a larger step than it can accept. See load-step response.
- Define who leads frequency in each mode. See grid-forming coordination.
- Define how the system behaves if either source trips.
Savings and limits
Fuel savings come from running at efficient load and from reducing idle running. Claims of savings should be checked with measured fuel use and load data, not assumed. The battery also adds capital cost, so evaluate payback with the actual duty.
Transition tests
- Generator trip while battery is discharging.
- Battery trip while generator is at high load.
- Loss of communication between controllers.
- Utility return and resynchronisation.
Illustrative example
A 500 kVA site with a base load of 150 kW and 300 kW spikes from crushers once ran a large generator at low load most of the time. With a battery covering the spikes, a smaller generator runs near a middle loading band, charges the battery in the lulls, and the control keeps a reserve so a crusher start never stalls the set.
Why combine a generator and a battery
| Strength of the generator | Strength of the battery |
|---|---|
| Long duration, fuel stored | Instant response, no start delay |
| High energy per unit cost | Excellent at fast load steps |
| Efficient at moderate to high load | Efficient at any load |
Together, the battery covers transients and low-load periods while the generator runs at its best operating point.
Dispatch strategies
- Peak shaving: generator runs at a set fraction of rating, battery covers peaks above it.
- Generator off at low load: battery supplies night load; generator starts when state of charge falls to a threshold.
- Generator best-point charging: generator runs at 70 to 80 percent rating, load plus battery charging, then stops.
Fuel saving example (illustrative)
A generator lightly loaded at 20 percent uses a high fuel rate per kWh and risks wet stacking. If a battery lets it run at 75 percent for shorter periods, fuel use per kWh falls and hours reduce. Real savings depend on load profile, so model before investing.
Control cautions
- Avoid frequent generator starts: set minimum run and rest times.
- Prevent the battery from charging faster than the generator can support.
- Define behaviour when the battery is full or empty.
- Define what happens if the generator fails: battery runs to its limit then load is shed.
See BESS control and EMS architecture.
The master engineering question
With the battery at its lowest allowed state of charge and the generator at its loading limit, can the plant still ride through its worst credible step?
What to send us
Send load profile and equipment details. Send your application details. See also hybrid generator modernisation.
Questions engineers ask
Does a battery always cut fuel use?
Only for suitable load profiles. Check with data.
Can the generator be smaller?
Sometimes, if the battery covers the steps, but validate with a study.
What if the battery is empty?
The generator must be able to carry critical load alone.
Who sets priorities?
The control philosophy owner, with limits enforced by the PMS.
Values are illustrative. Hybrid dispatch needs project-specific design.