A Practical Guide to Controlling Solar, Battery, Grid, and Generator Sources
A power system with solar energy, battery storage, the utility grid, and a standby generator does not always require a dedicated microgrid controller.
The correct solution depends mainly on the system architecture.
The key question is:
Does the generator connect through a compatible hybrid inverter, or do the generator, battery system, solar inverters, and grid operate as independent sources on a common AC bus?
In a simple system, the hybrid inverter may control the complete operating sequence.
In a larger or more complex installation, a dedicated microgrid controller may be required to coordinate synchronization, load sharing, protection, circuit breakers, and load shedding.
A typical source priority may be:
Solar → Battery → Grid → Generator
However, this priority must be supported by a clear operating philosophy and appropriate control equipment.
1. Two Ways to Control the System
Option One: The Hybrid Inverter Is the Main Controller
This approach is suitable when the installation includes:
- One compatible hybrid inverter or inverter-charger system.
- One battery system.
- One utility grid connection.
- One standby generator.
- A dedicated generator input on the inverter.
- Loads within the inverter’s supported backup capacity.
In this arrangement, the generator normally does not operate as an independent source in parallel with the battery inverter.
Instead, the generator connects to the dedicated AC or generator input of the hybrid inverter.
The inverter then performs functions such as:
- Sending a dry-contact signal to start the generator.
- Waiting until generator voltage and frequency are stable.
- Accepting the generator as an AC input source.
- Supplying the loads through its internal transfer path.
- Limiting the power drawn from the generator.
- Controlling the battery-charging current.
- Disconnecting the generator when the stopping conditions are met.
- Sending the final generator stop command.
This architecture is generally simpler, less expensive, and easier to commission.
Option Two: A Dedicated Microgrid Controller
A microgrid controller becomes necessary when the power sources operate independently on a common AC bus.
This may include:
- Grid connection.
- One or more generators.
- Battery Energy Storage System.
- Several solar inverters.
- Bus-coupler circuit breakers.
- Critical and non-critical load groups.
- Medium-voltage distribution.
- Sources from different manufacturers.
The controller monitors the complete electrical system and coordinates:
- Generator start and stop.
- Source synchronization.
- Circuit-breaker operation.
- Active and reactive power sharing.
- Battery charging and discharging.
- Solar-power limitation.
- Load shedding and restoration.
- Grid-connected and island operation.
- Resynchronization with the utility grid.
Basic Decision Rule
When one generator connects to the dedicated generator input of one compatible hybrid inverter, the inverter settings may be sufficient.
When several independent sources connect to the same AC bus, the installation is a real microgrid and normally requires a dedicated controller.
2. Operating Logic Using a Hybrid Inverter
When the Utility Grid Is Available
The system may operate according to the following priority:
- Solar power supplies the loads.
- Excess solar energy charges the battery.
- The utility grid supplies any remaining demand.
- A minimum battery reserve is maintained for emergencies.
- The generator remains stopped.
The battery does not necessarily need to discharge whenever solar production is lower than the load.
Its operation may depend on:
- Electricity tariffs.
- Peak-shaving requirements.
- Required emergency reserve.
- Expected grid outages.
- Battery cycle-life strategy.
For example, the system may prevent the battery State of Charge from falling below 40% during normal grid operation.
This reserve remains available if the grid fails.
When the Utility Grid Fails
The inverter detects that grid voltage or frequency is outside the permitted limits.
It then:
- Disconnects the installation from the utility grid.
- Prevents unsafe power export to the failed grid.
- Changes to island or backup operation.
- Uses the battery inverter to establish voltage and frequency.
- Allows compatible solar power to continue supporting the loads.
- Uses the battery to cover the difference between the load and solar production.
For this arrangement to work, the inverter must support functions such as:
- Island Mode.
- Backup Operation.
- Grid-Forming.
- Black Start, where required.
- Solar operation during an outage.
- Battery-management communication.
A normal on-grid solar inverter may shut down during a grid outage unless it is integrated into a compatible backup or microgrid system.
3. When Should the Generator Start?
The generator should not necessarily start immediately after every grid outage.
Short interruptions may be covered completely by the battery.
The generator can be started according to one or more operating conditions.
Low Battery State of Charge
The generator may start when the battery SOC falls below a defined limit.
Example:
- Generator start request at 35% SOC.
- Critical battery reserve at 25% SOC.
- Generator stop request at 80% SOC.
These figures are examples only. Actual values depend on the battery technology, required autonomy, load profile, and manufacturer limits.
High Load
The generator may start when the load approaches the maximum continuous capacity of the inverter or battery system.
Example:
- Start the generator when inverter loading exceeds 85% for 30 seconds.
- Start immediately if the inverter reports an overload risk.
- Shed non-critical loads if the generator cannot start in time.
Long Grid Outage
The system may delay generator starting to avoid unnecessary operation during short outages.
Example:
- The battery supports the load for the first 10 minutes.
- If the grid does not return, the inverter starts the generator.
- The battery continues supporting the load during generator starting and warm-up.
Low Solar Production
A sudden reduction in solar production may cause the battery to supply a large amount of power.
The generator may start when:
- Solar production remains low.
- Battery discharge exceeds a defined limit.
- The expected battery runtime becomes insufficient.
- The battery SOC continues falling rapidly.
Battery or Inverter Fault
The generator may start immediately following:
- Battery Management System fault.
- Battery overtemperature.
- Inverter or PCS fault.
- Loss of communication with the battery.
- Insufficient battery capacity.
- Inverter overload condition.
Generator starting should therefore depend on several parameters, not only battery SOC.
4. Generator Starting Sequence
The generator should not be connected immediately after receiving the start command.
A correct sequence normally includes the following stages.
Step 1: Start Request
The inverter or controller closes a dry contact connected to the generator control panel.
This contact sends the remote-start command.
Step 2: Generator Start and Warm-Up
The generator starts without load.
The controller waits until:
- Engine speed is stable.
- Oil pressure is normal.
- Generator voltage is available.
- Frequency is within limits.
- No common fault is active.
Step 3: Source Qualification
The inverter checks the generator input.
It may verify:
- Voltage.
- Frequency.
- Phase sequence.
- Stability time.
- Voltage and frequency tolerance.
The generator should remain stable for a programmed qualification period before being accepted.
Step 4: Generator Acceptance
In a simple inverter-based system, the inverter accepts the generator through its dedicated AC input.
In an advanced microgrid system, the generator may first be synchronized with the common bus before its circuit breaker closes.
Step 5: Load Ramp
The generator should not receive the full load in one sudden step.
The inverter gradually increases the power drawn from the generator.
This reduces:
- Frequency dips.
- Voltage dips.
- Generator overload.
- Unstable transfer.
- Repeated acceptance and rejection of the source.
Step 6: Battery Charging
Battery charging should begin only after the generator and loads have stabilized.
The charging power must be limited to avoid overloading the generator.
The generator supplies:
- The operating loads.
- Inverter conversion losses.
- Battery-charging power.
- Any additional emergency loads.
5. When Should the Generator Stop?
The generator should not stop immediately when the battery reaches a single SOC value.
The stopping logic should verify that the complete system can continue operating safely.
Typical stopping conditions include:
- Battery SOC has reached the stop level.
- Solar production and battery capacity can support the load.
- The generator has completed its minimum running time.
- No high-load condition is active.
- The battery and inverter are healthy.
- The utility grid has returned and remained stable.
- No operating mode requires the generator to remain available.
Generator Stopping Sequence
- Reduce the power drawn from the generator gradually.
- Transfer the load to solar and battery operation.
- Disconnect the generator input or open its circuit breaker.
- Allow the generator to run without load for the required cooling period.
- Send the stop command.
Why Two Different SOC Limits Are Required
The generator may start at 35% SOC and stop at 80% SOC.
This difference is called hysteresis.
Using the same or very close start and stop limits can cause repeated generator cycling.
For example, if the generator starts at 40% and stops at 45%, a small change in load or solar production may cause repeated starting and stopping.
The system should also include:
- Minimum Run Time.
- Minimum Stop Time.
- Warm-Up Time.
- Cooling Time.
- Start-failure alarm.
- Maximum number of start attempts.
6. What Must the Hybrid Inverter Support?
Before relying on the inverter alone, verify that it supports the required functions.
Generator Interface
The inverter should provide:
- A real dedicated generator input, not only a general AC input.
- Generator start and stop dry contacts.
- Programmable generator start conditions.
- Programmable generator stop conditions.
- Generator voltage and frequency qualification.
- Warm-up and cooling timers.
Battery Control
The inverter should support:
- Minimum SOC.
- Generator-start SOC.
- Generator-stop SOC.
- Maximum charging current.
- Maximum discharging current.
- Communication with the Battery Management System.
- Battery fault and temperature monitoring.
Power Control
Required functions may include:
- Generator input-current limit.
- Battery charger-current limit.
- Load-priority settings.
- Solar-priority settings.
- Grid-import limitation.
- Export limitation.
- Solar curtailment.
- Peak shaving.
Backup Operation
The inverter should support, where required:
- Grid-forming operation.
- Island mode.
- Black start.
- Seamless or fast transfer.
- Operation of compatible PV inverters during grid failure.
- Critical-load output.
- Automatic return to the utility grid.
The term “hybrid inverter” alone does not guarantee all these functions.
The manufacturer’s wiring diagrams, operating manual, and approved system architecture must be reviewed.
7. When Is a Microgrid Controller Required?
A dedicated microgrid controller is normally the better choice when the installation includes:
- Multiple generators.
- Generators operating in parallel.
- Generator and battery sharing the load on a common bus.
- Several battery or solar systems.
- Equipment from different manufacturers.
- Medium-voltage switchgear.
- Bus-coupler circuit breakers.
- Multiple utility incomers.
- Automatic load shedding.
- Automatic load restoration.
- Source synchronization.
- Closed-transition transfer.
- Active and reactive power management.
- Complex protection coordination.
- High-reliability industrial processes.
In these systems, one inverter cannot see or control every source, breaker, and load.
The microgrid controller receives data from:
- Power meters.
- Protection relays.
- Generator controllers.
- Battery PCS.
- Battery Management System.
- Solar plant controller.
- Circuit breakers.
- ATS panels.
- PLC and SCADA systems.
It then sends operating commands and power setpoints to the appropriate equipment.
8. Hybrid Inverter or Microgrid Controller?
| Project Condition | Recommended Solution |
|---|---|
| One inverter, one battery, and one generator connected to the inverter generator input | Hybrid inverter settings may be sufficient |
| Several compatible inverters from one manufacturer | Manufacturer’s master controller or energy manager |
| Generator and battery operate independently on one AC bus | Microgrid controller |
| Two or more generators operate in parallel | Microgrid or generator power-management controller |
| Sources are supplied by different manufacturers | Microgrid controller |
| Automatic multilevel load shedding is required | Microgrid controller |
| Medium-voltage switchgear or bus couplers are involved | Microgrid controller |
| Simple building without source paralleling | Hybrid inverter is generally simpler |
| Only automatic generator start at low battery SOC is required | Inverter dry contact may be sufficient |
| Synchronization and real-time load sharing are required | Dedicated controller |
9. Practical Example
Consider a system with:
- 300 kW load.
- 200 kW solar system.
- 250 kW battery inverter.
- 500 kWh battery.
- One standby generator.
- One hybrid inverter system.
- Utility grid connection.
Grid Available
- Solar supplies 180 kW.
- Grid supplies 120 kW.
- Battery remains at its emergency reserve level.
- Generator remains stopped.
Grid Failure
- The inverter disconnects the utility grid.
- Battery operation establishes the local voltage and frequency.
- Solar supplies 180 kW.
- Battery supplies the remaining 120 kW.
- Generator remains stopped.
Solar Production Falls
Solar production falls from 180 kW to 40 kW.
The battery must now supply 260 kW, which is higher than its 250 kW rating.
The system may:
- Start the generator.
- Shed a non-critical load.
- Temporarily limit the load.
- Use a combination of these actions.
Generator Starts
- The inverter sends the start command.
- The generator reaches stable voltage and frequency.
- The inverter accepts the generator input.
- Generator power is increased gradually.
- The battery discharge falls.
- Battery charging begins later at a limited rate.
Generator Stops
The generator stops when:
- Battery SOC reaches the selected stop level.
- Solar and battery can support the load.
- Minimum running time has been completed.
- The generator is unloaded gradually.
- Cooling time has been completed.
Conclusion
A dedicated microgrid controller is not required for every hybrid-energy installation.
A compatible hybrid inverter may control the complete system when:
- There is one generator.
- The generator connects through the inverter’s dedicated input.
- The battery and solar system are compatible with the inverter.
- No complex source paralleling is required.
- The inverter supports the required start, stop, charging, and backup functions.
A microgrid controller becomes necessary when independent sources operate on a common AC bus and require synchronization, load sharing, load shedding, circuit-breaker control, or advanced power management.
The correct decision should begin with:
- The single-line diagram.
- The operating philosophy.
- The load profile.
- The required backup performance.
- The capabilities of the selected equipment.
Do not select a microgrid controller simply because it is more advanced.
Do not select an inverter-only solution simply because it is less expensive.
Select the simplest architecture that performs the required functions safely.
Prepared by Eng. Mohamed Badr