In short. A PLC suits plants where several generators, breakers and loads must be coordinated. Good power PLC design keeps safety hard-wired, separates fast and slow tasks, structures the program as clear sequences and tests every failure mode before the plant depends on it.
What a power PLC does
- Reads breaker status, source health, kW, kVAr and alarms.
- Runs sequences: start, synchronise, transfer, shed, restore. See state machines.
- Sends set points and commands to generator and breaker controllers.
- Reports to SCADA.
Architecture choices
| Item | Consideration |
|---|---|
| Scan time | Fast enough for the task; protection-speed functions belong in relays, not PLC |
| I/O type | Hard-wired for breaker commands and permissives; networked for measurements |
| Power supply | Dual supplies from independent sources with battery backup. See control power |
| Redundancy | Single, hot standby or distributed. See redundant PLC |
| Communications | Industrial networks with defined protocols and segmentation |
Program structure
- Separate I/O mapping from logic.
- One function block per device type (generator, breaker, source).
- Sequences written as states with entry conditions, actions, timeouts and exits.
- Timeouts on every step, with an alarm and a safe fallback.
- Clear alarm and event naming.
- Version control and a documented change history.
What not to rely on a PLC for
- Fault protection, which needs relay speed and reliability.
- Last-line interlocks such as preventing two sources closing on a dead bus.
- Emergency stop circuits.
Testing
Test normal operation, then failures: lost input, wrong feedback, communication loss, power cycle, watchdog trip and operator error. Simulate before site. See FAT and SAT.
Illustrative example
A three-generator plant uses a PLC for start and stop sequencing and load shedding. The sync check relay and breaker interlocks remain hard-wired. During a power-cycle test, the PLC restarts in a defined state, reads breaker positions and resumes without closing any breaker unexpectedly.
Why a PLC for power systems
A PLC runs repeatable logic reliably: sequencing, interlocks, load shedding, mode control. For electrical power it must also be fast enough, robust enough and integrated with protection devices.
Architecture choices
| Element | Considerations |
|---|---|
| CPU scan time | Tens of ms is typical; fast load shedding may need dedicated hardware |
| I/O | Hardwired for critical signals, networked for monitoring |
| Power supply | Redundant, fed from battery-backed DC |
| Communications | Modbus, Profinet, Ethernet/IP or IEC 61850 gateways |
| Environment | Temperature, dust, vibration, EMC in switchgear rooms |
Good programming practice
- Structured code with a function block per equipment type (generator, breaker, feeder).
- State machines for sequences, not tangled latches.
- Watchdog and heartbeat between controllers.
- Defined safe state on CPU stop or power loss.
- Version control and a documented change procedure.
What the PLC should not do
It should not be the only trip path for electrical protection. A protection relay trips without it. It should also not hold the only copy of settings or logic.
Testing
Simulate failures: loss of input, loss of communication, CPU stop. Confirm the plant goes to the planned safe state. See FAT and SAT framework.
The master engineering question
After a PLC restart or fault at any point in a sequence, does the plant stay safe and does the PLC recover into a state that matches reality?
What to send us
Send the functions you want automated. Send your application details.
Questions engineers ask
Which PLC brand is best?
One your team can support, with proven power-system libraries and spare availability.
Can a PLC synchronise generators?
Dedicated synchronisers are more usual. Use the PLC to coordinate them.
How many I/O points?
Count from the cause-and-effect list, then add spare capacity.
How are PLC programs protected?
Access control, backups, change logs and restricted remote access.
This guide gives general principles. Design to the manufacturer’s rules and applicable standards.