Solarna Systems Hybrid Power Controls

Engineering design example

Hybrid mini-grid controls

A controls example for a PV, battery, generator, and load system, covering operating states, permissives, interlocks, alarms, HMI structure, fail-safe responses, and testing.

Control philosophy example · Representative ratings, operating modes, and interfaces.

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Operating states

Define system behaviour before writing control logic

Control logic should begin with a clear operating philosophy. This state model defines normal operation, generator support, battery protection, and fault handling.

Standby PV priority Battery support Generator-supported Fault lockout

State conditions

StateEntryExit
StandbyDispatch disabled; sources in the permitted standby conditionEnable request with valid inputs, bus reference, and readiness checks
PV priorityGenerator not connected; PV meets demand and surplus is stored or curtailed within limitsPV deficit, generator connection, or required fault response
Battery supportGenerator not connected; load exceeds PV and SOC/PCS limits permit dischargePV recovers, discharge floor is reached, or generator connects
Generator-supportedGenerator connected under the approved bus and transfer schemeReplacement source ready and stop criteria met; unload, open, and cool down
Fault lockoutProtection trip or invalid safety-critical inputCause cleared, authorised reset, and readiness checks repeated

Control philosophy

  • Prioritise available PV while bus conditions, equipment limits, and battery constraints remain healthy.
  • These are operating modes, not a fixed sequence. Fault lockout overrides dispatch; disabled dispatch selects standby.
  • A connected generator takes dispatch precedence. Otherwise PV covers demand or the battery supplies the deficit within its limits.
  • Generator start, warm-up, connection, unloading, and cool-down are separate equipment phases. An unsupported deficit requires the approved load-shed or standby response.
  • Form generator requests from enabled, validated SOC, load, schedule, or approved manual conditions; apply timing and hysteresis to each relevant request.
  • Prevent unsynchronised closure, unintended backfeed, and generator/BESS paralleling unless the approved design permits it.
  • Log every automatic state change for commissioning review.

Automation logic

Interlocks, alarms, and I/O mapping

Interlock

Generator request and sequence

  1. Request: enabled low-SOC condition OR load-support request OR scheduled exercise OR approved manual request. The model’s 180-second delay applies only to the low-SOC branch.
  2. Start readiness: request AND generator available/remote AND required auxiliaries healthy AND no emergency stop, protection trip, or start inhibit; breaker proven open for this example.
  3. Crank with bounded attempts, prove running, then complete warm-up. A failure to start raises a fault and follows the approved retry rule.
  4. Connect only on a permitted connection request, valid speed/voltage/frequency, and approved synchronisation or dead-bus conditions with interlocks and breaker proof.
  5. Stop only after request-clear and minimum-run conditions, replacement-source readiness, unloading, breaker opening, and cool-down. Scheduled exercise may remain unloaded.
Inputs

I/O map snapshot

TagTypeUse
BAT_SOCAnalogGenerator start/stop logic
GEN_READYDigitalStart permissive
INV_FAULTDigitalFault lockout
LOAD_KWAnalogPeak load support
Alarms

Alarm matrix

AlarmSeverityAction
Low SOCHighStart generator sequence
Communications lossHighInhibit affected transitions and enter the defined fail-safe response
Breaker mismatchHighBlock transfer
OverloadCriticalLoad shed request

Operator screens

HMI structure for operations and fault review

The HMI shows the active source, the reason for a transition, and the conditions that must be cleared before reset.

Mini-grid dashboard concept

82 kWSite load
67%Battery SOC
PV priorityActive state
0Critical alarms

AC-coupled concept. Site design defines the grid-forming source, earthing, switching and permitted source combinations.

Screen list

  • Overview screen with source status, load, SOC, generator state, and alarms.
  • Energy screen with PV, battery, generator, load, and daily totals.
  • Alarm screen with severity, timestamp, acknowledgement, and operator note.
  • Settings screen with protected thresholds and commissioning records.

Commissioning handover

Test requirements before site commissioning

Factory test plan

Simulate SOC, PV availability, breaker status, generator readiness, and communication loss.

Site acceptance test

Verify state transitions, interlocks, alarms, HMI values, manual reset, and event logging.

Operator handover

Provide alarm response notes, reset rules, operating limits, and support escalation path.

Next step

Discuss a similar controls project

A project controls package is developed from the approved operating philosophy, equipment interfaces, protection scheme, and test plan.

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