Generator ATS Integration: Automatic Transfer Switch Design and KNX Supervision
An automatic transfer switch (ATS) is the critical interface between the utility mains supply and a standby generator — ensuring a controlled, interlocked transfer of load to the generator on mains failure and safe retransfer when mains is restored. KNX integration adds real-time supervision and automated load shedding to the sequence.
ATS function and transfer sequence
The ATS performs five sequential functions during a mains outage and five in reverse during mains restoration. Each step has time constraints that must be set correctly to avoid false transfers, generator damage, or load interruption exceeding the design brief.
Transfer sequence — mains failure to generator
Step 1 — Mains failure detection: Undervoltage relay monitors mains voltage (L1, L2, L3) Threshold: voltage falls below 85% of nominal (≈195V on 230V) Time delay: 0.5–3 seconds (prevents transfer on momentary dips) Step 2 — Generator start command: ATS controller sends dry-contact start signal to genset Genset cranking: up to 3 attempts × 10 seconds each Between attempts: 5-second rest (protect starter motor) Step 3 — Generator stabilisation: Wait until genset output meets acceptance criteria: → Voltage: 90–110% of nominal (207–253V on 230V) → Frequency: 47.5–51.5 Hz Stabilisation time: typically 10–30 seconds after engine fires Step 4 — Transfer to generator: Open mains contactor (K1), pause 100ms, close genset contactor (K2) Mechanical and electrical interlock prevents both closing together Step 5 — KNX notification: ATS auxiliary contact → KNX binary input → GA 8/2/0 (Genset ON) Trigger load shedding scene for non-critical loads
Retransfer sequence — mains restoration
Step 1 — Mains restoration detection: Voltage and frequency within limits for 30 seconds (prevent retransfer during unstable mains recovery) Step 2 — Retransfer to mains: Open genset contactor (K2), pause 100ms, close mains contactor (K1) Step 3 — Generator cool-down: Engine continues running unloaded for 3–5 minutes Prevents thermal shock to turbocharger and exhaust manifold Step 4 — Generator stop: ATS withdraws start signal → engine stops KNX: GA 8/2/0 returns to 0, GA 8/2/1 (Genset Hours) logged
ATS types: open-transition, closed-transition and static
The three ATS switching technologies differ in how they handle the moment of transfer — specifically whether there is a supply interruption, a momentary parallel connection, or no moving contacts at all.
| Type | Transfer interruption | Synchronisation required | Typical application |
|---|---|---|---|
| Open-transition | 100–500 ms interruption | No | Most commercial and industrial ATS — standard choice |
| Closed-transition | < 100 ms (momentary parallel) | YES — voltage, phase angle, frequency | Hospitals, data centres; requires synchronisation relay |
| Static transfer switch | < 1 cycle (< 20 ms) | No moving contacts | Critical IT loads, UPS bypass — high cost |
Closed-transition warning: momentary parallel operation between mains and generator requires the generator AVR and governor to synchronise within ±0.5% frequency and ±5° phase angle before transfer. Without a synchronisation check relay, a closed-transition transfer on an unsynchronised generator will produce a high-current transient that can damage the alternator windings and trip downstream MCBs.
Genset start sequence and timing
The start sequence timing must balance fast transfer against false starts from momentary mains disturbances. Incorrect timing is the most common commissioning error in ATS installations — too short causes nuisance generator starts; too long means the load is without power for an unacceptable period.
Timing settings reference
Mains failure delay (T1): Commercial / industrial: 3 seconds Hospital / critical: 0.5 seconds Residential: 5–10 seconds Purpose: ride-through momentary dips without starting genset Start signal duration: Continuous until genset accepting criteria met Crank cycle: Per cycle: 10 seconds crank Rest between cycles: 5 seconds Maximum attempts: 3 (then lockout alarm) Genset acceptance window: Voltage range: 90–110% nominal Frequency range: 47.5–51.5 Hz Stabilisation delay (T2): 10–30 seconds after engine fires Mains restoration delay (T3): 30–60 seconds stable mains before retransfer Prevents retransfer during unstable mains restoration Cool-down period (T4): 3–5 minutes unloaded running after retransfer
ATS panel design: dual contactor and relay settings
The ATS panel contains the mechanical and electrical interlock between the mains contactor (K1) and generator contactor (K2), plus the voltage and frequency monitoring relays that drive the transfer logic. Panel design must ensure it is physically impossible for both contactors to be closed simultaneously.
ATS panel component list and relay settings
Mains contactor (K1): Rating: ≥ 125% of full load current (AC3 duty) Auxiliary contact: 1NO used for interlock, 1NO to KNX BI Generator contactor (K2): Rating: same as K1 Auxiliary contact: 1NO used for interlock, 1NO to KNX BI Mechanical interlock: Mechanical coupling bar between K1 and K2 (mandatory) Prevents simultaneous closure regardless of control logic Electrical interlock: K1 NC auxiliary in K2 coil circuit — K1 open required for K2 to close K2 NC auxiliary in K1 coil circuit — K2 open required for K1 to close Undervoltage relay (mains monitoring): Dropout voltage: 85% nominal (≈195V on 230V, ≈360V 3-phase) Pick-up voltage: 90% nominal (≈207V) Time delay: set to T1 (mains failure delay above) Overvoltage relay: Trip at 110% nominal (≈253V on 230V) Prevents transfer to overvoltage mains Frequency relay: Mains loss: < 47 Hz or > 51 Hz Genset acceptance: 47.5–51.5 Hz
KNX integration: binary inputs and group addresses
ATS auxiliary contacts provide the status signals that the KNX system needs for supervision, load shedding and notification. The following group address map covers a complete ATS-KNX integration with logic for automated response to power events.
ATS KNX group address mapping
Signal: Mains contactor K1 open (mains lost) Source: K1 NC auxiliary → KNX binary input GA: 8/2/0 — Mains Lost Alarm DPT: 1.005 (alarm) — 1 = mains off Signal: Generator contactor K2 closed (on generator) Source: K2 NO auxiliary → KNX binary input GA: 8/2/1 — Generator Running DPT: 1.001 (switch) — 1 = on generator Signal: Load on generator (K2 closed + stabilised) Source: ATS controller "load transferred" output GA: 8/2/2 — Load On Generator DPT: 1.005 (alarm) — 1 = load fed from genset ETS6 logic on 8/2/0 (Mains Lost) = 1: → Activate scene "Emergency": dim non-critical lighting to 20% → Send OFF to HVAC group address (shed 15–25kW) → Send OFF to EV charger group address → Send alarm notification to visualisation (Gira X1 popup) ETS6 logic on 8/2/0 = 0 (mains restored): → Delay 60 seconds (wait for retransfer to complete) → Restore normal lighting scene → Restore HVAC (staggered 30-second delay to limit inrush)
Generator manufacturer ATS modules
Major generator manufacturers offer proprietary ATS and control modules that replace the custom panel relay design. These integrate genset control, metering and Modbus TCP communications in a single panel-mount unit.
| Module | Manufacturer | Communication | Notable feature |
|---|---|---|---|
| MPAC 1500 | Kohler | Modbus TCP, relay outputs | Integrated ATS with remote monitoring via OnCue Plus |
| PowerCommand 3.3 | Cummins | Modbus RTU/TCP, InPower | Full genset parameters: oil pressure, coolant temp, fuel level |
| EMCP 4.4 | Caterpillar | Modbus TCP, CAN, relay | Paralleling-capable, SCADA integration, event log 200 entries |
Modbus TCP to KNX: connect Cummins or Caterpillar Modbus TCP output to a KNX Modbus gateway (Intesis, MDT or WAGO 750-362) to publish genset parameters — engine hours, oil pressure, coolant temperature and fuel level — as KNX group addresses for BMS visualisation and predictive maintenance alerts.
Testing programme
ATS and generator installations require a structured test programme to demonstrate readiness and identify degraded performance before a real outage. All tests must be recorded in the plant O&M manual and signed by a competent person.
Test programme schedule
Weekly — No-load test (automated via genset controller): Start engine, run 5–10 minutes at no load Verify: starts within 3 crank attempts Check: oil pressure, coolant temp, no fault alarms Log: engine hours counter Monthly — Transfer test (manual or automated): Simulate mains failure at ATS (test button or open K1) Verify: genset starts, stabilises, load transfers within T1+T2 Verify: KNX GA 8/2/1 (Generator Running) = 1 Verify: load shedding scene activates (HVAC, EV charger OFF) Restore: re-close mains, verify retransfer sequence Log: transfer time, genset voltage/frequency at acceptance Annual — Full-load test: Connect resistive load bank equal to 75–100% of genset rating Run at full load for minimum 2 hours Record: voltage, frequency, fuel consumption, exhaust temp Compare fuel consumption with manufacturer spec (±10%) Test battery: genset battery load test (cranking current) Record: test date, load (kW), duration, pass/fail
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