STS and Seamless Transfer: How to Achieve Near-Invisible Switching
Summary: Built-in STS <10ms, standalone module <20ms. How STS works, transfer timing, sizing advice and when STS is mandatory.
Switching time: loads barely notice
Built-in STS <10ms; standalone module <20ms (per BOM). STS provides "supply continuity" — it does not generate power.
Three energy-flow states and transfer timing
- ① Grid-tied (GFL, PQ): grid via ATS supplies; PV/battery self-consume/arbitrage per EMS; surplus to grid.
- ② Grid loss (STS transfer): STS/ATS switch fast; system isolates from grid; critical loads stay powered.
- ③ Off-grid, low SOC (diesel): grid-forming PCS holds reference; diesel via ATS; exit when recovered.
Sizing advice
Not every load needs seamless transfer — confirm "how long an outage is acceptable" before specifying STS. VGRID STS200/400/600 is a 200–600kW fast transfer module with Local EMS and RS485/Modbus TCP/CAN/4G; many PCS and all-in-ones offer built-in STS versions.
FAQ
How do STS and ATS divide work?
STS does millisecond grid/off-grid transfer for critical loads; ATS switches grid↔diesel and interlocks.
Which loads must have STS?
Outage-sensitive loads (data centres, precision manufacturing, medical, comms) should; ordinary loads can relax by allowed interruption time.
Built-in vs standalone STS?
Small/mid single units often use built-in STS; large systems or multi-path transfer use standalone modules (e.g. 600kW) for flexibility.
How does diesel coordinate during transfer?
Grid loss → STS off-grid; low SOC → EMS auto-starts diesel; recovered → exit; EMS owns start/stop.
