# STS and Seamless Transfer: How to Achieve Near-Invisible Switching 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](/en/product/sts200-400-600) is a 200–600kW fast transfer module with Local EMS and RS485/Modbus TCP/CAN/4G; many [PCS](/en/product/sp125hcps) and all-in-ones offer built-in STS versions. ## 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.