UPS Systems

A UPS is the bridge between utility and the load during the seconds-to-minutes that the genset takes to start, transfer, and stabilise. Its sizing, topology, and battery technology decide whether the bridge is fit for the load it carries — or quietly fails the day everything depends on it.

Three topologies exist per IEC 62040-3. VFD (offline / standby) — load runs on raw mains; UPS only engages on failure with 4–10 ms transfer. VI (line-interactive) — adds a buck/boost autotransformer to handle voltage swings without battery use. VFI (online / double-conversion) — load is permanently fed by inverter from rectifier+battery DC bus. VFI is the only acceptable choice for mission-critical IT, medical, telecom, and most data-centre loads.

Sizing in kVA / kW is more complex than it looks. Servers are mostly active power (kW); ageing PSUs draw at PF 0.7–0.8; modern PSUs at 0.95–0.98. A UPS rated 10 kVA / 8 kW will overload on a server cabinet drawing 9 kW even though apparent power is fine. Always match kW to load demand — kVA is secondary.

Battery sizing follows runtime requirements. Runtime (h) ≈ (Capacity Ah × Voltage × DoD × inverter η) ÷ Load W. A 480 V × 100 Ah string at 80% DoD with 95% inverter efficiency will run a 30 kW load for ≈ 1.2 hours. VRLA batteries lose 4–5% capacity per year of float; lithium-ion lose < 2% — but capex is 2.5–3 × VRLA.

Battery technology is shifting. VRLA (sealed lead-acid) AGM remains common for cost; lithium-ion (NMC or LFP) for longevity, weight, and faster recharge. LFP is the preferred chemistry now — thermal runaway resistance, 10-year design life, 3,000–5,000 cycle endurance. NMC is denser but riskier; banned from many data-centre rooms.

Modular UPS (Schneider Galaxy VS, Eaton 93PM, Vertiv Liebert APM, ABB Conceptpower) lets you scale capacity in 25–50 kW slices, hot-swap modules, and design N+1 redundancy without doubling capex. Single-block UPS still has a place in small server rooms but loses on serviceability.

Static bypass is the crucial fallback that lets a faulted inverter route power directly from utility to load without dropping the load. Test the static bypass annually under controlled conditions; an untested static-bypass on the day of an inverter failure is no bypass at all.

Harmonic distortion of the input current matters because UPS rectifiers are large non-linear loads. Older 6-pulse rectifiers produce 25–30% THDi — punishes upstream gensets and trips PF-correction. 12-pulse and IGBT input rectifiers cut THDi to <5%. Specify low-THDi for any UPS > 100 kVA.

Crash-cart / EPO (Emergency Power Off) wiring is one of the most-mis-installed UPS components. A latched EPO that is wired through the same control circuit as the room shutdown will trip the UPS along with the rest of the room — defeating its purpose. EPO must be a dedicated normally-closed loop tested at commissioning.

Maintenance: monthly battery monitoring (impedance / float current), quarterly thermography and inspection, annual full-load battery test (or use connected battery monitor system to avoid downtime). Battery replacement at end-of-life is non-negotiable; in a critical-load environment we recommend replacement at 80% capacity rather than waiting for failure.