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.

The UPS and the standby generator have to be commissioned as one system, and this is where most sites come unstuck. A UPS rectifier is a non-linear load that the genset sees as harsh, and a genset is a source whose voltage and frequency wander for the first seconds after it takes the load. Three settings decide whether the pair works: rectifier walk-in, which ramps the UPS input current up over several seconds instead of slamming the genset; the input current limit, which caps how hard the UPS charges its batteries while on generator; and the bypass frequency window and slew-rate tolerance, which determine whether the UPS will stay synchronised to a source that is still hunting. Get these wrong and the UPS refuses to accept generator power, runs the batteries flat while the genset sits there loaded only by the building, and drops the load anyway. Where a 6-pulse rectifier is involved, plan on the generator being materially larger than the UPS rating rather than matching them one to one.

Two local realities shape UPS specification in Kenya. The first is heat: VRLA battery life is roughly halved for every 8–10 °C of sustained temperature rise above the 20–25 °C design point, so a battery room without dedicated cooling turns a five-year string into a two-year string, and the customer experiences that as "the UPS failed" rather than "the room was hot". Air-conditioning the battery space is almost always cheaper than the replacement cycle it prevents. The second is that a UPS is only ever a bridge. Kenyan outages routinely outlast any economically sized battery, so the honest design question is not "how many hours of battery" but "how quickly does the generator start, and what must stay alive in between". Sites that try to buy their way out of long outages with battery capacity alone spend several times what a correctly configured UPS-plus-generator pair would cost.