In short. A UPS bridges the gap between loss of supply and the arrival of the generator, and conditions the supply to sensitive loads. A resilient UPS architecture decides topology, redundancy, bypass paths and battery arrangement together, and checks how the UPS behaves on a generator supply.
What a UPS does
- Carries the load on battery during a short outage.
- Conditions voltage and frequency so loads see a clean supply.
- Provides a defined transition when the source changes.
Common topologies
| Topology | How it works | Typical use |
|---|---|---|
| Online double conversion | Always converts to DC and back to AC | Critical loads needing consistent output |
| Line interactive | Regulates and switches to battery on loss | Smaller or less critical loads |
| Offline or standby | Switches to battery after detecting failure | Non-critical small loads |
Critical systems generally use online double conversion. See online UPS systems.
Redundancy arrangements
- Single UPS (N): simple, but the UPS is a single point of failure.
- Parallel redundant (N+1): extra capacity so one unit can fail or be serviced. See modular redundancy.
- Isolated redundant or distributed: separate UPS systems feeding different paths.
- 2N: two complete UPS systems and distribution paths for dual-corded loads. See dual-cord loads.
Bypass paths
A static bypass lets the load transfer to the mains if the inverter fails or overloads. A maintenance bypass lets the UPS be isolated for service without dropping the load. Neither should be a single point of failure in its own right. See bypass arrangements.
Batteries
- Autonomy time is chosen from the generator start time plus a margin. See battery autonomy.
- Multiple parallel strings avoid one string failure losing all autonomy.
- Battery monitoring detects ageing before an outage exposes it.
Interaction with generators
UPS rectifiers can draw non-sinusoidal current and may need a soft-start or filter so the generator voltage stays stable. Allow for the generator sizing effect and the interaction of regulator and UPS input. See UPS and DG integration.
Worked illustration
A 200 kVA load is fed by two UPS modules in parallel. If each module is rated 200 kVA, either can carry the full load alone, so the pair is redundant. If each is rated 100 kVA, both are needed to carry the load and a single failure drops it, so the pair is not redundant, however many cabinets are installed. Rate each remaining unit against the full load to test redundancy.
UPS topologies
| Topology | Operation | Efficiency | Protection |
|---|---|---|---|
| Offline / standby | Mains direct, inverter on failure | High | Basic, with a transfer gap |
| Line-interactive | Mains with voltage correction | High | Moderate |
| Double-conversion (online) | Always through rectifier and inverter | 94 to 97 percent typical | Full isolation, zero transfer time |
| Eco-mode double conversion | Bypass when mains is clean | 98 percent or more | Fast switch back on disturbance |
Key sizing points
- Rate in kW as well as kVA; many modern UPS units are rated at unity power factor.
- Allow for load growth and overload capability.
- Check input power factor and harmonic distortion, since these affect the generator.
- Choose the battery autonomy against how long the generator takes to carry the load.
Interaction with the generator
A six-pulse rectifier UPS produces substantial input current distortion, which stresses an alternator; an active-front-end or filtered input is friendlier. The UPS should walk in gradually when the generator comes on. See UPS and nonlinear loads.
Maintenance bypass
Provide a maintenance bypass so the UPS can be serviced without dropping the load, with interlocks to prevent wrong operation.
See modular UPS and battery autonomy.
Common mistakes
- Parallel UPS units that share one battery string or one output breaker.
- Maintenance bypass that cannot be operated without a brief break.
- Ignoring the UPS input behaviour on generator supply.
- Letting batteries age without monitoring.
Testing a UPS system
Beyond the factory test, site acceptance should include transfer to battery and back, a discharge test to confirm autonomy at the real load, transfer to bypass and back, module failure and re-insertion in a parallel system, and operation from the generator. Record the output voltage and any transfer disturbance so you have a baseline for later comparison.
Life and replacement
Batteries are usually the shortest-lived part of the system and their end of life is a common cause of UPS failure during a real outage. Plan replacement by age and measured health, not only after a failure.
What to send us
Send the load, the outage tolerance and the supply arrangement. Send your application details. See also UPS capacity guide.
Questions engineers ask
What is the best UPS topology for critical loads?
Online double conversion is the usual choice because it isolates the load from supply disturbances and gives a defined transfer.
Is a single UPS acceptable?
It can be where the load tolerates the risk, but the UPS is then a single point of failure. Critical sites often use parallel redundant arrangements.
How is UPS rating chosen?
From the load in kVA and kW, power factor, growth and redundancy needs. See the UPS capacity guide.
Do UPS units cause problems for generators?
They can, through input current distortion and step loading. A study or manufacturer check avoids surprises.
This guide gives general principles. UPS selection should be confirmed with the manufacturer’s data.