In short. When utility, generator, UPS and battery storage share a site, each can help or harm the others. The study defines every operating mode and transition, then tests voltage, frequency, fault current, harmonics and control behaviour in each. The weakest transition usually decides reliability.
Sources and their roles
| Source | Strength | Limit |
|---|---|---|
| Utility | Stiff, low cost energy | Outages and sags |
| Diesel generator | Long-duration energy | Start delay, step limits, higher impedance |
| UPS | No-break for critical loads | Short battery time, rectifier harmonics |
| BESS | Fast response, peak support | Limited energy, limited fault current |
Modes and transitions to study
- Normal utility supply.
- Utility loss, UPS on battery, generator start.
- Generator takes load, UPS rectifier restarts.
- Return to utility with retransfer, open or closed.
- BESS peak shaving, islanded operation and grid support.
- Failure cases: generator fails to start, ATS fails, UPS bypass, BESS trips.
What to check in each mode
- Capacity and step: does the source carry the load and the step? See generator transient performance.
- Fault current and protection: low fault current from generator or inverter changes tripping time. See inverter fault behaviour.
- Harmonics: UPS rectifier distortion on generator supply. See harmonic studies.
- Synchronising: closed transition needs matched voltage, frequency and phase. See synchronising studies.
- Control hierarchy: who leads on frequency and voltage, and what happens if the leader fails.
Control hierarchy
Only one source should set frequency in an island. Two grid-forming sources fighting for it cause oscillation. Define which is grid-forming, which follow, and what the fallback is.
Illustrative example
A data hall has two UPS systems, two generators and a battery system for peak shaving. The study finds that on generator, the UPS rectifier input distortion is high and the battery inverter, set as grid-forming, competes with the generator governor. The fix sets the generator as the leader in island mode, the battery as follower, and adds a soft-walk-in on the UPS rectifier.
Four sources, four behaviours
| Source | Role | Strength | Limit |
|---|---|---|---|
| Utility | Primary energy | Strong fault level, low cost | Outages, voltage events |
| Diesel generator | Long-duration backup | Duration limited only by fuel | Start delay, weak source |
| UPS | Instant ride-through | No break | Battery minutes, low fault current |
| BESS | Energy shifting, peak support, longer ride-through | Fast response | Energy limited, converter limits |
The UPS protects the instant; the generator protects the duration.
Studies in scope
- Sequence of operation during mains failure: time from loss to generator on bus, and battery energy used meanwhile.
- UPS input behaviour on generator supply, including walk-in, input power factor and harmonics.
- BESS control mode (grid-following or grid-forming) and interaction with the generator.
- Fault levels and protection in each combination.
- Retransfer and recharge loading.
Battery energy check (illustrative)
A 400 kVA, 0.9 PF UPS carrying 300 kW needs about 300 kW × 10 min ÷ 60 = 50 kWh from the battery during a 10 minute generator start window, plus efficiency losses. If the generator starts in 15 seconds, the battery need is tiny and the margin large; if a generator fails to start, the UPS autonomy is the time to shut down safely.
Interaction hazards
- UPS recharge adds load right after generator pick-up.
- Generator voltage and frequency windows must suit the UPS input.
See UPS and nonlinear loads and ATS study.
Common mistakes
- Studying each source alone.
- Ignoring retransfer.
- No defined control leader.
What to send us
Send the one-line diagram and the operating philosophy. Send your application details. See also ATS service.
Questions engineers ask
Can a battery replace a generator?
Only for the duration its energy allows. Long outages still need a source of energy.
Why does the UPS cause trouble on generator?
Rectifier harmonics and large steps stress the generator.
What is grid-forming?
A source that sets voltage and frequency for the island.
Is a closed-transition retransfer worth it?
It avoids a second outage, but needs synchronising checks and protection.
This guide gives general principles. Integrated systems need a project-specific study.