UPS And Generator Integration: Why It Fails (And How To Get It Right)
Pairing a diesel generator with a UPS is unforgiving. The UPS may drop to bypass, alarm or shed load if generator voltage, frequency or THDv stray outside limits. Typical root causes include:
- Undersized alternators creating high source impedance and THDv
- Slow or unstable governors causing frequency dips and overshoot
- Poor AVR tuning or no PMG excitation under harmonic stress
- Incorrect ATS logic or wide sync windows
- No rectifier walk‑in or unmanaged battery recharge current
In data centres, healthcare, manufacturing, utilities and construction, that means stoppages. PowerTech Generators delivers end‑to‑end critical power: design, generator installation, commissioning, generator maintenance and 24/7 generator servicing across the UK. Start with our guide to choosing an industrial generator and review outcomes in our generator case studies. We supply new and used diesel generators, industrial generators for sale, and generator hire with practical, UK‑wide support.
Understanding UPS Loads: Rectifiers, THDi And Dynamic Behaviour
Rectifier design dictates harmonic current (THDi) and how hard the generator must work to hold voltage shape:
- 6‑pulse: ≈ 30–35% THDi
- 12/18‑pulse: ≈ 8–15% THDi
- IGBT/Active front end (AfE): ≈ 3–5% THDi
Higher source impedance amplifies voltage distortion (THDv). Partial load, high crest factor and non‑linear downstream loads can worsen waveform peaks and regulation.
After a mains failure, battery recharge adds a hidden step load. Enable rectifier walk‑in (typically 5–30 s) so the UPS ramps smoothly and avoids frequency dips. Static bypass windows are commonly around ±10% voltage and ±2 Hz frequency before transfer, but always follow the UPS manual. For AVR fundamentals and testing, see our guide to automatic voltage regulators (AVRs) and generator load testing best practice.
Specifying The Generator For UPS Loads: Alternator, Engine And AVR
Right‑sizing controls THDv and keeps the UPS on inverter. Rules of thumb for alternator capacity versus UPS kVA (to hold ≲ 5% THDv):
- 6‑pulse UPS: 2.0–2.5 × UPS kVA
- 12/18‑pulse UPS: 1.5–2.0 × UPS kVA
- IGBT/AfE UPS: 1.2–1.5 × UPS kVA
- Favour alternators with low subtransient reactance (X″d) to reduce source impedance and distortion.
- Use a PMG‑excited AVR with three‑phase sensing and anti‑harmonic/fast response features.
- Specify fast, stable governors that meet ISO 8528‑5 transient limits (class G2/G3 as required).
PowerTech Generators is brand‑independent across Cummins, Perkins, FG Wilson and Baudouin, supplying robust commercial diesel generators for demanding UPS loads.
This image was generated with AI and may not always represent the product or service exactly.
Harmonics Management: THDi, THDv And Filtering Options
Set a practical generator‑bus target of ≤ 5% THDv (≤ 8% for some legacy systems). To achieve this:
- Oversize the alternator to reduce impedance and distortion.
- Add input line reactors or passive 12/18‑pulse front‑ends where needed.
- AfE rectifiers give the cleanest input but require correct firmware settings and commissioning.
- Check for resonance with AVR control loops and site impedances.
- Use K‑rated transformers and size neutrals for triplen harmonics.
- Verify performance with a power quality analyser during commissioning.
For quick troubleshooting, see common AVR issues and fixes.
Voltage And Frequency Windows For UPS Ride‑Through
Design to keep the UPS on inverter:
- Steady‑state frequency within ±1% (±0.5 Hz) and voltage within ±10%.
- On load steps, aim for ≤ 10–12% voltage dip with recovery ≤ 3 s.
- During synchronisation, hold frequency slew at ≈ 0.5–1.0 Hz/s to avoid chasing UPS phase lock.
Coordinate rectifier walk‑in and soft‑start after generator pick‑up so steps are small and predictable. Validate all setpoints against the UPS manual. Align ATS controls and tolerances with the UPS sync logic using quality ATS panels.
Control Strategy And ATS Settings For No‑Break Transfer
Closed‑transition ATS or synchronising controls enable make‑before‑break transfers. Typical sync limits:
- Phase angle difference ≤ 10°
- Voltage difference ≤ 1%
- Frequency difference ≤ 0.2 Hz
Use a 4‑pole ATS to switch the neutral and prevent earth leakage and RCD issues. Recommended sequence:
- Start and warm‑up
- Stabilise voltage/frequency
- Enable UPS rectifier walk‑in and set recharge limits
- Synchronise within defined windows
- Close on tolerance and verify load on inverter
- On return to mains, soft‑unload and re‑sync
PowerTech Generators delivers the controls, documentation and training to make this repeatable.
Worked Sizing Examples: From 200–800 kVA UPS
Example A: 300 kVA 6‑pulse UPS
- Assumptions: UPS efficiency 95%, rectifier PF 0.95, recharge allowance 15% (45 kVA).
- Running kVA ≈ 300 + 45 = 345 kVA (check kW, PF and crest factor on the UPS datasheet).
- Alternator sizing for THDv: ≈ 2.2 × 300 = 660 kVA (confirm low X″d).
- Select a genset with governor/AVR performance to meet dip/recovery targets and suitable fuel profile for duty.
Example B: 500 kVA IGBT/AfE UPS
- Assumptions: THDi ≈ 3–5%, recharge 10% (50 kVA).
- Running kVA ≈ 500 + 50 = 550 kVA.
- Alternator sizing for THDv: ≈ 1.3 × 500 = 650 kVA typically meets ≤ 5% THDv.
- Confirm dip/recovery against UPS ride‑through and ISO 8528‑5. Use site studies for N+1, diversity and future growth.
This image was generated with AI and may not always represent the product or service exactly.
Commissioning And Proving The System
Before live tests:
- Verify protection settings, 4‑pole ATS wiring, governor/AVR parameters and UPS setpoints.
- Confirm rectifier walk‑in and recharge current limits.
- Prepare RAMS, permits and clear communications.
On site:
- Run resistive/reactive load bank tests and step‑load profiles.
- Log THDv/THDi, voltage dip and frequency response per ISO 8528‑5 methods.
- Prove closed‑transition transfers both ways and record tolerances.
- Provide operator training, handover packs and a preventative generator servicing plan.
For best practice, see understanding generator load testing and why it matters.
Operational Resilience: Maintenance, Monitoring And Upgrades
- Plan generator maintenance for engines, alternators and ATS panels, with 24/7 call‑out.
- Schedule harmonic surveys; re‑verify firmware and control parameters after UPS/AVR updates.
- Monitor fuel quality, cooling, exhaust and ventilation to protect performance.
PowerTech Generators provides remote monitoring, generator servicing and nationwide support with export capability. Our installation and maintenance team keeps critical power stable through seasonal loads, site changes and compliance checks.
Procurement Paths: New, Used, Hire, Export And Decommissioning
We offer new and used generators with finance, temporary generator hire during UPS upgrades, export generators with logistics, and compliant decommissioning services. As an independent supplier, we match Cummins, Perkins, FG Wilson and Baudouin builds to your specification.
Every set is factory‑tested with full documentation and fast delivery. For immediate support on industrial generators for sale or project‑ready packages, contact PowerTech Generators for a fast quote or site survey. We align scope, tolerances and controls to your operational risk profile.
FAQs
How big should my generator be for a 6‑pulse UPS?
As a starting point, size the alternator at 2.0–2.5 × the UPS kVA to hold THDv ≤ 5%. Confirm with a harmonic study and UPS ride‑through limits.
Do I need a 4‑pole ATS with a UPS?
Yes, in most cases. Switching the neutral avoids earth leakage issues, neutral shift and unwanted RCD trips during transfers.
Can a generator support an IGBT/AfE UPS without oversizing?
You still need headroom, but less than with 6‑pulse. Alternator ≈ 1.2–1.5 × UPS kVA is typical if X″d is low and the AVR is PMG‑excited.
What tests prove no‑break transfer?
Closed‑transition transfers in both directions within tight sync windows, plus step‑load tests logging frequency, voltage and THDv/THDi. Document all setpoints.
Will battery recharge current trip my generator?
It can if not managed. Enable rectifier walk‑in and limit recharge current after pick‑up to avoid large load steps and frequency dips.
Can I hire a temporary generator during a UPS upgrade?
Yes. We provide generator hire with ATS integration and commissioning support to maintain continuity while works proceed.
What information do you need for a quote?
UPS type and rating, THDi targets, site voltage/earthing, step loads, transfer method, space, noise and fuel constraints. We can survey and advise.