Harmonics And VFD Loads: Specifying Generators For Modern Plants
Harmonics are unwanted frequency components that distort the electrical waveform. THDi is current distortion created by non‑linear loads; THDv is the resulting voltage distortion seen at the generator terminals. VFDs, UPS rectifiers and LED drivers chop the waveform, pushing rich harmonics back into the alternator and AVR.
Left unmanaged, distortion causes overheating, reduced motor torque, voltage instability and nuisance trips. The commercial impact is downtime, shortened asset life and compliance issues. PowerTech Generators delivers end‑to‑end support across supply, generator installation, generator maintenance, generator servicing and generator hire for diesel generators UK and critical power solutions. For buying context, see choosing industrial generator for your business or review our range of commercial diesel generators.
THDi Vs THDv: Practical Limits, Measurement And Risk
THDi is the total harmonic current distortion drawn by the load. THDv is the total harmonic voltage distortion at the point of common coupling (PCC). In islanded generator systems, a practical target is THDv below 8–10% at the PCC. Six‑pulse drives can draw THDi of 35–50% without mitigation. Use IEEE 519 as guidance; ENA G5/5 applies to public networks, so for island mode follow alternator OEM limits.
Measure with a Class A power quality analyser, using true‑RMS, logging across load steps, motor starts and any regeneration events. Risks include:
- Heating in windings and neutrals
- AVR hunting and flicker
- Protection mal‑operation and trips
For test methods, see understanding generator load testing and why it matters. Planned generator maintenance and generator servicing keep distortion within limits.
Alternator Excitation: PMG, AREP And Self‑Excited—What To Choose For VFDs
Self‑excited alternators power the AVR from the stator, so heavy distortion can upset the regulator. PMG and AREP systems feed the AVR from a separate source, giving stable control even when the main output is distorted. The result is stronger short‑circuit response, better motor starting and cleaner voltage under non‑linear loads.
For VFD‑heavy sites, specify: PMG/AREP, digital AVR with true‑RMS sensing, waveform filtering, 2/3 pitch, Class H insulation, a low‑reactance design, and an oversized alternator (typically 1.25–1.6x). Consider short‑circuit ratio (SCR) and X″d for step loads. We supply independent builds using Cummins/Stamford, Leroy‑Somer, Perkins, Baudouin and FG Wilson. Learn more in a simple guide to automatic voltage regulators AVRs.
AVR Setup Under Non‑Linear Loads
Choose AVRs with true‑RMS sensing or effective harmonic filtering. Tune gain and damping to avoid hunting; set robust under‑frequency roll‑off so the AVR behaves during engine transients. For parallel sets, wire and calibrate the droop CT correctly. Coordinate protection so distorted fault currents still trip correctly.
Commissioning checklist:
- Confirm sensing points and polarity
- Record AVR parameters and U/F roll‑off
- Prove stability with step and drive‑inrush tests
- Log THD and steady‑state regulation
We document settings and sign‑off for safety and compliance. For ongoing support, see installation maintenance.
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Sizing kVA And Alternator Heads For Drives
Rules of thumb for VFD loads: six‑pulse without mitigation typically needs 1.6–2.0x generator kVA to driven motor kW; with DC chokes or line reactors, 1.3–1.6x is common; 12‑pulse or AFE can be 1.1–1.3x, subject to measured THDi. Oversize the alternator frame by 10–25% relative to the engine for thermal headroom and lower THDv.
Worked example: 400 kW of VFD motors using six‑pulse drives with DC chokes. Assume 1.4x → ~560 kVA genset, with a larger alternator head to reduce X″d and improve waveform. Expect THDv ~6–9% if cabling is tight and step loads are staged. Check fuel implications with load factors. Use our kVA calculator when comparing industrial generators for sale and diesel generators UK.
Mitigating Harmonics: Practical Options That Work
Start simple: input line reactors or DC chokes often cut THDi to ~25–35% and reduce voltage distortion. Passive filters can help further but must be tuned and reviewed for leading vars. Active filters dynamically cancel multiple orders and suit mixed loads, with a known footprint and loss. AFE drives offer very low THDi and regeneration but need EMC and protection review. Multi‑pulse (12/18‑pulse) with phase‑shifting transformers is robust when space allows.
Effective mitigation can reduce generator kVA and alternator oversizing while improving stability and fuel efficiency.
Commissioning, Testing And Maintenance For Reliability
We prove performance with a documented process: specification review, FAT where available, then SAT with representative step tests. Where possible, use non‑linear load banks or staged live testing while logging THDv, voltage regulation and AVR behaviour. Deliverables include settings sheets, PQ reports and certificates.
To keep THD low over life, schedule generator maintenance and generator servicing: tighten terminations, inspect alternators, clean cooling paths, update firmware, and repeat PQ surveys annually. See our case studies for results in critical power solutions.
Brands, Alternators And Build Options We Supply
We regularly specify Perkins, Cummins, FG Wilson and Baudouin engines with Stamford or Leroy‑Somer alternators, including PMG/AREP options, enhanced cooling and extended frames for harmonic resilience. Controls integrate with Deep Sea or ComAp, plus ATS and paralleling gear to suit your switchboard.
PowerTech Generators is independent, sourcing any brand or specification across new, used and hire fleets. Options include industrial generators for sale, used generators UK and rapid generator hire packages, all built around your THD limits and uptime goals.
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Project Delivery: End‑To‑End Critical Power Solutions
Our turnkey service covers surveys, harmonic studies, specification, generator installation, FAT/SAT, commissioning certificates, O&M manuals and training. We work to safe systems with method statements, RAMS and compliance testing.
We also provide decommissioning services, removal and export generators with crating and shipping. Flexible finance and instant pricing tools support planning and procurement. You get one accountable team, 24/7 support and clear documentation.
Sector Snapshots: VFD‑Heavy Sites We Support
Manufacturing and utilities: pumps, compressors and conveyors on six‑pulse drives. We specify PMG alternators, DC chokes and slightly oversized heads. Result: THDv 6–8% and stable restarts after dips.
Healthcare and data centres: chillers, HVAC and UPS front‑ends. We prefer AREP/PMG, digital AVR with true‑RMS and active filtering on key feeders. Result: tight voltage regulation and clean PQ logs for compliance.
Construction and temporary power: tower cranes, welfare and pumps. Sets with robust AVRs, line reactors and conservative kVA headroom. Result: reliable starts, low flicker and fewer nuisance trips on site.
FAQs On Harmonics And Generator Selection
Do I Need PMG For My Drives?
For high THDi, step loads or parallel operation, PMG or AREP is strongly recommended. It stabilises the AVR supply when the stator waveform is distorted. For light, well‑filtered loads, a quality self‑excited alternator can suffice.
What THDv Is Acceptable On An Islanded Generator?
As a practical target, keep THDv below 8–10% at the PCC. Always check alternator OEM limits and verify with PQ logs during worst‑case operation.
Can A Used Generator Handle VFD Loads?
Yes, with the right alternator and controls. Upgrades such as a PMG/AREP kit, digital AVR, and input reactors or filters allow many used sets to perform reliably with drives.
Talk To PowerTech: Book A Harmonic Review
Get a complimentary pre‑assessment from our engineers. Send your load list, SLD, drive make/models, existing THD data and any protection settings. We’ll propose a survey, measurement plan, specification and programme.
We can provide temporary generator hire while we upgrade your plant, then supply new or used units and switchgear. Trade‑in and export options are available. Speak to PowerTech Generators today via contact for a clear, costed plan.
FAQs
How Do I Check THD On My Site?
Use a Class A power quality analyser at the PCC and key feeders. Log across load steps, motor starts and any regeneration events to capture worst case.
Will Harmonic Filters Reduce My Generator Size?
Often, yes. DC chokes, passive or active filters lower THDi and THDv, which can justify a smaller kVA or alternator head. Always confirm with measurements or a study.
What Happens If THDv Exceeds Limits?
Expect overheating, flicker, reduced torque and nuisance trips. It also risks non‑compliance with OEM limits and shortens asset life.
Can I Parallel Generators With Heavy VFD Loads?
Yes, with PMG/AREP alternators, correctly set droop CTs and tuned AVRs. Verify stability and sharing with witnessed step testing.
How Often Should THD Be Re‑Tested?
Annually as a minimum, or after changes to drives, cables or switchgear. Include THD checks within planned maintenance and before warranty milestones.