How to size a genset: Proper generator set sizing requires analysis of parameters and loads

Overview

Inputs fall into two groups. Project parameters are duty, fuel, minimum load, permissible voltage and frequency dips, and altitude and temperature. Load characteristics are power factor, motor inertia and starting, non-linear loads such as VFDs, UPS and battery chargers, regenerative loads and single-phase imbalance. Cummins does not recommend running sets below 30% of rated load and suggests load banks to supplement.

Duty type matters: standby ratings generally carry no overload capability, while prime ratings generally allow at least 10%. Three-phase sets are rated at 0.8 PF, so lower PF needs a larger alternator. Lightly loaded UPS input filters are a common leading-PF source, and a set can carry up to about 10% of rated kVAR capability as leading-PF load without losing voltage control.

For non-linear loads the paper gives numbers. VFD load should stay below about 50% of generator capacity to keep THD under roughly 15%. Battery chargers require alternator oversizing from about 2.5 times running load (3-pulse) down to about 1.15 times (12-pulse). Across-the-line motors over 50 hp draw roughly 6 times running current at start, causing voltage dip, and sequencing load steps can reduce set size.

Key points

1. Running below 30% of rated load is not recommended; load banks supplement.
2. Standby ratings generally have no overload; prime ratings generally have at least 10%.
3. A set tolerates up to about 10% of rated kVAR as leading-PF load before losing voltage control.
4. Three-phase sets are rated at 0.8 PF; lower PF requires a larger alternator.
5. VFD load below about 50% of capacity keeps THD under about 15%.
6. Battery-charger alternator oversizing runs from about 2.5x (3-pulse) to about 1.15x (12-pulse).
7. Across-the-line motors over 50 hp draw about 6x running current at start.

Using it in design

• Give the sizing engine inputs for minimum load %, maximum voltage and frequency dip, altitude and ambient derate, and per-load type (UPS rectifier pulses, VFD share, motor starting method).
• Warn when VFD share exceeds 50% of set kVA.
• Warn when leading kVAR exceeds 10% of alternator kVAR capability, as with lightly loaded UPS filters.
• Warn when normal load is under 30% of rating.

Who should read it

Electrical engineers sizing a data-centre generator for the first time, or checking the output of sizing software.

Related standards

• ISO 8528-1
• NFPA 110

Publication details

Authors

Jim Iverson

Publisher

Cummins

Year

2007

Document number

Power Topic #7007

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation