Transient Performance of Generating Sets

Overview

The ISO 8528-5:2018 limits are tabulated. Transient frequency deviation on a BMEP load increase is no worse than -15 % for G1, -10 % for G2 and -7 % for G3; on a 100 % load decrease it is +18/+12/+10 %. Voltage deviation on load increase is -25 % (G1), -20 % (G2) and -10 % (G3), and +35/+25/+10 % on a 100 % decrease; voltage recovery is 10/6/4 s. Frequency recovery is within 10 s (G1), 5 s (G2) and 3 s (G3), with a steady-state frequency band of +/-2.5/1.5/0.5 %.

Load acceptance is tested in BMEP-dependent steps (up to five power stages, at most six steps), while load rejection is always 100 % in one step. BMEP [kPa] = P x nr x 1000 / (Vd x N); a 13 L engine at 470 kW and 1500 rpm gives 2892 kPa. Specifications, however, often phrase requirements such as 'G3 with a 60 % step'. Typical customer 'G3-equivalent' load-acceptance limits for diesel are -7 % frequency and -15 % voltage with 3 s (f) and 4 s (V) recovery.

On the load side, a static UPS tolerates +10/-15 % voltage and +/-5 % frequency and is sensitive to a frequency slew above 0.5 Hz/s; VFDs are sensitive above 1 Hz/s (from T-030). A DOL motor start draws about 4.5-8.5x FLC, while star-delta reduces it to about 2x FLC. The paper discusses why motor starting method dominates transient sizing and why full G3 on 100 % load rejection is hard to achieve.

Key points

1. ISO 8528-5:2018 frequency deviation on load increase: G1 <= -15 %, G2 <= -10 %, G3 <= -7 %; on 100 % decrease +18/+12/+10 %.
2. Voltage deviation on load increase G1 -25 %, G2 -20 %, G3 -10 %; on 100 % decrease +35/+25/+10 %; voltage recovery 10/6/4 s.
3. Frequency recovery <= 10 s (G1), 5 s (G2), 3 s (G3); steady-state band +/-2.5/1.5/0.5 %.
4. Load acceptance tested in BMEP-dependent steps (up to five power stages, max six steps); load rejection always 100 % in one step.
5. BMEP [kPa] = P x nr x 1000 / (Vd x N); 13 L engine, 470 kW at 1500 rpm gives 2892 kPa.
6. Typical 'G3-equivalent' diesel limits: -7 % frequency, -15 % voltage, recovery 3 s (f) and 4 s (V).
7. Static UPS tolerates +10/-15 % V and +/-5 % f, sensitive above 0.5 Hz/s; VFDs sensitive above 1 Hz/s.
8. DOL motor start draws about 4.5-8.5x FLC; star-delta about 2x FLC.

Using it in design

• Store per generator the ISO 8528-5 class achieved, BMEP at rated load and the maximum single load step (%) that meets the stated dip limits.
• When a data-hall spec calls for G3, check the planned load-step sequence (UPS rectifier walk-in, cooling plant motors) against the set's tested step capability rather than the class label.
• Let users define load steps and starting methods, and warn when a step exceeds the set's BMEP-based capability or the UPS slew-rate tolerance (0.5 Hz/s).

Who should read it

Electrical engineers sizing data-hall generators against load steps, UPS behaviour and motor starting.

Related standards

• ISO 8528-5:2018
• ISO 8528-1

Publication details

Authors

Pedro Ponte

Publisher

Cummins

Year

2025

Document number

Bulletin 5544386 (Rev. 2/25; earlier Bulletin 5544421)

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