Considerations when Paralleling Generating Sets

Overview

The paper explains how groups of like or unlike generator sets synchronise and share load, either in island mode or in parallel with the utility. Its first conclusion is that with dissimilar sets, the first-priority load is capped by the smallest set that may close first: a 1500 kW plus 500 kW system can accept only 500 kW as first-priority load.

A manageable system keeps the smallest set at no less than 30% of the largest set's capacity. For synchronising, the transient current into the incoming set at breaker closure should stay below 50% of its rated full-load current. The suggested close window is frequency within 0.1 Hz, rate of change of 0.1 Hz/s, voltage within less than 1% and phase angle below 10 degrees, as the record lists it.

For first-start arbitration, random access uses random 1-8 ms bursts over 800 ms and retries after 2.5 s, and a set must be at 90% voltage and frequency before it is ready to load. Dead-bus (dead-field) paralleling is not supported by standard PowerCommand controls, needs a third-party module, and is considered less robust than random access.

The most reusable section covers circulating neutral currents. 2/3-pitch alternators produce no third-order harmonics, so paralleling 2/3-pitch machines with connected neutrals avoids triplen circulating currents, while mixing them with 5/6-pitch machines needs care. Alternator voltage waveform guidance is no more than 5% THD line-to-line and line-to-neutral and no more than 3% in any single harmonic.

Key points

1. With dissimilar sets, first-priority load is limited by the smallest set that may close first (500 kW in a 1500 kW + 500 kW system).
2. Manageability guideline: smallest set at least 30% of the largest set's capacity.
3. Transient current into the incoming set at closure should not exceed 50% of its rated current.
4. Suggested sync window: 0.1 Hz frequency, 0.1 Hz/s rate of change, voltage within under 1%, phase angle under 10 degrees.
5. Random-access arbitration: 1-8 ms bursts over 800 ms, retry after 2.5 s, set ready at 90% voltage and frequency.
6. Dead-bus paralleling needs a third-party module with PowerCommand and is judged less robust.
7. 2/3-pitch alternators give no triplen harmonics; mixing with 5/6 pitch requires mitigation of neutral circulating current.
8. Waveform guidance: THD up to 5%, any single harmonic up to 3%.

Using it in design

• Check smallest-to-largest set ratio against the 30% guideline and flag designs below it.
• Compute the first-priority load limit from the smallest set's rating whenever set sizes differ.
• Before interconnecting neutrals in a 4-wire system, require identical alternator pitch or specify neutral reactors or switching.
• In a paralleling switchgear template, make the first-start scheme and tie-breaker options explicit choices.

Who should read it

Electrical engineers and controls integrators designing low- or medium-voltage paralleled standby generator plants.

Related standards

• ISO 8528-5
• IEC 60034-1

Publication details

Authors

Robert Patrick

Publisher

Cummins

Year

2017

Document number

Power Topic #5410679

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