Totally Integrated Power Technical Series Edition 6 - Special Application: Short-circuit Protection for the "Isolated Parallel" UPS System

Overview

The note builds on the isolated-parallel concept, where several diesel rotary UPS units feed a common IP bus through reactors (chokes). The arrangement balances load between units without extra UPS units or static transfer switches, and it can be applied at low or medium voltage. Siemens presents it together with a matching distribution chain for a high-availability data centre.

The reference chain runs from 8DJH gas-insulated MV switchgear and a GEAFOL cast-resin transformer to a SIVACON S8 LV main board, UNIBLOCK UBT+ diesel rotary UPS, a UPS main distribution and SIVACON 8PS busbar trunking. The document is therefore less about one device than about a product line seen through the lens of fault protection.

The core subject is short-circuit behaviour. For a fault downstream of the IP system there are always two reactors between the fault and the other loads and UPSs, which limits its effect. The worst case is a short circuit on the IP bus, so that bus is built with fully insulated busbars, making such a fault practically impossible.

Even so, the IP reactors are rated so that an IP-bus fault produces an initial voltage dip of no more than 30%. The ITI (CBEMA) curve tolerates that dip for 500 ms, and the selective protection holds the effect on connected loads to under 50-70 ms. Settings are project-specific and derived from short-circuit studies; the note explains the approach rather than publishing a setting table.

Key points

1. The IP system couples several UPS units to a shared IP bus through reactors, balancing load without extra UPS units or static transfer switches, at LV or MV.
2. For a fault downstream of the IP system, two reactors always sit between the fault and the other loads or UPSs.
3. A short circuit on the IP bus is the most critical case; fully insulated busbars practically rule it out.
4. IP reactors are rated to limit the initial voltage dip to 30% even for an IP-bus fault.
5. The ITI (CBEMA) curve tolerates the dip for 500 ms; selective protection limits the effect on loads to under 50-70 ms.
6. Reference chain: 8DJH GIS, GEAFOL transformer, SIVACON S8, UNIBLOCK UBT+ rotary UPS, UPS main distribution, SIVACON 8PS busbar trunking.

Using it in design

• Add "isolated-parallel rotary UPS" (N UPS + IP bus + chokes) as a UPS topology template.
• Require a short-circuit study per operating mode and verify the voltage dip at healthy buses for an IP-bus fault, targeting 30% or less.
• Compare clearing time against the 500 ms limit of the ITI curve.
• Recommend a fully insulated busbar for the IP bus, and treat the chokes and the IP bus as shared elements in the reliability model.

Who should read it

Electrical designers working on LV/MV distribution and protection coordination for high-availability sites with diesel rotary UPS.

Related standards

• ITI (CBEMA) curve
• IEC 61439
• IEC 62271-200

Publication details

Authors

Siemens

Publisher

Siemens

Year

2012

Document number

Technical Series Ed. 6

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