Parallel UPS Systems

Overview

Paralleling is common: at least 50-60% of large UPS systems (300 kVA and up) are configured in parallel, and up to 40% of systems around 10 kVA. In the example, an 800 kVA load is protected by three 400 kVA modules in N+1, each carrying one third. In the N+1 alarm logic, five 100 kVA modules alarm when load exceeds 400 kVA.

The paper contrasts master-slave or communication-dependent paralleling schemes, which create single points of failure, with a peer-to-peer approach that needs no inter-module control wiring. Failure of the control system to switch to a redundant path is cited (GE) as the leading cause, 32%, of mission-critical power system failures.

Unbalanced load sharing (for example 40/40/20 in a three-module system) stresses some modules and can drive the system to bypass. When the paralleled modules together exceed one unit's internal static switch rating, a separate bypass cabinet with its own static switch is needed.

Parallel limits by range are given as examples: the 9395 at 200-1200 kVA up to 4800 kVA (N+1) and the 93PM at 20-400 kW up to 1200 kW. Tie cabinets are among the deployment considerations.

Key points

1. At least 50-60% of large UPS systems (300 kVA and up) are paralleled; up to 40% of ~10 kVA systems.
2. Example: an 800 kVA load on three 400 kVA modules in N+1, each carrying one third.
3. Combined modules above one unit's internal static switch rating need a separate bypass cabinet.
4. Control-system failure to switch to a redundant path is cited as the leading cause (32%, GE) of mission-critical power failures.
5. Unbalanced sharing such as 40/40/20 stresses some modules and can drive the system to bypass.
6. N+1 alarm: five 100 kVA modules alarm when load exceeds 400 kVA.
7. Parallel limits: 9395 200-1200 kVA up to 4800 kVA (N+1); 93PM 20-400 kW up to 1200 kW.

Using it in design

• Add maxParallelUnits, paralleling architecture (distributed or centralised bypass) and static switch rating to the UPS catalogue.
• Compute redundancy-aware capacity (N+1 usable = (n-1) x module rating) and alert when design load exceeds it.
• Require external bypass cabinet sizing when the combined output exceeds a single static switch rating.
• Warn when the parallel scheme relies on a single master controller.

Who should read it

Electrical designers choosing a paralleled UPS arrangement for capacity or N+1.

Related standards

• IEC 62040-3

Publication details

Authors

Ed Spears

Publisher

Eaton

Year

2020

Document number

WP153026EN

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