Cost, Speed, and Reliability Tradeoffs between N+1 UPS Configurations

Overview

The paper compares three 1 MW UPS designs: a 1N modular frame with four 250 kW modules, an internally modular N+1 frame with a fifth module sharing one backplane and battery plant, and parallel-redundant N+1 using three 500 kW units each with its own batteries. It defines isolated-redundant, parallel-redundant and internally modular N+1, then quantifies capex per kW (materials plus installation), deployment schedule, operating load percentage and qualitative downtime risk for module, DC-bus and maintenance events.

Capex per kW of rated capacity is $453 for 1N, $482 for internal modular N+1 and $660 for parallel-redundant N+1, so internal N+1 is $178/kW (26.9%) cheaper than parallel. The breakdown (1N / internal N+1 / parallel) is UPS $148/$177/$214, bypass cabinet $36/$36/$70, 7-minute battery $98/$98/$148 and installation $163/$163/$213 per kW. The premium is 6.5% from 1N to internal modular and 36.8% from internal modular to parallel. A typical 1 MW UPS installation spans about 6-8 weeks; parallel systems take 1-2 weeks (25-30%) longer because of extra terminations, start-ups, load-bank tests and paralleling checks, and need about 25% more space even with an ideal layout.

At 800 kW load (80% of 1,000 kW), module loading is 80% for 1N (4x250 kW), 64% for internal N+1 (5x250 kW) and 53% for parallel 2+1 (3x500 kW). In internal modular N+1 the common DC (battery) bus and output bus are single points of failure: a battery-bus fault or UPS maintenance puts the load on bypass without battery backup, unlike parallel redundant. The paper ends with a checklist of fault-tolerance features inside a single UPS frame: redundant power modules, fans, controller power supplies, battery strings, communication bus, control system, and a static switch sized above the maximum expected inrush and step loads.

Key points

1. Capex per kW: 1N $453, internal modular N+1 $482, parallel-redundant N+1 $660.
2. Internal N+1 is $178/kW (26.9%) cheaper than parallel.
3. Premiums: 6.5% from 1N to internal modular; 36.8% from internal modular to parallel.
4. A typical 1 MW installation takes about 6-8 weeks; parallel systems 1-2 weeks (25-30%) longer.
5. At 800 kW load: module loading 80% (1N), 64% (internal N+1), 53% (parallel 2+1).
6. Parallel redundant needs about 25% more space even with an ideal layout.
7. In internal modular N+1 the common DC and output buses are single points of failure; a battery-bus fault puts the load on bypass without battery backup.
8. Check for redundant modules, fans, controller supplies, battery strings, communication bus and a static switch above expected inrush.

Using it in design

• Distinguish 'N+1 internal modular', 'N+1 parallel redundant' and 'isolated redundant' instead of a single N+1 flag; for modular frames store module rating, max modules, shared battery bus and redundant controls/fans.
• Compute per-module load % for the chosen redundancy at design load and evaluate efficiency from the UPS curve at that point.
• Warn that internal N+1 with a common battery bus does not protect against DC-bus faults or UPS maintenance; flag parallel systems with large N for load-sharing risk; require static-switch rating above expected inrush.
• In the capex estimator use per-kW UPS, bypass, battery, assembly and installation lines plus a 1-2 week schedule adder and about 25% more plant-room area for parallel redundant designs.

Who should read it

Electrical designers and decision makers choosing a UPS redundancy configuration.

Related standards

• IEC 62040-3
• IEEE 3006.7
• Uptime Institute Tier Standard: Topology

Publication details

Authors

Wendy Torell

Publisher

Schneider Electric

Year

2017

Document number

WP 234

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