The Role of Isolation Transformers in Data Center UPS Systems

Overview

The paper first corrects a historical point: early UPSs contained transformers because inverter technology required them, not because galvanic isolation was the goal. Three basic mains configurations are defined: single mains, dual mains and single mains without bypass. Across them there are 32 transformer arrangements (8 single mains, 16 dual mains, 8 without bypass), rising to 92 when local versus remote placement is considered.

The main useful function of a delta-wye transformer is creating a new, isolated neutral; other benefits are voltage change, impedance and blocking triplen harmonics. The drawbacks are weight, cost, space and losses. Grounding system matters: IT equipment is always operated from TN-S; TT or IT supplies generally need a transformer to convert to TN-S, while TN-C is commonly converted without one.

In dual mains systems with RCD protection there must always be a transformer in one of the mains paths; omitting the rectifier neutral does not stop circulating currents. Single mains is used in over 50% of UPS installations and about 90% of those below 100 kW. Without a bypass, load fault current is limited to the UPS inverter output current, which can prevent downstream breakers from clearing. Preferred arrangements are tabulated in Tables 2a-2c, 4a-4d and 6a-6b, and none places a transformer in the inverter location; the inverter-output position used by legacy transformer-based UPSs is the least useful place. Related standards named are IEC 60364-1 and IEC 62040-3.

Key points

1. 32 transformer arrangements across three configurations: 8 single mains, 16 dual mains, 8 without bypass; 92 with local/remote placement.
2. Delta-wye benefits: neutral isolation, voltage change, impedance, triplen harmonic blocking; drawbacks: weight, cost, space, losses.
3. IT equipment always runs from TN-S; TT or IT supplies generally need a transformer, TN-C commonly does not.
4. Dual mains with RCD protection always needs a transformer in one mains path.
5. Single mains is used in over 50% of installations and about 90% below 100 kW.
6. No preferred arrangement puts a transformer at the inverter location.
7. Without bypass, fault current is limited to inverter output current and may not clear downstream breakers.

Using it in design

• Give each UPS a mainsConfig (single mains, dual mains, no bypass), an inverter-transformer flag and a per-input earthing system (TN-S, TN-C, TT, IT).
• Raise an error for dual mains with RCD and no transformer in any path.
• Require an isolating transformer when a non-TN-S supply feeds IT loads.
• For no-bypass designs check downstream breaker clearing against inverter fault current, and add each transformer's losses to the efficiency calculation.

Who should read it

Electrical designers deciding UPS mains configuration, earthing system and transformer placement.

Related standards

• IEC 60364-1
• IEC 62040-3

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2011

Document number

WP 98

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