The Role of Isolation Transformers in Data Center UPS Systems

Introduction

Every data center power system includes transformers, but their role has evolved significantly over the past 40 years. Modern uninterruptible power supply (UPS) systems have moved away from the bulky internal transformers found in older designs, offering substantial improvements in efficiency, weight, size, and cost. However, this evolution has created complexity around when and where transformers are needed. This article explains the key functions of isolation transformers in UPS systems, outlines the three main UPS configurations, and identifies optimal transformer arrangements for modern data center designs.

Why Older UPS Systems Included Transformers

A common misconception is that internal transformers were originally provided in UPS systems to deliver galvanic isolation between the input and output. In reality, they were a necessary component of the power inverter technology used in early UPS designs. The first UPS products, developed over 40 years ago, used ground-referenced battery systems that required two transformers – one on the input rectifier and one on the output inverter. Later design improvements eliminated the rectifier transformer, and the advent of high-voltage, high-speed power semiconductors has enabled modern transformerless UPS designs.

Three Basic UPS Configurations

The configuration of a UPS system falls into three basic categories, differentiated by the presence of a static bypass and how it is connected:

Single Mains

One mains connection supplies both the bypass and the UPS module. This is the most common arrangement, found in over 50% of all UPS installations. It is simpler and more cost-effective, eliminating many complexities relating to circulating currents and grounding.

Dual Mains

The bypass is fed from a second mains source different from the mains feeding the UPS rectifier input. This configuration allows for concurrent maintenance of distribution wiring and breakers, and is required in some redundancy architectures. However, it introduces complex grounding and circulating current issues.

Single Mains Without Bypass

Used primarily in environments with extremely poor mains power quality, where it is not desired to ever power the critical load from the mains. This is common in developing nations with stressed electrical grids.

Key Characteristics of Delta-Wye Transformers

In UPS applications, the delta-wye transformer is the type used almost exclusively. These transformers have both beneficial and undesirable characteristics:

Beneficial characteristics:

• Voltage change – stepping down from 480V to 208V, commonly required in North America
• Isolation of the output neutral from the source – the most important function
• Blocking triplen harmonic currents – historically useful, now largely obsolete as modern UPS and IT loads are power factor corrected
• Impedance – generally secondary and unimportant in modern data centres

Undesirable penalties:

• Weight, cost, and space consumption – transformers are large, heavy, and expensive
• Electrical losses – contributing to data centre inefficiency

Because the penalties are severe, transformers should only be used when their beneficial characteristics serve a genuine purpose.

The Crucial Function: Neutral Isolation

The most important characteristic of a transformer is its ability to isolate the output neutral from the source. When a delta-wye transformer creates a new neutral on the output, there is no electrical connection between the input and output – power is transferred through magnetic fields.

Since IT load equipment is grounded, the new neutral must be connected to ground, creating what is known as a "separately derived source". This isolation provides several essential functions:

1. Changing different mains grounding systems to the TN-S system required by IT equipment
1. Creating a new neutral when the mains neutral is unreliable or subject to disconnection
1. Combining two sources without connecting their neutral wires together
1. Preventing circulating currents that could cause Residual Current Detectors (RCDs) to activate unnecessarily

Transformer Arrangements for Single Mains

In the single mains configuration, transformers can be located in three positions: mains (input), inverter, and output.

The inverter transformer provides no function or benefit in the single mains configuration. Therefore, there is no advantage to using a transformer-based UPS in this configuration.

Preferred options:

1. Local input transformer – Used when the mains ground system is poor or not TN-S, or the mains voltage differs from the load voltage
1. No transformers – Used with high-quality TN-S mains where voltages are compatible
1. Remote output transformer(s) – Used in large data centres

Transformer Arrangements for Dual Mains

The dual mains configuration has the most complex grounding and transformer issues. In this configuration, transformers can be located in four positions: bypass, rectifier, inverter, and output.

Critical insight: When two sources are combined, connecting their neutrals together can create circulating currents. The inverter transformer provides neutral isolation, but this function is also provided by a rectifier or bypass transformer. Since the inverter transformer handles full load power, placing a transformer in the bypass path is more efficient.

Preferred options:

1. Local bypass transformer with remote output transformers – For large data centres
1. Local bypass transformer, rectifier transformer, and remote output transformers – For poor mains or different voltages
1. No transformers – For small data centres with high-quality TN-S and no RCD requirements
1. Local bypass transformer – For small data centres where RCD protection is required or the bypass is separately derived

Legacy Transformer-Based Designs

Transformer-based UPSs have the transformer in the inverter output location – the least useful place to locate a transformer for these reasons:

• It does not protect the UPS input
• It does not isolate the rectifier ground system from the mains
• It always dissipates heat because it handles full load power
• It cannot allow the output neutral to be isolated from the input due to the presence of the bypass

None of the eight preferred UPS/transformer arrangements have a transformer in the inverter output location. The historical transformer-based UPS is obsolete because its transformer is in the wrong location.

Conclusion

The shift to transformerless UPS systems represents a significant advancement in data centre power protection. Modern transformerless designs offer superior efficiency, smaller footprint, and lower costs while enabling transformers to be placed in the most beneficial locations when needed.

For system designers, the key takeaway is that the transformer location matters far more than whether a UPS is transformer-based or transformerless. In almost all cases, the transformerless UPS design is superior because it allows the transformer to be installed in a more optimal part of the power path.

This article summarises key concepts from "The Role of Isolation Transformers in Data Center UPS Systems" (White Paper 98) by Neil Rasmussen, Schneider Electric.

Download the full free white paper here: The Role of Isolation Transformers in Data Center UPS Systems – White Paper 98 (PDF)

Disclaimer: This article provides a summary of a third-party publication produced by Schneider Electric. While efforts have been made to accurately represent the content, please refer to the official white paper "The Role of Isolation Transformers in Data Center UPS Systems" for complete technical details. Final design, specification, and implementation decisions should be made by qualified electrical engineers and competent professionals in accordance with all applicable regulations and standards.