The Role of Isolation Transformers in Data Center UPS Systems

The Role of Isolation Transformers in Data Center UPS Systems

Every data center power system includes transformers. Isolation transformers have historically had a number of different roles in the power architecture of data centers

• Voltage stepdown from medium-voltage mains supply to the utilization voltage 

• Within a UPS, to act as an integral part of the power conversion circuits

 • To create a local ground-bonded neutral

 • Within power distribution units or a UPS, to reduce harmonic currents 

• To provide taps to accommodate abnormally high or low mains voltage 

• To eliminate ground loops with multiple generators or mains sources

• Within power distribution units, for stepdown from the data center distribution voltage of 480V or 600V to 208V (North America only) 

• Within power distribution units, to provide additional utilization voltages (e.g., 120V in North America and 100V in Japan) 

UPS systems have historically had one or more permanently installed internal isolation transformers to provide one or more of the above functions, depending on the design of the data center power system. Newer UPS systems do not require power transformers as part of their circuits, improving efficiency and reducing weight, size, and cost. Instead, transformers are added to a transformerless UPS as needed to achieve a desired function. 

In most data center power system designs, transformerless UPSs provide the same function and performance as older UPSs with internal transformers. Nevertheless, there are many cases where a transformer is required and must be either internal to the UPS or added externally. Even older UPS systems with internal transformers require additional external transformers in many cases. 

In almost all cases where a transformer is needed, the transformerless UPS design is superior because it allows the transformer to be installed in a more optimal part of the power path. First, this article  explains why older UPSs have transformers and the circumstances under which transformerless UPSs behave differently.

 It is widely believed that internal transformers were originally provided in UPS systems in order to provide galvanic isolation between the UPS input and output. This is false. The reason internal transformers were used in early UPS systems is because they were inherently required due to the power inverter technology used in the UPS design. 

The first UPS products were developed over 40 years ago and used a ground-referenced battery system. The grounded electronics and battery configuration required these systems to have two transformers for isolation from the mains – one on the input rectifier, and one in the output inverter. Later improvements in these designs, where the battery bus was moved to the neutral wire or electrically floated, eliminated one of the transformers, typically removing the rectifier transformer. Taking advantage of high-voltage, high-speed power semiconductors that did not exist 15 years ago, most recent UPS designs use newer designs that eliminate both the input and output transformer

In some data center power system designs, no transformers are needed in conjunction with the UPS; but there are many situations where transformers must, or are advised to be, installed in combination with the UPS. 

It is not possible to consider the use of transformers in a UPS system without understanding the important differences between the different options. The configuration of a UPS system falls into three basic categories, differentiated by the presence of a static bypass and how it is connected. 

These three categories are commonly called single mains, dual mains, and single mains without bypass. They are schematically illustrated in Figure 1.

UPS Configuration

In the single mains configuration, one mains connection supplies both the bypass and UPS module which are connected together at the UPS. This is the most common arrangement, and is the only arrangement supported in many small UPS systems. It is found in most smaller data center installations and also found in many large data center installations. The main benefits of this system are the simplicity and low cost of installation and the fact that many complexities relating to circulating currents and grounding are eliminated. The downside of this system is that the actual mains supply system cannot be taken down for maintenance without disrupting power to the critical load, although some of these disadvantages can be overcome with wraparound breakers on the mains. 

The dual mains configuration is required when the bypass is fed from a second mains that is different from the mains feeding the UPS rectifier input. The difference in the mains can range from minor (e.g., they are fed from different breakers on the same panel) to major (e.g. they come from completely independent sources with different ground systems and even different voltages). There are a variety of data center redundancy architectures that specify this type of configuration.

Another reason for the dual mains configuration is to allow either of the two mains to be taken down for maintenance while providing power to the critical load. Note that this configuration can be used, but is not required, when a generator is used, since the generator is typically connected to the mains bus upstream of the UPS with an automatic transfer switch (ATS) so that it can provide power to other loads, such as chillers, in addition to the UPS. The dual mains configuration is required in some data center architectures, and is chosen by preference in many larger data centers in order to allow for concurrent  maintenance and/or to slightly improve the overall system reliability by preventing the wiring and breaker upstream of the UPS from being a single failure point for the power system. 

The final configuration, single mains without bypass, is mainly used in environments where the mains power quality is considered to be extremely poor, to the point where it has been determined that it is not desired to ever power the critical load from the mains via a bypass. This can occur in industrial situations, shipboard, or on small islands, where the mains frequency (50 or 60Hz) is not the same as the IT load frequency, or in stressed electrical grids in developing nations. In some countries (the United States, for example) this is an extremely uncommon approach, but in others (India, for example) it is quite common and may be the majority of installs in some regions. Note: A complete analysis comparing the three configurations above would involve tradeoffs in price, complexity, reliability, maintainability, and power quality; such an analysis. 

Figure 2 shows the possible locations of transformers in the three UPS configurations.

UPS Configuration
Fig 2

The various transformers shown in Figure 2 other than the inverter transformer are all options that are either optional within a UPS or can be installed externally to the UPS enclosure. 

The distinction between a transformerless UPS and a transformerbased UPS is the presence of the inverter transformer. All of the other transformers that might be used in a UPS system are optional and can be used with either a transformerbased or a transformerless UPS. Note that in any installation there are upstream transformers providing power to the UPS system and other loads. 

The mains, bypass, and rectifier transformers in the diagrams in Figure 2 represent transformers that are specifically dedicated to the UPS system; they are separate from the transformers that step the voltage down from medium voltage. In each of these three UPS system configurations any combination of transformers may be present – from none to all. 

For the single mains configuration, there are 8 possible transformer arrangements; for dual mains there are 16 arrangements, and for single mains without bypass there are 8 arrangements, for a total of 32 possible arrangements. Furthermore, the mains transformers and output transformers can be located either locally or remotely from the UPS, which affects the grounding system. This adds an additional 60 variations, for a total of 92 ways transformers can be installed with a single UPS. Virtually all of the 92 transformer installation variations have been used in real installations. However, not all transformer arrangements are logical, and there are a few that offer a superior combination of performance, economy, and efficiency. To understand when the use of a transformer is required or why various transformer locations exist for the three UPS configurations, we first must consider the effect of transformers on the neutral and ground wiring.

This article was extracted from "The Role of Isolation Transformers in Data Center UPS Systems" by Schneider Electric

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