Classification and Types of UPS – Part Seven

In the previous article “Classification and Types of UPS – Part One”, we stated that UPS is classified according to:

  1. Voltage range,
  2. No. of phases,
  3. Mobility,
  4. Technological design,
  5. Physical Size/capacity,
  6. Form factor/ configurations,
  7. Topology,
  8. Distribution Architecture,
  9. Use of transformers.


We already explained all classifications in the Previous articles (see table in the end of Article) except classification according to the use of transformer with the UPS which we will explain it today using the papers from Schneider Electric – Data Center Science Center.



Three Basic Configurations Of Mains And Bypass For A UPS System





The configuration of a UPS system falls into three basic categories, differentiated by:

The number of mains (single or dual) and The presence of a static bypass and how it is connected.  These three categories are commonly called:

  1. Single mains,
  2. Single mains without bypass,
  3. Dual mains,

 They are represented in Fig-1.

Fig-1 Three basic configurations of mains and bypass for a UPS system





1- Single Mains 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.





2- Single Mains without Bypass 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.





3- Dual Mains Configuration


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. There are a variety of data center redundancy architectures that specify this type of configuration.

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).

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.





Eighth: According to Use of transformers with the UPS




Every data center power system includes transformers. Isolation transformers have historically 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 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 transformer-less UPS as needed to achieve a desired function.




Therefore, UPS products are often described According to Use of transformers with the UPS to 3 types:

  1. Transformer based,
  2. Transformer less UPS,
  3. Transformer less UPS with external input/ output transformer.


The Three basic configurations of mains and bypass for a UPS system (Single mains, Single mains without bypass, Dual mains) can include one or more transformers in the power path. Fig-2 shows the possible locations of transformers in the three UPS configurations.


Fig-2 Possible location of isolation transformers in the three basic UPS system configurations

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


Therefore, the distinction between a transformer less UPS product and a transformer based UPS product 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 transformer based or a transformer less UPS product.


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 Fig-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.




Possible location of isolation transformers in the three basic UPS system configurations




In each of The 3 basic configurations of mains and bypass for a UPS system (Single mains, Single mains without bypass, Dual mains) 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.




Why using transformers with UPS Systems?




There are different types of transformers, but the transformer used exclusively in data center applications is the “delta-wye” configuration, which is the type used in almost all UPS applications. Delta-wye transformers have a number of characteristics, both good and bad, that impact their use in UPS systems.

The Good characteristics are:

  1. Voltage change (for example, 480V to 208V)
  2. Impedance that limits fault current or acts as a noise filter
  3. Blocking the 3rd, 9th, 15th, and other multiples-of-three harmonic currents
  4. Isolation of the output neutral from the source.


The bad characteristics are:

  1. Weight, cost, consumption of natural materials, and taking up space
  2. Electrical losses and contribution to data center inefficiency

The beneficial characteristics will be briefly explained in below paragraphs.




1- Voltage change


This is necessary in applications where the mains voltage is not the same as the voltage used by the IT equipment. This is a common condition in North America where the mains voltage is 480 or 600V in larger data centers. In most of the world, the 400/230V three-phase mains voltage is the same voltage used by the IT load equipment, so this function is not required.




2- Impedance


This is generally secondary and unimportant in the modern data center. Most designs do not require additional impedance, and if it were required it would be more effective to create it with a power inductor (sometimes called a “choke”), which is smaller, lighter, and more efficient than a transformer.




3- Blocking harmonics


This was historically a useful function to prevent the harmonic currents created by the UPS from affecting the mains, and to prevent IT-load harmonic currents from affecting the mains via the UPS bypass.

However, two major changes have changed this situation: both the modern UPS and modern IT loads are “power factor corrected”, which means their harmonic current generation has been dramatically reduced to the point where no additional filtering is necessary.

Therefore, the use of transformers to reduce harmonic currents is no longer a necessary function in the modern data center.




As explained above, the first three beneficial characteristics have limited or obsolete value, which leaves the fourth characteristic – isolation of the neutral from the source which is by far the most important.




4- Isolation of the output neutral from the source


This is the most important characteristic of a transformer – it is this characteristic that causes transformers to be useful, necessary, or even legally mandated under certain conditions.

Transformers are typically represented by the overlapping double-circle symbol shown in Fig-2. However, this symbol is a simplification of the actual wiring diagram of a transformer, which is shown in Fig-3.


Fig-3 Wiring diagram showing input and output connections to a power isolation (delta-wye) transformer


The example shown is a “delta-wye” transformer. Although other types such as delta-delta, wye-delta, or wye-wye are possible, the delta-wye transformer shown has special advantages and is used almost exclusively in data center applications. The technical reasons for this can be found in many textbooks on power systems and are not discussed here.

The secondary, or output, is connected in the “wye” (Y-shaped) configuration and consists of three power phases and a center point, or neutral, connection1 . There is no electrical connection between the input and output; the power is transferred through magnetic fields between the input and output. What is important to note is that there is no neutral connection on the input. Even if the supply circuit has a neutral connection, it is not used with a delta-wye transformer.

The transformer “makes” a new neutral on the output – a new neutral that has no electrical connection to any neutral on the input. In fact, the whole output circuit is at an indeterminate voltage with respect to the input or ground, which is referred to as “floating”.

Since IT load equipment is grounded, it is never appropriate to supply floating power at an indeterminate voltage because this could cause insulation failure and other hazards. Therefore, the new neutral on the output of the transformer is connected to ground in virtually all data center applications.

When an isolation transformer has a grounded neutral, its output circuit is often referred to as a “separately derived source”. Grounding the output neutral can be achieved by:

  1. Directly connecting the neutral to the nearest grounded metal (equipment enclosures, ground rods, water pipes or ground wire known to be grounded), or
  2. it can be connected to an existing neutral wire known to be grounded, or
  3. it can be connected to ground through a grounding resistor (this is only used on high power busses and not on final distribution to the IT loads).

 All three of these techniques are used in data centers.

Considering the above isolation properties of a transformer, we can now describe the key beneficial – and sometimes necessary – functions resulting from isolation:

  • The first function: Changing different mains grounding systems to the system required by data center IT equipment
  • The second function: Creating a new neutral connection when the mains neutral has serious power quality problems or the neutral is subject to disconnection when upstream 4-pole circuit breakers are used (as required in some countries)
  • The third function: Combining two sources without the need to connect their neutral wires together
  • The fourth function: Preventing circulating currents that could cause Residual Current Detectors (RCD)or other safety systems to activate unnecessarily.

Since these functions must be understood in order to understand how and why transformers are used and where to apply them, each function will be briefly explained in below paragraphs.




The first function: “changing a mains grounding system in a data center to the grounding system required by IT equipment”


This function is clearly an essential function. IT equipment in a data center is always operated from a TN-S grounding system and we will have three cases:

  • Case#1: the mains provides a TNS system, so no change is required.
  • Case#2: the mains provides a TT or IT grounding systems, require conversion to TN-S by use of a transformer before they can be utilized by IT equipment. This grounding conversion can occur before the UPS or after the UPS.
  • Case#3: In some countries such as the USA and Great Britain, TN-C grounding systems are common and are converted to TN-S without a transformer.



The second function: “creating a new neutral when the mains neutral has serious power quality problem”


This function is used when the provided mains neutral be one of the following:

  1. It is shared with other customers,
  2. It is generated a distance from the data center, or
  3. It is deemed unreliable to the point where it might either be interrupted or become disconnected from earth.

In developed countries and in most large new buildings, the TN-S neutral source is within the customer premises and typically close to the data center. In this case the quality of the neutral would be considered excellent and the second function is redundant.

But in other situations the neutral to ground bond may be outdoors, possibly distant, shared, and part of a degraded or overloaded distribution system. Under these conditions the neutral may have significant offset or noise voltage with respect to ground, or worse it might lose its ground connection or become interrupted. This problem is made worse in tropical climates where it can be difficult to maintain low impedance metal-to-metal bonds over time. If a mains source under a loss of neutral condition were passed directly to the IT equipment, massive equipment failures could occur due to the higher voltage. These problems are common in developing countries and are the reason why a data center power system design often requires additional power transformers when deployed in emerging markets.




The third function: “combining sources without the need to connect the neutrals”


This function is unique to emergency power systems that have backup sources, such as commonly used in a data center.

A data center may be fed from a combination of multiple mains services and generators that are combined with switches to assure power continuity to the critical load. The bypass path within a UPS is itself an alternate power path from the UPS module that is essentially “combined” at the output of the UPS.

Whenever two sources are combined through a switching arrangement, a situation may arise where there are two input neutral connections and a single output neutral connection. This leads to the problem of how to connect a single output neutral given two input neutrals as shown in Fig-4.


Fig-4 How to create a single output neutral given two input source neutrals

Since switching between neutrals supplying an IT load creates a momentary open-neutral condition which can be hazardous or destructive, the neutral to the critical load should never be switched. This means if two alternate sources are combined in the UPS, they must have their neutral wires permanently connected to each other. However, connecting the input neutrals together to the output neutral can create circulating currents between the input neutrals as shown in Fig-5.


Fig-5 Circulating current created by connecting two separate input neutral wires to create a single output neutral

While these circulating currents are a minor nuisance when the bypass and rectifier come from the same source, this can be a hazard if the two input neutrals come from different sources. The connection of two independently derived neutral sources together is universally not permitted by law.



The insertion of a transformer in series with one of the two sources solves this problem. Therefore, whenever a dual mains UPS is supplied by separately derived neutral sources, a transformer is required.



some dual mains systems have the two inputs supplied by sources with a commonly derived neutral and these don’t require a transformer.




The fourth function: “preventing circulating currents that could cause RCDs or other safety systems to activate unnecessarily”

This function is also related to the situation where sources are combined such as in a dual mains configuration. Circulating currents between neutrals always occur when separately derived neutrals are connected to each other, but as pointed out in the previous paragraph, this is not permitted by law so a problem should not be created.

But circulating currents can also occur even when a UPS is supplied from two inputs that are derived from the same neutral. Therefore, in any system where both a rectifier and bypass neutral connection are provided to the UPS, any RCD protection on the supply circuits will activate unnecessarily.



An isolation transformer located in either the rectifier supply, the UPS module output, or the bypass is needed to prevent RCD activation.



At first it seems that if the rectifier input neutral connection could be omitted, the circulating current problem should be solved. In fact, all UPS systems designed for dual mains are designed to operate without a rectifier neutral connection; the UPS input rectifiers draw power between the input phases and do not require a neutral connection to function.

As long as the rectifier source is known to be grounded, the rectifier neutral need not be provided. Since there is no longer any rectifier neutral connection, it seems that circulating current should not be possible. Unfortunately, although this it is widely believed that the absence of a neutral connection on the rectifier eliminates circulating currents, it is not true.





Fig-6 shows the dual mains UPS system configuration with the rectifier supply neutral not connected. The red line shows the flow of circulating current which still exists but flows through the UPS module instead of the rectifier neutral. Any UPS inverter module that has an output neutral will inject current onto the output neutral bus in excess of any neutral current required by the load. This “excess” neutral current is a side-effect of inverter operation and is caused by reactive loads, non-linear loads, and imbalances in the load currents.


Fig-6 Circulating current can still exist even if the rectifier input neutral is not connected

This “excess” neutral current is not consumed by the IT loads and returns to the mains via the bypass neutral. This current may appear small under normal conditions but can become large under various loads or mains voltage imbalances.

If the bypass mains supply includes RCD protection (mandated in some conditions in some countries), then these protective devices will sense this neutral current as an unexpected current and misinterpret it as a ground fault, possibly shutting down the system. This leads to a very important principle of data center power system design:

In DUAL MAINS systems with RCD protection, there must ALWAYS be a transformer present somewhere in one of the mains paths and removing the rectifier neutral connection is NOT sufficient to prevent circulating currents.




Single Mains VS dual Mains


It should be clear at this point that the dual mains configuration has, by far, the most complex issues relating to grounding and transformer use. Many mistakes are made in the application of transformers and the appropriate grounding of dual mains systems, mistakes which often result in intermittent problems and unexpected downtime.

These problems are simplified in single mains systems (with or without bypass). Often a dual mains system is chosen without consideration of these complexities, and a single mains system might have been a better choice because there are fewer things that can go wrong in the design and the installation.

The single mains configuration can be a reliable and cost-effective choice because the theoretical reliability advantages of a dual mains configuration are not always realized in practice. This is why the single mains configuration is often used even in very large ultrahigh availability data centers, especially when redundancy is achieved by dual path or N+1 UPS configurations.



In the next Article, I will explain Transformer arrangements in practical UPS systems. So, please keep following.


Subject Of Pervious Article


Applicable Standards for UPS Systems

What is a UPS?

Why do we need a UPS?

UPS Rating

Classification of UPS:

1-Voltage range,

2-No. of phases,

3- Mobility,

4- Technological design,


Classification and Types of UPS – Part One

5- Physical Size/capacity,

6- Form factor/ configurations:

6.1- “N” System Configuration

Classification and Types of UPS – Part Two

6.2- “N+1” System Configuration, which includes:

Isolated Redundant Configuration (N +1)

Parallel Redundant Configuration (1+1)

Parallel Redundant Configuration (N +1)

Parallel Redundant Configuration (N +2) and so on


6.3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1)


Classification and Types of UPS – Part Three


6.4- Parallel Redundant with STS Configuration

Parallel Redundant Configuration (1+1) + STS

Parallel Redundant Configuration (N+1) + STS


6.5- System plus System 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N


Classification and Types of UPS – Part Four



7- According to UPS Topology

7.1 Off-line or Standby UPS,

7.2 Line Interactive UPS,

7.3 Standby-Ferro UPS,

7.4 Online Double Conversion UPS,

7.5 The Delta Conversion On-Line UPS.


Classification and Types of UPS – Part Five




8- According to UPS Distribution Architecture

8.1 Centralized UPS Configuration,

8.2 Distributed (Decentralized) UPS Configuration,

8.2.1 Distributed UPS-Zonewise Configuration

8.3 Hybrid UPS Configuration.

Conventional (Monolithic) Vs Modular UPS System:

1- Deploy UPSs in parallel,

2- Deploy UPSs in Series,

3- Use modular UPS products.


Classification and Types of UPS – Part Six



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