In the previous article “Classification and Types of UPS – Part One”, we stated that UPS is classified according to:
- Voltage range,
- No. of phases,
- Mobility,
- Technological design,
- Physical Size/capacity,
- Form factor/ configurations,
- Topology,
- Distribution Architecture,
- 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
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Three
Basic Configurations Of Mains And Bypass For A UPS System |
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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:
 Fig-1 Three basic configurations of mains and bypass for a UPS system
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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. |
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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. |
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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. |
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Eighth:
According to Use of transformers with the UPS |
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Every data center power system
includes transformers. Isolation transformers have historically a number of
different roles in the power architecture of data centers:
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. |
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Therefore, UPS products are often described According to Use of transformers with the UPS to 3 types:
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. |
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Possible
location of isolation transformers in the three basic UPS system
configurations |
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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. |
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Why
using transformers with UPS Systems? |
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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:
The bad characteristics are:
The beneficial characteristics will be briefly explained in below paragraphs. |
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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. |
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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. |
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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. |
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Note: 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. |
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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. %20transformer.jpg){kind=spacer} Fig-3 Wiring diagram showing input and output connections to a power isolation (delta-wye) transformer Note: 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:
Considering the above isolation properties of a transformer, we
can now describe the key beneficial – and sometimes necessary – functions
resulting from isolation:
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. |
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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:
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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:
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. |
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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. Solution: 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. Note: some
dual mains systems have the two inputs supplied by sources with a commonly
derived neutral and these don’t require a transformer. |
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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. Solution: An
isolation transformer located in either the rectifier supply, the UPS module
output, or the bypass is needed to prevent RCD activation. Note: 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. |
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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. |
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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.
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Subject Of Pervious
Article |
Article |
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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 |
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5- Physical Size/capacity, 6- Form factor/ configurations: 6.1- “N” System
Configuration |
Classification and Types of UPS – Part Two |
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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 |
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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 |
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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 |
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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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