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). In addition, in the last classification “Use of transformers”, we already described the functions and benefits of the transformers in the first two basic configurations (Single mains and Dual mains).
Today we will continue describing
the functions and benefits of the transformers in the third basic configuration
(Single mains without bypass) using the papers
from Schneider Electric – Data Center Science Center.
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3- Transformer
options for “single mains without bypass” |
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The single mains without bypass
configuration is commonly used when:
The diagram of the “without
bypass” configuration, with the possible transformer locations, is shown in Fig-1.  Fig-1
Possible transformer locations in the Single Mains Without Bypass configuration
When the mains power is of very poor quality
or the grounding system is unstable, it can be undesirable to ever expose the
IT load to the mains via a bypass, so no bypass is provided in this system. This configuration is almost exclusively
used in developing nations, where poor power can be quite common. In
developed countries with stable power grids, the bypass is a valuable feature and either the single mains
or the dual mains systems predominate; the “without bypass” configuration is
quite rare. An inspection of the diagrams shows that a single mains configuration with bypass appears to become equivalent to the without bypass diagram if the bypass function is disabled. It is commonly assumed that a system can be
installed with a bypass and then toggled back and forth between the “with”
and “without” bypass configurations. This mode of installation is only viable if
either an input or an output transformer is present. The reason is that if
the bypass is wired, the neutral of the inverter output transformer cannot be
connected to ground if the bypass neutral is already grounded. If the only
transformer present is the inverter transformer, and the output is separately
derived by connecting the inverter transformer neutral to ground, then the
bypass must not be wired and cannot be used. This limitation applies to mechanical
“wrap-around” bypass as well as to automatic “static” bypass. The inverter transformer, if present, is
always integral to the UPS, but the mains and output transformers can be
either located at the UPS or remotely located. Table-1 describes the function of each transformer, when it is
needed, and the issues related to its physical location (local vs. remote). Table -1
Characteristics of the different transformer locations for a SINGLE MAINS
WITHOUT BYPASS system
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There are 8 possible combinations of
transformer arrangements in this system. When remote vs. local location
options for the mains and output transformers are considered, the number of
options grows to 18. Table-2a and Table-2b summarize the preferred transformer options
for the single mains without bypass configuration, with guidance as to where
each approach is used. Table-2a SINGLE
MAINS WITHOUT BYPASS preferred option 1: Local input transformer and remote
output transformer(s)
Table-2b SINGLE
MAINS WITHOUT BYPASS preferred option 2: Local input transformer
The use of this configuration, single mains
without bypass, is common in countries with very poor power. However, in most
cases where it is used, the single mains configuration with a bypass would be
a better choice. The reason why many customers choose not to
use the bypass is because it potentially subjects the load to raw mains
power. This is true even with transformer-based UPS systems, because they
don’t isolate the bypass. This is a problem because in the older
transformer-based UPSs, the transformer is in the wrong location. If the transformer is moved to the input,
instead of at the inverter output, then it can protect both the UPS and the
bypass path (this is described in more detail in the next section). A major problem with the single mains without bypass configuration is that the load fault current is always limited to the UPS inverter output current, which is typically much lower than the mains available short-circuit current. This can cause difficulty clearing breakers downstream of the UPS, and can result in a total load drop when a downstream fault does not clear, or even if it does not clear quickly. In contrast, in a configuration with a bypass, output faults activate the bypass and the mains current is available to help clear output faults. When designers and users understand all the options, the use of the single mains without bypass configuration can be expected to decline in use. The single mains with bypass, when combined with an input transformer, is the same cost, weight, and efficiency but preserves the additional option of using the bypass, which can be beneficial. In general, there are common historical configurations using transformer-based UPSs that will be replaced by newer configurations. This will be described in the next section. |
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Transformer-based
UPS designs |
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Transformer-based UPSs have the
transformer after the output of the inverter and before the bypass connection
to the UPS output. This is the least useful place that a transformer can be
located in a UPS for these reasons:
It is important to understand
that the transformer exists in this location because it was required as part
of the UPS inverter circuits, and was not added to solve these other
problems. It does provide one key
function, of isolating the inverter from the output, but this function can
also be provided by transformers placed in other, and better, locations. Even
though transformer-based UPSs have the transformer in a sub-optimal location,
the transformer is part of the circuitry and not optional, so many early data
center designs took advantage of it. There are two main designs that
take advantage of the integral transformer in a transformer-based UPS, which
are summarized in Table-3a and Table-3b along with recommended
improved replacement designs. Table-3a
DUAL MAINS configuration with inverter transformer
It is important to recognize that in the
above situation the legacy inverter transformer was providing a function of
isolating the inverter from the bypass neutral. In this case it is better to
locate the transformer in the bypass path for the reasons provided. However,
as stated in the earlier sections describing the single mains and dual mains
recommended systems, not all installations require isolation between the
inverter and the bypass neutral. In designing a new facility, the recommended designs described in the earlier sections should be considered first; the examples here are only to show how a few popular designs should be improved. Table3b SINGLE
MAINS configuration with inverter transformer
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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 |
|
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:
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
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:
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Classification and Types of UPS – Part Six |
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Three Basic Configurations Of Mains And Bypass For A UPS System:
9-According to Use of transformers with the UPS
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Classification and Types of
UPS – Part Seven |
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Transformer Arrangements in Practical UPS Systems: 1-Transformer options for the “single mains” configuration. 2-Transformer Options for the “Dual Mains” Configuration. |
Classification and Types of UPS – Part Eight |
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