Classification and Types of UPS – Part Eight ~ Electrical Knowhow

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) and today we will explain Transformer arrangements in practical UPS systems related to the last classification “Use of transformers” using the papers from Schneider Electric – Data Center Science Center.

Transformer Arrangements in Practical UPS Systems

As stated in previous article “Classification and Types of UPS – Part Seven”, 92 different arrangements of transformers are possible with a single UPS, even before redundancy configurations are considered.

Instead of attempting to describe and compare each of these arrangements, we will describe the functions and benefits of the transformers in the three basic configurations:

Single mains,
Dual mains, and
Single mains without bypass.

From this, the preferred arrangements of transformers will be deduced and described.

1- Transformer options for the “single mains” configuration

The single mains system is the most common UPS configuration, used in over 50% of all UPS installations. In smaller systems (below 100kW), it is even more dominant, making up approximately 90% of all installations.

The advantage of this system is that it is relatively simple to design and install, yet it provides a bypass that gives a variety of redundancy and maintenance benefits.

Small UPS systems below 10 kW typically only have a single input and must be used in the single mains configuration.

UPS systems over 10 kW usually provide for dual mains input but are converted to single mains by simply connecting the two mains inputs together. The majority of UPS systems shipped with dual mains input connections actually end up configured for single mains input.

In the single mains configuration, transformers can be located in three positions as shown in Fig-1. Any combination of these transformers, including none or all three, might exist in a UPS installation.

Fig-1 Possible transformer locations in the “single mains” UPS configuration

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 three SINGLE MAINS transformer locations

Transformer

location

Functions

provided

When needed

Local / remote

issues

Mains

• Change mains voltage

• Change grounding system to TN-S

• Isolate UPS and the output from poor quality mains neutral

Mains voltage must be changed to match the UPS voltage

• Mains grounding system not compatible with the load AND there is no output transformer

• If transformer is local to the UPS, a high quality TN-S grounding system is

created at the UPS output

• Locating this transformer remotely from the UPS requires that the TN-S grounding system is created at the

transformer location, which is not ideal

Inverter

• This transformer provides no functions in the single mains

configuration, but may be present due to the design of the UPS

Never needed in this configuration

• On older UPSs, this transformer may be

integral to the UPS design and cannot

be removed, but it is not providing any

useful function

Output

Change the UPS output voltage

• Change the grounding system to TN-S, if the UPS is

not using TN-S

• Improve the quality of the TNS grounding system if the input transformer is not

present or is remotely located

• UPS voltage must be changed to match the IT load voltage

• UPS grounding system is not

compatible with the IT load AND there is no input transformer

• Input grounding system is compatible

with the IT load but the quality of the

neutral circuit is poor OR the input

transformer, if present, is remotely

located

• If the IT loads are far from the UPS,

locating the transformer remotely

(closer to the IT loads) has the

advantage of creating the TN-S ground near the IT load, improving the quality of the grounding system

Notes to Table-1:

The following observations can be made from a study of the functions of the transformers in the single mains UPS configuration:

If voltages must be changed, transformers must be installed. For example, if the mains voltage is 600 V, the UPS voltage is 480 V, and the IT load voltage is 208 V, then both input and output transformers are required. This is a fundamental requirement and is the first issue to be considered in determining where to put a transformer.

If the mains and/or UPS ground system are not TN-S, then transformers must be installed. For example, if the mains grounding system is TT, and UPS grounding system is IT-HRG, then both input and output transformers must be installed. This is a fundamental requirement, but some aspects of this problem are under the control of the system designer. For example, a designer could ensure that a high quality TN-S grounding system is provided at the data center, so no ground system changes are required.

The inverter transformer provides no function or benefit to the single mains UPS system. Since the presence of the inverter transformer defines the difference between a transformer-based UPS and a transformer-less UPS, it follows that

There is no advantage to using a transformer-based UPS in the single mains configuration.

There are 8 possible transformer arrangements in a single mains UPS configuration. Tables -2a to 2c below summarize the preferred transformer options for the single mains UPS configuration, with guidance as to where each approach is used.

Table-2a SINGLE MAINS preferred transformer option 1: Local input transformer
SINGLE MAINS Local input transformer

Application: Small data centers where the mains ground system is poor or not TN-S, or the mains voltage is not the same as the IT load voltage
Features	Notes
Mains ground systems: TN-S, TT, IT-HRG, TN-C Input voltages: UPS and load voltage must match Power range: Up to 1MW, but limited in North America to less than 100kVA due to mains and UPS being 480V or 600V above this power rating Efficiency: High IT load neutral bond: Established at the UPS output	• If the input transformer is local to the UPS, then its neutral connection is not connected to ground at the transformer, but brought to the UPS where neutral is connected to ground at the UPS output (preferred method) • If the input transformer is remote from the UPS, then the neutral may be required to be connected to ground at the transformer, and the UPS neutral must be connected to this neutral and cannot be connected to ground at the UPS

Table-2b SINGLE MAINS transformer option 2: No transformers
SINGLE MAINS No transformers

Application: Small data centers with high quality TN-S mains ground system, where mains and UPS and IT load voltage are all the same (400/230V, 380/220V, 208/120V, or 200/100V)
Features	Notes
Mains ground systems: TN-S only Input voltages: Same as IT load Power range: Up to 1MW, but limited in North America to less than 100kVA due to mains and UPS being 480V or 600V above this power rating Efficiency: Highest IT load neutral bond: Established at the TN-S source	• The use of a UPS with a built-in inverter transformer offers no benefits over a transformer-less UPS in this arrangement, because the output neutral must be derived from the TN-S source due to presence of the bypass

Table-2c SINGLE MAINS transformer option 3: Remote output transformer(s)
SINGLE MAINS Remote output transformer(s)

Application: Large data centers
Features	Notes
Mains ground systems: TN-S, TT, IT-HRG, TN-C Input voltages: Mains and UPS voltage must match Power range: Any, but not the most cost effective alternative in smaller data centers Efficiency: High, but lower if multiple output transformers are used IT load neutral bond: Established at the output transformer	• The UPS must be rated to operate with the specified mains ground system. Not all UPS can operate with an IT-HRG ground system, and some UPS require an input neutral connection • There is no neutral connection between the UPS and the output transformer(s) in this system • The output transformer may be a single transformer or its function may be divided among many PDU transformers

Another option for large data centers is to add an input transformer to the “single mains with remote output transformer” arrangement, to allow for use with very poor mains grounding systems or a mains supply with an incompatible grounding system, but this would be an uncommon situation.

The single mains configuration has fewer options and is simpler to design for and deploy than the dual mains system described in the next section. In practice, this means there are fewer design and installation mistakes made. This configuration will remain the most common, and should be considered for both large and small data centers.

Transformer Options for the “Dual Mains” Configuration

The dual mains configuration is used in many, but not all, larger installations. Fig-2 shows four options for transformer location in a dual mains system

Fig-2 Possible transformer locations for the DUAL MAINS configuration

There are two core reasons that the dual mains system is used:

The first reason:

The dual mains approach must be used when the system architecture prescribes that the rectifier and the bypass supplies come from different sources – for example, separate utility substations or in highly specialized redundancy architectures. This requirement is actually quite uncommon and is restricted to architectures designed for extreme redundancy. Note that in most systems with alternate supplies, such as diesel generators or secondary utility mains supplies, an automated transfer switch is provided upstream of the UPS because there are other loads in addition to the UPS – such as cooling plants – that must be backed up.

The second reason:

The dual mains design is used is to allow for concurrent maintenance of the distribution wiring and breakers feeding the UPS. If the breakers and wiring supplying the two mains inputs are separate, either can be shut down without shutting down the critical load, which will be powered from the remaining path during the maintenance. While this is a common reason for specifying this arrangement, there are alternative ways to allow for concurrent maintenance without using this approach, such as a dual power path architecture or concurrently maintainable parallel breakers.

Inverter transformer, if present, is always integral to the UPS, but the rectifier, bypass, and output transformers can be either located at the UPS or remotely located. Table-3 describes the function of each transformer, when it is needed, and issues related to its physical location (local vs. remote).

Table-3 Characteristics of the four DUAL MAINS transformer locations

Transformer

location

Functions

provided

When needed

Local / remote

issues

Bypass

Isolates the bypass neutral from currents generated by the inverter

• Changes bypass mains voltage

• Changes bypass grounding system to TN-S

• Isolates bypass from poor quality

neutral

• Protects the UPS from open neutral conditions caused by

upstream transfer switches or breakers

• The bypass voltage must be changed

to match the UPS voltage

• The bypass grounding system is not compatible with the load AND there is

no output transformer

• RCD protection is used on the mains

supplies AND neither a rectifier or local inverter transformer is present

• 4-pole breakers are used on the supply to the bypass (open neutral can occur)

• The output neutral of the UPS is always

the same as the bypass neutral, unless there is an output transformer present

• If this transformer is local to the UPS,

and either a rectifier or inverter

transformer is provided, or the rectifier is capable of operating without an input neutral connection, a high quality TN-S grounding system is created at the UPS

output

• Locating this transformer remotely from the UPS requires that the TN-S grounding system is created at the

transformer location, which is not ideal

Rectifier

• Change rectifier mains voltage

• Isolates the UPS input from an

incompatible mains grounding system

• Mains to UPS voltage change is

required

• This transformer does not completely

protect the UPS from open neutral conditions caused by upstream transfer

switches because the UPS neutral normally is derived from the bypass input

Inverter

• Isolates the bypass neutral from currents generated by the inverter

• The bypass is from a separately derived source and no rectifier transformer is present

• This transformer provides the same

neutral isolation as a rectifier transformer,

but cannot perform a voltage change

• The load power always flows through this transformer, so isolation between mains inputs is better provided in the bypass path since it increases system efficiency

Output

Change the UPS output voltage

• Change the grounding system to

TN-S, if the UPS is not using TN-S

• Improve the quality of the TN-S grounding system if the input transformer is not present or is remotely located

The UPS voltage must be changed to

match the IT load voltage

• The UPS grounding system is not compatible with the IT load AND there is no input transformer

• The input grounding system is

compatible with the IT load but the quality of the neutral circuit is poor OR the input transformer, if present, is remotely located

• If the IT loads are far from the UPS,

locating the transformer remotely from the UPS and closer to the IT loads, has the advantage of creating the TN-S

ground near the IT load, improving the

quality of the grounding system

Notes to table-3:

If voltages must be changed, transformers must be installed. For example, if the mains voltage is 600 V, the UPS voltage is 480 V, and the IT load voltage is 208 V, then all three – bypass, rectifier, and output transformers – are required. This is a fundamental requirement and is the first issue to be considered in determining where to put a transformer.

If the mains and/or UPS ground system are not TN-S, then transformers must be installed. For example, if the mains grounding systems is TT, and UPS grounding system is IT-HRG, then all three – bypass, rectifier, and output transformers – must be installed. This is a fundamental requirement, but some aspects of this problem are under the control of the system designer. For example, an architect could ensure that a high quality TN-S grounding system is provided at the data center, so no ground system changes are required.

The inverter transformer provides the function of breaking the neutral connection between the rectifier and bypass inputs, but this function is also provided by either a rectifier or bypass transformer. The inverter transformer function is redundant if either a rectifier or bypass transformer is installed for another purpose. Since the presence of the inverter transformer defines the difference between a transformer-based UPS and a transformer-less UPS, it follows that:

In many cases, there is no advantage to using a transformer-based UPS in the dual mains configuration.

There are 16 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 56. Tables-4a to 4d below summarize the preferred transformer options for the dual mains UPS system, with guidance as to where each approach is used.

Table-4a DUAL MAINS transformer option 1: Local bypass transformer and remote transformer(s)
DUAL MAINS Local bypass transformer Remote output transformer(s)

Application: Large data centers
Features	Notes
Mains ground systems: TN-S, TT, IT-HRG, TN-C, inverter and bypass grounding systems can be independent Input voltages: Mains and UPS voltage must match Power range: Any, but impractical below 100kW Efficiency: High, but lower if multiple output transformers are used IT load neutral bond: Established at the output transformer	• Equivalent functionality can be provided by moving the bypass transformer to either the rectifier or inverter locations but efficiency is degraded • The UPS must be rated to operate with the specified mains ground system. Not all UPS can operate with an IT-HRG ground system, and some UPS require an input neutral connection • There is no neutral connection between the UPS and the output transformer(s) in this system. • The output transformer may be a single transformer or its function may be divided among many PDU transformers

Table 4a DUAL MAINS transformer option 1: Local bypass transformer and remote transformer(s)
DUAL MAINS Local bypass transformer Rectifier transformer Remote output transformer(s)

Application: Large data centers with poor mains ground system, different mains and UPS grounding systems, or different mains and UPS voltage
Features	Notes
Mains ground systems: TN-S, TT, IT-HRG, TN-C, inverter, bypass, and UPS grounding systems can be independent Input voltages: Any combination Power range: Any, but impractical below 100kW Efficiency: Lowest, and even lower if multiple output transformers are used IT load neutral bond: Established at the output transformer	• There is no neutral connection between the UPS and the output transformer(s) in this system • The output transformer may be a single transformer or its function may be divided among many PDU transformers • A transformer-based UPS with a transformer in the inverter location provides no additional functionality and degrades efficiency.

Table-4c DUAL MAINS transformer option 3: No transformers
DUAL MAINS No transformers

Application: Small data centers with high quality TN-S mains ground system, where mains and UPS and IT load voltage are all the same (400/230V, 380/220V, 208/120V, or 200/100V) and RCD is not required
Features	Notes
Mains ground systems: TN-S only, bypass and mains common Input voltages: Same as IT load Power range: Up to 1MW, but limited in North America to < 100kVA due to mains and UPS being 480V or 600V above this power rating Efficiency: Highest efficiency IT load neutral bond: Established at the TN-S source	Both the bypass and the rectifier must be TN-S and both neutrals must be derived from a common ground connection • RCD protection on the bypass and rectifier inputs cannot be used in this configuration, because the inverter will inject some current into the bypass neutral

Table 4d DUAL MAINS transformer option 4: Local bypass transformer
DUAL MAINS Local bypass transformer

Application: Small data centers with high quality TN-S mains ground system, where mains and UPS and IT load voltage are all the same (400/230V, 380/220V, 208/120V, or 200/100V) and RCD protection is required or the bypass is separately derived
Features	Notes
Mains ground systems: TN-S on rectifier input, bypass can be any type Input voltages: Mains, UPS and load voltage must match Power range: Up to 1MW, but limited in North America to < 100kVA due to mains and UPS being 480V or 600V above this power rating Efficiency: High IT load neutral bond: Established at the bypass transformer	This method is used if the bypass and inverter supplies are separately derived (separate neutral bonding connections), or RCD protection is used • The transformer could have been placed in the rectifier or inverter location to achieve RCD compatibility, but this would reduce the UPS mode efficiency • If the input transformer is remote from the UPS, then the neutral may be required to be connected to ground at the transformer, and the UPS neutral must be connected to this neutral and cannot be connected to ground at the UPS

Note that none of the preferred arrangements have a transformer in the inverter location. Most transformer-based UPSs have the transformer in the inverter location, yet this is the least useful place to have a transformer. Therefore that the historical transformer-based UPS is obsolete because its transformer is in the wrong location. The better solution is to use a UPS where the location of the transformer (if any are needed) can be established in the optimal location during the system design.

The most important characteristic of the dual mains design is that there are many options and the interactions between the options and other equipment such as RCD and circuit breakers is quite complex.

In the next Article, I will explain Transformer options for “single mains without bypass”. So, please keep following.

Subject Of Pervious Article

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,