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2 UPS METERING, POWER MODULE LEVEL

2 UPS METERING, POWER MODULE LEVEL

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©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Uninterruptible Power Supply (UPS)



must be measured and power calculated manually or derived from

upstream switchboards. The units must communicate their status either

visually or via contact closures. The estimated measurement accuracy of

this method is 3% to 5%.

13.2.2



Best Practical Measurement



The best practical measurement includes a local display of voltage,

frequency, and current measurement for both input and output.

Calculated output values such as apparent load (kVA), real load (kW),

crest factor, and percent load might be available at the module level, but

such values are more common at the system level. Input power factor

and harmonic current may be available. Communication is usually direct

to the system controls, via open source or proprietary systems to a

system management network for manual control. Data can be accessed

via LAN or WAN. The estimated measurement accuracy of this method

is 2% to 4%.

13.2.3



State-of-the-Art Measurement



The state-of-the-art measurement involves all of the features of the

best practical measurement, as well as the ability to integrate with facility

management systems for automatic control of the UPS operation. A

graphical user interface (GUI) display of power flow and operating status

may be included on each module. Remote monitoring capabilities allow

remote manual module operation via system-level controls. Power

modules can be automatically added-to or deleted-from the parallel bus

to achieve optimum performance. The estimated measurement accuracy

of this method is 1% to 3%.

13.3

13.3.1



UPS METERING, SYSTEM LEVEL

Overview



The UPS ―system‖ includes all of the distribution switchgear,

paralleling switchgear, static bypass, maintenance bypass, and possibly

output distribution (See Figure 13.3). The primary consideration is for

the output side of the UPS system (i.e., what is being delivered to the IT

169



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



loads). For multi-module UPS systems, data points are usually measured

at the paralleling switchgear or output distribution switchboard.

Calculated values may be accomplished at the power equipment itself or

calculations might take place at a remote intelligent monitor.



Figure 13.3 – UPS ―System‖ Within

Power Distribution Scheme



170



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Uninterruptible Power Supply (UPS)



13.3.2



Minimum Practical Measurement



The minimum practical measurement typically provides output

voltage phase-to-phase (and phase-to neutral where applicable), current

per phase, and status indicators. Control panels display power flow on a

mimic buss. Under this scheme, power may have to be calculated

manually. The units must communicate their status either visually or via

contact closures. The estimated measurement accuracy of this method is

3% to 5%.

13.3.3



Best Practical Measurement



The best practical measurement includes local and remote display of

voltage phase-to-phase (and phase-to-neutral where applicable),

frequency, current each phase (and neutral where applicable). Calculated

values include output apparent load (kVA), real load (kW), power factor,

crest factor, and percent load. A GUI mimic display shows the operating

condition of all system elements including power modules. Alarms and

thresholds are programmable, both locally and remotely.

Communication may be via open source or proprietary systems to a

system management network, and can be in user-selected language. Data

can be accessed via LAN or WAN. Protocols include HTTP, HTTPS,

SMS, SMTP, SSL, and TCP/IP. Typical interfaces are RJ-45 10 / 100 /

1000 Base-T. The estimated measurement accuracy of this method is 2%

to 4%.

13.3.4



State-of-the-Art Measurement



The state-of-the-art measurement involves all of the features of the

best practical measurement, as well as the ability to integrate with facility

management systems. Control capabilities allow remote automatic or

manual operation of power modules, system controls, and circuit

breakers or switches. Power modules can be automatically added or shut

down to achieve optimum performance. The estimated measurement

accuracy of this method is 1% to 3%.



171



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



14.1



OVERVIEW



Transformers can be used throughout the critical power path. For

purposes of this section, transformers integrated into other equipment are

not discussed. For instance, UPS systems usually contain one or more

transformers, but they are not metered separately; instead, they are

factored into the overall performance of the UPS. This chapter looks

specifically at three-phase transformers introduced into the system as

stand-alone devices, or as Power Distribution Units (PDU) that include

distribution with the transformers. From an efficiency and first-cost

perspective, transformers should be eliminated whenever possible.

However, there may be reasons of cable installation costs or safety for

keeping transformers in the system. One way to eliminate transformers

may be to match the IT input voltage to the UPS output voltage (for

example, 240 volt UPS output and 240 volt server input).

While it is intended that this chapter be as standalone as possible, it

is also recommended that the reader review Chapters 1 – 4 for supporting

material.

14.2



STAND-ALONE TRANSFORMERS



Transformers are almost always used to convert from one voltage to

another, although this is not a 100% rule. Auto-transformers, used

exclusively for voltage conversion, are small and highly efficient, but

they lack any power conditioning. Transformers can also be used for

power conditioning. The most common is a Delta-Wye isolation

transformer (three-phase input and three-phase plus neutral output).

Utility and off-the-shelf supplied distribution transformers will often



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



exhibit maximum efficiency at loads between 40% and 60% of their

kVA rating; their efficiency will drop gradually when loaded above 60%

of their rated load, and will drop significantly when loaded below 40% of

their rated load.

Table 14.1 gives an overview of the minimum practical, best

practical, and state-of-the-art measurement levels.



14.2.1



Minimum Practical Measurement



The minimum practical measurement for stand-alone transformers,

which covers the great majority of transformers, typically involves no

metering at all. Most stand-alone transformers rely on handheld

instruments, or upstream and downstream devices external to the

transformer for metering.



174



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Computer Room Transformer & Power Distribution Unit (PDU)



Figure 14.1 - Typical Transformer

With No Metering (Side Panel

Removed)



14.2.2



Best Practical Measurement



The best practical measurement involves measuring voltage and

current, typically by a customized meter for the particular transformer.

Estimated accuracy for these measurements is between 2% and 4%.

14.2.3



State-of-the-Art Measurement



The state-of-the-art measurement involves measuring input voltage

from phase to phase, output voltage from phase to phase and phase to

neutral, and current for each phase. Calculated values can include Kfactor, total apparent load (kVA), total real load (kW), power factor, and

transformer temperature.

Two-stage over-temperature alarm and

contacts may be utilized to communicate the unit’s status via network

communications. Estimated accuracy for these measurements is between

1% and 2%.

14.3



COMPUTER ROOM

UNITS (PDU)



POWER



DISTRIBUTION



A PDU is a cabinet used to distribute power within a computer room.

Connection to the PDU usually marks the demarcation between facility

wiring and the IT equipment. The PDU contains one or more power

distribution panels, in which circuit breakers connect via interconnect

175



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



cables to equipment racks, cabinets, or freestanding IT devices. PDUs

can be stand-alone devices or they can be integrated into a distributed

UPS system. PDUs come in two versions: with or without a transformer.

Figure 14.2 shows a floor-mounted PDU with a transformer and

distribution panelboards.

PDU WITH TRANSFORMER - The most common use of a power

distribution unit is to step down voltage from a centralized UPS system

to IT equipment utilization voltage (e.g., 480 volt input to 208 / 120 volt

output). The transformer is usually an isolation transformer, but it can

sometimes be designed for additional power quality features such as

creation of a common ground, creation of a separately derived neutral,

and harmonic cancellation. Unlike some power supplies, today’s

transformers are typically most efficient at around 50% load.

PDU WITHOUT TRANSFORMER - In distributed UPS

applications it is common for the PDU to have no transformer, in which

case it might depend upon upstream fuses or circuit breakers for overload

protection. Metering may be reduced or totally absent, relying instead

upon upstream monitoring (e.g., at the UPS).

As described above, the main components of a PDU are a

transformer and one or more distribution panelboards. They can range in

rating from 15 kVA to over 300 kVA and are most commonly used to

step down voltage and distribute power in the computer room when fed

from centralized UPS systems located in separate equipment rooms. A

typical application is for a branch circuit to connect to a rack-mounted

PDU (RPDU) device serving an equipment rack. PDU controls can

operate one or more panelboards or, in some cases, individual branch

circuit breakers. More than one PDU can be supported by a single UPS

system. PDU control panels are the prime source of information about

power in the computer room. Figure 14.3 shows the power flow through

a basic PDU with transformer.



176



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Computer Room Transformer & Power Distribution Unit (PDU)



Figure 14.2 - Floor-Mounted Power

Distribution Unit (PDU)



Figure 14.3 - Power Flow Through A

Power Distribution Unit



177



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Real-Time Energy Consumption Measurements in Data Centers



A variation of a PDU is a remote power panel (RPP). This is usually

nothing more than one or more distribution panel boards in an enclosure

that is sub-fed from an upstream PDU in order to add more branch

circuits closer to the load(s). RPPs usually have no additional

monitoring, but could be a point of measurement with a clamp-on meter.

Table 14.2 gives an overview of the minimum practical, best

practical, and state-of-the-art measurement levels of a PDU.



14.3.1



Minimum Practical Measurement



The minimum practical measurement will include analog input

voltage from phase to phase, output voltage from phase to phase and

from phase to neutral, output current for each phase, and total apparent

178



©2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org).

For personal use only. Additional reproduction, distribution, or transmission in either print or

digital form is not permitted without ASHRAE's prior written permission.



Computer Room Transformer & Power Distribution Unit (PDU)



load (kVA). Two-stage over-temperature alarm and contacts may be

utilized to communicate the unit’s status via contact closures and RS232

ports. Estimated accuracy for these measurements is between 2% and

5%.

14.3.2



Best Practical Measurement



The best practical measurement involves measuring true RMS input

voltage from phase to phase, input phase rotation, output voltage from

phase to phase and phase to neutral, frequency, output voltage total

harmonic distortion (THD), output current for each phase, K-factor, total

apparent load (kVA), total real load (kW), energy consumption (kWh),

power factor, percent load, and transformer temperature. Out of

threshold alarms may be communicated via network communications.

Estimated accuracy for these measurements is between 1% and 2%.

14.3.3



State-of-the-Art Measurement



The state-of-the-art measurement involves all of the features of the

best practical measurement, as well as the current measurement and on /

off control per output branch circuit. Network communications will

enable the use of a facility management interface. Data can be accessed

via LAN or WAN. Protocols include HTTP, HTTPS, SMS, SMTP, SSL,

and TCP / IP. Typical interfaces are RJ-45 10 / 100 / 1000 Base-T. The

estimated accuracy of these measurements is 1%.

14.4



RACK-MOUNTED POWER DISTRIBUTION UNIT

(RPDU)



An RPDU is a device typically mounted in an equipment rack or

cabinet into which IT equipment connects (usually by an equipment

power plug). There can be several RPDU devices mounted in a cabinet

and each is usually powered from a different upstream UPS or alternate

power source. Instrumentation in an RPDU can range from no metering

at all to limited metering to metering with limited controls. Current per

phase, when available, enables a technician to load balance within a rack.

An RPDU can interface with environmental monitoring devices to give a

total picture of the power, temperature and humidity in a single

equipment rack or cabinet. Most RPDU current sensors are low cost

179



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