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| UPSonNet NewsLetter June 2011 |
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Editorial Topic by Meir Portnoy: Effect of Blade Servers and Other Leading Power
Factor Loads on UPS Operation in Data Centers.
Latest green trend changing Data Center environment is evidenced mainly by introduction of new
computers, such as Blade Servers, which offer higher efficiency, lower PUE figure, and smaller
foot print. To gain the merits which these new computers and Blade servers offer, suitable UPS
systems designed to work with Active and Leading Power Factor loads are required.
In order to see how the land lies, work colleagues and I have visited during the last week the
premises of a medium sized ISP site comprising about 2000 servers. We were accepted by the
Company's facility manager, who is also in charge of data center power equipment including UPS
systems and data center's conditioning.
This ISP has several server locations; generally each equipped with two 300kVA UPS systems.
Each individual UPS powers half of total installed local servers, supplying continuously up to
120kVA (40% of rated UPS load). In case of malfunction in one of the UPS systems a common
transfer switch connects its load to the healthy UPS.
To eliminate the possibility of common point failure, 2N (1+1) architecture is implemented. Each
UPS has two separate power feeders, from main building transformer to its Rectifier and Static
Bypass inputs. Separate battery banks, one per each UPS, are located in an adjacent battery room.
Most UPS's are ten years old Double Conversion systems; working with total AC to AC efficiency
of about 85%.
Since UPS installation, most computers were gradually replaced with new Blade Servers, loading
the UPS systems, as indicated on the UPS display panel, with a leading 0.95 power factor load.
These figure however, as well as other figures displayed on the UPS front panel, seem dubious,
because the displayed input power was about 30% lower than the output power.
The facility manager wasn't sure about the reason for violation of the energy conservation
principle; his estimation was that the false power readings may stem from improper interpretation
of measured data by a UPS designed to work with lagging PF load, when connected to a leading
PF load. As most servers were replaced before his nomination, he couldn't opine on any additional
effects, caused by replacing the old servers with Blade type.
This example illustrates the advancement of UPS technology over the last decade, as well as the
need to reconsider the ability of existing installation to meet new data center needs. UPS
manufacturers made a giant step concerning efficiency, and are now able to offer efficiencies of
94% to 96% in double conversion mode, which may be enhanced to 98% as described in our
previous newsletter titled : The Hunt for More Efficient UPS Solutions, but most still fail to keep up
with the need to offer a solution for loads with leading power factor.
A decade ago most computer servers were designed to comply with IEC 61000-3-2 standard titled
" Limits for harmonic current emissions of equipment input current ≤ 16 A per phase", first
published on 1995. Power Supplies of most servers until that time converted the AC mains input
to the DC needed to drive server's circuits, by means of a simple diode rectifier with capacitor
filter, which drew a pulsed current from the utility generating harmonics beyond the said
standard's requirements.
Meeting the IEC standard was generally accomplished by installing in the server a simple low pass
LC filter between AC input line and power supply's rectifier bridge. The high impedance of filter's
choke in series with the line, and low impedance of the filter's shunt capacitor in parallel with the
diode bridge input, provided a return path for harmonics current through the capacitor, limiting the
input line harmonics to values allowed by the standard.
Such LC input filter in server's power supplies presents at UPS output a load which has a lagging
power factor characteristic. The lagging current is compensated by the leading current of UPS's
output capacitor, needed to filter out the voltage harmonics, which are generated in the process of
converting the DC buss voltage of the double conversion UPS to AC.
Thus, introduction of the LC filter caused the net Volt –Ampere loading on the UPS to decrease,
compared to a resistive load. UPS inverter with a resistive load has to supply the active power to
the load and the reactive power, drown by the current through the output capacitor; whereas an
inverter feeding the same resistive load with an LC input filter, supplies same active power but less
reactive power, because the reactive power of server's input inductance component is subtracted
from the reactive power of UPS output capacitor.
On May 2009 new Energy Star standard, titled " Power Factor Requirements for Computer Server
Power Supplies" became effective, requiring computer AC to DC servers to meet input power
factor rating of at least 0.8 at 20% load, 0.9 at 50% load and 0.95 at 100% load. Introduction of
power factor correction circuit was therefore required, which can best be implemented by means
of an Active Power Factor Correction (APFC) circuit.
The APFC circuit solves the requirements of both Energy Star Power Factor standard and IEC
harmonic limitation standard. Servers with a power supply equipped with APFC circuit have
almost unity input power factor and almost sinusoidal input current with total harmonics not
exceeding a few percent. In addition, mitigation of input harmonics increases the efficiency of the
server and reduces the upstream losses from mains or from UPS output.
Introduction of APFC circuit in the servers had several impacts on UPS technology.
Firstly, UPS Wattage rating, not the VA rating becomes the dominant factor. Most new three phase
UPS systems are therefore designed with almost unity output PF rating.
Secondly, servers with APFC circuits are designed to work with sinusoidal input voltage
waveform and may malfunction when connected to voltages supplied by non sinusoidal inverters
used in Standby UPS systems and in most Line Interactive systems. This subject was treated in
UPSonNet June 2010 newsletter titled " UPS Inverter Waveform & New Computer Servers".
Thirdly the introduction of server power supplies with APFC circuits' causes in certain cases a
leading input power factor at UPS output, which may effect UPS ability to work correctly, as
evidenced during our said visit in the Data Center premises.
A power supply with APFC draws sinusoidal input current from the mains achieved by modulating
the width of high frequency carrier pulses, with a sinusoidal waveform. These pulses, unless
filtered out, pollute the input power line with radio frequency harmonics. Electro-Magnetic
Compatibility ( EMC) consideration require practically adding a shunt filter capacitor of about 10uF
per kW at the input of APFC circuit, in order to limit the conducted radio frequency energy, which
power supplies inject into the input power lines, to the limits of EMI standards such as EN61204-
3, or equivalent FCC standards.
Thus, a 100kVA/100 kW UPS, which powers computer servers with APFC, supplies reactive
power to the equivalent of 1000uF capacitor, or 16.6 kVAR, in addition to the 100kW active load.
The capacitive load in this case is in parallel with UPS output capacitor, and the UPS is required to
supply 101.4 kVA load, (square root of the squared sums of active and reactive components), with
a leading power factor of 0.986 (100/101.4).
The main effect of the leading power factor, depends however not so much on the required power
which the UPS supplies but on the ratio of the reactive leading current to the active current which
the UPS inverter has to supply, in addition to the leading current of UPS's internal output
capacitance.
The situation becomes much more severe if the server's active power is decreased. The power
supply of a blade server for instance is designed for maximum load in the enclosure, when all
blades are populated. Practically, the amount of inserted computing blades is dictated by the
present requirements. Reducing blade server's power consumption to 50% of the load in the
described example, decreases servers input power factor to 0.95; but its main effect lies in
increased ratio of leading VA to Active Power, and leading current component to active current
component from 16.6% to 33.2%.
The increased capacitive current requirements may influence dramatically the operation of the
UPS, if it is not designed for leading PF loads, and may affect the following UPS design
considerations and parameters:
1. UPS Inverter might be required to supply currents beyond the designed values increasing its
probability of failure, and overload characteristics.
2. Leading power factor may affect UPS Control loop stability causing unstable, operation or
marginally stable operation evidenced at load transients.
3. Stability of parallel operation of UPS system with other systems may be affected due to loop
characteristics not considered during the design stages.
4. UPS ability to detect and isolate a failed UPS in a parallel redundant configuration may be
affected, eventually leading to total failure of all UPS parallel systems.
5. DC Buss capacitors may overheat and fail due to high AC current component.
6. Battery lifetime, if connected to the DC buss, may decrease due to high AC ripple current.
7.Measured and monitored power data may be affected providing false and misleading information,
affecting UPS ability to control its operations.
The described visit in the Data center facility indicates an additional problem associated with
implementation of blade servers in data centers. The blade servers were installed in an existing site
already equipped with UPS systems. It seems that nobody considered the ability of the UPS to
work with the leading power factor loads before changing the servers to blade type. The UPS
systems apparently succeeds to power the partial load without actuating alarms.
In addition, it reveals the need to reconsider the entire UPS system power protection strategy
whenever a load with a leading power factor is introduced. The redundancy offered by the 2N
configuration in the described site, doesn't solve problems caused by malfunction of a UPS due to
leading power factor load. Upon failure of one UPS the second UPS assumes the total power; the
ratio of reactive to active power doesn't change, but the increased Power on the healthy UPS
makes the situation more severe.
I welcome remarks. Please send to: meirp@upsonnet.com
UPSonNet Website News
Google uses a numerical value called Page Rang (PR) to decide page importance. On 27th June
2011, Google has updated its Page Rating. The new rating appears to be more content oriented.
Nobody outside Google seems to know the exact algorithms used by Google's to derive the PR
number; it is important to know however, that Google uses the PR rating to decide which pages to
present when a search term is used.
Until now the PR rating of our internal pages was always lower than the home page, which aims
mainly to direct customers to the relevant topics. The new algorithm rates certain internal pages
with higher PR rating than the home page, thus directing users to the relevant page. This may be of
interest to companies looking for relevant locations to advertise their products on UPSonNet
website. Interested parties are invited to read: " Why to Advertise on UPSonNet " and contact us.
UPS Industry News
Several News items briefed on UPSonNet News during June 2011 offer important information of
interest to users and industry participants:
A brief about Emerson's survey which deals with top concern of data center managers. The
survey generates responses from more than 158 data center, facility and IT managers across the
United States. It considers topics of interest to data center managers as well as to data center
vendors and service providers, such topics as availability ability to manage data center
infrastructure, heat, densities, energy efficiency, and more.
More and more cases are published, where Lithium Ion batteries are preferred over lead acid
batteries. These batteries seem to offer longer lifetime, increased backup time and environment
friendliness. A News brief in June presents Mega Watt installation project where Li Ion batteries
replacing Lead Acid; whereas a market report brief indicates that Lithium Ion batteries are gaining
share in the data center market.
Factor Loads on UPS Operation in Data Centers.
Latest green trend changing Data Center environment is evidenced mainly by introduction of new
computers, such as Blade Servers, which offer higher efficiency, lower PUE figure, and smaller
foot print. To gain the merits which these new computers and Blade servers offer, suitable UPS
systems designed to work with Active and Leading Power Factor loads are required.
In order to see how the land lies, work colleagues and I have visited during the last week the
premises of a medium sized ISP site comprising about 2000 servers. We were accepted by the
Company's facility manager, who is also in charge of data center power equipment including UPS
systems and data center's conditioning.
This ISP has several server locations; generally each equipped with two 300kVA UPS systems.
Each individual UPS powers half of total installed local servers, supplying continuously up to
120kVA (40% of rated UPS load). In case of malfunction in one of the UPS systems a common
transfer switch connects its load to the healthy UPS.
To eliminate the possibility of common point failure, 2N (1+1) architecture is implemented. Each
UPS has two separate power feeders, from main building transformer to its Rectifier and Static
Bypass inputs. Separate battery banks, one per each UPS, are located in an adjacent battery room.
Most UPS's are ten years old Double Conversion systems; working with total AC to AC efficiency
of about 85%.
Since UPS installation, most computers were gradually replaced with new Blade Servers, loading
the UPS systems, as indicated on the UPS display panel, with a leading 0.95 power factor load.
These figure however, as well as other figures displayed on the UPS front panel, seem dubious,
because the displayed input power was about 30% lower than the output power.
The facility manager wasn't sure about the reason for violation of the energy conservation
principle; his estimation was that the false power readings may stem from improper interpretation
of measured data by a UPS designed to work with lagging PF load, when connected to a leading
PF load. As most servers were replaced before his nomination, he couldn't opine on any additional
effects, caused by replacing the old servers with Blade type.
This example illustrates the advancement of UPS technology over the last decade, as well as the
need to reconsider the ability of existing installation to meet new data center needs. UPS
manufacturers made a giant step concerning efficiency, and are now able to offer efficiencies of
94% to 96% in double conversion mode, which may be enhanced to 98% as described in our
previous newsletter titled : The Hunt for More Efficient UPS Solutions, but most still fail to keep up
with the need to offer a solution for loads with leading power factor.
A decade ago most computer servers were designed to comply with IEC 61000-3-2 standard titled
" Limits for harmonic current emissions of equipment input current ≤ 16 A per phase", first
published on 1995. Power Supplies of most servers until that time converted the AC mains input
to the DC needed to drive server's circuits, by means of a simple diode rectifier with capacitor
filter, which drew a pulsed current from the utility generating harmonics beyond the said
standard's requirements.
Meeting the IEC standard was generally accomplished by installing in the server a simple low pass
LC filter between AC input line and power supply's rectifier bridge. The high impedance of filter's
choke in series with the line, and low impedance of the filter's shunt capacitor in parallel with the
diode bridge input, provided a return path for harmonics current through the capacitor, limiting the
input line harmonics to values allowed by the standard.
Such LC input filter in server's power supplies presents at UPS output a load which has a lagging
power factor characteristic. The lagging current is compensated by the leading current of UPS's
output capacitor, needed to filter out the voltage harmonics, which are generated in the process of
converting the DC buss voltage of the double conversion UPS to AC.
Thus, introduction of the LC filter caused the net Volt –Ampere loading on the UPS to decrease,
compared to a resistive load. UPS inverter with a resistive load has to supply the active power to
the load and the reactive power, drown by the current through the output capacitor; whereas an
inverter feeding the same resistive load with an LC input filter, supplies same active power but less
reactive power, because the reactive power of server's input inductance component is subtracted
from the reactive power of UPS output capacitor.
On May 2009 new Energy Star standard, titled " Power Factor Requirements for Computer Server
Power Supplies" became effective, requiring computer AC to DC servers to meet input power
factor rating of at least 0.8 at 20% load, 0.9 at 50% load and 0.95 at 100% load. Introduction of
power factor correction circuit was therefore required, which can best be implemented by means
of an Active Power Factor Correction (APFC) circuit.
The APFC circuit solves the requirements of both Energy Star Power Factor standard and IEC
harmonic limitation standard. Servers with a power supply equipped with APFC circuit have
almost unity input power factor and almost sinusoidal input current with total harmonics not
exceeding a few percent. In addition, mitigation of input harmonics increases the efficiency of the
server and reduces the upstream losses from mains or from UPS output.
Introduction of APFC circuit in the servers had several impacts on UPS technology.
Firstly, UPS Wattage rating, not the VA rating becomes the dominant factor. Most new three phase
UPS systems are therefore designed with almost unity output PF rating.
Secondly, servers with APFC circuits are designed to work with sinusoidal input voltage
waveform and may malfunction when connected to voltages supplied by non sinusoidal inverters
used in Standby UPS systems and in most Line Interactive systems. This subject was treated in
UPSonNet June 2010 newsletter titled " UPS Inverter Waveform & New Computer Servers".
Thirdly the introduction of server power supplies with APFC circuits' causes in certain cases a
leading input power factor at UPS output, which may effect UPS ability to work correctly, as
evidenced during our said visit in the Data Center premises.
A power supply with APFC draws sinusoidal input current from the mains achieved by modulating
the width of high frequency carrier pulses, with a sinusoidal waveform. These pulses, unless
filtered out, pollute the input power line with radio frequency harmonics. Electro-Magnetic
Compatibility ( EMC) consideration require practically adding a shunt filter capacitor of about 10uF
per kW at the input of APFC circuit, in order to limit the conducted radio frequency energy, which
power supplies inject into the input power lines, to the limits of EMI standards such as EN61204-
3, or equivalent FCC standards.
Thus, a 100kVA/100 kW UPS, which powers computer servers with APFC, supplies reactive
power to the equivalent of 1000uF capacitor, or 16.6 kVAR, in addition to the 100kW active load.
The capacitive load in this case is in parallel with UPS output capacitor, and the UPS is required to
supply 101.4 kVA load, (square root of the squared sums of active and reactive components), with
a leading power factor of 0.986 (100/101.4).
The main effect of the leading power factor, depends however not so much on the required power
which the UPS supplies but on the ratio of the reactive leading current to the active current which
the UPS inverter has to supply, in addition to the leading current of UPS's internal output
capacitance.
The situation becomes much more severe if the server's active power is decreased. The power
supply of a blade server for instance is designed for maximum load in the enclosure, when all
blades are populated. Practically, the amount of inserted computing blades is dictated by the
present requirements. Reducing blade server's power consumption to 50% of the load in the
described example, decreases servers input power factor to 0.95; but its main effect lies in
increased ratio of leading VA to Active Power, and leading current component to active current
component from 16.6% to 33.2%.
The increased capacitive current requirements may influence dramatically the operation of the
UPS, if it is not designed for leading PF loads, and may affect the following UPS design
considerations and parameters:
1. UPS Inverter might be required to supply currents beyond the designed values increasing its
probability of failure, and overload characteristics.
2. Leading power factor may affect UPS Control loop stability causing unstable, operation or
marginally stable operation evidenced at load transients.
3. Stability of parallel operation of UPS system with other systems may be affected due to loop
characteristics not considered during the design stages.
4. UPS ability to detect and isolate a failed UPS in a parallel redundant configuration may be
affected, eventually leading to total failure of all UPS parallel systems.
5. DC Buss capacitors may overheat and fail due to high AC current component.
6. Battery lifetime, if connected to the DC buss, may decrease due to high AC ripple current.
7.Measured and monitored power data may be affected providing false and misleading information,
affecting UPS ability to control its operations.
The described visit in the Data center facility indicates an additional problem associated with
implementation of blade servers in data centers. The blade servers were installed in an existing site
already equipped with UPS systems. It seems that nobody considered the ability of the UPS to
work with the leading power factor loads before changing the servers to blade type. The UPS
systems apparently succeeds to power the partial load without actuating alarms.
In addition, it reveals the need to reconsider the entire UPS system power protection strategy
whenever a load with a leading power factor is introduced. The redundancy offered by the 2N
configuration in the described site, doesn't solve problems caused by malfunction of a UPS due to
leading power factor load. Upon failure of one UPS the second UPS assumes the total power; the
ratio of reactive to active power doesn't change, but the increased Power on the healthy UPS
makes the situation more severe.
I welcome remarks. Please send to: meirp@upsonnet.com
UPSonNet Website News
Google uses a numerical value called Page Rang (PR) to decide page importance. On 27th June
2011, Google has updated its Page Rating. The new rating appears to be more content oriented.
Nobody outside Google seems to know the exact algorithms used by Google's to derive the PR
number; it is important to know however, that Google uses the PR rating to decide which pages to
present when a search term is used.
Until now the PR rating of our internal pages was always lower than the home page, which aims
mainly to direct customers to the relevant topics. The new algorithm rates certain internal pages
with higher PR rating than the home page, thus directing users to the relevant page. This may be of
interest to companies looking for relevant locations to advertise their products on UPSonNet
website. Interested parties are invited to read: " Why to Advertise on UPSonNet " and contact us.
UPS Industry News
Several News items briefed on UPSonNet News during June 2011 offer important information of
interest to users and industry participants:
A brief about Emerson's survey which deals with top concern of data center managers. The
survey generates responses from more than 158 data center, facility and IT managers across the
United States. It considers topics of interest to data center managers as well as to data center
vendors and service providers, such topics as availability ability to manage data center
infrastructure, heat, densities, energy efficiency, and more.
More and more cases are published, where Lithium Ion batteries are preferred over lead acid
batteries. These batteries seem to offer longer lifetime, increased backup time and environment
friendliness. A News brief in June presents Mega Watt installation project where Li Ion batteries
replacing Lead Acid; whereas a market report brief indicates that Lithium Ion batteries are gaining
share in the data center market.
UPSonNet
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| Spread Your Word On the Globe |
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UPSonNet
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| UPS operation with Blade Server Loads |
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