AC versus DC Power Distribution in the Data Center

AC versus DC Power Distribution in the Data Center November 2013/AT328 by Kristopher Jones Make the most of your energy SM Revision #0 0/00

Summary Introduction... p 3 Background... p 4 Efficiency... p 6 Reliability... p 8 Cost... p 11 Standards & Safety... p 13 Standards... p 14 Safety... p 15 Integration... p 18 Conclusion... p 19 References... p 20 White paper on data centers 2

Introduction Since their emergence as a critical internet-enabling tool, data centers have been highly dependent upon the AC (alternating current) electricity required by power intensive servers, cooling, and communications equipment. Since the majority of the end-use equipment in a data center uses various levels of DC (direct current) voltage, the discussion of AC versus DC for power distribution has garnered significant interest over the past ten years. One major stumbling block to effectively utilize DC power in the distribution world was the inefficiency and difficulty required to go from one voltage level to another. Improvements in power electronics have enabled engineers to develop circuits capable of inexpensively transforming DC voltages. This raises the question, Why not DC? Get connected to power The discussion of AC versus DC in the data center has centered around five major areas: efficiency, reliability, cost, safety, and the integration of other sources. Each of these aspects will be examined in this paper. White paper on data centers 3

Background Data centers have many different methods of distributing electricity. The first step is to identify the basic layout of the equipment used to distribute data center loads to the IT equipment. The block diagrams below show the typical distribution method when utilizing AC and DC equipment. DC PDU PSU IT Loads 480VAC Source AC/DC Rectifier DC/DC Battery Figure 1 DC Distribution AC UPS PDU PSU IT Loads 480VAC Source AC/DC Rectifier AC/DC Rectifier AC/ DC Battery Figure 2 AC Distribution To keep the scope of this paper manageable, it is necessary to limit the discussion to the legacy system design and the most promising alternatives. There are many different proposals for changing methods of delivery that center around various voltages. Currently, all options start with a 480V source on the input side to an Uninterruptable Power Supply (UPS). Typically there are 5 different power distribution concepts that are either in use or being proposed today: 1. Legacy 480VAC to 208/120VAC distribution 2. 415/240VAC distribution 3. 380VDC distribution 4. 240VDC distribution 5. 48VDC distribution The legacy distribution system represents the majority of the data center distribution configurations in use today. In this layout, the AC UPS has a 480V output to the PDU. The PDU then includes a 480V: 208/120V transformer to distribute power to the power source of the IT equipment. White paper on data centers 4

Outside of North America, the prominent distribution voltage is 415/240VAC. The higher distribution voltage allows for smaller wire sizes and it eliminates the need for a transformer in the PDU. While there are many available possibilities of utilizing different DC voltages, the current push is to move towards implementing 380VDC as the standard. China telcom and cloud providers are currently deploying 240VDC architectures due to its ability to use stander server power supplies. This architecture is viewed as intermediate, since more savings can be realized at the higher DC voltage. Some early discussion centered on using a 48V DC architecture similar to designs already in use within much of the telecom industry. One IEEE white paper (1) noted that the cost of cabling a typical 380VDC site can be 10% of the cost to do a similar 48VDC installation. This makes 48VDC a prohibitive solution. Therefore, this paper will focus primarily on a 380V DC topology as opposed to other possibilities. White paper on data centers 5

Efficiency Typically the discussion of AC versus DC in the data center starts with efficiency. Since a data center draws a significant amount of power, a relatively small increase in efficiency can lead to a reduction in operating costs. Concerns over efficiently utilizing the currently available power sources are also driving this topic. This makes efficiency improvements very attractive to data center operators. The proponents of DC power typically point to 2 prominent studies; the Lawrence Berkley National Labs (LBNL) project (3) and the EPRI/Duke Energy Data Center project (4). The LBNL study constructed AC and DC distribution systems for IT servers in order to measure the efficiency differences between the two architectures. These studies are referenced here because they provide quantitative data that can be easily referenced. The LBNL study reports an expected improvement of 4.7% to 7.3%. This study also projects a hypothetical 28% improvement in overall facility efficiency due to a lighter loading requirement on the cooling systems. The EPRI project reports a 15% improvement by utilizing a 380VDC architecture. The majority of these gains are a result of efficiency improvements found in the UPS and server power source equipment that are common components in almost every data center. However, more recent studies have revealed that these figures may not be entirely accurate. The efficiency gains for DC over AC reported in recent Schneider Electric and The Green Grid white papers (5 and 6) provide a much different view of the possible efficiency gains. The Schneider Electric study shows a 0-1% improvement and The Green Grid shows a 1% improvement. The primary differences in the findings between the Schneider Electric and Green Grid study and the LBNL and EPRI study are the efficiency values used for the UPS and IT Equipment Power Source for both the AC and DC equipment and the use of transformers in the distribution system. One peer review (7) recommended that the measured 5% - 7% be used since these were measured values and not the hypothetical 28% efficiency gain. For both the LBNL and EPRI reports the majority of the discrepancies in efficiency gains are a result of the definition of best in class equipment, For example, the AC UPS used in the LBNL and EPRI studies used a 90% efficiency rating for the UPS when 95% efficient equipment was available. A more thorough discussion on the differences found between these studies can be found in the Schneider Electric White Paper 151 (8). The chart below was presented and summarizes the efficiency findings from these 4 studies. Table 1 Comparison of Efficiency Expectations Study DC Benefit Cited Lawrence Berkley National Lab (LBNL) 1 28% Electric Power Research Institute (EPRI) study conducted at Duke Energy 2 15% The Green Grid 3 1% Schneider Electric 4 0% 1% White paper on data centers 6

Recently, there have been a number of gains in efficiency for AC equipment. State of the art UPS are espousing efficiencies of up to 97%. One of the major sources of inefficiency has been the power supplies used to convert power from AC to the necessary DC voltages in the server racks. The 80-Plus program implemented by Ecova, a total energy and sustainability management company, is a performance specification requiring 80% or greater efficiency at various amounts of loading for a power supply. Averaging the published efficiencies provided by their website for 5 companies that have at least 80 different power supplies listed result in average efficiencies of all levels 85% across all loading percentages. This is a significant increase in efficiency over older modules. Table 2 Power Supply Efficiencies 10% 20% 50% 100% All Loading Percentages Dell 82.95% 86.79% 89.07% 86.42% 86.31% Antec 81.34% 84.93% 86.77% 83.63% 84.17% Be Quiet 81.82% 85.59% 87.49% 84.57% 84.87% Delta Electronics 81.49% 85.39% 87.52% 84.93% 84.83% Sea Sonic 84.24% 85.37% 87.53% 85.00% 85.53% Averages 82.37% 85.62% 87.67% 84.91% 8514% The Green Grid White Paper #4 (9) compares a wide variety of distribution voltages and concludes that the 415/240VAC and 380VDC configurations offer a significant potential for efficiency gains over the typical 480/277VAC designs currently in use today. As with most new design ideas there are challenges that need to be overcome. A recent issue in using 415/240VAC in North America is the large amount of available fault current seen at the rack level equipment that causes these items to be inadequately rated for the available fault current. To prevent this problem, smaller substation transformers have been required or a 480/415V transformer has been connected in series with the UPS. This will reduce the efficiency of this design. Since 380VDC is still in the early stages of development, it is expected that it has the most potential for developing greater efficiency in the future. However, there is a push for improvements in AC equipment as demonstrated by the 80-plus program. Therefore, the efficiency conversation is ongoing with both approaches demonstrating strong cases for being able to produce a highly efficient system. Due to improvements in UPS and power source equipment efficiencies and the differences in data used when comparing equipment, it would appear the discussion on efficiency is a draw. The DC architecture does not appear to have a significant advantage over AC. White paper on data centers 7

Reliability Having a reliable data center can be at least as important as or even more important than running an efficient data center. Depending upon the data center s specific application, downtime can be costly to the facility owner and will be avoided at all costs. Consequently, anything that increases reliability can be of great benefit to the operators of a data center. The majority of the end-use equipment on the data center floor, particularly the servers, operates on DC power. For this equipment to utilize AC power it must go through a conversion process from AC to DC and then to various DC voltage levels, depending upon the different types of equipment utilized. A common measure of reliability, Meant Time Between Failure (MTBF), is used when assessing equipment reliability. It stands to reason that the MTBF for equipment will increase as the number of power conversion stages decrease. Like a car, the fewer moving parts that exist in an engine, the less chance of that engine failing. The Typical AC system requires converting AC to DC at the UPS and then back to AC for distribution. A second conversion from AC to DC occurs at the IT power supply. Removing the inverter (DC to AC stage) from the AC UPS and the AC to DC converter from the IT power supply will simplify the equipment, resulting in fewer points of failure. From a theoretical point of view this is an advantage of DC equipment over AC. Reliability is not only a function of the individual equipment utilized but also incorporates design, application, operation, and maintenance. The technology being developed to allow DC distribution in data centers is exciting and offers many possibilities for improvements. Developing a new overcurrent protective device that utilizes the AC/DC conversion stage as a part of the protection scheme is one possibility. The DC approach has the advantage with the available possibilities that come with developing new technologies and methods. The disadvantage to reliability is the learning curve associated with first generation equipment. Many of the unknown and unanticipated problems that occur with new processes, equipment, and designs have been identified and mitigated over the many years AC power has been in use. Higher-voltage DC designs do not have this background and therefore it is to be expected that the reliability of using a DC design will be reduced when the concept is relatively new. As mentioned above, reliability is not only a function of the individual equipment used in a facility. Because 380VDC technology and delivery methods are relatively new, there is not enough historical information to compare DC system to AC system reliability ratings over time. One white paper (10) compared two different distribution topologies utilizing AC and DC components. Each topology was examined using a modular DC UPS, modular AC UPS, or monolithic AC UPS. The monolithic UPS consists of a system that contains a single rectifier, inverter, and static switch. A modular UPS consists of multiple individual modules that contain the rectifier, inverter, and static switch. Each module operates independently of each other. In a monolithic UPS failure of the static switch removes the ability of the unit to bypass the conversion stages. In a modular UPS, the failure of the static switch in one module allows the other separate modules to continue operation. The paper calculated the reliability and availability for a total of 4 different scenarios: the DC modular UPS, AC modular UPS, and two different AC monolithic UPS s. The outcome indicated the use of both modular AC and DC UPS systems have a significantly greater reliability than monolithic AC equipment regardless of topology. The graphs below summarize the results of the reliability analysis presented in this paper. White paper on data centers 8

1.00005 1 Reliability 0.99995 0.9999 0.99985 0.9998 0 876 1752 2628 3504 4380 5256 6132 7008 7884 8760 Time (h) AC Monolithic AC Monolithic AC Modular DC Modular in ECO Mode Only Figure 3 Reliability of Topology 1 1.00005 1 Reliability 0.99995 0.9999 0.99985 0.9998 0 876 1752 2628 3504 4380 5256 6132 7008 7884 8760 Time (h) AC Monolithic AC Modular DC Modular Figure 4 Reliability of Topology 2 As shown, the reliability of the DC and AC modular UPS designs are very similar while the AC monolithic designs are significantly lower. The increased reliability is due to the ability to remove the single points of failure inherent in a monolithic UPS design, such as the static switch. The difference in reliability between topology 1 and 2 emphasize the affect that system design can have on a data center. Simply having the most reliable equipment does not guarantee a reliable data center. The method of operation and system design has a great affect on reliability as well. White paper on data centers 9

Many proponents of DC systems have pointed to the reliability and uptime of the 48VDC systems used in the telephone industry. The requirement of telephone systems to maintain uptime is similar to the uptime demands put on data centers. The ability to contact a first responder in an emergency situation has depended upon the telephone system remaining operational during all types of events. A large portion of that reliability is also due to the stringent requirements for equipment to meet the Network Equipment-Building System (NEBS) standard. The NEBS requirements consist of a series of standards designed to reduce the cost of deployment and maintain reliability of the telecom networks. It also requires testing of these systems to ensure the installation and operation of the facility will meet the necessary reliability standards. If a similar standard were to be developed and accepted by data center designers then the reliability comparison is definitely valid. If not, then it is difficult to justify translating the success of the telecom industry to a 380VDC data center environment. With 380VDC system efforts still very much in their infancy, it is reasonable to assume that future designs will increase reliability as refinements are made to equipment. Removing the DC to AC inverters in UPS equipment and the AC to DC converters in the IT power supplies increases the reliability of DC equipment. Coupled with the ability to simplify the connection of battery backup sources to the DC bus and an overall simplified electrical distribution system allows for a more reliable DC system as opposed to an AC system. White paper on data centers 10

Cost With any system, the cost can be broken into two categories, operating expenses (OPEX) and capital expenses (CAPEX). There are 3 major areas of possible cost reduction that are generally brought up in this discussion: a reduced utility bill due to a lower power draw made possible by more efficient equipment, a lower cost of DC UPS because there are fewer components required when compared to AC equipment, maintenance cost, and the smaller footprint requirement for DC equipment. Cost Reduction Due to Efficiency Improvement As discussed previously, the industry consensus is that DC is 0-1% more efficient than a modern AC system without the use of eco-mode. Expectations for a reduced energy bill need to be tempered by understanding that operating expenses can be greatly affected by the design of the system as a whole. Most of the inefficiency in data center power distribution equipment is due to heat loss generated during the power conversion process. Making equipment more efficient results in a corresponding reduction in the amount of heat generated. Installing more efficient equipment can result in less heat being generated which would allow for less cooling equipment in a data center. Without a significant increase in efficiency, it can t be assumed that there will be an opportunity to significantly reduce the amount of cooling equipment required. Cost Reduction Due to Lower Equipment Cost Eliminating unnecessary equipment in the data center distribution equipment will result in a lower capital expenditure when building new data centers. The legacy 480V: 208/120V systems required the PDU s to contain a transformer capable of lowering the voltage for use with the server power systems. Using a 380VDC or a 400/230VAC distribution architecture will allow the removal of the transformer used in the PDU. Utilizing a common overhead bus will also reduce the amount of floor space required by moving the PDU from a floor mounted location to an overhead location. This can be done using AC or DC rated busway. One advantage enjoyed by AC systems is the availability of multiple different options for circuit protection. The AC circuit breaker is a time tested option for providing circuit protection on electrical distribution systems. Multiple manufacturers produce a variety of devices that provide a wide range of options at a relatively low cost. The DC circuit breaker does not enjoy the history of the AC circuit breaker or the wide range of options that are currently available on the market. This limited availability can increase the cost of a DC electrical distribution system. White paper on data centers 11

Cost Reduction Due to Lower Maintenance Costs With a simplified electrical distribution system with fewer components, it is expected the overall maintenance costs of a DC will be reduced, over more complicated AC systems. This factor is not proven but should become clearer as the DC products and architectures mature in the coming years. Cost Reduction due to a smaller footprint There are opportunities for removing equipment from the data center footprint using the 380VDC and 400/230VAC design. Both architectures allow for this possibility. White paper on data centers 12

Standards and Safety Data centers have some unique challenges when it comes to designing a system that maximizes uptime while enabling typical maintenance practices. Traditionally, data center designers have been able to rely on existing standards that govern and guide design practices. Whenever the discussion of maintenance is broached, the typical accompanying talk turns to safety. Data centers typically have multiple sources of power, therefore increasing the inherent electrical hazard. White paper on data centers 13

Standards AC systems have the benefit of over 100 years of operation and experience that have lead to the development of electrical and safety standards covering multiple areas. Two organizations, IEEE and NFPA, have assembled and produced a number of written standards and safety documents concerning all aspects of AC power and installation requirements. The hazards of AC power have been extensively studied over time. The iterative process of publishing a standard, implementing these practices into real world environments, and then rehashing these documents with lessons learned has resulted in safety documents that not only address the obvious safety concerns but also address many of the unknown questions that don t arise until a system is actually put into operation. Two common examples of this type of document are the National Electrical Code (NFPA 70) and the Standard for Electrical Safety in the Workplace (NFPA 70E). No such standard currently exists for DC voltages above 48VDC. That being said, there is currently a strong push by organizations such as the Emerge Alliance to develop these standards. The Emerge Alliance is a consortium of various companies involved in the production of data center equipment dedicated to promoting the use of DC power in facilities and providing support for the creation of standards in this field of study. They have recently released a white paper entitled 380VDC Architectures for the Modern Data Center that is developed to aid data center engineers in designing DC systems. There are also efforts to provide standards addressing proper wiring methods, grounding methods, etc that will support the DC power community. DC powered architectures do not have the depth of support enjoyed by the AC systems. Consulting firms and design engineers are heavily dependent upon using these standards because they provide a level of assurance that a system will operate as expected. White paper on data centers 14

Safety Which system is inherently safer, AC or DC? The concern over electrical safety is generally broken down into two main topics: electrical shock and arc flash. Electrical shock is the result of an individual becoming a part of the current path in an electrical system. Arc flash is the result of a sudden release of energy that causes an explosion of varying degrees. Electrical Shock During an electric shock, current flows from one point on the human body to another. The most immediate danger is when current flows across the human heart causing fibrillation. Since AC power operates on an oscillating frequency of 60 Hz in the US, it will disrupt the heart s natural rhythm causing ventricular fibrillation. DC power does not oscillate and therefore has the possibility of causing asystole, or no electrical activity in the heart. Both outcomes can result in death. It must be pointed out that defibrillators use a DC source to shock the heart when it is in ventricular fibrillation in order to stop the erratic heartbeat and allow the heart to resume its natural rhythm. A second concern is respiratory arrest due to electrical shock. According to The Laboratory Manager s Professional Reference, it may take 170 ma of DC current and 30mA of AC current to cause respiratory arrest (12). The other hazard of receiving an electrical shock is internal and external burns. AC power causes muscle spasms due to the oscillating nature of the system. DC power causes a muscle to contract and hold steady. When comparing the two systems, it should be easier to escape an AC shock versus DC because the alternating nature of AC creates a natural point to escape the situation. This would reduce the length of time current flows through the body, thereby reducing the damage. The implication is that AC current is safer than DC when the concern is damage caused through the release of heat in the body. Arc Flash The other concern involving safety and electricity is the possibility of an electrical worker getting burned as a result of an arc flash event. Both AC and DC voltages can cause an arc flash. The severity of an AC event has been studied and can be estimated using the arc flash calculations found in the IEEE-1584 and the NFPA 70E. These methods and calculations were developed over the last 12 plus years involving testing of possible events. The NFPA 70E contains a set of formulae for DC powered arc flash events as well. These have not undergone the same amount of scrutiny that the AC calculations have experienced and are relatively new. Is a DC system or AC system safer from an arc flash perspective? There are many variables that come into play when assessing the severity of an arc flash event. Voltage level, amount of available current, trip time, and physical location of equipment are all parameters that can affect the arc flash calculation. As with much of the comparison to this point, it depends upon the electrical distribution system. The following comparison of AC arc flash values to DC arc flash values attempts to keep the aforementioned variables as consistent as possible. White paper on data centers 15

Voltage plays a significant factor in the arc flash calculation. The DC voltage discussed to this point has been 380VDC. This voltage level will be used in the arc flash calculation. A 480VAC system has been found to produce high arc flash incident energy values and is very common in many data center distribution systems. So, it is the natural voltage to use for comparison purposes. The arc flash calculation is greatly dependent upon the amount of available fault current. The amount of fault current in an AC system is highly dependent upon the system layout, location of the source, and size of the source. For purposes of comparing arc flash values between AC and DC systems, the following example will determine the amount of fault current available in a DC setup and use the same value in the AC calculations. For the DC system, the primary source of power is the AC/DC converter. A converter is not a pass through device with an uninterrupted path for the incoming power to flow through the device. There are no contacts that need to open as with circuit breakers. When a fault occurs, the converter controls should be able to sense the overcurrent and simply stop conducting or limit the current produced to a specified amount. Reducing the available fault current and the time to clear a fault are two major benefits when limiting arc flash incident energy. For the purposes of comparing AC versus DC, any contribution from the AC/DC converter will be ignored since it is possible to use internal controls to significantly limit the output. The other major source of energy from the DC system is the backup battery system. Many data centers have very large battery banks capable of producing significant amounts of short circuit current. The basis of the comparison will be an IT equipment load of 100A at 380VDC with a requirement for the backup system to operate for 2 hours. Table 3 below shows the values used to develop a battery bank capable of delivering these requirements. Table 3 Parameters for DC Bank Number of Cells in Series 32 Battery Voltage 12 Battery Amp Hours 20 AHr Battery Internal Resistance 11 mohms Battery Cable Connection 2/0 Battery Cable Connection Length 1 foot Battery Cable Connection Resistance 0.11 mohms Number of Rows in Parallel 10 Fault Current 11,171 Amps Since the resistance of the cables connecting the battery terminals is minimal, this value was excluded from the short circuit calculation. The worst case scenario for a fault at the output side of an AC UPS is when it operates in bypass mode. The amount of available fault current in this situation can vary widely. Therefore, it will be assumed the AC source generates as much fault current as the DC battery source. This assumption leaves the only true variable in this situation to be whether the system is AC or DC. White paper on data centers 16

The major contributor to the severity of an arc flash event is the amount of time the event lasts. This is highly dependent upon the upstream overcurrent protective device. These times can vary widely depending upon the type of breaker. Therefore, the calculations will be made to cover a range of clearing times:.02 seconds,.1 second, 1 second, and 2 seconds (the NFPA 70E recommended cut-off). Using a range of clearing times provides a comparison of AC arc flash to DC arc flash while eliminating the possibility of coming to erroneous conclusions by only reviewing the incident energy for a single duration. The arc flash energy will be calculated using the IEEE-1584 equations for the AC source and the NFPA 70E appendix D for the DC source. Since the majority of live work occurs on equipment that is installed in an enclosure, these calculations are done using the in the box formulae. Table 4 shows the results of this comparison. Table 4 Arc Flash Comparison Results Fault Time AC AFIE HRC DC AFIE HRC 0.02 sec 0.42 cal/cm 2 Cat 0 0.61 cal/cm 2 Cat 0 0.1 sec 2.2 cal/cm 2 Cat 1 3.05 cal/cm 2 Cat 1 1 sec 21 cal/cm 2 Cat 3 30.46 cal/cm 2 Cat 3 2 sec 43 cal/cm 2 Dangerous 60.92 cal/cm 2 Dangerous Table 4 demonstrates that the DC values are comparable to the AC values when it comes to the arc flash hazard. The calculation method for AC arc flash has been refined. The equations for calculating DC arc flash are conservative in nature. Equipment Concerns DC equipment has a unique challenge when it comes to disconnecting energized equipment. AC is alternating the voltage and current waveforms go through a natural zero crossing during as it cycles. When an item is unplugged or a set of contacts open, there may be a brief spark created that is soon extinguished once the waveform passes through the natural zero crossing. Since DC does not have a natural zero crossing the spark created will exist until the resistance becomes too great to allow current flow and it is extinguished. At lower DC voltages, the resistance needed to quench the arc is relatively small and the arc is quickly extinguished once the connection points are separated by a short distance. At higher voltages the resistance value needed to quench the arc becomes greater. This requires specific equipment connectors that prevent an arc from occurring. It also requires specific DC circuit breakers that are rated for the available fault current and DC voltage being used. These items can be purchased through manufacturers but are not found in the same variations and operating ranges as AC equipment. When it comes to safety, there are some differences between AC and DC power. AC power being cyclical in nature causes the muscles in the body to spasm which can be exceptionally dangerous with organs such as the heart. DC power tends to cause a single strong contraction of a muscle which may lead to prolonged exposure to an electrical shock. Both sources of power have similar arc flash hazards. AC power has the advantage of over 100 years of study and standards development that DC is just now beginning. White paper on data centers 17

Integration An increase in technological development has lead to an increase in the complexity of an electrical distribution system. Buildings have multiple sources of power to include generators, solar cells, and wind turbines. It has become necessary to plan for methods of including them into an electrical system. Which system architecture has a better ability to incorporate these sources of power, AC or DC? More emphasis is being placed upon utilizing alternate sources of renewable energy. Wind and solar power are the primary focus as these elements can be deployed almost anywhere in the world. Solar panels and wind farms produce power at a DC voltage. This has traditionally been converted to AC in order to be integrated into the power system. On a 380VDC distribution system, a direct connection can be made with these sources making them more efficient and easier to use. On an AC grid it is necessary to convert these sources from DC to AC and match the system characteristics. This is a definite advantage of a 380VDC design. Many data centers rely on having multiple separate power sources available should the primary source experience problems. In order to execute a closed transition from one AC source to another, it is necessary to match the phase angle, voltage level, and frequency in order to connect them together. If the two sources were both DC it would simply require matching voltage levels. This is a significant advantage for DC power over AC. Fewer parameters need to be monitored, making the system simpler. In the end, the ability for DC power to simplify the process of paralleling two sources and incorporating the available renewable energy packages gives it the advantage over AC power. White paper on data centers 18

Conclusion In the competition to develop a better data center there are many factors to consider. When considering efficiency, there is no obvious advantage between a DC and an AC distribution system. The majority of the studies comparing efficiency of these two architectures shows conflicting results. Reliability favors a DC design standard due to the ability to have fewer components involved in the UPS and power sources. Utilizing a 380VDC or a 400/230VAC architecture will provide the ability to reduce equipment cost by removing components from the PDU s. This allows for a cost reduction due to shrinking the physical size of the data center and by eliminating the cost for transformers. AC systems enjoy a long history of standard development. DC systems operated at the 380VDC level have support organizations in place that are in the beginning stages of producing and developing similar documents. Both DC and AC electrical systems contain inherent safety concerns when considering electrical shock and arc flash potentials. DC presents its own set of challenges with the need to develop appropriate connection and overcurrent protection equipment. Integrating renewable energy sources into a DC system is much simpler than converting those sources for use with an AC system. As can be seen, each architecture has its own benefits and limitations. In the end, the winner in the data center discussion depends upon the final goal of the data center owner. White paper on data centers 19

References 1. Analysis of Wiring Design for 380-VDC Power Distribution System at Telecommunication Sites by Toshimitsu Tanaka, Keiichi Hirose, Didier Marquet, BJ Sonnenberg, Marek Szpek 2. Why Are Electricity Prices Increasing produced by The Brattle Group for the Edison Foundation in June 2006 3. DC Power for Improved Data Center Efficiency by My Ton of EOS Consulting, Brian Fortenberry of EPRI, and William Tschudi of Lawrence Berkley National Laboratory 4. Duke Energy - EPRI DC Powered Data Center Demonstration Executive Summary, contact Don Kintner, EPRI communications manager. Downloaded from: 5. AC vs. DC Power Distribution for Data Centers White Paper 151 by Neil Rasmussen of Schneider Electric 6. Quantitative Efficiency Analysis of Power Distribution Configurations for Data Centers, White Paper 16 by the Green Grid 7. Peer Review of Lawrence Berkeley National Laboratory (LBNL) Study on Direct Current in the Data Center by Lynn Simmons 8. A Quantitative Comparison of High Efficiency AC vs. DC Power Distribution for Data Centers by Neil Rasmussen of Schneider Electric 9. Qualitative Analysis of Power Distribution Configuration for Data Centers, White Paper 4 by The Green Grid 10. Telecom Datacenter Power Infrastructure Availability Comparison of DC and AC UPS, by Domagoj Talapko 11. 380VDC Architectures for the Modern Data Center, the Emerge Alliance 12. http://go.hrw.com/resources/go_sc/gen/hs2lmr07.pdf, by Holt, Rinehart, and Winston White paper on data centers 20

Schneider Electric USA, Inc. 1415 S. Roselle Road Palatine, IL 60067 Tel: 847-397-2600 Fax: 847-925-7500 www.schneider-electric.us Document Number AT328 This document has been printed on recycled paper November 2013 sf 2013 Schneider Electric Industries SAS, All Rights Reserved.