Edison vs Tesla: A rematch in the telecom data center

Edison vs Tesla: A rematch in the telecom data center Paul Smith Technical Marketing Manager GE Critical Power Plano, TX USA Paul.D.Smith@GE.COM Abstract For over a century, since the invention of the telephone, the telephone network has relied on DC Power has used DC battery reserve provide "continuous dial ne", even during commercial power outages. The debate between proponents of George Westinghouse's AC Thomas Edison s DC commercial power distribution schemes has raged just about as long as the telephone has existed. The debate over the use of AC DC Uninterruptible Power Supplies (UPS's) is more recent. Powering Telecommunications equipment has long been dominated by DC UPS's while Computer equipment has traditionally been backed up by AC UPS's. I. A. DC Architectures Low DC Voltage High Current 2 Conducrs (Sized @42 V) DC 48 V The metamorphosis of telecommunications networks in Information Communications Technology (ICT) networks with their reliance upon digital technologies has blurred the distinction of these systems the choice of UPS technology reflects this. Changing requirements advances in battery chemistries, power system efficiencies software controls are all affecting choice of power system architecture. Primary 4 Figure 1 Classic DC UPS Architecture This paper examines the changing requirements of ICT network equipment their effect on the various AC DC UPS architectures that are available power them. Changing requirements have opened up opportunities for different power architectures utilizing new battery chemistries, while advances in power conversion equipment allow power systems be placed in different locations, improving distribution efficiency. Continuous pressure for operational cost reduction is fueling the need for more efficient "end end" systems, as well as reducing capital costs of equipment. Total Cost of Ownership (TCO) is now a key ingredient in the choice of power architecture alongside the questions of reliability, availability, reserve time, redundancy scalability. The impact of all these key ingredients is examined. The DC UPS as depicted in Figure 1 has been used power telecom systems since the invention of the telephone early in 1870 s. Telecom central offices have traditionally been required provide 4-8 hours of battery reserve, depending on the availability of a generar specific regulary requirements. Many of day s telephone switching offices have gradually morphed in data centers, which may or may not require eight hours of power reserve. Some equipment is still subject regulary compliance may still require 824 hours of backup power between batteries generars. Much of the data center equipment however may not require this length of reserve time, especially if service can be maintained through spatial redundancy of server operations. It may only be necessary have 10-15 minutes of reserve power allow transfer of server operations, or allow generar(s) start synchronize their operation. This paper examines these advances with a view better understing the trends in power architectures implementation. This may also allow us a glimpse in the "crystal ball" of future developments trends. Keywords AC UPS; DC UPS; Rectifier; Power; Efficiency; Phase Balance; Data Center B. Rectifier Advances Rectifier technology has made tremendous improvements over the last few decades, with rectifier sizes reduced by one two orders of magnitude. Efficiencies have improved dramatically, with losses reduced by as much as 75%. This is illustrated in Figure 2 1 P a g e 978-1-4799-6582-3/15/$31.00 2015 IEEE DC UPS 495

1970 Ferro 200A Volume: 14,400 in3 Density: 0.8 W/in3 Weight: 725 lb. Efficiency: 88% 1990 SMR 220A (Gen 1) Volume: 2,883 in3 Density: 4.42 W/in3 Weight: 59 lb. Efficiency: 92% 2008 SMR 220A (Gen 2) Volume: 1,668 in3 Density: 7.64 W/in3 Weight: 50.5 lb. Efficiency: 96% Gravimetric Energy Density (Wh/kg) 200 2014 SMR 100A (Gen 3) Volume: 218 in3 Density: 26.6 W/in3 Weight: 4.1 lb. Efficiency: 97% Li-Polym mer 150 Li-ion 100 Na-Ni-Cl 50 H Ni-MH Ni-CD Lead Acid 0 0 100 200 300 400 500 600 Volumetric Energy Density D (Wh/l) Figure 5 Density Chemistries Figure 2 - Rectifier Advances With reduced reserve time requirrements advanced battery technologies, it now becomees practical put reserve power equipment on the upper floorrs with a reasonable expectation that the average floor iss capable of supporting it. This has enabled the development of o some alternative (distributed) DC UPS architectures as shown in Figure 6. Figure 3 illustrates a direct comparisoon of Advanced Switch Mode Rectifier (SMR), older generaation SMRs Ferro Resonant Rectifier efficiency performancce. AC Voltage DC Primary 3 Figure 3 Rectifier Efficiency Comparison Figure 6 Distributed DC UPS Architecture C. Advances Advances in battery chemistry are also trannslating significant reductions in battery size weighht. Flooded lead acid batteries providing 8 hours have traditionaally been located in the basement of large switching offices becaause of the their size weight. This architecture takes advantage of the smaller sizes in both rectifiers batteries placee smaller DC UPS s in the lineup with sections of the load equiipment. Benefits of this architecture include: c closer the Efficiency - By moving power conversion load, DC distribution losses are dram matically reduced. They are replaced by much smaller AC lo osses, due the higher voltage lower current of the AC C distribution system. This improves the end--end efficienccy will result in a reduction in a facility s OpEx. Copper Cable - The reduced diistance that the lower DC voltage has travel requires many times smaller conducrs d distances, resulting in carrying current over much reduced enormous reductions in CapEx requ uired purchase install the copper cable. Cooling - Similarly, improvements in conversion efficiency will reduce wasted energy y further reduce OpEx. Improvements in efficiency also red duce the heat dissipated in the power conversion equipment, which w in turn reduces the load on the HVAC system, further reducing r OpEx. It can be seen in Figure 5 that advanced batttery chemistries are capable of providing a two eight times reeduction in both volume weight. Flooded Flooded Lead Acid VRLA Valve Regulated Lead Acid Sealed Ni-Cd Nickel Cadmium Li-Ion Lithium Ionn Na-Ni-Cl Sodium Nickel Chloride Figure 4 - Types Chemistries 2 P a g e 496

Flexible Energy Reserve - By distributing the power conversion equipment feed a portion of the load equipment, it is possible provision reserve time appropriate that set of load equipment. This means that if one load needs a longer reserve time, it can be provisioned without encumbering the entire facility with the cost of the longer reserve time. The reserve time can be matched the load in appropriate increments. Reliability Grouping smaller loads with smaller power plants decreases the size of a given failure group, due power issues, resulting in an overall increase in reliability. Scalability Power equipment is added incrementally along with load equipment, minimizing initial power equipment installation costs (CapEx). Each of these strategies has its own strengths weaknesses may be appropriate for different applications, depending on needs. System + System AC UPS II. The AC UPS has also been around for many years, supporting AC powered equipment by cleaning up poor quality utility AC riding through utility AC outages. Catcher System The representation in Figure 7 is drawn in a way intended highlight the similarity with the Classic DC UPS shown in Figure 1, it does not show elements such as bypass or dual corded loads frequently utilized. Figure 8 Redundancy in AC UPSs A detailed study of the different redundancy strategies is the subject of a separate paper. [1] A. AC Architectures 550 VDC III. 550 VDC 240 VAC Low AC Voltage High Current 240 VAC 2 Conducrs Primary ALTERNATIVE UPS ARCHITECTURES A. High Voltage DC Architecture In the architectures shown so far, power has be transferred from one place another at some electrical voltage. In the DC architecture of Figure 1 power is moved from the central power plant the load location at 48VDC. When the load power level is high this can introduce significant losses, since the current will be high. High currents require large copper conducrs minimize losses. The cost of copper makes the Capital Expense (CapEx) high keep losses low. If the power were transferred at a higher voltage the conducrs could be significantly smaller for the same loss level, CapEx reduced. At 380VDC the currents are 12.6% of those required at 48VDC for the same power. Figure 9 shows the equivalent of the centralized 48VDC UPS scaled 380VDC. DC Redundant Parallel (N+1) 2 Figure 7 Classic AC UPS Architecture B. Redundancy Strategies in AC UPS Most AC UPS s are constructed as high power blocks, not utilizing the redundant rectifier type of architecture found in most DC UPS systems. Redundancy the associated improvement in reliability are incorporated in AC UPS systems at the block level as shown by some of the examples in Figure 8. Illustrated are: 1. The full System + System, where one complete system (3 blocks) is redundant. 2. A catcher system, where a single block is redundant, switched support any module that fails. 3. A redundant parallel configuration, where N+1 blocks operate in parallel. High Voltage DC Low Current Small Cables Low Losses 2(3) Conducrs DC 380 V Figure 9 High Voltage DC UPS Architecture Losses in the conducrs will be lower because of the higher voltage, but losses in the converter blocks will be similar because we have not eliminated any conversions, there 3 P a g e 497

are still 2 major conversion stages; the rectifier downstream DC/DC converter. B. High Voltage AC Architecture Transferring power from the input switch gear the load cabinet at 480VAC has the same, or better, improvement in losses as high voltage DC, the architecture looks like that shown in Figure 10. The rectifier stage battery are now moved all the way downstream the load cabinet. This is made possible by the aforementioned improvements in rectifier battery technologies. Designing the rectifier accept 480VAC 3 phase power output 12VDC directly removes a conversion step from the end end power train, further reducing losses. Figure 10 High Voltage AC Architecture High Voltage AC Low Current Small Cables Low Losses 3 Conducrs DC This architecture is a logical progression from the distributed DC architecture of Figure 6, which moved the rectifiers close the load, in a cabinet at the end of a row of load cabinets. In this case the rectifier moves all the way in the load cabinet, further reducing the distance from the rectifier the load, the losses associated with that low voltage loop. We have changed the rectifier output from 48V 12V also, which removes the need for an additional DC /DC conversion step, but has a consequence within the cabinet: The low voltage loop within the cabinet is now 12V rather than 48V, so the currents are proportionally higher, this warrants paying some attention minimizing the distance between rectifier load even within the cabinet. C. Optimizing within the Cabinet Installing the rectifier inside the load cabinet has been conventionally done by mounting in the p or botm of the cabinet space, as shown in Figure 11 The distance from the power unit the farthest load shelf may be as much as 7 ft with this arrangement, the 12V cables or bus bars must be sized keep the voltage drop within the load s acceptable limits under worst case conditions. Worst case conditions for voltage drop are when the unit is operating from the battery, when the battery voltage has depleted its lowest value. Low Voltage Power Low Voltage Power Figure 11 High Voltage AC Architecture Cabinet Depending on the tal load in the cabinet, the size of the rectifier, it may be advantageous split the power battery shelves in blocks which are spread throughout the cabinet, as shown in Figure 12. This significantly reduces the maximum distance between rectifier / battery associated load. DC Low Voltage Power Low Voltage Power Low Voltage Power 12V DC Rectifier 12V DC Rectifier Figure 12 High Voltage AC Architecture Horizontal In cabinets with space on the side, or if this can be engineered in the cabinet, the arrangement in Figure 13 can further reduce the distance travelled by the 12VDC power. In this vertical arrangement, 12V power flow in the vertical dimension of the power shelves is minimized, since each rectifier feeds a horizontally adjacent load, vertical flow is limited that which is needed balance load between adjacent rectifiers. 4 P age 498

A. Cabinet or PDU? o Figure 15 is designed The PDU shown in the left side of fit in many types of cabinet, or can be b designed in a cusmized cabinet. 12V DC Rectifier Edge Single Row PDU Edge Siingle Row PDU installed in a cabinet Low Voltage Power Low Voltage Power Figure 15 The Edge PDU a cabinet implementation. i One of the key advantages of thiis architecture is the ability place cabinets with different load d voltage requirements (AC DC) in the same line up, efficiently fed from the same 480VAC source. Figure 13 High Voltage AC Architecture Vertical (Eddge) IV. AN OPTIMIZED CABINEET B. 12V DC powered loads equipment that can accept 12VDC directly provides a the minimum the optimum power processing by allowing number of conversion stages, one. The use of 12VDC around the caabinet requires careful design because of the relatively high h current levels required at this voltage. The use of the verticall edge mounted PDU proposed minimizes conduction lossses. C. 48V legacy equipment compatiibility While we have seen that converssion directly from 480VAC 3 phase 12VDC is optimal for miinimizing the overall number of conversion steps con nduction losses, there may be occasions where the load equipm ment still requires the traditional -48VDC. This is easily accommodated a using 48V rectifiers battery modules in thee same Edge cabinet design. This has the advantage of lo ower currents in the cabinet, but there will be another po ower conversion in the load shelf transition from 48VDC th he final voltage (12V). Figure 14 Cabinet designed accomadate vertical Edgge power. The Edge cabinet shown in Figure 14 is fitted with a power distribution unit (PDU) in the left side, which accepts 480VAC at the p, has slots accept recctifiers, batteries or other power modules as needed. Key featurres include: 1. 480V AC input (or 380VDC) containedd within the PDU, enclosed for safety of personnel 2. Energy Srage Near Space 3. Turnkey Solution with 42U of Usable S 4. Flexibility of load voltage 5. Flexibility of reserve time 6. Flexibility of cooling (load power sseparate) 7. Infrastructure for load shelves plug inn DC bus 8. DC voltage determined by rectifier, 12V V, 48V etc 9. Cabinet can be facry integrated for quuality D. AC output from inverter in samee space UPS in a box. Inclusion of an inverter in one off the Edge slots effectively converts the cabinet pow wer infrastructure a mini AC UPS, housing rectifier, battery a inverter in the edge space. This allows simple mixing of o load cabinets with differing requirements, in this case AC A powered loads. Again this introduces more power conversions, but does allow the powering of legacy AC loads in the same line up with DC loads. E. Phase Balance owered by single phase AC Data center loads traditionally po present phase balance issues the facility f operar, since the load on each phase has be manag ged achieve phase 5 P a g e 499

balance. This is a significant burden as the data center grows loads are added. The use of a true three phase rectifier eliminates this issue by aumatically balancing the load between phases every time additional load rectifiers are added. F. Energy Reserve in an AC UPS We have shown how the battery can be moved all the way the load cabinet, while this offers several advantages, including reserve time flexibility, cabinet cabinet, there are applications where it may be more appropriate use the centralized AC UPS previously discussed. Figure 16 AC UPS feeding a single conversion step architecture. Figure 16 shows the combination of a centralized AC UPS with the 480VAC the load concept. Here the reserve is supplied by a centralized AC UPS system (the user can choose a redundancy scheme from Section II.B ) If the UPS is operated in bypass mode we still retain the single conversion step. capacity using AC UPSs is typically short (minutes) will be the same for all loads using this centralized architecture, however the battery reserve does not take up valuable space in the load cabinet. G. Energy Reserve in a High Voltage DC UPS If a longer battery reserve is required for all equipment, then the centralized high voltage DC architecture may be appropriate, combining the advantages of high voltage power distribution with the optimized cabinet layout, as shown in Figure 17. The optimized cabinet would be equipped with 380VDC 12VDC or 48VDC converters in this implementation. 480 V 3 Phase AC UPS High Voltage AC Low Current Small Cables Low Losses 3 Conducrs DC High Voltage DC Low Current Small Cables Low Losses 2(3) Conducrs requirements have converged allow changes in power architectures not previously practical. The use of a rectifier that converts 480VAC three phase directly the dc voltage required at the load, combined with an innovative load cabinet layout, minimize the losses from both conduction conversion. The result is a low initial cost low ongoing operating cost. This architecture ensures phase balance on the AC supply without owners having manage or assign loads individual phases, allows flexibility in the provision of energy reserves. The inclusion of appropriately sized power infrastructure in the load cabinet itself also allows cost minimization scalability at an unprecedented level. The optimized cabinet is also capable of supporting a high voltage DC input additional load configurations for different applications including a variety of DC AC loads enabling the support of transport equipment, data center loads legacy equipment. ACKNOWLEDGMENTS The author would like thank the following invenrs [2] for creative input assistance in preparing proof reading of this paper: Ed Fontana Mark Johnson Roy Davis REFERENCES Mike Steeves Bob Burditt Steve Stein [1] GEA-D1005 - Redundant Parallel Architecture(RPA) for UPS systems - http://apps.geindustrial.com/publibrary/checkout/gea- D1005?TNR=Data%20Sheets%7CGEA- D1005%7CPDF&filename=GEA-D1005-GB.pdf [2] Patents Pending: Systems methods for power conversion distribution (2014) Integrated power racks methods of assembling the same (2014). DC 380 V Figure 17 High Voltage DC UPS Architecture with Edge load cabinet V. CONCLUSIONS Advances in rectifier technology, size efficiency, battery chemistry size, changing reserve time 6 P age 500