Edison Redux

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  • 50 ieee power & energy magazine november/december 20121540-7977/12/$31.002012IEEE

    Digital Object Identifier 10.1109/MPE.2012.2212607

    Date of publication: 18 October 2012

    DDirect current (Dc) technology has changed significantly over the last 120-plus years since edison and Westinghouse publicly battled over dc versus ac in the War of currents. in the 19th century, ac became the dominant power distribution design because transformers cheaply solved the problem of obtaining power more than a kilometer from a centralized power station. even with more than a century of new technology, we never looked back.

    yet today, the world faces unprecedented issues with global climate change and the need for sustainability.

    Edison Redux

    380 Vdc Brings Reliability and Efficiency to Sustainable Data Centers

    By Guy AlLee and William Tschudi

    masterseries

  • november/december 2012 ieee power & energy magazine 51

    and the technology that eluded edison in the 19th century makes dc the distribution mode of choice for this millennium. if edison were here today, his data center would be dc.

    OverviewFor years, most data center operators focused primarily on providing enough computing capacity to meet demand and provide adequate cooling. But as businesses, organiza-tions, and governments have become increasingly depen-dent on data centers for practically everything they do (from day-to-day operations to electronic transactions and communications), the cost of powering these data centers has gained greater attention. equally worrisome, many facilities have outgrown the power capacity for which they were designed.

    the total power consumption of todays data centers is becoming noticeable. according to the u.s. environmental Protection agency (ePa), data centers and servers in the united states accounted for approximately 1.5% of total u.s. electricity consumption in 2006. to put this in per-spective, the ePa notes that this is more than the electricity

    consumed by the nations color televisions and is similar to the amount of electricity consumed by approximately 5.8million average u.s. households (or around 5% of the total u.s. housing stock). although data centers enable energy savings in the rest of the economy by allowing us to work smarter, e.g., through telecommuting and bank-ing online, their growing energy utilization is something the ePa thinks we can address by adopting more energy-efficient technologies. one prominent example is 380-Vdc electrical power distribution.

    since the 1960s, semiconductors (natively dc) have come to dominate our electrical devices such that soon the major-ity of the load base will be natively dc. Most carbon-free energy sources and energy storage systems are natively dc. Furthermore, they are being deployed in a distributed man-ner. the assumptions that gave us ac power distribution are therefore being reexamined, and a fundamental shift to dc is well under way.

    in this article, we look at low-voltage dc (lVdc) at 380 Vdc, the new industry specification and the single worldwide standard in data centers (and one envisioned for all future commercial buildings as well). starting

  • 52 ieee power & energy magazine november/december 2012

    with the motivationsand in the context of the changing environmentwe look at the development of 380 Vdc, its current implementation as a 190-Vdc distribution with midpoint ground (see Figures 1 and 2), and the resulting benefits. that leads to articulating the case for lVdc as well as debunking several common myths that no longer

    apply.Finally, we conclude with some notable examples of 380-Vdc data centers from around the world.

    The Wake-Up Callas a result of the electricity supply issues in california at the beginning of the 2000s, the california energy commission

    IT Equipment

    IT Equipment

    NOTE: All loads are to be connectedbetween L(+) and L().PE is not a power conductor.

    IT Equipment

    ac Mains

    acControls

    Isolation

    Rectifier

    Battery

    L(+) L(+)

    L()PE

    L(+)

    L()PE

    L(+)

    L()PE

    PDU

    L()1

    40k,

    5W

    , 1%

    40k,

    5W

    , 1%

    +19

    0V38

    0V

    380V

    19

    0V

    Generator

    380V

    380V

    1

    2

    2

    3

    3

    figure 1. Midpoint resisitive grounding for the 380-Vdc data center distribution at 190 Vdc (courtesy of EPRI).

    ac Panelboarddc Panelboard

    Power Strip

    Server Rack

    Rectifier RackPos

    Neg

    Battery

    FrGnd

    Fr Gnd

    Fr Gnd

    Fr Gnd

    ABCN

    PE

    To BuildingMain Ground Bar

    If Star Grounding ConceptTypically NA If MESH-BN Grounding Typically Europe

    Main Aisle Feeder Ground (Like 48 Vdc in United States) orMESH-BN Grounding

    R

    R

    MET (Main Earth Terminal)

    PosGndNeg

    PSU

    PosGndNeg

    PSU

    figure 2. Earthing practice for the 380-Vdc data center distribution at 190 Vdc (courtesy of Emerson Network Power).

  • november/december 2012 ieee power & energy magazine 53

    triggered a ten-year research initiative around high-tech buildings (data centers). the idea was to research, develop, and demonstrate innovative, energy-efficient technologies and use demonstration projects to accelerate the deployment of results.

    in 2004, lawrence Berkeley national laboratory (lBnl) began investigating the efficiency of power distri-bution in data centers. By 2005, it had studied the efficiency of power supplies in it equipment and uninterruptible power systems (uPss). the lBnl researchers noticed the poor efficiencies of these components and introduced the concept of eliminating some of the power conversions. this led to a dc data center demonstration sponsored by the california energy commissions Public interest energy research pro-gram. lBnl brought in the electric Power research insti-tute (ePri) as a subcontractor and started the investigation in earnest. By June of 2006, a public demonstration had been organized, with the help of 16 other organizations and 14 other stakeholders.

    the results were stunning: a 28% improvement in efficiency over the then-current average ac distribution efficiency in the united states. Figure 3 compares the two approaches, with ac shown in (a) and dc in (b).

    in an ac distribution, medium voltage (MV) is trans-formed into 480-Vac, three-phase power at the entrance to the data center facility. For reliability reasons, the power is typically then fed to a uPs. the 480-Vac, three-phase power is rectified to 540 Vdc to keep a minimum amount of energy in readiness to run the data center in the event of grid supply failure. the 540 Vdc is then converted back to 480 Vac (which also cleans up the quality of the power) and is distributed to the floor of the data center. Power distribution units (PDus) are scattered around the data center to convert the 480 Vac to 208 Vac, which is what runs each server. in high-reliability data centers, the power is run twice to each server; each server has redun-dant power supply units (Psus) inside, in a configuration called dual-corded.

    Finally, the Psu produces a 12-Vdc output and derives other voltages from that to run the server electronics at voltages that generally range from 13.3 Vdc. in doing so, the power supply internally converts the ac to unfiltered dc with a bridge. it takes that power and delivers it to a power factor correction (PFc) unit to minimize the reac-tive power load on the incoming supply, which also boosts it to 380 Vdc internally. Modern switching power supplies

    implement a dc-to-dc converter to take the 380 Vdc down to 12 Vdc very efficiently.

    all of the distribution voltage conversions across the data center and in the Psu are reflective of the practices developed over the whole of the 20th century and had never really been questioned. But herein lies the problem: every conversion wastes energy and produces heat. and while any single conversion may be in the mid-90% range, they dont add together, they multiply. and when you add in the energy needed to remove the heat, only about 50% of the energy at the wall of the data center actually winds up being used by the processors in the servers (see Figure 4). in many data centers at the time of the study, the power used for

    In 2004, Lawrence Berkeley National Laboratory began investigating the efficiency of power distribution in data centers.

    + +

    480 Vac

    480 Vac

    208 VacPDU

    UPS Rectifier

    480 Vac

    MV

    ACPSU

    12 Vdc(a) (b)

    Batt. Batt.

    380 Vdc

    DCPSU380 Vdc

    540 Vdc

    PFC

    figure 3. (a) Typical North American ac distribution versus (b) worldwide industry standard 380 Vdc dc distribution.

  • 54 ieee power & energy magazine november/december 2012

    cooling could approximate the direct losses along the dis-tribution path.

    the two primary secrets of efficiency are first to elimi-nate the conversions (less waste heat generated) and second to use as high a voltage as you can (to make the fixed losses as low an overhead as possible). in the 380-Vdc distribution shown in Figure 3(b), the MV was therefore converted to 480 Vac and then immediately rectified to 380 Vdc and kept there all the way through the data center distribution and even into the Psu.

    With the goal of being able to move quickly to imple-mentation, the study also focused on keeping the existing form factor and using standard high-volume components. it also made sure not to compromise the existing server

    performance, reliability, and safety. in addition, high value was assigned to compatibility with alternative energy solutions such as photovol-taic (PV), fuel cells, and so on. the