Designing Three-Phase Power Conversion and Distribution Solutions That Give Back Server Rack Space
At GE, we work with data center customers who share a common issue: how to increase computing capacity while improving power efficiency and reducing their power footprint. This power-versus-space challenge is especially true in settings like data centers and large industrial facilities that rely on three-phase AC power to run the network and other equipment.
At GE, we work with data center customers who share a common issue: how to increase computing capacity while improving power efficiency and reducing their power footprint. This power-versus-space challenge is especially true in settings like data centers and large industrial facilities that rely on three-phase AC power to run the network and other equipment.
As most power distribution engineers understand, converting each phase of the AC power coming into the facility into usable DC power presents phase balancing challenges. In a data center, for example, each phase may be powering a different set of servers within a rack. If one set of servers is pulling more power than another, the power distribution system is unbalanced. The data center power design, which splits each phase to feed a set of servers within each rack, has to ensure that each phase is evenly loaded.
To manage this three-phase load balancing challenge, traditional power conversion topologies use three separate AC-DC converters, or rectifiers, connected with an additional power distribution unit (PDU). This type of power conversion topology impacts power efficiency and occupies additional server cabinet real estate. For example, a typical server cabinet has 42 vertical rack units or about 74 inches of usable vertical space for server blades. Typical power conversion, however, consumes about four or five of these rack units – or about nine inches of vertical rack space. Because this space is wasted on power conversion, it cannot be used for data center computing or networking equipment.
Our challenge is reducing the size of the power conversion footprint to free up server cabinet computing space. To solve it, we use an approach we call Designing in the Negative Space, which finds ways to reduce the power footprint or locate power in unusable spaces.
In the case of three-phase AC-to-DC rectifiers, new products can replace the three single-phase conversion units for each phase with one unit that balances all three phases. These units employ sophisticated load-balancing algorithms and approaches so data center operators don’t have to manage load-balancing issues.
For example, two GE GP100 6-kilowatt (kW) rectifiers can deliver the same amount of power in half the rack space of previous-generation, three-phase power solutions, while consuming the same amount of space as competing single-phase power supplies (See Figure One). This means data center power designers can put 12 kW of three-phase power in a single rack unit of space, eliminating the need for a separate PDU to split the phases and connecting all three phases directly to the single power supply. In this way, a fewer number of three-phase, long-life rectifiers can share the electrical load of the rack while providing backup redundancy – all while greatly simplifying the electrical infrastructure within the rack.
Critical server bay space is therefore freed up inside the rack for extra computing capacity by simply getting the electrical equipment out of the way. Multiply that space savings times the hundreds of server cabinets in a typical data center and you can see how operators can save significant capital and operating expenses in infrastructure and facility costs. For example, if five vertical rack spaces can be freed up per rack in a data center containing 1,000 racks, then 5,000 vertical rack spaces have been vacated; a savings of 119 racks worth of space. That means 1,000 racks’ worth of computing capacity can be achieved in only 881 racks. If each rack consumes eight square feet of space, the savings equates to nearly 1,000 square feet of space, not counting the aisles.
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Figure One – Single Integrated Rectifier Power Conversion (click to zoom)
Looking inside the rack cabinet, if we apply a Designing in the Negative Space philosophy to power distribution within the server cabinet, we can eliminate three-phase power conversion equipment completely from the server rack. The few extra inches between the 19-inch interior width for equipment and the total 24-inch interior cabinet width, enable power designers to place the PDU and single, integrated three-phase-enabled rectifiers in previously unused space (See Figure Two). Putting power distribution and rectifiers into this unused space in the back corner of the server cabinet frees up the full 74 inches of vertical equipment space for data center equipment, improving power density and computing capacity.
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Figure Two – Using ‘Useable’ Server Cabinet Power Distribution Space (click to zoom)
In one real-world application, an Edge Cabinet PDU connects up to five rectifiers outside of the server rack. Each rectifier converts 380-480-volt three-phase AC power to the servers’ required 12-volt DC power or, alternatively, to a 48-volt DC distribution bus. This creates the same phase-balanced power conversion capabilities in half of the footprint of conventional single-phase rectifiers, enabling users to create a more efficient power system. Once converted at the rectifier, the 12-volt power is then delivered to servers via pairs of proven "bullet-style" power terminals. This configuration significantly reduces the distribution distance at 12 volts, minimizing loss and maximizing efficiency.
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By positioning power equipment in previously unused spaces, valuable rack space is freed up for additional networking or computing equipment. (click to zoom)
Using this unused corner and one side of the server cabinet also facilitates both the ease and safety of installing rectifiers, the PDU and cablings.
Finally, this innovative way of tucking power out of the way, enables much higher rack-level power densities without sacrificing an inch of server space within the rack. Densities approaching 100 kW in a single rack can be achieved easily and efficiently, further enhancing the overall space savings effect. Using the example above, by simply doubling the rack-level computing power density from 15 kW to 30 kW, the total number of racks required in the data center can be slashed, along with the required floor space.
Since infrastructure and power costs are such significant factors in the cost of data center operations, it only makes sense to minimize them as much as possible. Imagine pushing rack-level densities to 60 kW and beyond. What would that mean in terms of saving floor space and power distribution infrastructure within the data center?
As discussed, tackling the power-versus-space challenge for converting three-phase power opens up a range of power density design approaches that do more than just force more power into smaller packages. These new design approaches free up valuable server and rack real estate to improve capacity, provide higher power density and lower operating expenses. That’s Designing in the Negative Space.
