Maintaining a consistent flow of power to your IT equipment is crucial to keeping your data center running. Uninterruptible power supplies (UPSs) are a critical part of your facility’s electrical infrastructure, as are backup generators. But what about the devices that control which power source (e.g., utility-supplied power or backup generator power) feeds your servers and other IT equipment? The ability to switch quickly and cleanly from one power supply to another is required to keep your data center running smoothly. Static transfer switches are a solution that offers fast transfers, but are they the best choice for your facility?
The Function of Power Transfer Switches
Obviously, you can’t have someone sitting at a switch all the time, waiting to transfer power from the utility feed to the backup generator when an outage or other event occurs. And even if you could, that person wouldn’t have fast enough reflexes to prevent IT equipment from shutting down. That’s where a transfer switch comes into play. Depending on your data center design, the transfer switch can be located in one or more places, such as between UPSs to switch over from one to another in the even that one fails, between the utility feed and a backup generator, and at the rack level between two separate power buses. The transfer time, depending on the switch type, may range from a few milliseconds to about 15 milliseconds.
Deciding where to use a transfer switch in your data center’s power distribution design is a choice that must take into account your desired availability, the types of equipment you’re supplying (and, in particular, whether your servers are single or dual corded, for example), budget, scalability requirements and so on. Although these design choices are beyond the scope of this article, one important part of your decision is the type of switch you should use.
Automatic Transfer Switches Versus Static Transfer Switches in the Data Center
Power switches for use in data centers come in two major varieties: automatic transfer switches (ATSs) and static transfer switches (STSs). These two varieties differ fundamentally in the switching mechanism: ATSs are electromechanical, and STSs use semiconductor technology. Ideally, from a design perspective, one switch type would be superior to the other in every respect, making the design choice simple. Naturally, however, this isn’t the case: in fact, neither is always better than the other, although each one may have more benefits in a particular data center application. Thus, in choosing between the two, you must consider carefully where they will be employed in your design what functions they’ll serve. Here are some characteristics to take into account when deciding between ATSs and STSs for your data center.
- Switching speed. The faster the switch from a failed power supply to a backup or redundant supply, the less disturbance to your equipment. According to an APC by Schneider Electric white paper (“Powering Single-corded Equipment in a Dual Path Environment” http://www.apcmedia.com/salestools/SADE-5TNRLE_R1_EN.pdf), semiconductor-based STSs can switch in about 4 milliseconds, compared with 8–16 milliseconds for an electromechanical ATC. For the STS, that’s about a quarter of a cycle for 60Hz electricity, versus half to a full cycle for an ATS.
- Design complexity. Because STSs are more complex than ATSs, having more components and thus leaving more opportunities for failure, they are generally less reliable. The above-mentioned APC white paper estimates the mean time between failures for static transfer switches at 400,000 to 1,000,000 hours; for ATSs, this time between failures is significantly greater at 700,000 to 1,500,000 hours.
- Capacity. Owing to physical limitations, ATSs are only able to switch up to about 10kVA, whereas STSs can switch in the range of hundreds of kVA.
- Cost. Given their greater complexity and switching speed—and as you might expect—STSs are generally more expensive than ATSs. APC estimates initial cost in the case of rack-mount units at about $100 to $150 per kVA for an ATS versus $550 to $700 per kVA for an STS; that’s anywhere from 3.5 to 7 times more for an STS.
These are just a few considerations, but they illustrate the difficulty of this design choice: each option has benefits and drawbacks. A poor decision can potentially break the bank or provide insufficient capabilities. On some points, however, these differences may make your decision easier. If your budget is very tight, you might consider using ATSs simply to avoid the costs of their semiconductor-based alternatives. On the other hand, if you’re switching higher amounts of power for a large data center, STSs may be your only option.
One particularly noteworthy point is the reliability of the two switch types. Although STSs provide faster switching times, they are less reliable relative to ATSs, negatively affecting availability. This is a concern especially for a high-availability, mission-critical data center.
Faster Isn’t Always Better
An interesting conclusion of the APC white paper regards the use of switches in data centers that have a mix of single- and dual-corded equipment. To improve reliability, many servers, for instance, have two cords connected to two redundant power supplies. Thus, in the event of an outage to one cord, the server can switch to an alternative supply connected through the other cord. (Obviously, this system only works when the facility provides two separate power buses, one for each cord.)
But often, not all equipment in the data center offers two cords. Single-cord equipment is thus at the mercy of the lone power supply bus. Nevertheless, a power transfer switch can remedy this problem by switching between the two power buses, effectively obviating the need for redundant power supplies in the equipment (at least from the standpoint of preventing a single bus outage from taking down that equipment). Rack-mount transfer switches offer a number of benefits, such as modularity (their use is easily scalable according to need) and targeted use (they serve where power switching is specifically needed, rather than switching for the entire facility, even if this scope isn’t required).
According to APC, in this rack-mount application, ATSs actually tend to be the superior option relative to STSs, owing to their smaller size, lower initial cost and greater reliability, among other considerations. This superiority is despite the faster switching speed of an STS, thus illustrating that although one option may offer certain advantages that would seem to make it a no-brainer (an STS can handle higher power capacities and can switch faster), other considerations make the decision a little more difficult (an ATS is cheaper, smaller and more reliable).
Conclusions
The choice of using static transfer switches versus automatic transfer switches in your data center is not as simple as it might seem at first glance. Semiconductor technology offers some advantages by way of an STS, but the greater simplicity and lower cost of an ATS may make it the better option, even if it isn’t quite as flashy. Again, making the proper choice depends in large part on the intended use and placement in your power distribution design. Whatever your need, barring any overriding concern like high power capacity (which would tend to make an STS the only option), you should consider both STSs and the alternative—ATSs—as options for your data center.
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Photo courtesy of Yusmar Yahaya.