UPS systems in the data centre environment
Modern electronic equipment, and particularly modern servers, needs good quality, clean electrical power. Data centres and colocation houses exist to provide a benign environment for such equipment and Uninterruptable Power Supplies are a critical component of that. Nowhere is mains electricity sufficiently reliable for servers and the like to be connected directly to the mains. In remote environments or in environments where hurricanes or tornados occur regularly, outages may last several days. In more urban first-world environments, outages are less common but still occur.
At City Lifeline, a commercial colocation data centre in the heart of London, we regularly experience small sub-second outages, and, in the last five years have experienced two outages of eight hours each and one of sixteen hours. We have also experienced a major voltage surge when a utility substation in east London blew up in 2007, causing the closure of several rail stations and many offices. Lightning suppressors on all mains feeds as necessary to divert the energy contained in such surges to ground. To protect customer operations from the effects of such events, good quality, reliable, well maintained UPS systems backed up by modern diesel generators are essential.
Mains outages are not the only problem. Modern servers are built for a cost and, these days, also for energy efficiency. That has come to mean reduced resilience to mains fluctuations, to the point where some servers will operate at high temperatures but will not tolerate mains disturbances of under a cycle or even a half cycle. They do not tolerate voltage spikes or externally-generated harmonics on their power feeds and behave unpredictably when the mains voltage dips in a brownout. Connecting them directly to the mains will not result in consistently reliable operation.
UPS systems come in two main types. By far the most common is battery backed double conversion. This coverts ac to dc and uses it to charge a string of batteries in series. Essentially, these are the 12 volt batteries that go in any car, although their internal technology is optimised to suit this specific application rather than for sitting under a car bonnet. The resulting dc voltage, perhaps 480 volt, depending on the mains voltage and the specific design is then chopped up by IGBT devices (essentially a big bipolar transistor with an FET on the front to reduce the drive impedance) to produce a synthetic 50Hz ac waveform. This is used to supply the electronic equipment which forms the load.
The advantage of this is that the load is always supplied by the synthetic mains, so it is immune from mains outages, spikes and transients. The mains acts solely to charge the batteries, and the irregularities on the mains affect only the charging of the batteries. If there is a failure of the mains, the generation of the synthetic mains will continue for as longs as the batteries retain charge. Typically, in a data centre, that will be between 10 and 20 minutes, giving enough time for diesel generator backup to start and take effect – and, crucially, also allowing operators time to think and correct switching errors during maintenance.
Another type of UPS is a rotary system. This uses the mains to run a flywheel up to speed. When a mains outage occurs, the energy stored in the flywheel is transferred to the load equipment, giving the standby generators time to start before the flywheel slows down. Their main advantage over battery systems is the elimination of the batteries (although batteries are still needed to start the diesel generator). Although there are some UK installations, they have not so far found general favour in data centres and most UPS systems remain battery backed.
In common with all critical system installations, care and thought need to be put into configuration, installation and maintenance. Batteries are heavy, contain acid, give off explosive hydrogen gas and need to be handled with care. Floor loadings need to be checked. Methods of ventilation need to be thought about. Bypass systems are needed to allow maintenance without disrupting the load. If there are diverse feeds into the facility, as there will usually be in a data centre, then care needs to be taken with synchronisation of generators and mains during switching operations. Diesel generators, too, need to be thought about. The load presented to the alternator by a UPS is not linear and the alternator needs to be correctly sized to deal with the harmonics present at the interface.
UPS systems have been at the forefront of mission critical high reliability, high availability systems for many years, and that is likely to continue. Commercial data centres, like City Lifeline in London, will continue to install and operate UPS systems as an integral part of their strategy to deliver 100% uptime to their customers’ critical operations.
This was posted in Bdaily's Members' News section by Roger Keenan .
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