UPS systems - Uninterruptible power supplies – not just about power cuts
By Rob Morris, UK country manager, POWERVAR
There are two important reasons for computers users to install UPS (uninterruptible power supply) technology. The first, ensuring continuity of AC power when the mains supply goes down, is widely understood. The second, protecting sensitive electronic equipment, especially computers, extending their working life and guarding against the corrosive causes of sudden component failures is less widely appreciated.
Fortunately, power cuts are relatively rare occurrences but organisations need to give more thought to the vital benefits of a supply of clean, quality power uncontaminated by line noise, impulses, common mode disturbances and other problems inherent in the mains supply. In theory, a UPS system will give users the best of both worlds. But in reality, many UPS designs are just not up to scratch as acceptable power quality tools. A closer look at the issues involved can help preventing costly and usually, sudden, system failures. After all, what is the point of a piece of equipment that provides a reasonable degree of backup but is not delivering the day-to-day protection against damaging ‘gremlins' in the power supply - spikes, surges and other disruptions, things that are not only be invisible but tend to occur more often than power failures.
An important choice
Choosing between on-line UPS or standby UPS is key. This choice should not be influenced by how much protection is offered by a particular inverter design, but rather what type of technology the UPS system is protecting. Many industrial plants use a combination of both linear and switch mode power supplies.
On the one hand linear supplies contribute less noise to the electrical environment, yet switch mode supplies are popular because of their lower cost, higher efficiency, smaller size and low heat contribution.
Most linear power supplies lack the internal storage necessary for them function during the 4-6 msec needed for a UPS to switch from AC line to battery-powered inverter. What is more, linear supplies are sensitive to voltage, which means it is vital that they are well regulated.
By contrast , switch mode supplies are relatively immune even to large swings in line voltage and will perform satisfactorily as part of standby UPS systems. These criteria are crucial consideration in the initial choice of the most appropriate and effective UPS design namely, standby UPS for switch mode power supplies and on-line UPS for linear power supplies.
It is a myth that on-line UPS products deliver a higher level of power quality simply by virtue of their double conversion (AC-DC-AC) design. In fact, a well-designed standby UPS product (single conversion) can provide much better protection than more expensive poorly designed on-line systems.
Single or multiple units?
Another contentious point is whether it is best to install one large UPS or to use a number of smaller UPS products distributed throughout a facility.
Using a single large UPS offers the economies of scale. However, a number of small individual UPS systems offer more flexibility. A 15kVA system is, for example, likely to be less expensive than say, 30 individual 500VA units. And the ability to deliver more usable single-phase power from a centrally installed three-phase UPS than from several individual single-phase units can also be beneficial.
However, small individual UPS systems provide greater flexibility, require less distribution wiring, are easier to maintain and give the extra security of knowing that a UPS failure in one area will not affect the others. Ultimately, the user needs to make a decision that is consistent with their overall approach to maintenance and installation.
Clean Power
Power quality issues should be carefully considered when selecting a UPS. A system that is expected to protect against power outages should not be allowed to fail for other reasons either. The careful selection of a UPS system can deliver protection from a complete range of power problems, added to the fact that many UPS systems can be combined with other devices to further improve power quality.
Power quality involves more than having a clean, noise-free electrical supply. Ultimately it depends on the requirements of the equipment it is protecting. For industrial electronic systems some of the quality issues are:
- Impulses - They may be of short duration but these fast rise time, high energy events can pack significant destructive potential and can result in the catastrophic failure of semiconductor devices. Even when not immediately or apparently destructive, these impulses contain sufficient energy to erode or weaken semi-conductor junctions which can result in their eventual failure. The effect of these impulses can be mitigated by a surge diverter.
= Common mode voltage - This is any voltage measured with reference to safety earth. Since most modern computer systems use safety earth as their logic and communications reference, clean and quiet safety earths are essential. The kinds of problems that common mode voltage can cause computers include: processor lockup, lost or fragmented data, communication errors, or unexplainable ‘no trouble found' failures. Common mode voltage may occur in systems with daisy-chained or undersized neutrals, or when branch circuits become lengthy. Using an isolation transformer to create a separately derived power source eliminates common mode voltage.
- Noise - This is a result of the distribution and use of electrical power. RFI and EMI are generated by every device that uses electricity. Perversely, many of the disturbances that produce computer malfunctions are induced by the computer's own electrical system. Noise filters built from capacitive and inductive elements will divert disturbances to electrical system earth.
- Voltage regulation - This is more critical issue for linear power supplies than for systems powered by switch mode supplies. There are a number of methods of providing well-regulated voltage, including; tap-switching or ferro-resonant voltage regulators and buck/boost autoformers. An on-line UPS provides natural voltage regulation because of its built-in double conversion process.
- Outages. - These can only be mitigated with a UPS device that supplies reserve power from its batteries. These UPS systems can be an effective bridge until either the utility is restored or another source is brought on line.
- Frequency changes - These are caused by variations in the fundamental operating frequency of the electrical system. Unlike single conversion designs, double-conversion UPS systems can act as frequency regulators and high-quality on-line UPSs can regulate the output frequency tightly across a very wide range of input frequencies.
Clean, continuous power
While single-conversion UPS systems may not deal with all of the six elements described above, double-conversion systems will typically provide superior protection against normal mode noise and impulses.
Surge diverters, tap-switching voltage regulators and battery-powered inverters are found in single conversion designs but most do not include either a powerline noise filter or an isolation transformer.
Double-conversion UPSs offer better protection from normal mode noise and impulses and also produce an output free from voltage and frequency regulation problems. However, many do not include an output isolation transformer, which leaves the load vulnerable to disruptive common mode disturbances.
To ensure clean, continuous power for computing applications one should begin by examining the application itself and the technology to be protected. If linear power supplies are part of the application, the on-line UPS is a natural choice. If the application involves switch mode power supplies (as an increasing number do), either on-line or standby UPSs will provide backup power.
Other factors, such as personal preference, budget or other issues, can also come into play. If harmonics and power factor are factors there are on-line designs available with unity power factor input and low front-end harmonic contribution. These may be compelling reasons for preferring one technology over another.
Regardless of the design, it is important to ensure that a UPS delivers a high level of protection from noise, impulses and common mode disturbances. For this reason, noise filters, surge diverters and isolation transformers can be regarded as essential features of the UPS design.
Installation options
It should always be remembered that powerline noise and common mode disturbances are partly a function of branch circuit length. This means that while these disturbances may not exist in the output of an isolated UPS, they will gradually reappear as the output circuit length increases.
This is one disadvantage of a centralised UPS system. Even the best designed centralised UPS may prove incapable of providing clean power for a load located some distance from the UPS. So while the UPS portion of the overall power solution may be located where it can provide convenience and availability, the elements that ensure clean power (isolation transformers, filters, surge diverters etc) must be installed as close as possible to the load they are protecting.
As a result, industrial users are now specifying a less expensive central UPS and installing separate discrete power conditioners by each computer, control rack or cabinet.
Systems that must not fail when there is a power outage should also be protected from failures caused other power anomalies. But the fact is that not all UPS systems provide clean power. It is possible to select a UPS that does give complete protection. It is also feasible to combine a UPS with other devices to achieve clean, continuous power for industrial computing installations. The correct starting point is a close examination of the application and the technology that needs to be protected.
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