How to Avoid Common Mistakes in Power Conditioning

How to Avoid Common Mistakes in Power Conditioning

Traditional loads often generate objectionable power disturbances. When power quality problems cause high-technology systems to fail, the solution is usually the application of some form of power conditioning device.

Today's computer systems have different power protection requirements than their predecessors

Power quality problems frequently affect sensitive electronic systems in both industrial and commercial facilities. Buildings with numerous traditional loads often generate objectionable power disturbances that can result in a low-technology environment supporting high-technology systems. When power quality problems cause high-technology systems to fail, the solution is usually the application of some form of power conditioning device.

Users often make errors in the selection, application, and installation of mitigation devices, thereby preventing them from fully realizing the benefit of their power protection investment. However, an understanding of basic power quality needs for modern computer technology can help you avoid those mistakes.

What protection do you need? A discussion of the power protection requirements of today's computer systems invariably involves references to the CBEMA curve, named for the Computer and Business Manufacturers Association, which was renamed the Information Technology Industry Council in 2000 (Fig. 1 at right). Although this graph illustrates the degree to which electronic systems should tolerate voltage variations, its application has two fundamental shortcomings.

First, most modern computers use switched mode power supplies (SMPS) capable of tolerating voltage variations much higher than those shown in the curve.

Second, and more importantly, the curve addresses only variations to nominal voltage between primary power conductors or phase-to-neutral conductors, and ignores voltage disturbances between neutral and ground. SMPS technology doesn't incorporate the step down/isolation transformer that was a part of the linear power supply design predating the SMPS, resulting in a susceptibility to neutral-to-ground noise voltage disturbances for most modern computer systems. Recognizing this fact is critical in your selection and application of power conditioning technology and deciding where you should install the conditioning device.

What's available?

The most common protection technologies include transient voltage surge suppressors (TVSS), uninterruptible power supplies (UPS), and transformer-based power conditioners. An examination of some of the most common mistakes made with each type of device is helpful in avoiding problems.

The most common mistake made by users when selecting a TVSS is misinterpreting the rating as shown on the label. Most TVSS manufacturers label their product with a UL 1449 rating, and ratings of 330V L-N (line-to-neutral) and 330V N-G (neutral-to-ground) are common specifications. Many users believe this means the surge suppressor will clamp all voltage impulses up to 330V. In fact, the reverse is true: the rating states the lowest voltage level to which the TVSS device is capable of clamping high-voltage impulses. This is an important distinction because voltage impulses smaller than 330V will “slip by” the TVSS. If low-voltage impulses or high-frequency noise are the source of system malfunction, then TVSS technology isn't the appropriate solution.

Incorrect installation of TVSS products is equally problematic. Users often install TVSS devices to solve power supply failures only to find that poor placement can lead to system lockups, unnecessary service calls, and other problems. TVSS devices function by diverting surge current to the facility's grounding conductor. Plug-and-receptacle TVSS devices are frequently installed at long branch receptacles. The resulting surge current flowing through the impedance of the grounding conductor results in a substantial neutral-to-ground voltage at the receptacle. Since the computer's SMPS doesn't contain front-end isolation, the neutral-to-ground voltage often causes reliability issues.

In fact, plug-and-receptacle TVSS products have limited uses. The most effective deployment of TVSS technology involves the use of hardwired TVSS products at the facility service entrance. They can also be used at a distribution transformer where a neutral-to-ground bond is re-established per the requirements of the NEC. The clamping function of the TVSS also must not cause a neutral-to-ground voltage differential.

The first criterion to consider in the selection of a UPS is whether or not the device is equipped with an inverter. Next, you must look at the inverter's design. High-quality UPS inverters will convert stored DC energy into a low-distortion sine wave suitable for powering computers. However, UPS devices equipped with sine wave inverters are expensive, and many consumers may attempt to cut costs by selecting cheaper models with less effective modified inverters. Pursuing low-priced solutions is a common mistake. Where UPS products are concerned, an inverter capable of producing a low-distortion sine wave — the type of power provided by the electrical utility — is a far more appropriate choice.

Fig. 2 shows the inverter waveform of such a design. Fig. 3 and Fig. 4 on the same page show normal mode (line-to-neutral) disturbances and neutral-to-ground voltage disturbances generated by the inverter, respectively. It's important to note three of the details illustrated in these figures. First, the inverter voltage is 440V peak-to-peak, instead of a normal sine wave peak-to-peak voltage of 340V. Second, the inverter generates normal mode impulses of 178V and neutral-to-ground noise voltage reaching 2.32V. And finally, non-sinusoidal inverter waveforms are usually rich in harmonic content.

Larger UPS models — above 2kVA — generally come equipped with sine-wave inverters, eliminating the problems associated with converting DC energy. However, larger UPS products bring with them a different set of issues to consider prior to purchase and installation. Battery maintenance, serviceability, the need for maintenance/service bypass, compatibility and availability of input power, and distribution of output power are all important factors. It's also wise to remember that most UPS products are high-technology electronic systems that require a technology-friendly environment as well.

Selecting a UPS that can't meet the needs of the device it's meant to protect is the most common mistake. As mentioned earlier, SMPS are susceptible to neutral-to-ground noise voltage disturbances, so you must protect computers with a UPS that incorporates an output isolation transformer. Keep in mind, however, that if you're going to install the UPS at a remote location, such as a utility room, the output isolation transformer will provide little if any benefit. In such situations, it's preferable to install a non-isolated UPS and then provide some means of isolation at the point of use.

Don't install a UPS in a cramped, hot location with poor ventilation. Doing so will lead to shortened battery life and a higher cost of ownership. If you install one near the facility's electrical service entrance, you should consider protecting it from catastrophic voltage transients by installing a TVSS device ahead of it.

Similar to UPS products, a power conditioner should incorporate an isolation transformer and be located near the computer it's protecting. If it's too far from the load, the impedance of the power wiring between the conditioner and the computer will become problematic, and power that was noise-free at the output of the conditioner will become noisy again at the load.

Another common mistake is ignoring the design of the isolation transformer used in the power conditioner. For today's technology, the transformer should be low-impedance, on the order of 2%. This is critical because SMPS consume power from the main in large gulps for short periods of time near the peak of sine-wave rotation. Higher impedance transformers can't faithfully accommodate such nonlinear current requirements, starving the power supply. Their inability to respond to current demand from the load results in distortion and “flat-topping” of the voltage waveform. Low-impedance transformers tend to look “transparent” to the load with respect to current demand. This will allow the load to draw the necessary peak current without degrading the secondary voltage waveform.

A low-impedance design will also help prevent oversizing, another common selection mistake. Nonlinear loads are characterized by high crest factors. To avoid the aforementioned waveform distortion associated with impedance-transformers, size the transformer to the crest factor of the load rather than the rms current demand. Low-impedance designs are more expensive, but preventing the problems associated with oversizing will justify the cost. You'll also benefit from the smaller footprint and higher efficiency of the low-impedance design.

Unless a power conditioner incorporates a surge diverter in the form of a TVSS, and a high-frequency noise filter, it's little more than an isolation transformer in a box. These components will turn an ordinary isolation transformer into a bi-directional power conditioner capable of addressing power disturbances regardless of whether they originate at the line or load side.

Too many facilities managers continue to rely on outdated power protection tactics. Networked computer environments have rendered methods like dedicated/isolated electrical circuits useless, and the design of SMPS makes the use of voltage regulators unnecessary except for all but the most extreme cases. Proper selection and application of power quality solutions begins with an understanding of the equipment that must be protected. Only after you understand the system's power quality needs can you select the best solution and design the best implementation.

Jose is the engineering manager and Huss is the 3-phase engineering manager for Powervar, Inc. in Waukegan, Ill.

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