Uninterruptible Power Supplies (UPS) 1

Transcription

1 Uninterruptible Power Supplies (UPS) 1 In the not-so-distant past, good quality electric power received from the utility company could be properly defined as a power supply with reasonably good voltage regulation accompanied by relatively few and brief outages. This simple but adequate definition lost all validity with the launching of the solid-state electronic revolution - and most notably, with the proliferation of digital electronics. Power considered to be palatably clean by equipment such as motors, heating, and lighting can be deemed dirty and totally indigestible by solid-state electronic equipment. There are numerous types of power conditioners that eliminate or minimize power quality problems on an individual basis. Such equipment includes surge suppressors that effectively arrest transient spikes, voltage regulators that cope with problems of voltage deviation, and shielded isolation transformers that effectively screen out electrical noise, There are also hybrid conditioners that combine two or more of these individual functions. Depending on the severity of the power quality and extent to which it can be tolerated, such conditioners are often the most cost-effective solution. But when the problems are severe, and supplied systems and equipment have a low tolerance level for even occasional and minor power quality aberrations - and where operations must be maintained on total loss of power - only an uninterrupible power supply (UPS) suffices. A UPS is a very important device used with computer systems. It sits between your power source and the computer system and protects your components from harmful fluctuations (or anomalies) in electrical current. It provides battery backup power when current falls during a sag or blackout, and surge protection when it rises in a surge or a spike. If a blackout actually occurs, the UPS will keep your system running long enough for you to save your work and shut down gracefully (instead of your computer system suddenly dropping off the planet!). Some UPSes use their batteries to provide power to a PC at all times; others let the normal power source pass through, regulating it only when voltage fluctuations are detected. All static UPSs employ a solid-state inverter that converts direct current(dc) to alternating current(ac) at a voltage and frequency selected by the user. Depending on the particular type of system and its operating mode, dc is provided either by a rectifier fed from the utility supply or from a battery bank. The battery provides the power needed to sustain the system on loss of utility power. When interposed in operation between the ac power source and load, all static UPSs condition incoming power against any and all power quality aberrations. (More about the three basic types of UPSs later) 1 This article has been extracted from several articles in the Internet, hopefully it will provide information to enlighten our agents as to any questions concerning Uninterrruptible Power Supplies UPSs). ( If there are any questions contact Terry (303)

2 Ideally, the voltage flowing through your wiring should be represented as a perfect sine wave. Power conditioning helps regulate the steady flow of current by avoiding anomalies caused by outside forces. Five basic power events are considered anomalies: sags, blackouts, surges, spikes, and noise. A sag (or brown out) occurs when the voltage temporally drops below the normal level. This causes electrical components to work harder and can severely reduce the electrical equipment s efficiency and life span. A Blackout is a complete loss of electricity and can cause your system to shut down abruptly. A UPS senses a sag or blackout and kicks in its battery supply to pump the current back up to a normal level. A brief increase in voltage is called a surge. Surges often occur after shutting off a high-powered electrical device. When you turn off one juicy-hungry device, the electrical line will momentarily keep pumping at a higher rate and cause a surge. A spike is similar to a surge, only stronger and more severe, a spike can literally fry your hardware! The surge protection provided by most UPS units will also shield a PC from spikes. An irregular flow of current is called noise, or electromagnetic interference. The random flow of current caused by noise can corrupt program and data files on your system. Again, UPSes can compensate for those fluctuations. What causes these voltage irregularities? There are a variety of outside forces that may cause electrical current to deviate from its typical flow. For example, surges and sags occur frequently and are often caused by turning on or off other electrical devices - particularly large devices like air conditioners or photocopy machines - that share the same circuitry. Spikes are usually caused when lightning strikes a nearby phone or network line, or when power is suddenly restored after a blackout. What is a load? When you start shopping for a UPS, one term you ll have to be familiar with is load. The load represents the total wattage rating of all of the devices - such as your PC, external drives, or monitor - that you wish to protect with the UPS. To reduce your load and consequentially, the size of the UPS you need, you may wish to leave some of these components off the UPS. The batteries in a UPS provide DC power, but don t PCs use AC power? Yes, a PC runs on AC power, so the DC provided by a UPS battery wouldn t be much help. UPS s use a device called an inverter to convert DC to the AC power that your PC requires. Unfortunately, some energy is lost during the conversion process. Also, the size of the UPS battery determines the length of time that the UPS will function. Is there just one kind of UPS? No, there are three types of UPSes: standby (also called offline), line interactive and online. They each operate a little differently and vary in price.

3 1. Standby UPS s are passive systems that allow current to flow directly from the wall to the load (your PC and its peripherals) without providing any conditioning aside from basic surge protection. They have an offline inverter that kicks in when line power fails, transferring the power source from the wall to the UPS battery. When service is restored, the inverter returns to its offline state, allowing the current to flow directly from the wall to the load once again. Use of a standby UPS is limited to applications where the quality of power received from the utility is generally acceptable, and where loads can tolerate the brief power interruption incurred during transfer from utility to UPS. Standby UPSs are generally the least expensive. 2. Line-interactive UPS s (also known as a single conversion UPS) offer more advanced power conditioning than standby units. The term single conversion stems from the fact that utility power is not converted to dc for input to the inverter, but is fed directly to the critical load through a voltage-regulating transformer, usually of the tap-switching or ferroresonant type. A charger fed directly from the utility supply maintains battery charge. The term lineinteractive derives from the fact that with such units, the inverter interacts with the ac line to buck, boost, or replace incoming power as needed to maintain voltage control. The inverter interacts with the AC power line from the wall and is aware of an incoming power glitch before it reaches the load. This allows the inverter to kick in sooner, decreasing transfer time. Line-interactive UPSs increase and decrease the current to compensate for fluctuations. They provide a longer battery life and better protection for your equipment. 3. Online UPS s (also known as a reverse transfer, double-conversion, or rectifier/charger UPS) keep their inverters active at all times. Current for the load is drawn from the battery, which is constantly trickle-charged from the wall. During this process, the current has to be converted from AC to DC and then back to AC. This is known as a double-conversion process. Since the load is always powered by the battery, the UPS is able to achieve a zero-transfer time in the event of a power failure. Online UPSes provide a smoother flow of current and the best power conditioning for mission-critical operations. But because it is constantly in use, the battery s useful life is significantly shortened. Furthermore, the double-conversion process used by online UPS s results in less efficient energy use and a higher electric bill. While an on-line, double-conversion UPS provides the most comprehensive protection and power conditioning of the three basic UPS types, it also represents the most expensive approach - from both installed cost and operating expense standpoint. Refer to Table 1 for a listing of pros and cons of the different types of UPSs.

4 UPS Type Pros Cons Standby Inexpensive (typically less than $100) Limited power conditioning Surge protection Line Interactive Power conditioning Reasonably priced Surge protection Online Battery provides constant power to the load (zero transfer time) Power conditioning Surge protection Table 1. UPS Pros and Cons Not designed for mission critical operations that need a zero-transfer time for power. Most expensive, both in price and ownership cost Double-conversion process results in some energy loss and higher electric bill Which of these do I need? To protect your equipment from blackouts and surges, any of these UPSes will do. Modern power supplies on PCs can support blackouts of up to 28 milliseconds, which is longer than most UPS transfer times. For inexpensive protection from power failure, a standby UPS will do the job. If you want more advance voltage regulation, however, you ll need either a line-interactive or online UPS. For example, standby UPS s are ideal if you simply want to gain a few minutes to properly shut down your PC after a blackout, but if you need to protect a network running several processes and transferring gigabytes of data, you ll want the low-transfer-time battery power that a line-interactive or online UPS can provide. The lower ownership cost for line interactive UPS s makes them the most popular. What do I need to hook up a UPS? All that you need is a free serial port on your PC. A microprocessor in the UPS uses this serial connection to monitor power levels flowing to the PC and takes over when the voltage levels change. The UPS also uses the serial link to send instructions to your PC to perform operations such as displaying an alert and conducting an orderly shutdown. Do I really need a UPS? A UPS is essential for network servers. However, this doesn t mean that workstations hooked into the network can t reap the benefits of power conditioning. With the multi-gigabyte hard drive included in the average desktop system, every PC - whether or not it s connected to a network - has the potential to hold an invaluable amount of mission-critical information. If a UPS for every system doesn t fit into your budget, don t fret. Most new computers have power supplies that can handle moderate fluctuations, so a simple surge suppresser may be adequate. Also, many applications include auto-save features that enable you to retrieve unsaved changes in the event of an abrupt shutdown.

5 Your local wiring is a key factor, too. If your site s wiring is not up to industrial specs, you should have a UPS to protect your PC from the frequent sags and surges that will occur. Okay, I m convinced I need a UPS, what do I need to know? When purchasing a UPS, there are a few specifications you should consider to be sure you buy the right one. Above all, you ll want to check the UPS power rating to make sure it s able to support your load. Also, pay attention to its ability to handle surges and spikes, referred to as disturbance attenuation. Other areas that should be noted include the battery features, run-time, included software, notification means, transfer time and voltage regulation. What kind of power rating do I need, and how do I calculate it? Power ratings are expressed in volt-amperes (VA), which is simply the number of volts multiplied by the number of amps a UPS can deliver. The power rating necessary depends entirely on the load you wish to support, but there are a couple of ways to determine the power rating you need. The most scientific would be to measure your system s current draw using a device that plugs into a wall outlet. Alternately you can add up the power ratings from the nameplates on the backs of your computer equipment. This, however will give you an absolute worst-case scenario. With the exception of small units intended to serve a single PC workstation, PLC process controller, or other low-power consumption load, a UPS represents a sizeable investment. While one UPS manufacturer offers a 200-VA unit for $99 plus battery cost, other systems can cost hundreds of thousands of dollars. Cost of the support battery itself can readily amount to many tens of thousands of dollars, depending on the kva demands of the load and length of time that the battery is expected to support the load on loss of utility power. Following are some technical features of UPS systems that might help in determining the type of UPS that you need: Power Rating. Single-phase units are offered in ratings from 100 VA through 100 kva, and three-phase units from 10 kva through 4000 kva. It should be noted that for larger units, the maximums indicated for respective manufacturers are not absolute maximums for any given installation; higher-output systems can be attained by paralleling individual units. Enclosure Types. Some manufacturers offer small units in rack-mount versions. Such units can be handily applied to individual equipment such as programmable logic controllers. Free-standing units are offered in a variety of indoor and outdoor versions, including industrial grade enclosures approved for nuclear industry use. Cognizant of user awareness of the dollar value of floor space, some manufacturers are beginning to cite a small footprint as a significant feature of their product line.

6 Support Battery. Small UPSs are self-contained units in which the elements and support battery are housed in the same enclosure. For large UPSs, and those intended for extended battery operation on utility power loss, a free-standing battery bank is required. A variety of lead-acid and gel-cell battery types are available. Where a large, freestanding battery bank is required, it might be advisable to negotiate the battery purchase separately with various battery manufacturers. Battery Recharge Time. Time required to recharge the support battery following deep discharge incurred by an extended outage should be considered. One manufacturer, for example, cites some of its models as taking 4 hours to recharge to 80% capacity, some 4 hours to 90% capacity, and others 6 hours to 90% capacity. Transfer Time (Off-line Types). If an off-line unit will suffice in the application, transfer time required from utility to battery operation should nevertheless be considered. A choice must be made between an electromechanical or (faster) solid-state static switch. One manufacturer offers a make-before-break switch that eliminates a zero-voltage transfer condition when transfer occurs because the UPS has sensed that quality of incoming utility power has digressed beyond acceptable limits. Operating Energy Efficiency. All UPSs are energy consumers when in the operating mode, and energy efficiency can be an important consideration in an online UPS wherein the unit s power-consuming elements are in continuous operation. Efficiencies can vary considerably with the specific unit design. One manufacturer claims an overall efficiency or 94% for one model in its product line. Harmonic Contribution. Although UPS s screen out harmonics that are present on their input power supply and prevent those harmonics from affecting downstream equipment served from the UPS, all UPSs also generate harmonics that are reflected onto the upstream power system equipment. Some UPSs develop rather high total harmonic distortion (THD), but many units develop THD on the order of 5%, and one manufacturer offers a unit with less than 3% THD. Instrumentation. Instrumentation is offered as either standard or optional equipment. This includes ac input voltmeter, power rectifier and/or charger dc output voltmeter, rectifier/charger dc output ammeter, battery output voltmeter, battery output ammeter, system output ac voltmeter, system output frequency meter, and bypass source ac voltmeter. Indicators and Alarms. Status indicators and alarm functions include ac power failure, bypass source available, bypass source failure, bypass source high voltage, bypass source low voltage, transfer switch inoperative, in synch pilot light, ac available to charger, high dc voltage, low dc voltage, battery breaker open, battery ground fault, battery reverse polarity, negative/positive to ground, cooling fan failure, overtemperature, indicating lamp test push-button, blown fuse indication or alarm, high or low ac output, and inverter failure visions for remote monitoring of status indicators and alarm functions via modem connection (phew, that list sure was long!) Also

7 available is a monitoring/ alarm function that monitors condition of individual battery cells, and discloses when a cell is in need of replacement. Software Options. Software available with some systems automatically closes and saves all electronic files and performs an orderly computer shutdown in event of an incoming utility power interruption, and provides a list of all saved files on system reboot. Software is also available to maintain a log of the utility service record. Another software feature interrogates the equipment running on UPS battery power, and calculates and projects battery time remaining on a per-node basis. Warranty. Warranties vary widely, but a number of manufactures offer those that cover not only damage to the UPS, but also extend to damage suffered by equipment served by the UPS in event the UPS fails to properly do its job. One such warranty covers all damage to the UPS itself, and pays up to $25,000 to repair damaged equipment served by the UPS. Conclusion The final question comes down to How much is your data worth? If it is determined that you perform adequate daily backups and this data is not time critical to the minute, then you might get by with simple surge suppressers. If it is determined that you can t lose a single amount of data (such as a bank ATM machine), then you might need an Online UPS. The question of the severity of the lose of data will have to be answered before going out shopping for your UPS.

8 Surge Causes: Solution: Handy Reference Guide Lightning, power line switching, motors, copiers, laser printers. Surge suppressors, isolation transformers, line conditioners, uninterruptible power supplies (UPS). Over and Undervoltage Causes: Nearby utility substations, utility fault, generator fault, load increase/decrease. Line conditioners, uninterruptible power supplies (UPS). Harmonic distortion Causes: Process equipment, power conversion equipment, computers, computer monitors. Line Noise Causes: Brownout Causes: Blackout Causes: Filtering circuits, specialized transformers, line conditioners, uninterruptible power supplies (UPS). Arching contacts and switches, machine tools, poor grounding, fluorescent lighting. Surge suppressors, isolation transformers, line conditioners, uninterruptible power supplies (UPS). Reduced power supply voltage by utility, heavy loads, low generator output. Increase size of transformer supplying facility power, load management strategies, line conditioners, uniterruptible power supplies (UPS). Storms, accidents/malfunctions involving utility equipment, generator failure, blown fuse/tripped circuit breaker. Uninterruptible power supplies (UPS), backup generators