Power Quality on a Budget
Power quality is a hot topic in today's computer-rich environment. The cost of lost productivity, lost product or damaged equipment is potentially high. For a major corporate or financial data-processing center, the cost of downtime can be millions of dollars per hour. On the other hand, when it comes to general business, school or other non-critical operations, the cost of downtime is not quit...
Power quality is a hot topic in today's computer-rich environment. The cost of lost productivity, lost product or damaged equipment is potentially high. For a major corporate or financial data-processing center, the cost of downtime can be millions of dollars per hour.
On the other hand, when it comes to general business, school or other non-critical operations, the cost of downtime is not quite so extreme. But even though these facilities cannot justify a high level of investment in power quality, there are basic, budget-friendly measures that can improve power quality throughout a facility—and reduce equipment damage and downtime.
The PQ Culprits
The ultimate goal of power-quality strategy is to provide all of a building's utilization equipment with power that is free from outages, voltage transients and other distortions. A power outage might be an overall system outage from the utility supply, or an isolated outage in the facility due to an unnecessary event. Transients are momentary disturbances where the system voltage is out of tolerance, either high (spikes) or low (notches). Voltage spikes can be caused by events such as lightning strikes or utility-line capacitor switching, and are generally a short burst of energy passing through the system. Voltage spikes can damage equipment by breaking down equipment insulation on wiring, especially on printed circuit boards where electrical tolerances are tight.
Voltage distortions in a power-distribution system are longer-term disturbances such as brownouts—which are low voltage conditions—ring waves or harmonics. These distortions generally cause overheating or other problems with the operation of protective devices. For example, motors draw power according to the mechanical torque on the shaft. When the voltage is reduced at the motor, more amps are drawn to maintain the power output. With more amps passing through the motor windings, additional heat is created.
Harmonic voltages and currents also result in overheating of wiring, especially in equipment such as transformers and motors, and can cause improper voltage detection and regulation in variable-frequency drives and generators. High harmonic currents are probably most evident where combined neutrals are used for multiple branch circuits, because the triplen harmonics add on the neutral, instead of canceling like the fundamental 60-Hz frequency current.
Studies indicate that approximately 90% of all power quality problems in a facility are generated within the facility. Harmonic voltage and current distortion is created by facility loads. Switch-mode power supplies used in electronic ballasts, computers and office equipment take power nonlinearly, which creates harmonics. Control motors in copiers and commutation contacts in motors create small voltage spikes and noise on the power system. In essence, the power system must be protected from itself.
The PQ Budget Plan
Mission-critical facilities incorporate equipment and methods to improve all aspects of power quality. Power outages are the highest profile problem and require the highest-cost equipment to fix. Uninterruptible power supply (UPS) systems are employed with backup generation systems to ride through power outages, and with rising demands for total uptime, they are being designed as systems with ever- increasing redundancy.
The highest level of design to protect against voltage transients is to employ cascaded surge-protection devices (SPDs) to protect at each distribution level. The equipment requirements are reduced as they move away from the service entrance location, until the lowest level is the surge protection receptacle. In order to reduce harmonic voltage distortion on the system, a variety of products and designs may be employed, including harmonic mitigating transformers, k-rated transformers and harmonic filters.
Designing for power quality on a tighter budget requires incorporating similar equipment with a prioritized approach, and using design and installation techniques to provide the greatest effect for the least dollar. Most utility power outages are only momentary and can be ridden through without backup generation. A common approach is for a facility owner to identify the highest priority equipment—a centralized server or phone switch, for example—and provide individual UPS protection for the equipment. In fact, a good design practice is to identify this important equipment and provide dedicated power circuits from the branch panel to avoid nuisance trips caused by other equipment.
Two means of protecting against voltage transients are: SPDs and circuit segregation. SPDs can be deployed at critical locations and save the cost of a total cascaded system. The highest energy, and hence most damaging transients enter the distribution system from the utility due to local lightning and capacitors. Therefore, protection should be provided at the service entrance. The system should also be designed to segregate mechanical system load from all other loads to provide protection between the two systems. In most small- to medium-sized facilities, the major mechanical loads can be served from the main service switchboard or a dedicated distribution panel or motor control center, where the SPD located at the service entrance also provides protection between systems. In larger facilities, additional consideration should be given to taking 480Y/277-volt utility service. But the system architecture should also include an additional SPD at the 208Y/120-volt main distribution panel, which will provide greater protection of sensitive equipment from utility and facility generated transients.
Circuit segregation should be provided between critical equipment circuits and general equipment branch circuits on the same panelboard in order to provide as much circuit impedance as possible from transient-generating sources. Where there are enough branch circuits of critical equipment to warrant it, a separate branch panelboard should be provided just for critical equipment. Additional consideration should also be given to installing SPDs at these separate branch panelboards, especially for functions like computer labs or server rooms. Loads for office equipment such as copiers, staplers and pencil sharpeners should always be as far from electrically sensitive equipment as possible.
The Ubiquity of Harmonics
As mentioned earlier, the most common voltage distortion nuisance comes from within a facility, in the form of harmonic voltage and current distortion. Harmonics generally cause problems due to overheating or nuisance circuit-breaker trips. Two general design approaches for dealing with harmonics are:
Oversizing to account for heating.
Cancellation or filtering to reduce harmonics.
Generally, oversizing methods include: the use of k-rated transformers and installing 200% rated neutrals to branch panelboards. Also, limiting the number of devices or equipment connected to a circuit is, in effect, a way of oversizing the branch circuit. Finally, using dedicated neutrals for branch circuits—and not allowing a combined neutral for three single branch circuits—reduces overheating problems.
Use of any oversizing method will mean additional system cost, but this approach generally costs less than employing filtering techniques. However, harmonic mitigating transformers can be installed within certain medium and large distribution system designs for minimal to no additional cost and provide greatly reduced harmonic currents on the upstream side of the transformer. Cancellation of harmonic currents also provides energy savings. Downstream of a harmonic mitigating transformer, the same circuit sizing, segregation and separate neutral techniques should be employed.
The power-distribution system design for any modern facility needs to be conscious of power quality issues—regardless of the facility type and project budget. The designer should incorporate basic segregation techniques and marry the level of system segregation and protective equipment with project scope and budget. Basic segregation of critical or sensitive and non-critical equipment on branch circuits and use of separate neutrals is the starting point.
Additional segregation of the systems and critical locations to install SPDs becomes the intermediate cost vs. benefit decision. And finally, the use of specific equipment protection with SPDs, filters or UPS systems is determined by the cost of equipment failure vs. the cost of protection.
From Pure Power, Winter 2002
A Glossary of Power Events
The most common types of power events and their effects are described below. Underfrequency and overfrequency also are power-quality parameters, but they normally result from one of the following, or are directly related to generation problems:
Voltage sag is a momentary voltage dip of 15% or more and can last for a few hundred cycles. It is typically caused by fault conditions on either the transmission line or the distribution line, or it can be caused by large current in-rush due to large motor starting, large transformer magnetizing current and other large loads instantaneously applied to the distribution system. Electromechanical devices are somewhat immune to voltage sag and can continue to function throughout the duration, but the same sag can be disastrous for microcomputer-based equipment.
Voltage swell is similar to voltage sag both in magnitude and duration, but the voltage rises by at least 15%. The typical cause is a phase-to-ground fault on a three-phase, four-wire distribution system. The voltage swell occurs on the two unfaulted phases. The effect on electromagnetical devices is negligible. However, voltage swells can cause microprocessors to yield invalid output without causing physical damage to the devices.
Harmonics have always existed in AC power systems. Harmonics in both the voltage and current waveform components of power systems are simple integer multiples of the fundamental frequency (60 Hz, 180 Hz, 300 Hz). Switched-pulse mode power supplies—such as those used in personal computers and power conversion devices similar to variable-frequency drives—create nonlinear loads that no longer result in smooth sinusoidal waveforms.
The current component of harmonics is of particular concern due to neutral heating and significantly distorted voltage waveforms. Transformer cores become saturated by the distort current, and the consequent heating of neutral conductors, transformers, switchboards and panelboards has a significant impact on a building's entire electrical distribution system. Unmitigated neutral heating limits overall system capacity, shortens equipment life and can damage the panelboard neutral bar.
Transient-voltage surge or voltage spike typically consists of a very-high-magnitude pulse of extremely short duration. Lightning is the primary source; however, utility circuit-breaker operation or capacitor switching can cause the same effect. Microprocessor-based and other electronic devices are susceptible to significant damage. Electromechanical equipment can be damaged—due to insulation breakdown—but this is unlikely.
Undervoltage is a condition where the available voltage is 90% of the nominal voltage. The duration of the condition can be from a few minutes to several hours. Undervoltages usually result from system overload or utility-initiated brownout. Microprocessor-based devices can accommodate undervoltage with few or no significant problems, but electromechanical devices may be forced to shut down.
Overvoltage is excess voltage of 10% above nominal for a sustained duration of a few minutes to several hours. The primary cause is improper voltage regulation by the utility, usually due to poor control of voltage regulators or capacitor banks. This condition generally does not cause significant problems for either microprocessor-based devices or electromechanical devices, but it does shorten incandescent lamp life and may damage some electronic fluorescent ballasts.
Power outages are, of course, complete interruptions of power lasting from seconds to days. Obviously, the impact of a power interruption, both for microprocessor-based and electromechanical equipment, can be a tremendous loss of business—and of considerable revenue—for commercial and industrial facilities.