AUB Can Assist in Solving Problems
There is a solution for most every power quality problem. But you must be sure to choose the correct option to avoid either addressing only a symptom or, worse yet, creating a new problem. Does your facility's problem require surge suppressors, power conditioners, or regulators? Or does the solution lie in filters, transformers, and breaker panels?
Recent studies conclude that 80 to 90 percent of power quality issues result from unforeseen onsite problems ranging from improper grounding and bonding to code violations and internally generated power disturbances.
AUB teams up with TVA to offer valuable power quality audits that can help you home in on the real problem and then fashion an effective solution through consideration of cost-effective techniques and equipment. The investment in a solution now can pay off over the years as well by making your building and equipment more supportive of future use changes and demands.
What is Power Quality?
The term means different things to different people. Basically, power quality has to do with the relative frequency in the power supplied to electrical equipment from the customary, steady, 60 Hz, sinusoidal waveform of voltage or current. A power quality problem generally means there is sufficient deviation from norms in the power supply to cause operational problems with equipment. Good power quality would be a reliable supply of sinusoidal, 60Hz waveforms resulting in few operational anomalies.
Some pieces of equipment are more sensitive to such deviations than others. A variation that may be a power quality problem for one device may go virtually "unnoticed" by another.
With today's widespread computer use and computer-based equipment, variable-speed drives, large startup motors, arc welders and other arc devices, and uninterruptible power supplies, power quality is particularly at issue. People in industry are concerned not only about the physical effects on equipment, but by downtime and lost productivity that may come as a result.
Power Quality Problems
Surges and spikes (overpowering), transients, blackouts, noise and sags (under powering) are common names given to power quality problems. Poor power quality can yield repeated equipment failures, safety hazards, process interruptions, and shutdowns.
The vast majority of power quality problems in a building originate within that very building. (According to the Electric Power Research Institute, as much as 80% of power quality problems relate to inadequate wiring or grounding.) Power quality problems frequently can be avoided entirely through careful design of building systems. In existing buildings, they are sometimes alleviated or eliminated through simple, often inexpensive, changes.
Known as spikes, swells, and surges, transients are the most common power quality problems. Luckily, they are often the easiest to correct. But, they can be hard to detect because they show up only as short-duration changes in voltage. Swells may be 5 to 10 times normal voltage levels-enough to wipe out stored data and create myriad computer equipment problems. And swells can significantly reduce the life of equipment. Low-energy swells are caused by the switching on and off of the electric motors that power items such as air conditioners, power tools, furnace ignitions, electrostatic copiers, arc welders and elevators. Lightning usually causes larger swells. But a direct hit is not required. A lightning strike several miles away can travel through transmission lines and send a voltage swell into your facility. Electrical noise is another, milder transient power anomaly that may only cause computer glitch as opposed to equipment failure. Electrical noise is created when one piece of equipment interacts negatively with another, or with building grounding or wiring. Loose connections or the equipment itself can be responsible for noise. Known noise-generating equipment includes everything from computers, radios and fluorescent lights to fax machines, welders and light sockets.
Lights flickering? It could be the result of voltage fluctuations in your facility's electrical system. High- and low-voltage conditions can result in equipment damage, data loss and erroneous readings on monitoring systems. Overloaded power circuits are typically the cause behind under-voltage conditions. Heavily loaded motors such as air conditioners can result in intermittent low voltages. Less common but more damaging are over-voltage conditions, which can be seen in facilities that have rapidly varying loads.
Harmonics are not a new phenomenon. But the excessive amounts of certain odd harmonics generated by today's technology can cause good neutral cables and transformers to overheat. Harmonics can be particularly disruptive for companies that rely on communications, process control or information technology. Slowed data transmissions, higher than normal operating temperatures, excessive motor vibrations and equipment malfunctions, all can be indicative of harmonic distortion.
If you've investigated such problems but have detected no transients or voltage fluctuations, harmonic distortion may be the real culprit. It comes into play when voltage frequency increases five percent beyond the normal 60 Hz power coming from your power supplier, such as AUB. Harmonic distortion is almost always created by a customer's in-house equipment such as variable speed drives, computer supplies, electronic ballasts, dimmer switches, and so forth, many of which themselves are sensitive to harmonic distortion.
Use Double-Size Neutrals, or Separate Neutrals per Phase.
- Harmonics are more than an inconvenience or source of equipment malfunction. They can be a serious safety concern. Fortunately, they can be easily handled by using double-size neutrals. Alternatively, separate neutrals can be used for each phase conductor.
- At least one cable manufacturer makes a Type AC or MC cable with oversized or extra neutral conductors built-in. The additional cost of over sizing the neutral is minimal. And the safety provided will be functional, even allowing for changes in the equipment that affect the frequencies involved.
- Filters are sometimes most cost effective in an existing structure where rewiring is difficult or costly. Filters block or trap the offending currents, lessening the harmonic loads on the wiring. But the filter design is dependent on the equipment on which it is installed and may be ineffective if the particular piece of equipment is changed. Filtering characteristics need to be carefully designed for a given installation. Seek professional design advice, as filters are also fairly expensive on a per-kVA basis.
Shielded Isolation Transformers
- Shielded isolation transformers are filtering devices that lessen feed-through of harmonic frequencies from the source or the load. They are a plausible retrofit technique where power problems have already been encountered, but are also quite expensive per-kVA.
- K-rated transformers have beefed-up conductors (and sometimes cooling) to safely handle harmonic loads. Alternatively, standard transformers are sometimes de-rated to allow for the extra heating brought on by harmonics. Depending on the conditions encountered, a load limit of as little as 50% of the nameplate rating is observed. While this may be adequate to handle harmonics, it lowers effective transformer efficiency. A careful comparison of the relative costs of K-rated versus de-rated standard transformers should be made.
Harmonic-Rated Circuit Breakers and Panels
- Overheating due to harmonics is the danger here, and beefed-up components used in these elements offer protection. Neutral buses should be rated for double the phase current.
The term "ground" refers to the earth, or a large body that serves in place of the earth. The term "grounded", then, refers to a system in which one of the elements is purposely connected to "ground."
Electrical systems need not be grounded to function. Indeed, not all electrical systems are grounded. But the voltages referenced when talking about electrical systems are usually voltages with respect to ground. Ground, therefore, represents the reference point, or zero potential point, to which all other voltages refer. As computerized equipment communicates with other equipment, a zero reference voltage is critical for proper operation.
The ground (earth), then, is a good choice as the zero reference point in most cases since it surrounds us everywhere. When one is standing on the ground, one's body is approximately at the voltage potential of the earth. If the building is metal framed, the metal components of the building structure, or the water piping (if metallic), are approximately at ground potential.
Why Grounded Systems Are Preferred
The primary purpose of grounding electrical systems is to protect personnel and property if a fault (short circuit) were to occur. In simple terms, if one of the three hot legs (phases) of an ungrounded electric service becomes grounded, intentionally or accidentally, nothing happens. No circuit breaker trips, no equipment stops running. Ungrounded electrical systems were popular in industrial buildings of the first half of the 20th century precisely for the reason that motor-driven loads, which were the most common at the time, would not stop simply because of a short.
But a consequence of this type of system is that it is possible for the frame of a piece of equipment to become energized at some voltage above ground, and present a shock hazard for personnel who may be touching the equipment and a grounded component of the structure simultaneously.
A second purpose of a grounding system is to provide a controlled, low impedance path for lightning-induced currents to flow to the earth harmlessly.
Sensitive Electronic Equipment
With the proliferation of PCs and other microprocessor-controlled equipment, most of today's factory environments are computer-controlled.
Concurrent with the proliferation of these sensitive devices, the devices themselves have been changing in ways that make them more sensitive to power irregularities. Operating speeds continue to increase (in the radio frequency range), making the circuits more susceptible to (and emit) electromagnetic interference. Miniaturized circuits, with less space between adjacent conductors on a circuit board, increase susceptibility to over-voltages and increase adjacent-channel interference. Microprocessor chips themselves have become smaller and more densely packed. This decreases heat dissipation, and makes them less robust. Operating voltages continue to decrease to allow for this miniaturization. A digital "1" may be in the vicinity of 3.5 - 5.0 volts or less, and a "0" in the range of 0 - 1.5 volts. So smaller over-voltages from transient conditions may result in operating errors.
It is easy to see the importance of keeping transient over-voltages and high frequency harmonics away from the microcircuits.
Among the types of equipment that both can cause power quality problems, and are susceptible to them, are:
- Uninterruptible Power Supplies
- Variable Frequency Drives
- Battery Chargers
- Large Motors During Startup
- Electronic Dimming Systems
- Lighting Ballasts (esp. Electronic)
- Arc Welders, and Other Arc Devices
- Medical Equipment, e.g. MRIs and X-Ray Machines
This type equipment breaks a smooth sine wave into stepped increments, for control of the downstream device, by varying the voltage or frequency of the output.
Arc-operated devices, including general purpose "universal" motors with brushes, arc welders, and even arc-discharge lighting (fluorescent or HID) can be a strong source of electromagnetic interference. (The arc itself is rich in energy of all frequencies.) This interference can be picked up by improperly shielded or improperly grounded wiring, and then conducted into sensitive equipment.
Switched Mode Power Supply
As miniaturization of components became the norm, a new type of power supply was developed. Called the "switched mode" power supply, it offers dramatic weight and component savings, a necessary step to development of smaller, lighter and less costly computers.
Historically, devices requiring DC (direct current) to operate (as all electronic circuits do) had bulky power supplies that typically had a step-down transformer supplying a low voltage to a half-wave (simple diode) or full-wave (bridge) rectifier. More recently, the lighter and more efficient "switched-mode" power supply was developed. It has a full-wave bridge rectifier directly connected to the incoming 120 V AC line.
The switching circuit draws stored energy from a capacitor in short pulses (thus quasi-square waves) before sending the now pulsed DC on to the transformer. The transformer is now operating on high frequency, pulsed DC, instead of 60 Hz AC. This operating change allows for a smaller, lighter transformer than was possible in the 60Hz, 120-volt version. Thus, overall power supply efficiency is greatly improved, from about 50% in standard power supplies to about 80% for switched-mode.
Equipment can now be made smaller and lighter. Power consumption decreases, and batteries for portable models can last much longer. But there is a downside. The pulsed output contains a fairly high level of harmonics, which can flow back out onto the power distribution system, adversely affecting other equipment and even the wiring itself.
The Effects of Non-Linearity on 3-Phase Systems
The net result of harmonic and transient generation is possible operational anomalies of sensitive electronic equipment, and overheating of phase and particularly neutral conductors. How does this happen?
In a balanced 3-phase circuit (equal linear load on each phase), operating with a smooth 60 Hz sine wave voltage on each phase, the neutral carries the vector sum of the three phase currents, which is zero. But if one or more of the phase conductors is also carrying significant currents at harmonic frequencies (multiples of the 60 Hz fundamental), they may not cancel by vector addition, but may add in the neutral. Standard test instruments cannot even measure them.
If the harmonic currents are sinusoidal then, mathematically, even multiples cancel. But the odd multiples, because they are in phase, are additive, and appear in the neutral, where they can cause overheating. The current in the neutral can actually be higher than that in any one of the phase conductors. (Fires in fact have been reported that resulted from harmonics.) If the fundamental or harmonics are non-sinusoidal, such as square waves that may be caused by a pulsed power supply, mathematical analysis becomes very difficult.
The phase wires themselves may now be carrying a sinusoidal or non-sinusoidal 60Hz fundamental, plus non-sinusoidal, high frequency, pulsed currents, which may result in overheating of the phase conductors. As predicted by Ohm's Law, these distorted currents will cause distorted voltage waveforms in the building wiring system, which can, in turn, cause equipment failure in other equipment. There arises a situation where some equipment is creating problems that can affect other equipment in the building.
Separation of Sensitive Electronic Loads From Other Equipment
A dedicated "computer" circuit in each office is a good idea, at least back to the branch circuit panel. A better idea, and required in some cases, is to power sensitive equipment from separate branch circuits emanating from separate panel boards, fed from separate feeders back to the main service entrance.
The neutrals and grounding conductors need to be kept separate also. A dedicated circuit means separate phase wires, a separate neutral, with a separate grounding conductor, run in its own separate metal conduit, back to the source. See the section on conduit (below) for further discussion.
Avoid having sensitive equipment on the same circuits, or even panel boards, as motor loads. Such equipment as laser printers, copying machines and fax machines should be kept separate from computers.
Limited Number of Outlets per Circuit
- Three to six outlets per circuit is recommended instead of the thirteen allowed by Code on a 20-amp circuit. This will minimize the number and variety of sensitive equipment sharing circuitry, tend to minimize voltage drop, minimize the chance for interaction, and leave some room for later growth or equipment changes.
- Metal conduit, properly grounded, provides shielding of the conductors from RF energy. However, do not omit the grounding conductor, irrespective of the conduit material. It is needed for safety, as well as assurance of a continuous, low impedance path to ground. The grounding conductor is run inside the metal conduit, not outside.
- All connections should be made properly and maintained to avoid possible rectification of RF at poor joints. Corrosion and joint loosening need to be addressed on a regular maintenance schedule to ensure low impedance electrical continuity at all conduit joints.
- According to the IEEE Standard 142 (Green Book), rigid steel conduit offers better performance as a grounding conductor than aluminum, if a separate copper grounding conductor is not used. But the best advice is to always use a separate, full-size copper grounding conductor, irrespective of the conduit material, due to the concern for corrosion and loosening.
- Although the NEC allows up to a 3% voltage drop in a branch circuit, recommended practice is to design for no more than a 1% voltage drop at full load on branch circuits feeding sensitive equipment. Feeder voltage drop should not exceed 2%.
- That means conductor gauges should often be larger than required as code minimums. But a side benefit of larger conductor gauges is that larger conductors frequently save enough energy, due to their lower resistance, to compensate for higher initial cost, with a short payback.
- Another factor to be considered in computing voltage drop is the crest factor (ratio of peak to average value of the wave shape.) In a sine wave, the crest factor is 1.414, and most tables, formulae, and codes are based on this common traditional waveform. But a non-sinusoidal waveform, containing harmonics and irregular shapes, may have a crest factor of 3, 4, or higher.
- Thus, the voltage drop at the current peaks may be several times higher than usually expected from the sinusoidal case. The question arises as to the value of current to employ when computing the voltage drop, as well as the value of circuit impedance at the higher harmonic frequencies. One engineer has suggested using three or four times the nameplate loads of the connected equipment to account for this increased crest factor and to compensate for the skin-effect and higher inductive reactance of the higher frequency components of current that may be present. This degree of conservatism may not be required in most cases, but prudence would suggest that phase conductors not be loaded to their published ampacity limits.
- The combination of upsizing conductors beyond the gauge needed for the load, combined with a 1% design voltage drop limit, should preclude excess voltage drop in the branch circuit in most cases. Again, it is a case of the extra materials being an inexpensive part of the overall installation cost during construction.
- The chances of problematic connections that could cause voltage fluctuations in mild cases, and catastrophic failure in extreme cases, are decreased with the use of copper conductors. Copper is the standard conductor metal against which all other conductor materials are measured. And for good reason. It has lower electrical resistance for given gauge size. That means smaller gauges and conduit sizes for a given load requirement. Copper oxide is a relatively good conductor, whereas aluminum oxide is an insulator. Special installation precautions are not needed, and maintenance requirements are reduced when using copper. Special corrosion inhibitors are not needed. Because of its superior connectability, there is less risk of a power quality-related failure.
- All metal objects that enclose electrical conductors, or are likely to become energized in the event of a fault or electrostatic discharge, should be effectively grounded to provide personnel safety, as well as equipment performance. It is best to use solidly grounded AC supply systems.
- All metal enclosures, raceways, equipment grounding conductors and earth grounding electrodes should be solidly joined together into one continuous electrically connected system. All structural building steel should be bonded into a single electrically conductive mass, and connected to the required electric service ground at the service entrance, as well as the equipment grounding conductor system and the metallic cold water system. Ground in accordance with Article 250 of the NEC.
Isolated Grounds (IG)
- Isolated grounding is a loosely defined technique that attempts to reduce the chances of "noise" entering the sensitive equipment through the equipment grounding conductor. The exact methods used in IG wiring vary somewhat from case to case, and there is no defined standard method.
- In a typical branch circuit, the grounding conductor of the equipment is connected to the metallic outlet box through the connection of the grounding conductor screw to the mounting yoke (mounting strap), as well as to the green grounding conductor for that circuit. It is then further connected to the metallic panel board enclosure where the branch circuit originated. There, it can pick up noise from adjacent circuits sharing the panel board.
- In the case of an IG receptacle, usually orange-colored and identified with an orange triangle symbol on its face, the grounding pin is not electrically connected to the device yoke, and so is not connected to the metallic outlet box. It is, therefore, "isolated" from the green wire ground. A separate conductor, green with a yellow stripe, is run from the insulated grounding pin of the outlet to the panel board with the rest of the circuit conductors, but usually is not connected to the metallic enclosure. In some cases, the isolation may terminate here. Instead it is insulated all the way through to the ground bus of the service equipment or to the ground connection of a separately derived system, i.e., an isolation transformer.
- In the opinion of many designers, the IG wiring method sometimes helps reduce power quality problems, and sometimes it makes them worse! Thus, one may consider installing the IG conductor, to be available if needed, but experiment with reverting to a solidly grounded method if proven superior.
- A buried exterior ground ring is a technique to help achieve low impedance from the building's grounding system to the earth itself, and a convenient means to connect various grounds leading from the building. One recommended approach is to bury a bare copper conductor (minimum gauge is permitted to be as small as #2 AWG, but sizes of 4/0 and 250kcmil are more often specified, and 500kcmil sometimes used), at a depth below the frost line (36"-42" in most of the US). Larger gauges increase the contact surface area, helping lower resistance. The ring is set in a trench a few feet offset from the building's footprint, and completely surrounds the structure. Ground-enhancing backfill materials (bentonite, a natural clay material, or other proprietary materials) may be used to enhance earth conductivity.
- To this buried ring is connected the building steel, the lightning protection down-conductors, the grounding electrode system, any metal piping systems crossing its path, and any other grounding electrodes present. Sometimes, vertical ground rods further supplement the ground ring.
- The grounding resistance should be checked upon installation, using a ground resistance checker, and checked again periodically, depending on experience encountered, annually or semiannually. Significant changes in readings require further investigation as to cause and needed corrective action.
- Even though the National Electrical Code alludes to a "desired" ground resistance of 25 ohms or less, that standard is based on the level of ground resistance deemed adequate to cause the over-current device (circuit breaker) to trip under a fault condition. Proper operation of sensitive electronic equipment is not a consideration of the Code. Indeed, if the 25-ohm level is not achieved at first, the Code allows the installer to place a second ground rod, do no further checking, and stop there. The resultant ground resistance may be 100 ohms, 200 ohms, or whatever.
- Many telephone and telecommunications companies specify a ground resistance of 5 ohms or less. There is no one figure that will guarantee trouble-free operation of all equipment but, in general, the lower the figure the better, with 10 ohms or less being a reasonable target for most soil conditions. During the construction phase, while the site is excavated and personnel are on the scene, it is prudent and economical to install the best grounding electrode system possible for the site.
Depth of Grounding
- Where there is insufficient real estate to work with, or under conditions of unusually high ground resistivity, deep grounds may be required. Long copper pipe-type ground rods, sometimes tens or hundreds of feet long, in bored holes, are not unheard of in rare cases. In mountaintop locations, for example, in order to achieve the target ground resistance value, it may be more economical to bore a deep ground than to spread out a shallow ground system over rocky terrain or steep slopes.
- Generally speaking, deeper ground rods are more effective than shallow rods, so a twenty-foot rod is preferred to a ten-foot rod, etc. Resistance falls quickly as rod length increases, due to more stable temperatures and increased moisture at lower depths.
- Electrode spacing is also important. The general rule of thumb is that multiple rods should be spaced apart at least twice the length of one rod. That is, two ten-foot rods should be placed no closer than twenty feet apart.
Lightning Protection Systems
- In simple terms, if part of the "path of least resistance" to ground the lightning sees is through your wiring or equipment, that is where it will flow. Lightning produces very high currents, for a short time interval, but enough to cause fires or to destroy microcircuits even miles away. The idea of air terminals, or lightning rods as commonly known, goes back to Benjamin Franklin. The purpose is to provide a convenient, controlled point for lightning to strike, and then be safely conducted to ground. To provide the least resistive path, heavy-gauge copper wire should be employed in the leaders and down conductors.
Grounding of Lightning Systems
- The down conductors tie directly to the ring ground described above, or other grounding electrode system, along with all building steel and electric service grounds. Use heavy-gauge copper conductors to minimize impedance.
- Detailed design considerations covering lightning systems are found in the National Fire Protection Association's Code #780, Code For Protection Against Lightning.
During construction or major renovation, when structures are exposed and workmen are on-site, the cost of extra materials or larger conductors is minimal. The potential savings in lost production and downtime make these precautions a good investment.
In cases where power quality problems are encountered in an existing facility, a careful study will be necessary to determine the best course of action. Solutions may be as simple as moving some loads between branch circuits, some minor rewiring, or additional branch circuits. In some cases installation of shielded isolation transformers or harmonic filters may be the best course of action. In difficult cases, professional engineering assistance is recommended.
Ring grounds, combined with vertical rods, are recommended for new construction. They are usually not practical for retrofits, especially in urban areas or where there is limited space. In those retrofit cases the best solution may be a lengthy vertical ground rod or a chemically enhanced ground rod (or rods). Make sure any chemicals or backfill materials placed in the earth are environmentally acceptable and approved by such organizations as the National Sanitation Foundation and the relevant state environmental agency.
In diagnostic testing, be sure to use test instruments capable of accurately measuring harmonic frequencies (usually called "True RMS Meters").