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Power Quality
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.
Transients
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.
Voltage Fluctuations
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.
Handling Harmonics
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.
Electrical Grounding
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.
General Wiring
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
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.
- Voltage Drop
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 gages should often be larger than required as
code minimums. But a side benefit of larger conductor gage 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 gage 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.
- Conductor Material
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 gage size. That means smaller gages 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.
Grounding Considerations
- Metallic Enclosures
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.
- Ground Rings
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 gage is
permitted to be as small as #2 AWG, but sizes of 4/0 and 250 kcmil are more
often specified, and 500 kcmil sometimes used), at a depth below the frost
line (36"-42" in most of the US). Larger gages 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.
- Grounding Resistance
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
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").
*Detailed information on power quality obtained from the Power
Quality Primer, by David Brender, P.E, of the Copper Development Association.
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