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Electricity has a magical and mystical nature because it is an invisible
form of energy that can only be “seen” when converted by an electrical
device to another form of energy such as light, heat or mechanical motion.
In stark contrast, gasoline and motor oil can be seen and smelled and
easily measured by the liquid level in a container or the amount that
is pumped and metered through a dispenser. The quality of this gas and
motor oil cannot be easily determined without sophisticated testing equipment,
but is assumed by the general public (with their brand selection) to be
consistently good and readily available.
Determining the quality of electrical power also requires sophisticated
testing equipment and is likewise assumed by the general public to be
consistently good and readily available. However, this is not always the
case. When this invisible electrical power source is disturbed or disrupted,
one can definitely see bad things happen to the operation of any business
due to equipment damage, down time, unhappy customers and the associated
loss of revenue.
Unlike gasoline and motor oil, the quality of electrical power cannot
be tightly controlled from the production source, through the distribution
channels and all the way to its final point of use. There are many ways
that the quality of electrical power can be compromised in each of the
stages from the initial production at the electrical utility generator,
through the electrical service grid, through the electrical distribution
system of the facility and ultimately to its final point of use at the
electrical device.
The term “power quality” is a very simple concept, yet the search to
achieve it reveals a very confusing combination of unique power problems
and equipment technology solutions. The focus of power quality studies
should be to provide a matching level of power and protection for each
primary electrical and electronic system so that business operations and
cash flow are maintained. Matching up the correct solutions with these
different problems is the basis for a practical power quality program,
which should be one of the primary goals of all business operations.
Basic terms
In an effort to make the mysterious electrical power issues more understandable,
several household terms have been adopted to describe the behavior (and
misbehavior) of electricity. These terms are not literal or scientific
descriptions of electrical patterns. They are simply figurative descriptions
of what an oscilloscope looks like when observing the wave forms of electrical
voltage and currents.
An oscilloscope is a sophisticated metering device which allows the user
to observe these electrical wave forms in extremely short time frames
to see both the normal voltage patterns and the disturbed patterns when
quality problems occur.
The terms “clean power” and “dirty power” are hopelessly overused metaphors
for describing the difference in the desirable pure and uninterrupted
voltage sinewave (clean) and the undesirable distorted and/or interrupted
voltage sinewave (dirty). Just like the household terms, there are many
different types of dirty power requiring different types of treatment
to clean it up.
Figure 1 illustrates what an oscilloscope shows when plugged into a standard
electrical receptacle in either the home or business facility. In this
typical 120 volt, AC circuit, the clean or pure voltage wave form alternates
up and down in a cyclical pattern. The AC stands for alternating current
that is descriptive of the shape which technically is referred to as a
sinewave. This sinewave pattern alternates at a rate of 60 times per second
(60 hertz) so the single cycle shown in Figure 1 only takes 1/60th of
a second to complete. Note also in Figure 1 that the peak voltage of the
sinewave at the top of the cycle is actually 170 volts above the zero
crossover point and the voltage actually goes to zero volts on its way
to negative 170 volts at the bottom of the sinewave. Then why is it considered
120 volts? Because the net effective voltage of the sinewave form itself
(called RMS or root mean squared) is 120 volts, which is well below the
peak voltage.
The terms “spikes” or “surges” are used to describe a condition when
this smooth sinewave pattern is disturbed with a high amplitude and short
duration pulse. Figure 2 shows an example of this. On the oscilloscope,
it appears to look like a spike in the voltage pattern. The technically
correct term for this type of event is a voltage transient. This type
of transient is definitely an unwelcome visitor to your business and can
damage or destroy both electrical and electronic equipment in very short
order.
The terms “swell” or “over voltage” are used to describe a condition
when the voltage rises above the nominal 120 volts by 10 percent (132
volts) for more than one cycle. This is not to be confused with the previously
mentioned surge, spike or transient, which is a very short pulse that
lasts only milliseconds and has a much higher amplitude of at least 30
percent above the nominal 120 volts (156 volts). The term “sag” or “brownout”
is used to describe a condition when the voltage drops below the nominal
120 volts by 10 percent (108 volts). Figure 3 shows both a swell and a
sag relative to the normal amplitude of the 120 volt sinewave.
The term “dropout” is used to describe a condition when voltage goes
to zero, or “flat lines,” for less than one cycle. The term “blackout”
or “outage” is used to describe a condition when the voltage flat lines
for more than one cycle.
The term “noise” is used to describe a condition when this smooth sinewave
pattern is disturbed with a lower amplitude and high frequency voltage
that “rides” on top of wave form. Figure 4 shows an example of this pattern
which creates a fuzzy appearance (not a technical term) on the oscilloscope.
This type of noise is not audible, but it can make you yell if it locks
up your POS terminal in the middle of a transaction.
A tale of two problems
The source of the power problems described above can be divided into two
primary categories that are distinctly different and require various types
of defensive strategies:
A. Externally generated disturbances from the
electrical utility power distribution grid. In general, the quality of
electrical power provided from the electrical utilities in the United
States is excellent. The upcoming deregulation of electricity to provide
a selection of providers may change this somewhat, but in reality, this
will be more of a marketing and billing issue as opposed to a change in
the operation of the electrical power grid.
The electricity produced at the output of a utility’s generators is rarely
a problem. However, in the transmission of electricity across the country,
the hardware that distributes this power (such as cable, wire, transformers,
towers and poles) is subject to the effects of temperature, water, lightning
strikes and accidental damage, which can disturb and/or disrupt the normal
flow of electrical power.
The electrical power grid, which connects all electrical generation facilities
to each other and ultimately to all end users, also has huge switches
to direct power to where it is needed the most. The power switching process
can disturb the normal flow both when these switches are turned on and
turned off.
By the time the electricity reaches the user’s facility, there can be
significant voltage transients, swells, sags, dropouts and blackouts.
The frequency of these events is directly dependent on weather patterns,
proximity to heavy construction and substation switches and unpredictable
events such as accidents that damage the power distribution lines.
However, blaming the electrical utility and its incoming power as the
source of all problems is misguided, because many other culprits can impact
the power quality in every user’s facility.
B. Internally generated disturbances from the
electrical equipment and/or wiring inside the users’s facility. The other
source for power disturbances is the electrical equipment and wiring that
is operating inside the user’s facility itself. Internally generated events
are more frequent, although often less dramatic, than externally generated
disturbances.
Much of the electrical equipment inside the user’s facility is defined
as an “inductive load,” which means at least part of the device’s function
involves passing electrical current through a coil of wire that induces
an electromagnetic field. Inductive loads draw more current when they
are turned on and release energy when they are turned off. This type of
load can generate voltage transients, noise, swells and sags that couple
directly with the entire electrical distribution system in the facility.
Examples of inductive loads are refrigeration compressors, heating and
air conditioning compressors and fans, fluorescent lighting ballasts,
submersible turbines and car wash motors. The majority of these types
of loads also cycle on and off during their normal operation which generates
more power disturbances than a continuously running load.
The electromagnetic fields that are generated by inductive loads can
also create problems with other equipment wiring that is run in close
proximity to it, such as in the same conduit or wireways. These electromagnetic
fields can easily penetrate the plastic insulation of wire to induce unwanted
currents on the secondary wiring. An example of this would be running
data cabling above the ceiling tiles directly on top of the fluorescent
light fixtures. The ballasts in these fixtures can induce currents on
the data wiring which can disrupt the proper operation of the communication
equipment at both ends of the wire.
Improper electrical distribution and wiring in the user’s facility can
also lead to problems due to overloading, overheating and improperly grounded
equipment or receptacles. The number one rule of thumb with a persistent
data communication problem on a single piece of equipment is to check
for proper system grounding and induced currents from other equipment.
A double-edged power sword
Convenience stores are one of the most challenging retail environments
for power quality because of the tremendous growth in the use of electronic
equipment in close proximity with a large number of inductive loads that
are the primary source for internally generated disturbances.
Typical electronics at the user’s facility include computers, POS terminals,
scanners, card readers, Multi-Product Dispensers, data interfaces, automatic
teller machines, telephone and satellite communications equipment, security
systems and tank monitoring systems. With many of these systems networked
together, additional problems can arise from the routing of the network
cables and the possible disturbances on multiple power receptacles. Convenience
stores combine this high density electronic environment with a high density
inductive load environment, which produces a double whammy on the pursuit
of power quality.
No magic bullets
Another reason that power quality is so badly misunderstood is that different
types of building equipment require different types of power protection
from different types of power disturbances. However, most manufacturers
promote the quick-fix magic bullet that doesn’t always address the proper
application of equipment at the right place.
Hopefully, if you have made it this far through the article and have
gotten a feel for the challenges in developing a power quality program
in the convenience store environment, you will also realize that there
are definitely no magic bullets. Different types of electrical disturbances
require different kinds of electrical technology applied in the correct
manner.
One of the most important platforms for beginning a power quality program,
or for solving some specific problems that already exist, is to verify
the grounding system in the facility. Single point grounding refers to
the National Electrical Code requirement for a single ground reference
at the electrical service entrance to the facility, where the neutral
and ground terminals are bonded together and then bonded to a driven ground
rod or other approved grounding method. Multiple ground rods should never
be used unless they are all bonded together. Not having the proper neutral-to-ground
bond at the service entrance is a Code violation and can cause problems
with equipment operation.
Isolated ground receptacles (the infamous orange receptacle) on dedicated
circuits are required by most electronic equipment manufacturers in an
attempt to provide some isolation from the other building circuits. However,
these isolated ground receptacles often get installed incorrectly and
rarely provide true isolation.
Verifying a proper grounding system typically will require the services
of a licensed electrical contractor, but this is money well spent because
the other types of equipment that are used to provide power quality solutions
will not operate properly without a good grounding system in place.
Surge protection
The most recognizable, yet most misunderstood, category of hardware solutions
is Transient Voltage Surge Suppression (TVSS). The term “suppression”
is a little misleading because this type of equipment doesn’t really suppress
the transient voltage but instead diverts it away from the protected load
to another electrical location. The most commonly used phrase for TVSS
equipment is “surge protection,” for which there are many different technologies
and applications available.
The most common electrical component used for surge protection is the
Metal Oxide Varistor (MOV). The MOV is installed in parallel with the
protected load and when normal voltage is present, the MOV has an extremely
high impedance which makes it appear electrically to be an open circuit.
When a large voltage transient appears on the wave form, the MOV is designed
to rapidly become a short circuit and divert the transient away from the
protected load. The voltage at which the MOV begins to become a short
circuit is designed with the AC voltage sinewave in mind and is typically
25 percent above the nominal voltage, which, for the 120 volt RMS example,
would be 150 volts RMS.
The advantage of MOVs is that they can conduct large amounts of current
when they divert voltage transients, which is a very important factor
when designing a surge protection system. MOVs come in many different
sizes with regard to the design voltages where they begin to conduct and
the maximum amount of current that they can carry. Typically, MOVs are
used in parallel arrays to increase the surge current rating for the entire
system.
Other component technology that is used in some hybrid designs are Silicone
Avalanche Diodes (SADs) and gas tube diodes. These components complement
MOVs by increasing response time and providing high end current capacity.
Inductive chokes, which look like ring shaped magnets, are also used around
the primary conductors of surge protection equipment to resist rapid changes
in current and to assist the other components in the system.
These components begin to conduct at the design voltage levels, but the
only way to determine the effectiveness of surge protection equipment
is to know the “clamping voltage” of the entire system. This is essentially
the maximum voltage that the surge protection system will limit to the
load. The only way to determine this is by testing in the lab under live
conditions where specific wave form transients are intentionally produced.
Surge protection can be packaged in either parallel or series configurations,
but they all utilize this parallel component technology even if they are
wired in series to the protected load. In selecting surge protection equipment,
be sure that it has been tested to UL 1449 (2nd Edition) standards and
that it has a high surge current rating for the application and a low
clamping voltage rating (330 volts is the lowest possible category).
This is worth repeating: a higher surge current rating is better and
a lower clamping voltage rating is better when comparing products.
However, if surge protection equipment is not installed properly, these
ratings that worked in the lab will not perform up to the rating in the
field application. The key here is that the surge protection equipment
should be installed as close to the protected load as possible, with a
minimum amount of wire. The impedance of the wiring used to install surge
protection can directly diminish the performance of the system by increasing
the actual voltage that gets to the protected load.
Depending on the size of the conductor used, the net clamping voltage
can increase by 20 to 80 volts per foot of wire. Just 10 feet of wire
can increase the net clamping voltage by 200 to 800 volts. This means
that your 330 volt rated system now only performs at 530 to 1130 volts.
The lower the clamping voltage the better; so be sure to install the surge
protection equipment as close as possible to the load that you are protecting.
Externally generated voltage transients, such as lightning strikes on
the power grid, can best be stopped right at the source, which, in this
case, is the electrical service entrance to the facility. Install a circuit
breaker in the main distribution panel of the facility and install the
surge protection as close to this breaker as possible using the minimum
amount of wire. Any wire bends that are required should be a smooth radius
and not a sharp right angle bend. This application protects the entire
facility from the external transient by diverting most of it to ground
before it enters the building. Figure 5 shows a typical service-entrance-type
surge protection system installed on the main panel.
Electrical subpanels, which provide power to outside loads, such as dispensers,
submersible turbines and exterior lighting, are also good candidates for
surge protection to create a fire wall between the inside loads and the
exterior loads to limit the magnitude of transients going in either direction.
Install a circuit breaker in each subpanel feeding exterior loads and
install the surge protection as close to this breaker as possible using
the minimum amount of wire. Ditto on the wiring practice. Typically, this
subpanel application doesn’t require as high a surge current rating as
the one used at the electrical service entrance. Figure 6 shows a typical
subpanel type surge protection system installed.
Plug-in surge protection strips and hardwired surge protection boxes
are designed for individual loads within the facility and are normally
designed in series wiring configurations for convenience only. (Remember
they are still using parallel surge protection technology with the addition
of a fuse or breaker for overcurrent only.) However, the electronic loads
that typically get plugged into or wired to these devices usually have
more clean power needs than just protection from basic voltage transients.
Power conditioning and uninterruptible power supplies
Surge protection systems, when applied properly, provide excellent protection
from the large scale voltage transients that come from the electrical
utility grid or that are generated internally. However, surge protection
doesn’t do much for other power disturbances such as noise, sags, dropouts
and outages. These types of problems require another set of tools from
the power quality workshop
generally known as power conditioning and uninterruptible power supplies.
Power conditioners are true series-installed devices because they rely
on the use of an isolation transformer that carries the full load that
is connected to it. The purpose of the transformer is to isolate the connected
load from the rest of the electrical system and to allow for the re-establishment
of the neutral/ground bond on the output to eliminate common mode voltages
and noise that can disrupt communication in networked electronic systems.
Older power conditioner technology used ferroresonant-type transformers
that were good voltage regulators but were very inefficient and generated
a great deal of heat. The latest computer and POS technology has switch-mode
power supplies that are now very tolerant of voltage sags and swells but
much more susceptible to common mode disturbances. For this reason, most
power conditioner manufacturers now use low impedance-type transformers
which are very efficient in operation and are well matched with the switch
mode power supplies used in the protected electronic equipment.
Power conditioners also use some surge protection and line filtering
technology on the output of the transformer to enhance the performance
of the connected equipment. Since they are true series-connected devices,
power conditioners are sized according to the load that is connected and
are available in both plug-in and hard-wired configurations. Figure 7
shows both the plug-in and hard-wired types of power conditioners.
Load ratings are typically in volt-amp (VA) sizes and must be calculated
on the total rating of all connected equipment along with a safety factor.
To calculate VA, multiply the amp draw of the equipment times the voltage.
For example, a 3-amp connected load at 120 volts would be 360 VA; with
a 10 percent safety factor, it would be a 400 VA unit.
Applications for power conditioners include in-store electronics, such
as computers, POS terminals, scanners, card readers, data interfaces,
automatic teller machines, telephone and satellite communications equipment,
security systems and tank monitoring systems. Noticeably absent from this
list is Multi-Product Dispensers (MPDs), which are not good applications
due to their large load requirements and the long wiring distances from
the backroom electrical panels to the MPDs in the yard.
MPD power requirements have increased dramatically over the years and
contain such equipment as lights, heaters, solenoid valves and vapor recovery
systems that add to the load but do not really warrant power conditioning.
A typical dispenser that might have a total peak draw of 12 amps would
require a 1440 VA power conditioner (12 amps x 120 volts) and for a site
with 8 MPDs would require almost a 12,000 VA unit (8 MPDs x 1440 VA each).
This gets way too big and way too expensive to be practical and, even
so, would not be effective in protecting the MPDs because of the long
distances involved.
The best application for MPDs would be to install some combination of
power conditioning and surge protection in the MPD electronics head to
put the protection as close to the load as possible. This would require
close coordination with all the manufacturers of MPDs and would be driven
by the willingness of ownership to pay for the additional costs that would
come with the additional protection and operating benefits. As more internet
technology is added to MPDs, this change is inevitable.
Using properly applied surge protection equipment in conjunction with
power conditioners provides both the macro and micro electrical views
for the majority of power quality problems. However, neither technology
can provide protection from the power failure problems previously defined
as “dropouts” or “outages.”
Uninterruptible Power Supplies (UPS) provide a battery backup for these
types of power disturbances by converting the power from DC batteries
to some form of AC power to operate the equipment for a limited time (5
to 10 minutes) which is long enough to close out the transaction and turn
the load off.
Like power conditioners, UPS systems are also true series-installed devices,
because all the equipment load passes through the system. For this reason,
sizing the UPS is done in the same manner by calculating the connected
load in VA and adding a safety factor.
There is a huge disparity in the quality and functionality (and cost)
of UPS systems. Not every piece of electronic equipment requires a battery
backup for the operation of the business, but if it does, it is best to
consider both of these items in system design and product selection:
• Most UPS systems are battery backup only and do not have any built-in
power conditioning.
• Most UPS systems do not generate a pure sinewave voltagen the
battery backup mode creating noise problems.
Products are available in the marketplace that have power conditioning
built into the UPS and that provide a pure sinewave output during battery
backup. Figure 8 shows a photograph of a plug-in type conditioned UPS
system.
Products are also available that have the orange isolated ground receptacles
built into the back of the power conditioners and the UPS systems with
power conditioning that eliminates the need for wiring the orange receptacles
into the building wall. This is done by adding a controlled impedance
to the ground leg ahead of the power conditioners neutral/ground bond.
This essentially provides a portable isolated ground receptacle circuit
along with the power conditioning.
Be sure to lock the back door
Verifying proper system grounding and installing surge protection and
power conditioning equipment on the electrical system provides a solid
foundation for a power quality program. There are, however, a few other
items to consider that are not directly related to the electrical system.
These other links to the outside world include data and communications
wiring which can also be the source for voltage transients that can enter
the back door of your electronic equipment and damage modems and data
ports, even when the power sources are properly protected. Don’t forget
these other sources for possible problems:
• Telephone lines
• Cable TV
• Satellite networks
Beware of the red flags
This important quest for power quality has provided a major market opportunity
for manufacturers with a broad range of hardware solutions. Hundreds of
manufacturers are involved in developing and distributing products. This
competitive marketplace has yielded some excellent technology and outstanding
value to the end user.
However, as in any market, there is a segment of marginal players who
peddle a great deal of hype, fear and misleading claims. Even legitimate
manufacturers can have overly zealous sales reps who will look for any
opportunity to sell their products ahead of the competition. There are
a couple of “red flags” in this business that should provide just cause
to investigate product claims a little further before making purchasing
decisions:
• One size fits all. This goes along with the magic bullet theory
that one box fits all applications and will solve all problems. Be very
careful about this sales approach. It may sound good and be easy to understand
but usually is only a partial solution and usually not a good one.
• Vague UL listings and test ratings. When evaluating different
equipment, insist on published UL listings and appropriate test data.
If this is not readily available, the legitimacy of the product is questionable
at best. Surge protection equipment falls under UL 1449 (2nd Edition),
with the critical ratings being clamping voltage and surge current. Stand-alone
power conditioning systems fall under UL 1012, with the critical ratings
being common mode and normal mode voltage let-through. UPS systems (with
or without integral power conditioning) fall under UL 1778, with the critical
ratings being voltage tolerance range before switching to battery and
full load run time.
• Equipment damage insurance packages. Some manufacturers include
insurance policies that guarantee payment if any equipment is damaged
when using their power quality system. Most of these programs are marketing
gimmicks to make you feel better about making a decision and the insurance
cost is built into the product cost. Making claims on this type of invisible-third-party
insurance program requires extensive documentation that is nearly impossible
to obtain. You are better off discussing the existing comprehensive policy
on the facility with your existing insurance agent to see what coverage
is provided from lightning damage. You may also get a discount for installing
some preventive measures. If you insist on going with third-party insurance,
at least request a copy of the claim form up front so you can be aware
of what documentation will be required from you should a claim ever be
required.
• Claims that equipment is protected from a direct lightning strike.
No surge protection system can protect you from a direct lightning strike.
Claiming this is a sign of sales desperation. The only option here is
to look at installing lightning rods on top of the user’s facility or
canopy. Lightning rod systems provide an array of sharply pointed rods
pointed skyward which are all bonded together and to earth ground. These
systems are intended to prevent the accumulation of static electrical
charges on the facility which is the root cause of lightning strikes.
If there is a direct strike, it will theoretically hit the rod and go
directly to the ground without going through the building’s electrical
system. However, the effectiveness of lighting rod systems are unproven
and are very expensive; in the range of tens of thousands of dollars.
• Claims that surge protection saves money on electric bill. This
type of claim and related ones about installing power factor correction
capacitors are without merit in the convenience store environment. The
voltage transients that surge protection diverts are only milliseconds
in duration and are not even registered by electrical utility meters because
kilowatt demand (KWD) readings are typically averaged over a 15 minute
or 30 minute time period by the meter. Voltage transients are invisible
to the meter and do not increase the kilowatt demand charges on the electric
bill.
Similar claims about installing a standard bank of capacitors to correct
power factor and save kilowatt demand costs are also unfounded and a poor
application of technology (this is also a variation of the one-size-fits-all
claim). First of all, putting a fixed capacitance on a variable inductive
load environment is not the right approach, because when the inductive
loads are not on, the only result is a leading power factor instead of
a lagging power factor. However, the most suspect claims are about yielding
savings on kilowatt demand charges. Most electrical utility meters read
true kilowatt demand of power (not the higher KVA associated with leading
or lagging power factor), so even if the power factor was perfectly corrected
at all times to unity, it still wouldn’t reduce demand charges or dollars.
Some utilities charge their big industrial customers a power factor penalty
where special meters are installed, but not their small commercial convenience
store customers. Be sure to understand the local electrical utility rates
and metering systems when analyzing these claims and ask the opinions
of the electrical utility representatives.
Finding the balance point that works
The primary goal of a power quality program is protecting the investment
in electrical and electronic equipment and maintaining continuous business
operations. It is important to find a balance point on the equipment cost
versus the risk that works for the business. Divide weekly sales by the
weekly hours of operation to come up with an average hourly cash flow—this
can help gauge the hourly cost of downtime to the business. Estimate the
cost of repairing or replacing all of your major electronic systems—this
can help to determine the value of protecting this investment and what
should be budgeted for this. Check existing insurance policies on electrical
damage, deductibles and discounts that may be available for power quality
equipment.
It would be impractical and very expensive to put surge protection or
power conditioning on every single load in the building. The law of diminishing
returns states that, past a certain point, incremental investments bring
a reduced return. A better approach is to install high quality, heavy
duty systems at key points. Established manufacturers with quality products
will be around longer to support their products and warranties. Ask for
test results when comparing the ratings of different manufacturers.
Analyze the power patterns in your facility with plug-in metering equipment.
There are some excellent data recording meters on the market that plug
into the wall, record information for a week or two and then download
to laptop computers for processing.
A power quality program is usually a low priority compared to running
the day to day operations and investing in other equipment that provides
a more tangible and immediate return. However, it only takes a milli-second
to be shut-down and out of operation for hours or days on end with unhappy
customers and negative cash flow. Don’t wait until disaster strikes to
develop a power quality program because it is much more difficult to analyze
options and make decisions while doing a disaster recovery program at
the same time.
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