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Assessing the Cost of Voltage Disturbances
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The modern process industry depends on electric equipment that can be shut down by
severe disturbances on the incoming electrical power supply. These disturbances,
manifested as voltage dips and sags, can be caused by weather, accidents, or utility
equipment failure, and can last anywhere from cycles to seconds or more. The resulting
production outages, however, can last much longer.
While the quality of the incoming power supply voltage is fundamentally a technical
problem, it is ultimately necessary to make a business decision prior to implementing a
fix. Some solutions to voltage dip and sag problems require the use of exotic technology
and are expensive. The cost of applying a voltage dip or voltage sag solution to all the
power consumed in a facility may be prohibitive. However, if the vulnerability of the
process is understood, the system engineer may well determine that only selected critical
loads require protection. In some instances, the available solutions may be cost
prohibitive, and the right business answer is to do nothing. To decide whether the
solution can be justified, the engineer must first determine the cost of business
disruptions associated with voltage disturbances.
Assessing the cost of voltage disturbance problems consists of three major elements:
1. Assess the vulnerability of the system to determine the statistical likelihood of
interruption events.
2. Characterize the vulnerability to determine the nature of the sensitivity of the
system and the most suitable solution.
3. Quantify the value of correcting problems by calculating the expected annual cost
attributable to voltage interruptions.
1. Vulnerability Assessment
There are several mechanisms by which a voltage sag or dip can interfere with industrial
and commercial processes:
o
Control Error
Loss of control power results in the inability to control the
process. This may well be the most pervasive voltage interruption problem,
especially among commercial users.
o
Contactor Dropout
Many industrial controls employ magnetically-latched
contactors as motor control devices. A voltage sip or sag can cause a momentary
collapse of the magnetic field which holds the contacts closed. When the contacts
open, the motor stops. __________________________________________________________________________________________
2
o
Voltage Flicker
In the strictest sense, flicker is the repetitive variation in
intensity of lighting, and is more of a human irritation factor than a direct cause of
process disruption. However, it can also be used in a more literal sense to
describe a set of problems in which lighting is extinguished due to voltage dips.
o
Machine Dynamics
Since voltage magnitude is essential to transmitting power,
voltage dips and sags limit the ability of a power system to distribute power from
sources to loads. This limitation in power transfer can lead to generators not being
able to maintain stability.
o
Stall and Reacceleration
Motors will stall if the supply voltage is depressed for
a prolonged period. This may be a problem if the motor is not properly protected.
Furthermore, motors must reaccelerate when normal voltage is restored.
Reacceleration involves higher than normal motor currents which may result in
further voltage sag problems.
2. Characterization of Sensitivity
After the process sensitivity to voltage disruptions is understood, the engineer may
characterize the sensitivity in two ways. The first dimension of the characterization is the
distribution network topology, since the manner in which three-phase loads are served
determines the sensitivity of the load to unbalanced voltage dips and sags. Many voltage
dips and sags are due to faults on the supplying power system, and the majority of these
faults are single-phase-to-ground which cause unbalanced phase voltages. Certain
mitigation methods are particularly adept at solving these problems.
The second characterization dimension is duration. The causes of voltage disturbances
and the potential solutions are related, and the duration of the disruption is a critical
factor. Shorter duration problems tend to be related to a loss of voltage support, while
longer duration problems usually involve the system being unable to deliver energy.
The disturbances that cause these outages can be grouped into three general types:
o
Voltage sag is a partial reduction in the magnitude of voltage that often persists for
extended periods and is usually related to system loading conditions
o
Voltage dip is a significant reduction in voltage for a relatively short duration,
often caused by power system faults.
o
Voltage interruption is a complete loss of input voltage, lasting from seconds to
much longer.
Voltage dips and sags generally imply a solution that provides some means of supporting
voltage. Interruptions, on the other hand, usually require a source of energy to replace
the utility supply.
There are a number of examples where the sensitivity of critical equipment is best
described by a composite of the depth of the voltage depression and the duration. BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
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Quantification of this characteristic is difficult. The Computer and Business Equipment
Manufacturers Association (CBEMA) has prepared a curve (Figure 1) which defines
ideal maximum tolerance limits for process equipment and ideal minimum levels of
acceptable system performance in terms of a composite of depth and duration of voltage
depression. This curve is often employed in the power quality field as a performance
target, although not all power utilization equipment manufacturers design their products
to conform to the CBEMA limitations.

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Figure 1. CBEMA Guidelines
3. Economic Evaluation
The actual economic justification for prevent production interruptions due to voltage
disturbances must consider the following elements:
1. How vulnerable is the process to various types of voltage disturbances?
2. What is the net cost of production outages due to these disturbances?
3. How effective is a particular solution in avoiding these outages?
4. How does the cost of the solution compare to the savings which can be realized?
There are several elements of cost associated with a voltage interruption that should be
recognized and quantified in the economic evaluation.
o
Cost of Lost Production
In the simplest case, this is the incremental margin on
product that is not manufactured and therefore cannot be sold.
o
Cost of Damaged Product
If the interruption damages a partially completed
product, the cost of repairing that product must be recognized. In some cases, the
product cannot be repaired, so the value of the raw materials (including the
consumed energy up to the point where the disruption occurred) must be __________________________________________________________________________________________
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accounted for together with the cost of the incremental value added to the product.
In the commercial arena, a major source of concern is lost computer data.
o
Cost of Maintenance
The cost of reacting to a voltage disruption experience.
This includes everything involved in restoring production, including diagnosing
and correcting the problem, cleanup and repair, disposing of damaged product, and
environmental costs.
In some industries (e.g., plastics and electronics), an interruption for several hours
may result in the need to invest many days and thousands of dollars in cleaning up
the process system before it can be returned to service.
o
Hidden Costs
This factor may be the most difficult to quantify but it can easily
be the most significant. If the impact of the voltage dip or sag is control error, it is
possible that the impact on product may not be apparent until the product is in the
hands of the consumer. Product recall and/or public relations costs can be
significant.
The underlying events that lead to voltage interruptions are generally random. An
effective way to put risk and expenditure on a common basis is to assess the statistical
risk of a voltage interruption (this mathematical treatment is beyond the scope of this
document). Determine the statistical expectation of events that cause voltage interruption
and can be addressed with a given solution.
It is usually best to identify those problems that are responsible for