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Electrical Powe r System Design forIndustrial Facilities

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2008 Louie J. Powell, PE Page 1Rev 1

Principles of Electric Power System Design for Industrial

Facilities

Louie J. Powell, PE

Saratoga Springs, NY

Introduction

Industrial facilities utilize electric power in the process of manufacturing an industrial product.

The requirements of the industrial manufacturing process drive the specific requirements that the

facility must satisfy. Different industrial manufacturing processes have vastly different technical

requirements for electric power, but those requirements always come down to decision points

regarding a relatively small number of key considerations:

Safety all industrial workplaces are inhabited by people. In the early days of industrialization,

debilitating accidents and even fatalities were often accepted as part of the cost of doing business.

While tragedies for the immediate families of those affected, these events were viewed as the

unavoidable consequence of applying technology to activities that involved inherent risk. Today,however, safety is considered a prime directive in industry, and facilities have to be designed to

achieve the highest level of protection for employees. Occasional accidents still occur, but

injuries and fatalities are almost always investigated thoroughly to determine a root cause, and

corrective actions are undertaken to assure that similar events dont occur a second time. And

governmental agencies such as OSHA in the US are empowered to impose severe penalties on

employers who fail to properly protect employees from workplace injury.

Reliability A critical requirement in industry is reliability. Industrial facilities are almost

always designed to achieve a target production rate X automobiles per day, Y bottles of beer per

month, Z tons of finished paper per week, etc. Failure of the power system will cause a shortfall

in production, with direct economic consequences on the owner.

Maintainability industrial facilities also have to be maintained. Maintenance is not an objective

unto itself, but rather is required to assure that the safety and reliability designed into the system

at the time of its initial construction will continue to be there many years into the future. And

maintenance must be possible without compromising either safety or reliability. As a result, it is

necessary to anticipate maintenance practices at the time that the facility is first designed.

Economy the industrial workplace exists to produce a product, and in the vast majority of cases,

those products are sold in a competitive marketplace that focuses management attention on the

cost of production. Accountants have cleverly invented processes to amortize the cost of the

facility over the expected future production, and impose intense pressure on reducing per-product

cost in order to increase operating margins. As a consequence, every design decision is subject to

review on the basis of its impact of final cost, and only those decisions that can be supported interms of achieving reasonable safety or operating reliability are likely to be approved by

management. In this context, electric power, and the infrastructure required to supply electric

power, are viewed as simply elements in the cost of doing business, and in this context, it is

sometime helpful to identify that there are four elements of the cost of electric power:

1. The cost of the power itself, either in terms of the price that must be paid to a commercialpower supplier, or the equivalent in fuel used to generate the power within the industrial

facility.

2. The amortized cost of the infrastructure required to deliver electric power to the process.3. The cost of maintenance.


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4. The cost of adverse events associated with power. The cost of reliability will bediscussed in more detail further in this course. But there is also a cost associated with

safety the direct costs of lost time accidents, the legal and other indirect costs

associated with personal injury, etc.

And like any other component of cost in a business, there will be pressures to find ways to reduce

these costs.

The Basic Requirements of the Distribution System

One of the key considerations in the basic architecture, or structure, of the power system for an

industrial facility is differentiation between distribution and utilization of electric power.

Distribution is the process of moving electric power from its source to the point where it is

consumed. The distribution system is a utility structure within the industrial facility, just like

potable water, cooling water, natural gas, lubricants, raw material supply, byproduct handling,

and waste treatment are utilities.

A target production rate is always specified when new plants are initially designed. That

production rate typically will take into account the expected operating cycle of the plant. In

continuous process industries (stuffmakers), the usual expectation is that the plant will operate24/7. Paper mills and oil refineries typically operate continuously, shutting down only for major,

thorough rehabilitation once every couple of years. Discrete manufacturing (thingmakers)

typically assume a daily operating cycle expressed in terms of the number of shifts per day, and

that cycle may include an inherent maintenance period. For example, in the automotive industry,

a new facility might be intended to operate in normal production mode for two 10-hour shifts per

day, with a 4-hour maintenance window overnight, for five days per week, to produce the target

number of jobs, or finished automobiles, for which the plant is designed.

One of the challenges faced by the designer of an industrial power system is that while a target

production rate is specified at the time of initial design, the expected rate of production typically

will increase over time. Part of that increase is expected to come out of productivity. That is, the

expected electrical energy consumption per unit of product is expected to diminish over time asprocess engineers find more efficient ways to produce the product. But another part of the

increase in production will inevitably come at the prices of an increased total plant electrical

demand. That means that the designer of the facility has to anticipate that the electrical demand

will grow over time. But the economy objective also limits how much inherent expandability the

power system engineer can afford to build into the initial design to allow for process growth. And

eventually, that growth in production capacity presents a need for creativity on the part of the

power system engineer who will be challenged to find ways to increase the electrical capacity of

the infrastructure at minimum costs avoiding the need to replace otherwise functional

equipment, while retaining safety, reliability and maintainability.

The target production rate translates directly to a capacity figure for each of the utilities that

support the plant. For example, the electric power requirement in kWHr for a cement plant can be

related directly to the expected output of the plant in tons/day of final product. The conversion of

required infrastructure capacity to the product output target varies from one industry to another

aluminum smelting is very energy-intensive, paper and petrochemical slightly less so, while

discrete manufacturing tends to require less energy per unit of final product, etc. In addition, the

conversion will vary between product lines and between manufacturers. In fact, the rate of

energy conversion, and the techniques used to achieve that rate, are typically considered

proprietary information in that they directly relate to how individual companies achieve

competitiveness in their respective markets. As a result, that actual target energy requirement for

a proposed new facility must be obtained from the engineers who are responsible for designing


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the process to be embedded in that facility. However, it may be possible to use benchmark

numbers from similar, older facilities for preliminary planning purposes.

One of the most important steps in designing the power distribution system for a new industrial

facility is to clearly define the performance that system is expected to deliver. If the system is

described first in terms of performance, then the process of design can focus on arriving at cost-

effective solutions to deliver that performance. That will avoid questions about why certain

features are (or are not) included in the design.

Its not difficult to design a system that has inherent capacity for growth. The real problem faced

by the power system engineer is more likely to be either justifying capacity for expansion in the

initial design, or justifying the costs of modifying an existing system to accommodate load

growth. These challenges are often more political than technical. When the time comes to

address the issue of expansion, having a performance specification that defines the parameters

that were expected for the system at the time it was originally designed will help keep the

discussion about what should be done about expansion focused on performance, and help avoid

pointless arguments about what should have been known when the plant was first designed.

Fortunately for electric power engineers, the equipment used to actually construct the electrical

distribution infrastructure in an industrial plant has been highly commoditized to the point whereit is available in standard unit ratings. Specifically, engineers have to make rating choices with

respect to frequency, voltage, thermal capacity, and short circuit level.

Of these, frequency is the easiest choice to make because frequency is almost always a given

based on location. For example, for industrial facilities in North and South America, thefrequency is almost certainly going to be 60Hz (although a few older facilities operating at 25Hz

and 40Hz, dating back to the late 18th

and early 19th

centuries, still exist), while in Europe, Africa

and most of Asia, the standard frequency is 50Hz. The selection of frequency also then drives the

range and selection of voltage, thermal and short circuit capacities that are available for

equipment.

The other component ratings must be selected as part of the design of the facility. And a majoreffort in that design is to make decisions about the basic arrangement, or architecture of the

system. A key consideration, however, is that these component ratings and the so-called one-

line arrangement, or architecture, are not independent variables that have inherent significance

on their own, but rather are dependent variables that are driven by four key decisions pertaining

to expected system performance:

1. Reliability2. Steady-state performance3. Performance under expected dynamic conditions4. Protection

The discussion that follows presents some criteria in each of these areas that have proven to be

helpful in making design decisions. It must be emphasized that there is no formula that

automatically guarantees a perfect industrial system design. In fact, there may be no such thing

as a perfect design. Instead, design is a collection of compromises, and the criteria discussed here

may be helpful in sorting through the competing objectives to arrive a acceptable solutions for

individual cases.

Also, while there are a number of references available to the design engineer, there is no true

standard for the design of an industrial power system. The reference that comes closest to

being a standard is IEEE 141, Recommended Practice for Industrial Power Distribution for


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Industrial Plants, (the Red Book), but in reality that reference suggests alternatives rather than

one ideal solution to serving the needs of an industrial process.

It also should be stated that the criteria present here are all based on fundamental electrical

physics. However, in the interest of time it may not be possible to fully develop the underlying

basis for each criterion.

Reliability Considerations

Every component in an electrical system is a candidate for failure. There is a branch of electric

power engineering that specializes in the study of component failures, assessing the statistical

probability of failure together with the expected time required to repair components.

Maintenance is a major consideration in this analysis, both because projected expectations of

component reliability are predicated on the presumption that those components are maintained

regularly, and also because it is necessary to take equipment out of service to perform

maintenance, and those maintenance outages may have an impact on the overall availability of

the electrical system. Methods are available to use this information to make informed decisions

about alternate system arrangements based on achieving the highest statistical expectation of

electrical system availability.

Using the analytical approach, it is possible to achieve a very high overall system availability.

However, that high availability comes at the cost of significant capital investment and an

aggressive maintenance program. Whether those costs are justifiable depends on the nature of the

industrial process that the electrical system must support. And in particular, the key issue is how

the cost of an electrical service interruption impacts on the cost of doing business.

For example, in discrete manufacturing, the direct impact of an electrical interruption typically is

that products cannot be manufactured during the outage. The accounting cost of the interruption

is primarily the margin on product NOT produced during the outage. There is also an unapplied

labor cost (employees who cannot actually do anything productive but who are still collecting

wages).

The impact of an electrical service interruption on a continuous process application may be more

severe. Product in process may be spoiled and have to be discarded, and the cost of the raw

materials, labor and electrical energy invested in that unsalable product adds to the cost of the

interruption. There may also be a need to retune the manufacturing process when production

resumes, resulting in even more waste product.

A more insidious risk is that the electrical interruption could damage the process in some way a

good example might be a plastics extrusion process in which material freezes during the

interruption, requiring that a portion of the process be rebuilt before production can resume.

Finally, there is the risk that the electrical interruption could cascade into a catastrophic failure.

Some chemical processes have inherent instabilities during startup and shutdown, and anuncontrolled hard stop due to a power interruption could lead to an explosion. Or, if electric

power is required in the process to maintain environmental controls, an unexpected interruption

could result in release of hazardous materials with both legal and economic consequences.

While an analytical approach to assessing reliability is intellectually interesting, it may not be

justifiable in every instance. In fact, most industrial facilities can be adequately designed using

the n-1 concept.

The n-1 concept.


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n-1 is the notion that every component of an electrical system is a candidate for failure, then the

system must be designed in a way that allows it to deliver power to all important loads assuming

the failure of every component. Obviously, the likelihood that multiple components will fail

simultaneously is remote, so practical designs employ redundancy in a way that allows individual

components, or perhaps logical groups of components to fail, and still provide power to important

loads.

Fig 1 Simple single-ended substation

Fig 1 depicts a single-ended substation consisting of a primary switch, a transformer, a secondarybreaker, and a load bus. Failure of the primary switch, transformer or breaker results in an

inability to supply power to loads served from the bus. A simple, n-1 solution to enhancing the

reliability of this design is shown in figure 2.

Fig 2 Simple Double-Ended Substation

In this arrangement, any switch, transformer or breaker could fail, and power could still be

supplied to loads on the critical bus.

There are three important assumptions embedded in this simple example. One has to do with the

system that supplies power upstream of the primary switch. Ideally, the concept of redundancy

would extend as far back into that system as practical. That suggests that the double-ended

arrangement should also be applied to whatever primary system supplies this substation, and in

fact most industrial system do just that. Obviously, however, there is a limit to how far thatredundancy can go. For example, it would be very unusual for circumstances to provide two

completely independent sources of power in one geographic location.

The second assumption is that while each component has its own inherent failure characteristic,

practical designs group components. So while the transformer, the connection between the

primary switch and the transformer, and the connection between the transformer and the

secondary breaker are actually independent, for design purposes they are considered to be agroup. Failure of any one is addressed through redundancy of the entire group.


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Third, it is assumed that the load bus itself is not subject to failure, and of course thats not a

realistic assumption. Hence, a more practical design might be the arrangement shown in fig 3.

Fig 3 Double-Ended Substation with a Tie Breaker

In this instance, installation of a tie breaker in the load bus allows a failure to occur on one half of

the load bus without affecting the other half. Of course, that then raises two additional questions.

First, electrical failures involve arcing, and arcs generate a lot of combustible and potentially

conductive gas. Therefore, it is possible that a failure on one half of the load bus might actually

jeopardize the other half in spite of the fact that the breaker is present. Second, consideration also

has to be give to the possibility that the tie breaker itself could fail. Addressing these two

concerns leads to the arrangement of figure 4.

Fig 4. Double-Ended Substation with Dual Tie Breakers in Separate Vaults

In this design, the single time breaker has been replaced with two breakers, and the two halves of

the double-ended substation have been placed in separate rooms, or vaults (represented by the

dashed line), so that an ordinary electrical failure in one cannot communicate to the other.

This example show how an intuitive approach to reliability, considering the n-1 principle, can

evolve into an increasingly complex, and therefore increasingly costly, system design. As a

practical matter, the arrangements in figures 1 and 2 are relatively typical of electrical system

designs supporting commercial applications offices and shopping, for example. Figure 3 is

typical of what might be found in a great many discrete manufacturing and even continuousprocess industrial applications, while figure 4 might be reserved for a mission critical facility

such as a data center or a hospital surgical suite.


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Fig 5 A Double-Ended Medium Voltage bus feeding a double-ended low-voltage substation

The double-ended substation shown in figures 2-4 is very typical of low voltage1 substations in

industrial or commercial applications. Double-ended arrangements are also very commonly seen

in medium voltage2

systems. Figure 5 shows a typical double-ended medium voltage industrial

substation that also feeds multiple double-ended low voltage substations, and illustrates how theconcept of double-ending and redundancy can be extended upstream from the load toward the

source of electric power.

The designs shown here call for only one layer of redundancy. This is not a casual decision, and

in fact is based on an important principle that can be found through a more analytical approach to

reliability. That principle is that the amount of improvement caused by redundancy decreases

dramatically as the number of layers of redundancy increases.

Redundancy

1

10

100

1000

10000

100000

1000000

10000000

1 2 3 4

Number of Transformer s

MTTF

Fig 6 Improvement in Reliability due to Multiple Transformers in Parallel

Figure 6 shows the calculated mean-time to failure for a hypothetical system served by one, two,three or four transformers, and shows that adding a second transformer makes a dramatic

improvement in calculated mean-time-to-failure, but the improvement associated with a third or

fourth transformer is inconsequential.

1 Under ANSI standards, low voltage refers to applications operating at 1000 volts or below. Typical

low voltage industrial applications are at 480v.2 Under ANSI standards, medium voltage refers to applications operating at voltages greater than 1000

volts but lower than 72kV. Typical industrial medium voltage applications include 4.16kV, 13.8kV, and

(less frequently) 34.5kV.


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Most industrial applications are therefore served using designs that call for only one layer of

redundancy. Only in very usual cases (data centers, ethylene plants etc) is there a justification for

additional redundancy, and in those instances that additional redundancy is most commonly in the

form of local emergency generation that provides a source of energy that is independent of a

commercial utility supplier.

There are two special arrangements that need to be mentioned here for completeness. One is thelow voltage sparing arrangement. While the majority of industrial applications utilize simple

double-ended low-voltage substations, a special concern exists in applications where the

concentration of load is heavy enough to require multiple substations to meet thermal

requirements. In those situations, a design that originated in the automotive industry may have a

more attractive initial cost.

Fig 7 Low voltage sparing arrangement

In figure 7, one single-ended low voltage substation (shown in blue) has been set up to spare four

other single-ended substations. In theory, this arrangement could be extended to any number of

other substations if there is sufficient load in the vicinity to justify additional capacity. In this

arrangement, the breakers connecting the sparing bus to the individual load buses are normallyopen in order to keep the short circuit levels within reason.

In some industries, most notably pulp and paper but also petrochemical, there is a need to

integrate local generation. This generation is typically integrated into the thermal cycle of the

process (it is classic cogeneration in that steam is produced that is used to both generate

electricity and to perform some function in the process), so there is a desire for the electrical

output of the generator(s) to be utilized at a point in the process that is close to where the steam is

utilized; in that way, the impact of a maintenance shutdown is localized to the one area

immediately associated with both the steam and electrical loads on that boiler. The problem that

often occurs with generators is that their presence causes a dramatic increase in the short circuitcurrent available on the system, so an arrangement is require that permits the generator(s) to be

integrated while providing control over short circuit duties. The answer to this dilemma is themedium voltage synchronizing bus design.


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G

Fig 8 Medium Voltage Synchronizing Bus Arrangement

The synchronizing bus arrangement requires more careful consideration during the design phase,

and in particular more attention to concerns relating to steady-state loading, motor starting, short

circuit levels and system protection. But is can also be a very powerful tool in addressing designchallenges in system with cogeneration integrated with the process.

Steady State Performance Considerations

The power system in an industrial facility is largely passive, so it is important that careful

attention be given to several aspects of steady state operation.

The most important is that the thermal ratings of the components must be adequate to handle the

load that will be imposed on the electrical system by the process. And here the challenge of

predicting the future is important. Its not at all unusual for a facility to be commissioned one

year, and then have receive a management request to operate at 115% of design capacity one year

later. And thats just the beginning.

All electrical components have a fundamental thermal rating, usually expressed in terms of

continuous amperes or kVA. That rating describes how much power can flow through the

component without exceeding a design temperature rise in an environment where the ambient

temperature falls within the constraints defined by the standards against which the component

was designed.

It is important to recognize that the thermal stress on electrical system components is related to

the current flow through the electrical conductors making up those components. For

convenience, thermal ratings are expressed either in amperes or kVA; in the case of kVA rating,

there is an implied assumption of rated voltage. Occasionally, one will see a rating expressed in

kilowatts (or some multiple of kW). Most often this is associated with generators as is based on

the mechanical output of the prime mover. One should take care to use the actual kVA rating of

the generator rather than the kW rating of the prime mover.

In most industrial applications, loads are close enough to constant that one need not be concerned

with intermittent loading, thermal cycling and related issues. If those issues do arise, guidance

can often be found in standards. In extreme cases, it may be necessary to define the thermal duty

as part of the specification that the component manufacturer is expected to meet, and rely on the

manufacturers understanding of the fundamental physics of his equipment to offer assurances

that their proposed design is adequate.


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The fundamental principle is that loading should never exceed the rated thermal capacity of

individual system components under steady state conditions. One does not normally have to be

concerned about exceeding continuous thermal ratings under abnormal conditions such as motor

starting or other impact loading, or short circuits, unless these conditions occur frequently enough

to be considered a cycling load.

Some electrical components, most notably transformers, may be equipped with supplemental

cooling provisions that increase the continuous thermal ratings of the basic component carcass.For example, outdoor oil filled transformers typically have a fan-cooled rating that is 30 higher

than the base rating. Other transformers have other force-cooled ratings, and it is appropriate to

consult both manufacturers specifications and standards to determine what the actual force-

cooled ratings of a specific transformer will be.

Earlier, the double-ended substation (figure 2) was suggested as a reliability enhancement over

the single-ended substation (figure 1). If all of the components have the same rating, then that

double ended enhancement would have a cost of approximately twice that of the single-ended

design, and an initial assessment is that the incremental reliability comes at a price premium of

100%.

Fortunately, the ability to manipulate thermal ratings significantly changes that assessment, andalso offers the power system engineer a number of design options.

Consider the case of a low voltage unit substation that is designed to carry a projected initial load

of 800 kVA. There are several potential ways to serve this load:

o Solution A: Single-ended substation, 1000 kVA transformer rating

This solution provides 20% margin for future growth, but with a single-

contingency failure risk.

o Solution B: Double-ended substation, 750kVA transformer ratings

The total capacity of the two transformers is 1500kVA. Therefore, not only does

the design provide enhanced (n-1) reliability, it also has the ability to

accommodate 87% load growth. Since cost is approximately proportional to the

installed kVA rating, it will cost about 50% more than A.

o Solution C: Double-ended substation, 500kVA transformers with 50% force-cooledcapability

This design offers identically the same loading and reliability performance as B,

but it will cost only slightly more than A. The premium will be associated with

the addition of forced cooling, and the incremental installation cost of the second

unit.

o Solution D: Double-ended substation, 750kVA transformers with provision forfuture 50% forced-cooled capability.

This design offers enhanced reliability, an initial ability to accommodate 87%

load growth and at a cost that is only slightly more than solution B. But it also

offers the ability to expand the load capability to 281% of the initial requirement

with the addition of relatively inexpensive accessories and with no additional

footprint requirements in the future.


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In this example, it was assumed that the transformer(s) would be the most costly components of

any design, and that ratings could be chosen for other components (switches, low voltage

breakers, cables, etc) that could accommodate any of the rating options available with

transformers. In an actual example, of course, it would be appropriate to test these assumptions.

Voltage spread

The most common load served by the power system in an industrial facility is motors. Motors

typically have voltage ratings that are slightly lower than the nominal ratings of the corresponding

voltages in a system. For example, motors designed for application on a 480 volt system

commonly have a 460 volt rating.

The torque produced by an induction motor, the most common variety used in industry, is

proportional to the square of applied voltage. So if a 460 volt motor is applied on a system where

the applied voltage is 480v, the motor will produce about 109% rated torque. Or conversely, the

motor will be able to produce rated torque with up to 4% voltage drop in the feeder cables.

It is not necessary that motors always produce

100% rated torque. But it is necessary thatmotors produce more torque than is required

by the loads that they are driving. Fortunately,

most motors are oversized for their

mechanical loads. But it is still incumbent on

the power system engineer to design adistribution system that will deliver a

reasonable voltage to the terminals of motors

during all steady state operating conditions.

A design rule of thumb that has workedreasonably well is that systems generally can

withstand sustained voltages down to about95% of rated during normal operations. In

this context, normal means either the

normally intended mode of operation, or one

of several modes that might have been

envisioned as a possible mode of operation at

the time the system was designed. Thus, for

example, in the example above, solution C

involves a potential future load of 1500kVA

on two paralleled transformers, each with a

base rating of 500kVA. Therefore, it would

be necessary that the system deliver at least

95% rated voltage at the terminals of motors

for this heavy load condition for the design to

be considered adequate.

Figure 9 shows this condition in a power flow

solution. In this instance, the load was

represented as one 100 horsepower motor plus

an aggregation of other loads totaling

1393kVA at 93% power factor. The voltage

at the 480v bus with rated voltage on the.

Fig 9 Power flow solution for a fully-loaded

double ended substation.3

3

See appendix I for an explanation of the

information in this figure.


13/29


transformer primaries were depressed to 95% of rated, the voltage delivered to the secondary

would be 89.2% of rated, well below the threshold of acceptability.

This illustrates a critical point. While it may be possible to perform much of the analysis required

to determine the thermal ratings of system components using a simple spreadsheet, it is not

possible to gain complete insight into the performance of a power system without using a truepower flow model. That is especially true if the design involves significant lengths of feeder

cables or a medium-voltage synchronizing bus design. The reason for this is that motors and other

actual power system loads are non-linear. Motors have a constant kVA characteristic in response

to changes in bus voltage. As a result, increases in load tend to amplify voltage drop. The only

accurate way to assess these considerations is by computer simulation that can deal with the non-

linear equations required to describe the performance of those loads

One solution to the possible problem presented in this example is to take advantage of the voltage

taps that are usually present on transformers. Moving the transformer taps to the 2.5% boost

position would cause the secondary voltage in this example to be a completely acceptable 97.3%

with rated voltage on the primary. With the primary voltage depressed 5%, the 2.5% boost tap

would elevate the secondary voltage to 91.8% of the nominal 480 volt value. However, thatwould equate with 95.7% of rated motor voltage leading to about 92% rated torque. It might be

possible to arrive at a conclusion that if the combination of reduced primary voltage and heavy

loading is not likely to occur frequently enough to worry about this being unacceptable

performance.

.

Voltage spread is primarily a function of

reactive power flow through system inductive

reactance. That can be demonstrated quite

dramatically by adding a capacitor bank to the

system simulated in figure 9. This simulation

shows that a 500kVAR capacitor bank causes

the voltage at the 480v. bus to be higher. Thereactive demand of the 480v loads is unchanged;

because the capacitor bank supplies some of that

reactive locally (at the 480 v bus), it is not

necessary for the entire reactive demand to be

supplied from the primary, thereby reducing theamount of voltage drop experienced in the

transformers

It has been said that vars dont travel well.

Thats a profound consideration. Designing thepower system in a way that provides local

reactive sources helps improve the overallvoltage profile of a system.

BUS-11.000@

0.0

BUS-20.974@

-4.5

81.087(43.766)

1407.

517

(181.

845)

13.800

703.

759

(90.

923)

0.480

690.

543

(34.

543)

13.800

703.

759

(90.

923)

0.480

690.

543

(34.

543)

1300.000

(500.000)

0.000

(474.680)

Fig 10. Power flow solution with the

addition of a 500kVAR capacitor bank


14/29


If inductance and vars determine voltage drop and the magnitude of voltage, then resistance and

real power determine the angle of the voltage phasors. Voltage angle is normally not a concern in

the design of industrial power systems.

Steady-state supply and management of reactive power is also an economic consideration. Must

electric utilities impose a penalty on industrial customers with low power factor. That penalty

may be expressed directly in terms of power factor, or it may be formulated in other ways. Oneof the more subtle approaches is to define demand limit in terms of total apparent power (MVA),

while setting the power consumption measurement on real power component of consumed energy

(kilowatthours). Regardless of how it is done, it is important that the process of designing the

industrial system anticipate the need to manage reactive power demand on the host utility.

Ordinary capacitor banks are a relatively inexpensive way to supply and manage steady-state

reactive power, and with many typical reactive penalty arrangements, the economic payback is

less than one year.

The concern that is usually most vexing is designing the system to deliver at least 95% of rated

voltage everywhere and at all times. However, the designer also must be sensitive to the fact that

the sustained voltage cannot be too high. Its not unusual for system to operate with elevated

voltages, with a practical upper limit usually considered to be about 103% of rated. That is, theacceptable spread of steady-state voltage is commonly 95% to 103%.

Incidentally, a question that is often debated is what is the optimum location of power factor

capacitors. The answers are:

If the power factor capacitor is needed ONLY to reduce the reactive demand at the pointof metering, then the optimum solution is to install one capacitor bank on the load-side of

the revenue meter.

If power factor capacitors are needed to address steady state voltage spread and to relievethermal loading in cables and transformers, then the optimum solution is to apply

capacitors at the terminals of loads and arrange for them to be switched on and off with

their associated loads. It should be noted, however, that there are limitations in the

amount of capacitance that may be switched in combination with motors, and thoselimitations may necessitate the addition of other capacitor banks in order to achieve the

overall power factor improvement target for the system.

If there are sources of harmonic currents in the system such that there is a need for thecapacitors to be configured as harmonic filters, then the optimum solutions will usually

be to provide the smallest number of capacitor banks possible.

And there are exceptions to each of these general rules.

In recent years, industry has become much more concerned about efficiency and losses, and

operating economy clearly needs to be taken into account in designing industrial power

distribution systems. That said, there may not be a lot that can be done with the powerdistribution system itself to enhance operating efficiency. The fact is that the components that

make up the power system (mainly switchgear, transformer and cables) are quite efficient and areresponsible for only a very small fraction of the losses incurred by typical industrial systems.

Most losses are incurred in motors, and the greatest opportunity for improving overall operating

efficiency is in the way that motors are selected to meet individual process requirements.

Certainly, the opportunity exists to select between traditional motor designs and those with

reduced losses. In the late 1970s, when manufacturers started offering higher efficiency motors,

many industrials were tempted by the promise of economic savings. But when they did the

analysis, they found that the actual savings that would result from switching from existing


15/29


standard motors to higher efficiency motors of the same rating would not pay for the cost of

retiring the older machines and buying new motors. However, the use of higher efficiency

motors does appear to make economic sense when new systems are being built.

A more important consideration is that the motor must be sized properly for the load. Motors are

most efficient when they are operated at a point fairly close to their electrical ratings, and motors

that are oversized relative to the actual mechanical requirements of the load they drive will incurhigher energy losses. The principle that seems to be involved here is that the mechanical

engineer responsible for the driven load adds a margin when specifying the brake-horsepower

requirement of the load. Then, the application is handed of to an electrical engineer who adds an

additional margin to determine the theoretical rating of the motor. But because motors come in

standard frame sizes, the motor that is actually chosen will be larger than the theoretical rating..

The result is a process that almost certainly will lead to oversized motors. Obviously, a non-

parochial design process in which there is a single assessment of margin and economy would help

avoid this kind of problem.

Dynamic Performance Considerations

The third major set of considerations for the designer of an industrial electrical distribution

system relate to system performance during or in response to changes in dynamic changes on the

system. For typical industrial systems there are two general sets of concerns. One relates to

motor starting or other impact loads. The second is the pervasive concern for short circuits.

Impact Loading

Impact loading is a generic term for the concerns associated with sudden changes in electrical

load. The most common example of impact loading is motor starting, but there are other fairly

common examples in industry.

o In the mining industry, draglines have a dramatically cyclical characteristic. Realpower peaks as the shovel is digging into the bank, or crowding the bucket back

toward the cab, but then drops and may actually reverse direction as the bucket isemptied. Reactive power also goes through a cycle that may be a bit less extreme,

but that may actually be more significant because of the effect on voltage.

o In the metal rolling industry, the power input to a rolling mill increases sharply whenthe raw stock enters the mill. Again, the reactive swing may be of far greater concern

than the real power impact. In one rather notorious instance, the reactive demand of

a slab mill increased 70MVAR in a few seconds.

o The load associated with dynomometers (in the automotive industry) can swingbetween net-motoring and net-generating.

o Even some continuous loads have cyclical nature. Reciprocating compressors arenotorious for applying a pulsating demand on the electrical system.

The major concern with impact loading is for how the load affects voltage, and of course the loadthat is of interest is the impact load. One way to quickly approximate the effect of a change in

reactive demand is equation [1].

100

circuitshort

(%)changeVoltageMVA

MVAR[1]


16/29


In this equation, MVAshort circuit is the short circuit stiffness at the point where the impact load is

applied, and the voltage change is relative to the steady-state voltage that existed prior to

application of the impact. So, for example, a step change of 10MVAR would lead to a 2%

voltage change on a system with a 500MVA short circuit stiffness4.

If a more exact numerical assessment is required, then it is again necessary to employ a system

power flow model to simulate the conditions that exist before and after the application of animpact load.

More commonly, the concern is for starting of motors. The power factor of motor starting current

tends to be in the range of 15-20%, so the initial demand of the motor is predominantly reactive.

And the magnitude of the starting current is several times the rated full load current of the motor.

Typical, traditional induction motors have starting currents that are about six times normal full

load current, while the starting current of higher-efficiency motors can be as high as 8.5 times full

load. Synchronous motors tend to have lower starting currents perhaps four to five times full

load. Special soft start motors can be specified that have starting currents as low as three times

full load. And, of course, motors that are served through electronic power converters (typically,

but not always, for variable speed applications) have significantly lower starting impacts.

Using equation [1], it is possible to arrive at a quick estimate of the voltage dip that will

accompany starting of an induction motor. For the sake of consistency, consider the system

depicted in the power flow solution in figure 9 consisting of two 500kVA transformers supplying

a 100 horsepower motor.

The short circuit stiffness can be approximated by dividing the transformer kVA by the

transformer impedance. Standard low voltage unit substation transformers have an impedance of

5.75% on their own base, so

MVA17.4kVAStiffness ==

= 391,170575.0

5002[2]

When starting, a typical 100 horsepower motor will impose an impact load on the system of

MVARkvarkVAratedLoad 6.06006 === [3]

Therefore, from [1], the voltage change will be

%44.31004.17

6.0(%)changeVoltage [4]

Another way to arrive at an estimate of the voltage impact from motor starting is by performing

two successive power flow simulations, one without the motor, and the other with the motor

represented by its starting impedance.

4 In the past, it was routine to express short circuit levels in terms of MVA. Today, circuit breaker

standards define breaker short circuit ratings in amperes, and it is necessary to calculate short circuit levels

in compatible terms. However, it remains a convenience for system planners to think in terms of the

equivalent short circuit MVA level, where 3= kVkAMVA circuitshortcircuitshort .


17/29


BUS-11.00

[email protected]

BUS-20.950

@-4.0

1328.

198

(620.

300)

664.

099

(310.

150)

6

50.

000

(250.

000)

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099

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6

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000)

1300.000

(500.000)

BUS-10.99

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BUS-20.916

@-3.9

135.932

(536.374)

MSHp = 100

MSPF = 12%

699.

939

(597.

670)

6

77.

676

(

502.

691)

699.

939

(597.

670)

6

77.

676

(

502.

691)

1219.420

(469.008)

Before start After start

Fig 11 Steady-state power flow simulations of a system before and after starting a motor

Note that the simulations show that starting the motor will cause the voltage to dip from 95% of

rated to 91.6% of rated.- a 3.58% dip. Note also that this answer is very close to that calculated

by the quick approximation formula [1].

The problem of motor starting is a special case of the more general problem of impact loading.

Generally, the major issue involved in impact loading is the increase in reactive loading, although

there are instances in which changes in real power loading can be just as disruptive. It was noted

in the discussion of steady state loading that addition of power factor capacitors can be an

effective means to address voltage spread problems. That leads to the question of whether power

factor capacitors might also help with impact loading.

The answer, in general, is that capacitors are not helpful by themselves. To illustrate that point,

consider the example presented in figure 11, but with a 500kVAR capacitor bank applied on the

480v bus.


18/29


BUS-11.00

[email protected]

BUS-2

0.97

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1323.

163

(119.

979)

661.

581

(59.

989)

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000

(10.

580)

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(59.

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650.

000

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1300.000

(500.000)

0.000

(478.838)

BUS-10.99

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BUS-2

0.943

@-4.1

139.324(565.548)

MSHp = 100

MSPF = 12%

694.

368

(363.

506)

678.

223

(294.

627)

694.

368

(363.

506)

678.

223

(294.

627)

1217.121

(468.123)

0.000

(444.417)

Before starting After starting

Fig 12 - Steady-state power flow simulations of a system before and after starting a motor with a

capacitor bank supporting system voltage

This example shows that the starting the motor will cause the voltage to drop from 97.9% of rated

to 94.3%, or a change of 3.68%. There are two important points to note here.

First, the presence of the capacitor elevates both the pre- and post-impact voltage by about the

same amount compared with the example of figure 11. That is, the effect of the capacitor bank is

to change the baseline from which the dip occurs,

More significantly, the presence of capacitors appears to actually amplify the magnitude of the

voltage dip from 3.58% to 3.68%. This is not a numerical oddity, but rather is an actual

consequence of the physics of the electrical circuit. A power factor capacitor is passive shunt

device an impedance. The reactive flow in the capacitor is directly proportional to the square ofthe voltage applied across the capacitor and as the bus voltage is depressed by the impact load

of the starting motor, the capacitor will be less able to provide reactive support of voltage.

Hence, while a capacitor bank is most helpful in addressing steady-state voltage profile issues, it

may actually aggravate impact-related voltage issues.

Instead, dynamic source of reactive support is required to address voltage dips associated with

impact loading. A dynamic reactive source is one that provides progressively greater reactive

support as the bus voltage becomes lower. The simplest example would be to associate a


19/29


switched capacitor with the starting motor a capacitor that is applied at the instant that the

motor contactor is closed, and that is subsequently removed from the system after the motor

accelerates to rated speed.

Synchronous motors are also effective as dynamic sources of reactive support. Alternatively, it is

also possible to purchase solid-state reactive compensators that can react very quickly to changes

in reactive loading.

Transient voltage dips are an unavoidable consequence of step loading and motor starting.

Before opting for a solution to transient voltage disturbances, it is important to know whether

there is really a problem. There are really two separate issues involved in answering that

question.

Like steady-state voltage spread, transient depressions of system voltage cause motors to produce

less mechanical torque. Whether that is actually a problem really depends on how long the

transient voltage depression lasts. In most instances, a voltage dip that lasts only a few cycles of

time, or perhaps even a few seconds, will not cause any problems with operating motors.

However, there is one very notable exception to that generalization. The reduction in torque can

cause some slow-speed mechanical systems to slow down sufficiently that there is a significantdifference between the phasor angle of the residual magnetic field in the motor and the system

voltage, and this can result in significant torque transients when the voltage dip is corrected. The

most notable examples of this problem are very slow speed applications such as autogenous

grinders in the mining industry and cement kiln drives.

The other issue is that the instantaneous magnitude of voltage drops low enough to cause some

kind of magnetically-latched control device to drop out. The most traditional example of this is

that motor starter contactors will open if the applied bus voltage drops too low. The exact

threshold is a variable, but the traditional rule of thumb is that limiting the instantaneous

magnitude of voltage to no less than about 75% of rated should avoid contactor dropout. On that

basis, it is common to limit the lowest voltage under motor starting conditions to be no lower than

80% of rated system voltage. Computers and PLCs also have an undervoltage drop-outcharacteristic, but they may be more sensitive to voltage dips than magnetically latched

contactors and may require special provisions to assure ridethrough.

Short Circuits

Short circuits (faults) are relatively common occurrences on electric power systems. The currents

that flow into a short circuit are much higher than normal load currents. As a result, a fault can be

a destructive event that results in a significant energy release at the point of the fault and doing

considerable damage to the components that make up the power system. The primary means of

protective against short circuits is by means of protection devices sensors and relays incombination with circuit breakers. As the magnitude of the potential short circuit currents that a

system can produce increases, it is necessary to resort to larger (and therefore more expensive)circuit breakers that have the rated capability to interrupt those higher currents.

Every power system must be equipped with fault interrupting devices that are intended to

automatically open in the event of short circuits. And under the National Electric Code, those

fault interrupting devices must be rated to interrupt the short circuit currents that the system is

capable of producing.


20/29


In addition, in recent years engineers have become much more aware of the potential hazard to

individuals in the vicinity of a short circuit. Not only does the arc associate with a short circuit

throw off a significant amount of radiant heat, but there are also concerns with the concussive

impact of a fault, the audible noise that accompanies the short circuit, and the possibility of

shrapnel being projected away from the fault point.

As noted earlier, process load requirements dictate the size of motors and other cyclic loads, andthese in turn determine the minimum short circuit stiffness that the system must have to be able to

cope with those loads. But the desire to choose lower-rated (and therefore less costly)

switchgear, combined with the desire to minimize arc flash hazards, are factors that limit the

maximum short circuit stiffness. So designing a system to address short circuits is a challenge in

balancing these competing objectives.

There are a number of factors that have a bearing on the available short circuit current the

presence and ratings of motors and generators and the voltage are significant, but the ratings of

transformers is probably the most important. Ultimately, the magnitude of available short circuit

current comes down to the Thevenin equivalent driving point impedance at each point in the

system and lower impedances mean higher short circuit currents. Since the actual impedance of

a transformer is primarily determined by its rating, then the ratings of the transformers supplyinga system are critical.

But transformer rating is also closely tied to the load to be served by the system. That leads to an

important but seldom documented observation: in an ideal system, the available short circuit will

be in the neighborhood of 25 times the maximum load to be served by that system. While this

ratio can be expressed in terms of current (amperes), practiced system designers more typically

talk in terms of MVA in order to eliminate system voltage from consideration.

So, for example, if the load that must be served is 10MVA, one should design the system for a

maximum available short circuit level of about 250MVA.

There are two important implications from this observation. The first is that the magnitude ofload drives the selection of transformer ratings, and the available short circuit level is a

consequence of that selection. That is, one generally does not design a system to achieve a

desired short circuit level. Instead, one designs a system to serve identified loads, and then has to

deal with the short circuit level that is necessary to support those loads

That observation in turn leads to several conclusions about how loads should be served. First,

with regard to unit substations, there is an advantage to design for a larger number of smaller

substations instead of combining all loads on a small number of larger substations. That is

because substations built around transformers with lower ratings have lower available short

circuit currents. Obviously, there are both economic and real estate penalties in using a largernumber of substations, so the designer is faced with a tradeoff.

Second, as discussed earlier, the ability to start motors is a major factor driving the required short

circuit level (and hence, the required rating of substation transformers). While the size of

individual motors is driven by process non-electrical process considerations and generally has to

be accepted as a given, the power engineer may be able to influence the choice of starting means

for those larger motors. Motors applied through power converters (as adjustable speed drives)

have a significantly lower impact upon starting that do single-speed motors with across-the-line

start. In addition, in many instances the ability to throttle back the speed of the drive translates


21/29


into savings in energy consumption. Therefore, applying larger motors through adjustable speed

power converters may be an attractive option.

There are also other options for reducing the starting impact of larger motors. Motor starting

reactors and autotransformers are an old solution that still works quite well. On very large motors

(typically 10,000 HP or larger), it is not uncommon to see the motor applied on a dedicated

captive transformer. That arrangement transfers the starting impact of the motor to a highervoltage bus that almost certainly will have a higher available short circuit level, while also

reducing the magnitude of the impact. Obviously, these solutions require capital investment and

real estate, so there selection has to be part of an overall trade-off evaluation.

The second major observation derived from the 25:1 rule is that because the available short

circuit level is a consequence of the load that must be served, the ideal system voltage is also

determined by that load. Under ANSI standards, the availability of short circuit ratings in

medium voltage switchgear is tied to voltage level (Table 1), and it may be necessary to resort to

a higher voltage in order to select commercially available switchgear for a desired fault level.

Table 1 Typical Short Circuit Ratings of Medium Voltage Circuit Breakers under ANSI

Standards

Voltage RatingShort Circuit

Rating 5kV Class (4.16kV) 7.2kV Class (6.9kV) 15kV Class (13.8kV)

250 MVA X350 MVA X500 MVA X X750 MVA X1000 MVA X1500 MVA X

So, for example, if the load to be served is 10MVA, then the optimum fault level is 250MVA, and4.16kV might be a good voltage choice. But if the load to be served is 15MVA, the optimum

fault level will be 375MVA and one could choose between 6.9kV and 13.8kV.

There are a few special voltage considerations worth mentioning:

o 2.4kV used to be a relatively common system voltage, especially in the pulp and paperindustry. Today, that voltage is generally viewed as obsolete, and it is difficult to

purchase switchgear designed specifically for 2.4kV. However, 2.4kV may still be an

economically appealing voltage for motors, and could be a practical motor voltage when

the motor is applied through a captive transformer served from a higher-voltage bus.

o 4.8kV is occasionally seen as a distribution voltage. Under ANSI standards, 4.16kVswitchgear has a maximum voltage rating of 4.76kV and may not be applied at 4.8kV.

Therefore, 4.8kV requires at least 7.2kV class switchgear; most typically, 15kVswitchgear is used. It is generally not an ideal voltage for industrial distribution.

o The technical and economic tradeoffs involved in motor design result in the situationwhere the optimum machine voltages tend to range from 2.4kV through 6.9kV although

higher voltage ratings are seen on very large machines. While 6.9kV might appear to be

an ideal voltage for a system serving a large population of motors, that voltage has not

been well supported by switchgear manufacturers.

o 12.47kV and 13.2 kV are voltages used by distribution utilities in some areas. Typically,these voltages are only used in industrial systems when those systems are being fed


22/29


directly from the distribution utility at those voltages and without intervening

transformers.

Protection Considerations

The final aspect of system design is protection. The ultimate test of a design is whether it can be

easily and economically protected using simple, commercially available protection equipment.

There are four fundamental objectives that must be met by the protection system:

1. Economy protection must be achieved at the lowest practical cost. A rule of thumb isthat the protection system should contribute about 10% of the total cost of an industrial

electrical distribution system (and the electrical distribution system itself should cost

about 10% of the total capital investment in the industrial facility).

2. Reliability the protection system must work as expected, every time, even though thesystem may go for years with no events that require action by the protection system.

3. Selectivity while the protection system must isolate portions of the system that havefailed, but the portion is isolated must be as small as practical.

4. Speed the protection system should be as fast as possible while achieving the objectivesof reliability and selectivity.

Of these, economy, reliability and selectivity can generally be achieved by applying traditional,

well-tested protection solutions. These solutions are well documented in the technical literature,

and references such as IEEE 241 Recommended Practice for Protection and Coordination of

Industrial and Commercial Power Systems (the Buff Book) are highly recommended.

Speed is another matter. Speed of the protection

system is primarily a consequence of the design

of the system in combination with economy,

reliability and selectivity. Hence, the speed of

the protection system itself is a measure of the

quality of system design.

The most common form of protection in an

industrial system involves time overcurrent

devices, and the traditional rule is that it should

be possible to detect and clear a fault at any

location in one second or less.

Its important to recognize that this is a

traditional measure, and in todays world there

may be a desire for faster fault clearing times to

manage the energy release associated with arc

flash. That desire may lead to the application offaster protective functions that, in order toachieve selectivity, could also distort traditional

economics. Even so, the one-second criterion is

still a valid measure of system performance.

0.3 sec

0.3 sec

0.3 sec

time

current

A

B

C

D

Fig 13 Selective Coordination of

Overcurrent Protection Devices5.

5 By tradition, time-current curves are always

log-log plots.


23/29


Figure 13 illustrates an array of four protective devices, A, B, C and D, that are arranged

selectively. Device A is termed a primary protective device and includes an instantaneous

overcurrent element, while devices B, C and D are time-overcurrent devices that provide backup

protection in the event A fails to operate in addition to perhaps providing primary protection to

other areas in the system.. Note that at the maximum fault level, the time differential between the

devices is 0.3 seconds, so that the slowest device (device D) will operate in 0.9 seconds or less forthe maximum fault.

This time-current curve can then be used to

translate the one-second criterion into a

system design criterion if this curve

depicts the limit of acceptable system

operation, then the limit of acceptable

system design is one that can be adequately

protected by this array of protective devices.

Therefore, if the one-second rule is to be

met, the system must be designed to require

no more than three layers of time-overcurrent backup protection at any

voltage level.

Figure 13 could also be applied to a medium

voltage system with fundamentally the same

conclusion there can be no more than three

levels of time overcurrent backup protection

in a well-designed system. However, in that

instance the interpretation of the conclusion

might differ because of differences in the

kinds of equipment used to provide

protection.

Fig 14 depicts a typical double-ended low

voltage unit substation that might have

protection characteristics such as those in

Fig 14 Low Voltage Substation that might

correspond to the protection in Fig 13.

figure 13. Here, device A is in a motor control center, B is the substation feeder supporting the

motor control center, C is the tie breaker, and D is the secondary main breaker.

Note that it would not be possible to insert any additional time-overcurrent protective devices into

this system without causing the time delay associated with device D to exceed the one second

criterion. That consideration therefore discourages insertion of sub-buses or other complications

into this system.

Finally, there is one other criterion in the world of protection that provides insight into the quality

of design of a system. In Figure 15, the time current curves have been redrawn with devices A

and B having the same pickup sensitivity, and with devices C and D having different sensitivities.

Note that it is still possible to achieve the maximum one second clearing time but only if devices

C and D have a higher pickup sensitivity that devices A and B.

The pickup sensitivity of a protective device is related to the full load that it is designed to carry

for obvious reasons, the pickup sensitivity must be great than full load current. Therefore, the


24/29


message here is that in a well designed system, the backup breaker must have a significantly

higher full load rating than the largest feeder breaker. As a practical rule of thumb, the largest

feeder breaker should be no larger than one-third the rating of the main breaker in a substation,

and loads that would otherwise violate this rule should be viewed as candidates for special

treatment, eg, a captive transformer connection for larger motors.

0.3 sec

0.3 sec

0.3 sec

time

current

A

B

C

D

Fig 15 Feeders should have higher current ratings than mains.

The System Performance Specification

The performance specification does not have to be a long document, but it does need toexplicitly define the expected range of parameters that may either be calculated as part of the

design process, or that may be measured on the physical system after it is constructed. Critical

attributes of performance include:

Reliability n-0, n-1, or for systems demanding a more rigorous approach toreliability, a quantitative measure of reliability such as mean-time-to-failure (MTTF).

Note that statements of reliability in nines is vague and not at all meaningful when

designing a system. Note also that it may necessary to establish different reliability

targets for different components of the load.

Load to be served load should be identified by process elements. Power factor power factor is important in system performance, but the power factor of

loads will be what it will be, and power factor improvement will mainly be needed toaddress other system performance issues (eg, steady state voltage spread). However, if

the host utility imposes a power factor penalty at the revenue meter, the system

specification should identify the target power factor that the owner wishes to achieve for

the purpose of managing that penalty.

Expected voltage spread including minimum and maximum steady-state voltage levels. Thermal rating limits It would normally be assumed that power system components

would require continuous thermal ratings that exceed their expected continuous loading.


25/29


However, it may be appropriate to specify a minimum margin between loading and

component limits.

Specifying a minimum margin establishes the principle that there should be some margin,but more importantly, defining the margin at the outset eliminates second guessing about

what should have been anticipated about provisions for future expansion. Note that the

National Electrical Code may impose minimum required margins between electrical

loading and ratings, and the specification should clearly delineate between marginsmandated by the Code and additional margins that the owner/operator of the facility may

be able to utilize for future growth.

It is also helpful for the specification to define the minimum voltage level required formotors of various horsepower ratings. That is, it is helpful to define in the specification

the size of the largest motors that will expect service at, for example, 480v versus

4.16kV.

Limits should be established for the minimum voltage that can be tolerated underexpected impact load conditions.

While a limit on the minimum impact load voltage ultimately does provide guidance onhow large motors should be started, it is sometimes helpful to go one step further in the

system specification to delineate horsepower thresholds that at least meet, and possibly

exceed, the voltage dip specification. The National Electrical Code requires that circuit breakers be rated to interrupt the

maximum short circuit current that they will encounter in their application. That means

that the minimum margin legally required between short circuit current and short circuit

rating is only slightly greater than zero. Given the critical safety issues involved with

short circuits, most industrial system designers prefer to specify a finite minimum design

margin target (such as 10%).

In recent years, operators of industrial electrical systems have become acutely aware ofthe concerns associated with arc flash. The system specification should clearly state the

expected maximum radiated arc flash thermal energy rating for each voltage level in the

system. Obviously, this thermal energy level must be coordinated with the maintenance

procedures expected to be followed in the facility.

Various protection parameters need to be specified, including the maximum fault clearingtime, the target minimum coordinating time margin between fault protective devices,

whether protection will be applied on a per-phase basis (as opposed to an application in

which one protective device monitors multiple phases), etc.

This material has not addressed the issue of system grounding, but it is important that thesystem specification clearly define the expected modes of neutral grounding at eachvoltage level in the system.


26/29


Appendix I

Several power flow diagrams have been included as figures in this material. The following

defines the conventions used in preparing these diagrams.


27/29


Review:

Here are the major points you should remember from this course:

o The power distribution system in an industrial facility is the responsibility of a powersystems engineer. However, the role of that power system is to deliver electric power to

the process, and the power system engineer usually has little influence over how theprocess consumes electric power.

o In industry, electric power is a COST of doing business, and like any other cost ofbusiness, management will demand that the cost be managed and potentially reduced.

o The four major objectives in the design of an industrial power distribution system are1. Safety2. Reliability3. Maintainability4. Economy

o Industry constantly strives for growth, and the power distribution system for an industrialmanufacturing system will almost inevitably be called upon to support increases in the

production at that facility.

o Power distribution systems are built up using standard components, and the challenge tothe power system engineer is to make informed decisions about frequency, voltage,thermal capacity and short circuit capacity in selecting those components.

o The overall architecture, or one-line arrangement of the electrical system must be drivenby the requirements of the manufacturing process it serves. There are significant

differences in requirements between discrete manufacturing businesses and continuous

process businesses.

o The risk of failure can be minimized but never eliminated, and every component of thesystem is a candidate for eventual failure. Ultimately, what matters is whether the failure

of a component results in failure of the electrical system to deliver power to the process.

o The key to reliability is redundancy.o Single-point failure modes should be identified. Whether every single-point failure mode

must be eliminated really depends on the requirements of the business, and in particular,on the impact of an interruption in the supply of electric power to the process.

Ultimately, it comes down to whether the cost of addressing a single-point failure mode

exceeds the cost of living with that single-point failure if and when it does occur. This is

referred to as the n-1 principle.

o While assessing reliability numerically is interesting, the main advantage is to add insightinto the elements contributing to reliability. Except for special instances in mission-

critical facilities, it rarely contributes anything of value to the design process.

o In designing a system to address reliability, elements are often grouped in a way thatfailure of one element requires that there be redundancy for the entire group.

o In most instances, one layer of redundancy is sufficient. Adding one layer of redundancygreatly increases overall system reliability. Adding additional layers will add additional

theoretical reliability, but the incremental performance improvement generally does notjustify the added cost. The expression of that principle is that the most common

building block in designing an industrial power system is the double-ended unit

substation depicted in Figure 3.

o Predicting steady-state loading requirements while allowing for the uncertainties of loadgrowth is a major challenge.

o All electrical system components have thermal ratings that define the maximum loadingthey can withstand without overheating. While these ratings may ultimately be related to


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current flow in the conducting elements making up those components, it is typical for the

ratings to be expressed in terms of the total power (kVA or MVA) equivalent.

o Generator ratings are often expressed in terms of real power (kW or MW). Powerengineers need to recognize that these ratings may be the mechanical output ratings of the

generator prime mover (turbine, engine, etc), and need to also ascertain the actual MVA

rating of the generator itself.

o Some loads have cyclic characteristics. In those instances, it is usually necessary toconsult either the applicable industry standards, or perhaps the manufacturer, to

determine how cyclic loading relates to the continuous thermal rating of power

distribution system components.

o Some components (eg, transformers) may have forced cooling capabilities that increasethe inherent continuous thermal capacity beyond that of the basic component carcass.

Careful exploitation of these forced cooled ratings can result in considerable savings in

capital investment, and the ability to add forced cooling may be an option to address

future load growth.

o In most cases, a system can tolerate steady-state voltages between 95% and 103% ofrated. That criterion is based on the fact that the voltage rating of motors is typically less

that the voltage rating of the system that supplies those motors. For example, the motors

that would normally be applied on a 480v. system are rated 460v.o System voltage drop is a result of the interaction of the inductive reactance of the system

components, and the flow of reactive power to loads.

o The torque produced by a motor, in per unit of rated torque, is approximatelyproportional to the square of applied per unit terminal voltage. Most motor applications

include design margins that mean that the motor is not actually required to produce rated

torque. Unfortunately, those design margins are generally not known to the power

system engineer.

o Transformer voltage taps are helpful in reducing steady-state voltage spread.o The addition of power factor capacitors is another cost-effective way to address steady-

state voltage spread. The optimum location of those capacitors depends on the objective

for adding them to the system design Capacitors intended mainly to reduce reactive

demand at the point of revenue billing should be installed in one bank at a point adjacentto the point of metering. Capacitors intended mainly to remediate system voltage drop

should be distributed across the system adjacent to loads, and in a fashion such that

capacitors are switched on and off at the same time that their associated loads are

switched on and off.

o The only accurate way to predict voltage spread in advance of commissioning of a systemis by means of a steady-state power flow simulation that can correctly address the non-

linear relationship between load and voltage.

o The real-power energy losses in an industrial electric power distribution system aregenerally negligible. There are, however, often opportunities to improve efficiency at the

points of electric power consumption. In particular, the use of higher efficiency motors,and taking care to more carefully match motor ratings with the actual mechanical torque

requirements of their driven loads can result in an overall improvement in energyefficiency in a manufacturing process.

o The voltage dip associated with an impact load can be estimated from the formula:

100

circuitshort

(%)changeVoltageMVA

MVAR

In the specific case of motor starting, MVAR can be estimated to be six times the motor

horsepower rating.


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o The addition of power factor capacitors will change the baseline for the voltage dipassociate with starting motors or other impact loads, but in general will make the

magnitude of dip greater than would be the case without capacitors. Reactive solutions tomotor starting or other impact loads must be dynamic reactive sources that increase their

reactive output as the impact forces a decrease in bus voltage.

o Magnetically latched contactors drop out when the instantaneous voltage falls below

about 75%. A common rule-of-thumb is that the voltage dip associated with an impactload should not force the instantaneous voltage below 80-85% of rated system voltage

based on avoiding contactor dropout. Microprocessor devices may have a higher

undervoltage dropout level, and may require special provisions to ride through transient

voltage dips.

o Starting reactors and autotransformers may be useful in reducing the starting impact oflarger motors.

o Adjustable-speed power converters can significantly reduce the voltage impact associatedwith starting a motor, and the ability of these system to allow the speed of the motor to be

adjusted to match the requirements of the load may also result in enhanced energy

efficiency.

o Consideration should be given to applying very large motors (typically 10,000 HP are

above) in captive-transformer arrangements. The captive-transformer design minimizesmotor starting voltage drop, and also transfers the application of the associated large

motor to a higher voltage bus that is likely to have greater short circuit stiffness such that

the impact is less significant.

o The National Electric Code mandates that the short circuit rating of fault interruptingdevices be sufficient for the available short circuit currents that the system can produce.

o The loads to be served will determine the short circuit levels that the system must have tosupport those loads. An undocumented but effective criterion is that the available short

circuit stiffness should be about 25 times the magnitude of peak stead-state loading.

o In general, a modular design involving a larger number of individually-smaller unitsubstations is superior to a design that relies on a smaller number of larger substations.

o Application of the 25:1 rule is helpful in deciding the optimum voltage of a medium-

voltage distribution bus in an industrial system.o While the designer of a system has the freedom to choose any voltage that makes sense,

there are a few voltage choices that are not recommended:

2.4kV is obsolete as a distribution voltage although it may make sense as amachine voltage in a captive transformer application.

4.8kV is a poor choice of voltage for industrial distribution. 12.47kV and 13.2kV are utility distribution voltages and make sense for

industrial distribution only if the industrial system is to be fed directly from the

utility distribution system.

o The maximum fault clearing time using time overcurrent protection should be kept to onesecond or less.

o In general, the system design should not require more than three layers of time-

overcurrent backup protection at any given voltage levelo In general, the system design should result in a minimum 3:1 ratio between the pickup

threshold of a backup overcurrent device and the pickup threshold of the largest feeder

overcurrent device.

o Consideration must be given to the thermal energy released by faults and the resultingrequirements for personnel protection. That consideration may result in the desire to

reduce the available short circuit level, to reduce the fault clearing time, or both.