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How Does AMetal OxideDistribution

Arrester Work?

Reprint of a Series of Articles from Ohio Brass

POWER SYSTEMS, INC.

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The comments in this series apply toOhio Brass Type PDV and PVR

arresters.

The distribution class surgearrester is the most widely used of all

arrester classifications. This high volume

has held the cost to a level where, even

in areas of relatively low thunderstorm

activity, distribution surge arresters can

be used to protect every pole top

distribution transformer. In some casesof higher isokeraunic levels, distribution

class surge arresters are also used for line

protection and are installed periodically

along the distribution line.

Arrester design advances and progress

have allowed for continued improve-

ments in reliability and safety. The

polymer-housed MOV distribution class

Chapter One

How Does a Metal-OxideDistribution Arrester Work?One in a Series

MOV arresters depend on the nonlinear resistance characteristics of theirblocks for suitable discharge and continuous operating capabilities.

Figure 1

surge arrester is an excellent example ofthis trend toward higher reliability and

safety. The polymer-housed arrester

offers a lower failure rate and higher

safety because of its leakproof design

and its non-fragmenting characteristics.

Arrester FunctionThe reason for applying an arrester is

to provide overvoltage protection for

electrical insulation, thereby maintaining

high service reliability levels. It is

worthwhile to discuss how the distribu-tion class surge arrester performs this

function.

The principal of the polymer-housed

MOV distribution class surge arrester is

quite simple. It is a device which is

electrically connected in parallel with

insulation needing protection. The

polymer-housed distribution class metal-

oxide surge arrester is connected line toground in parallel with this equipment.

Therefore, it has a high resistance at the

arresters normal 60-cycle operating

voltage. As shown in Figure 1, the

resistance of the metal-oxide arrester

elements is a function of the voltage

which is applied to them. At normal

operating voltages, the resistance of the

metal-oxide blocks is extremely high.

The MOV arrester essentially behaves as

an insulator at these voltages.

Under a surge condition, the resistanceof the metal-oxide varistors drops

dramatically and the arrester permits the

surge to be diverted to the ground while

providing equipment protection.

The current which flows through the

arrester is the discharge current, and the

voltage which is developed across the

terminals of the metal-oxide arrester is

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called the discharge voltage. Since thearrester is in parallel with the insulation,

the discharge voltage of the arrester plus

the voltage drop in the arrester leads

equals the stress level to which the

insulation is subjected. The voltage

developed in the lead wires will be

discussed in a later chapter of this series.

After the discharge current has passed

through the arrester and the voltage

returns to normal system operating

voltage, the arrester again has a higher

resistance. The arrester then reverts tothe mode where it essentially behaves as

an insulator.

Arrester Designand Manufacture

The metal-oxide arrester components

are housed in an ESP rubber housing.

The rubber housing provides an external

electrical insulation for the internal

components, and it also protects them

from the effects of the elements. The

ESP rubber housing prevents moisture

from entering the arrester, causing an

arrester failure. This is accomplished by

a seal on each end of the arrester plus a

live silicone interface between the

internal elements and the rubber housing.

The amount of internal air space inside a

polymer-housed MOV distribution

arrester is quite small compared to a

porcelain-housed arrester. Therefore, the

possibilities of moisture ingress are

reduced or eliminated. Studies have

indicated that nearly 90% of allporce-

lain-houseddistribution arrester failures

have been a result of moisture ingress.

The metal-oxide varistors in the Ohio

Brass distribution class arresters are

manufactured in our high-volume facility

in Wadsworth, Ohio. This facility is

dedicated exclusively to the manufacture

of metal-oxide varistors.

The metal-oxide varistors consist

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primarily of zinc-oxide, with approxi-

mately 5% of the remaining material

consisting of various metal-oxides

forming a boundary around zinc-oxide

grains. These boundary regions give the

metal-oxide varistor its nonlinear

characteristics. (For more information on

this please contact Ohio Brass.)

The metal-oxide ingredients are

processed to a powder state and then this

powder is pressed to the necessary

diameter to match the application. The

metal-oxide varistors are fired in

controlled-atmosphere kilns at tempera-

tures over 2000F. The sides of the

finished metal-oxide varistors are

covered with an insulating electrical

collar and the ends are given a metalized

surface for electrical contact. This

finished arrester component is referred to

as a metal-oxide varistor.

Ohio Brass manufactures normal duty

polymer-housed MOV arresters (PDV-

65) and heavy duty distribution polymer-

housed MOV arresters (PDV-100). The

primary differences between these

designs are in the energy handling

capability and the discharge voltage

levels (protective levels) of these

designs. The PDV-65 uses a metal-oxide

disc 32mm in diameter and the PDV-100

uses a disc 40mm in diameter. These

differences will be discussed in more

detail in later chapters.

In the remaining chapters in this

series, we will discuss other factors

which are pertinent to distribution

arrester application including ANSI

Standard Terms and Tests, lightning

phenomenon, testing of metal oxide

varistors and assembled metal-oxide

distribution arresters both in the factory

and in the field, hardware accessories

which are available for distribution

arresters, protection of underground

distribution systems, the effects of lead

length, and other factors that should be

considered in the evaluation and applica-

tion of distribution class surge arresters.

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Chapter Two

How Does a Metal-OxideDistribution Arrester Work?One in a Series

PDV-100 Arrester, 9 kV Unit.

Design distribution arrestersfor protection against surges

To fully understand how distribution

and riser pole arresters perform their

functions, it is important to understand

something about the nature of surges on

the power system. The most common

surges on the distribution system are

from lightning. Therefore, in this

installment, we will examine lightning

and its characteristics.

Lightning, completely unpredictable

in most ways, is the most destructive of

all elements associated with thunder-

storms. Generated by massive thermal

instability of the atmosphere, thunder-

storms represent violent examples of

convection whereby huge layers of the

atmosphere are disrupted and overturned.

Isokeraunic maps published by the

government indicate the average number

of thunderstorm days per year for areas

of the United States. One thunderstorm

day is defined as "a day on whichthunder is heard." One thunderstorm day

could be one lightning stroke or it could

be hundreds of lightning strokes. On the

average, rural transmission lines in areas

with an isokeraunic level of 30 thunder-

storm days per year can expect to

experience approximately one lightning

stroke per mile per year.

There have been recent advances in

the field of lightning detection and

measurement. In fact, maps are now

being made that are useful in determin-ing the ground flash density. The entire

nation is being monitored by The State

University of New York Lightning

Detection Network.

Electric power lines are particularly

vulnerable to lightning. Utilities in areas

of average or high isokeraunic levels

often report lightning as the primary

cause of service interruptions and

damage to equipment. Conductors,

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Isokeraunic map prepared by the National Weather Service with hatched areas indicating thunderstorm days per year.

towers and poletop equipment all havethe attributes which make them attractive

targets for lightning. Lightning invari-

ably seeks the easiest path between

positive and negative charged centers of

the storm area, even if such paths add

substantial length to the strokes.

We know when lightning strikes a

power line, there is a zone extending to

each side of the actual stroke where the

lightning voltage may greatly exceed the

insulation level of the line and flashover

to ground will occur instantaneously.

Simultaneously, traveling waves are

generated in the conductors on either

side of the stricken point. These traveling

waves have two components: voltageand current. The voltage magnitude is

equal to the current magnitude multiplied

by the surge impedance of the line and is

less than the flashover voltage of the

system insulation. These surges travel

along the overhead line at about 1,000

feet per microsecond (the speed of light).

As much as possible must be known

about the wave characteristics of the

lightning surge in order to devise

effective protection. This is a field in

which scientists have made notable

progress, and design engineers are able

to separate lightning surges into a

distinct category in relation to the broad

spectrum of overvoltage surges. A typicallightning surge has an extremely steep

wave front, which means that its voltage

is rising at the rate of millions of volts

per microsecond; in fact, 15 percent of

strokes crest in less than one microsec-

ond. The steep wave front is followed by

a short wave tail, which means that after

crest voltage is reached, surge voltage

diminishes to half crest value in less than

200 microseconds and completely

dissipates in less than 1000 microsec-

onds.

The unpredictability of lightning

reasserts itself in attempts to classify

stroke dimensions, however, since it is

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an established fact that many lightning

strokes are actually multiple discharges,

one stroke following another along the

path of the initial stroke. In contrast to

the explosive short-duration stroke

described as typical, there are occasional,

relatively long-duration strokes.

The development, testing, and

correlation of insulation with lightning

protective devices has been facilitated by

adoption of a standard 1.2/50 voltage

wave as representative of impulse surges.

In the 1.2/50 wave, voltage crest is

reached in 1.2 microseconds and the

wave decays to half crest in 50 microsec-

onds. High-voltage testing laboratories,such as at Ohio Brass, have developed

surge generators which can stimulate

lightning strokes, producing not only the

1.2/50 waves, but also the steeper-front

waves with which arresters are tested for

equivalent front-of-wave as specified by

standards.

While lightning is usually considered

synonymous with extremely high

voltage, it is the current component in

Rural transmission lines in areas with 30thunderstorms days per year can expect toexperience approximately one lightning strokeper mile per year.

the lightning stroke which is the measure

of its effect on a stricken object. The

instant a voltage-sensitive device such as

a metal oxide arrester goes into a high

level of conduction, it becomes a

current-carrying path of relatively low

impedance for the duration of the surge

discharge. Major components of the

arrester's protective characteristics are

determined by its performance in

discharging the surge current.

Extensive and elaborate scientific

investigations have been made to

measure and record lightning stroke

currents. A tremendous range has been

reported, varying from lows of 1000amperes to highs of more than 200,000

amperes, again emphasizing the

unpredictability of lightning. Probability

patterns of lightning stroke currents have

been ably discussed in several of the

technical references of the industry; but

for the arrester application engineer, the

pertinent information can be consoli-

dated into a statistical graph which

compares stroke currents to transmission

lines and to towers with discharge

currents through distribution and station

arresters. This analysis shows that

currents through arresters are only about

one-tenth the total stroke currents, but it

is significant to note that less than five

percent of distribution arrester currents

exceed 10,000 amperes. Discharge

currents through distribution arresters are

noticeably greater than those recorded

through station arresters because of their

normal installation on unshielded

overhead lines.

The destructive power of lightning is

well documented. The use of distribution

and riser pole surge arresters provides ahigher power quality level to the utility

customer.

The surge arresters are not only used

to protect equipment such as transform-

ers and cables, they are also in use to

protect the air around line insulators on

unshielded lines reducing lightning

caused interruptions. This results in

better power quality and this is the

primary goal.

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Insulation flashover

and traveling waveon a power line.Traveling wavevoltage is equal tothe current magni-tude multiplied bythe surge imped-ance of the line.

Statistical datacompare towerstroke currentswith station anddistribution ar-rester currents.

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Chapter Three

How Does a DistributionClass Surge Arrester Work?One in a Series

ANSI/IEEE StandardC62.11 describes therelevant laboratory testsfor distribution classsurge arresters.

ANSI/IEEE Standard C62.11, developed

by IEEE, with input from users, produc-

ers, and those with general interest, is the

major industry reference document

pertaining to metal-oxide surge arresters.The major objectives accomplished by

C62.11 are:

1. The definition of terms unique to

the arrester field

2. Establishment of standard and

nonstandard service conditions

3. Requirement of uniformity in

certain construction aspects, such

as nameplate data and terminal

sizes

4. Description of electrical test by

which conformance to standardscan be demonstrated

5. Formulation of a group of design

tests which can be duplicated by

properly equipped electrical

laboratories to serve as a basis for

arrester ratings and classifications

6. Assignment of minimum ratings in

the design test categories where

such ratings are appropriate and

reasonable

The scope of C62.11 IEEE/ANSI

Standard is defined as applicable to surge

protective devices, having the capability

for repeated limiting of voltage surges on

50/60 Hz power systems by discharging

surge current and automatically resealing

against system continuous voltage. This

standard applies to station, intermediate,

Insulation Withstand Test

Requirement

A surge arrester is known as a "self-

protecting" device. During the discharge

of a surge the arrester limits the voltageto a level below external flashover level.

Therefore, an external flashover of a

surge arrester during a discharge is

prevented.

To ensure that the external insulation

Table 1Arrester Ratings in (kV) rms

Duty-Cycle Voltage

3

6

9

10

12

15

18

21

24

27

30

36

MCOV

2.55

5.1

7.65

8.4

10.2

12.7

15.3

17.0

19.5

22.0

24.4

29.0

distribution and secondary classes of

metal-oxide surge arresters. The portions

of the standard that relate to metal-oxide

distribution class surge arresters are

included in this discussion.

Service conditions as described in the

standard relate to both physical and

electrical aspects of the arrester. Condi-

tions are described as standard where the

ambient temperature does not exceed

40C and the altitude is not above 6,000feet and the power system frequency is

limited to 50/60 Hz. Conditions exceed-

ing these limits and including unusual

circumstances of contamination or

clearances are termed nonstandard and

require special consideration in the form

of recommendations from the arrester

manufacturer.

The standard defines MCOV (Maxi-

mum Continuous Operating Voltage)

ratings of the arresters as well as duty-

cycle voltage ratings. The standard alsodefines the relationship between the

duty-cycle voltage rating of the arrester

and the MCOV assigned to an arrester.

Table 1 shows the standard duty-cycle

voltage and MCOV voltage ratings of

distribution arresters.

The tests developed to evaluate

relative performance of distribution class

surge arresters are:

(1) Housing withstand test

(2) Power frequency sparkover test

(3) Discharge current withstand test

(4) Impulse sparkover voltage time

characteristics

(5) Discharge voltage test

(6) Duty-cycle test

(7) Radio influence and internal

ionization voltage test

(8) Disconnector test

(9) Contamination test

(10) Fault current withstand test

In this issue we will examine in detail

the housing withstand and the discharge

current withstand tests.

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of the surge arrester is commensurate

with the remaining insulation on the

system, IEEE/ANSI Standard C62.11

defines housing insulation withstand test

requirements.

The test standard specifies require-

ments for an impulse test and 60 Hz wet

and dry tests. The 60 Hz wet and dry

tests are especially important since the

arrester spends its service life under 60

Hz conditions.Under the test conditions outlined in

the standard, the voltage withstand test

of the arrester insulation demonstrates

that the assembled insulating

members of the arrester can withstand

the values listed in Table 2.

Discharge CurrentWithstand Tests

The discharge current withstand tests

are performed to demonstrate the

arrester's ability to discharge various

types of surges and remain physically

intact, thermally stable and capable of

performing its protective function. The

discharge current withstand tests are

critical to determining the durability of asurge arrester.

The two tests which make-up the

discharge current withstand capability

portion of the test are the low-current

long-duration test and the high-current

short-duration test.

The requirements for the discharge

current withstand portion of the standard

test series vary depending upon the

durability designation of the surge

arrester.

There are two types of distribution

class surge arresters. These are the

normal duty and the heavy duty surge

arresters.

The high current short duration test

requirements for these types are asfollows:

The normal duty arrester must

withstand two discharges of 65 kA

with a 4-6/10-15 wave.

The heavy duty distribution class

surge arrester must withstand two

discharges of 100 kA with a 4-6/10-

15 wave.

The test is performed on completearresters or on thermally prorated

sections of the arrester without 60 Hz

voltage applied.

The two discharges are spaced such

that the arrester section cools to ambient

between discharges.

Within five minutes of the second

discharge the surge arrester must be

energized at its maximum continuous

operating voltage (or higher if required

by the arrester design) and must demon-

strate thermal stability.

Thermal stability is demonstrated by a

decrease in temperature, resistive current

or watts loss. The 60 Hz voltage must be

maintained on the arrester for at least 30

minutes.

Table 2Insulation Withstand Test Voltages

rmsDuty-Cycle

Voltage Ratingof Arrester

(kV)

3

6

9

10

1215

18

21

24*

25

27

30

36*

Impulse Test1.2/50 Full Wave

(kV) Crest*(BIL)

45

60

75

75

8595

125

125

150

150

150

60 Hz rmsTest Voltage

(kV)

1 min Dry Test

15

21

27

27

3135

42

42

70

70

70

1 min Dry Test

13

20

24

24

2730

36

36

60

60

60

* Insulation values are not covered in standards.

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The low current long duration portion

of the discharge current withstand tests

are also performed on complete arrester

The oscillogram below shows the 60 Hz voltage and current wave shape at

the end of the thermal stability test.

Test Results

The following oscillogram shows the l00kA discharges. The downward

deflecting trace represents the current wave form and the upward deflecting

trace represents the prorated sample discharge voltage.

or on thermally prorated sections of the

surge arrester.

The parameters for the normal duty

distribution class surge arrester are 20

discharges of 75 amps with a duration of

2,000 microseconds and for the heavy

duty arrester 250 amps with a duration of

2,000 microseconds.

As with the high current short duration

test after the conclusion of the low

current long duration portion of the test

series the surge arrester must demon-

strate thermal stability.

The discharge current withstand test

series allows the arrester to demonstrate

that it has the capability to withstand

surge currents of both long and short

duration and remain intact and func-

tional.

In addition to the thermal stability

requirements there is also a requirement

for stability of protective characteristics.

To ensure the arrester protective ability

has not been impaired, the protective

levels of the arrester at 10 kA may notincrease by more than 10 percent at the

conclusion of this test.

Ohio Brass publishes booklets with

design test reports for PDV-65 normal

duty and PDV-100 heavy duty distribu-

tion class arresters.

These design test report booklets

include copies of oscillograms which

detail the results of the test described in

this section.

In the next issue we will continue to

examine the series of design tests whichapply to distribution class surge arresters

and will take a look at how these relate

to the arresters ability to perform its

function.

End of Thermal Stability (100 kA)

2.09 kV

0.25 mA

5 ms/majordivision

5.82 kVp

0.38 kAp(resistive)

0.49 mAp(total)

28.3 kV

104 kA

6/14

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Chapter Four

How Does a DistributionClass Arrester Work?One in a Series

ANSI/IEEE StandardC62.11-1987 DescribesRelevant LaboratoryTests For DistributionClass Surge Arresters

In this segment, we will continue ourlook at the design test requirements for

distribution class surge arresters as well

as how these test requirements relate to

the ability of the arrester to perform its

primary function. The primary function

of the surge arrester is protection of

utility equipment against overvoltages.

In this issue, we will examine the:

(a) The duty cycle test,

(b) The discharge voltage test,

(c) Impulse sparkover voltage timecharacteristic test.

Duty Cycle Test

The duty cycle test is performed to

ensure the arrester will support its duty

cycle rated voltage while discharging

lightning surge currents.

The duty cycle test voltage is a 60 Hz

voltage in excess of the MCOV rating of

the surge arrester.

The duty cycle test is performed byenergizing the surge arrester at its duty

cycle rated voltage and subjecting it to a

series of 20 discharges.

In the case of the normal duty distribu-

tion arrester, the magnitude of these

discharges is 5 kA with an 8/20 wave.

The discharges are spaced one minute

apart.

Discharge voltage test set up with

prorated sample in impulse generator.

For the heavy duty arrester, the surges

are 10 kA with an 8/20 wave followed by

an oven preheat to 60C. The arrester

then receives two additional discharges

of 40 kA, while energized at MCOV.

These additional discharges at the

higher current level are to ensure the

durability which users have come to

expect from a heavy duty product.

After completion of the duty cycleseries, the arrester is energized at the

MCOV rating and is monitored to ensure

thermal stability.

After the completion of the duty cycle

test, the protective characteristics of the

surge arrester are measured to ensure the

arrester will perform its function as

designed.

Discharge Voltage Test

A surge arrester protects equipment

from lightning surges. Therefore, the

measurement of the voltage developed

by the arrester when it discharges iscritical.

This measurement is performed during

the discharge voltage portion of the test

sequence.

The discharge voltage of a prorated

arrester section using the appropriate

diameter varistors is measured. A

prorating factor is then applied to the

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measured values to determine the

discharge voltage of arresters made from

this same type of varistor. This test helps

to assure the manufacturers cataloged

discharge voltages for the arrester will

not be exceeded.

The test is performed on the prorated

section by measuring the 1.5, 3, 5, 10, 20

and 40 kA discharge voltage using an 8/20 wave. In addition to the 8/20 wave

discharge voltage, the fast front charac-

teristic of the arrester is also measured.

This is done by using a current wave that

causes the arrester discharge voltage to

crest in .5 ,sec. For the heavy duty

distribution class arrester, this current

wave has a 10 kA magnitude, and for a

normal duty distribution class surge

arrester, it has a magnitude of 5 kA.

The discharge voltage (therefore, the

protective level) of the arrester solely

determines its protective characteristics

if the arrester is a gapless design.

If the surge arrester includes an

internal gap, then additional tests must

be performed. This testing includes

determining if the protective level is

defined by the gap sparkover or the

varistor discharge voltage.

Impulse Sparkover Test

The impulse sparkover test is per-

formed to ensure the gapped distribution

class arrester's protective level is

adequately defined for the end user.

The voltage impressed on the pro-

tected equipment is the higher of

the metal-oxide varistor element dis-

charge voltage or the gap sparkover.

The impulse sparkover test is per-

formed by taking a prorated sample of

arrester elements including the metal-

oxide varistors and the gap. The test is

conducted using various voltage wave

shapes and the sparkover of the gap

element is measured at these wave

shapes.

The protective level which then must

be used for insulation coordination is the

higher of the metal-oxide varistor

discharge voltage or the gap sparkover.

The end user is able to compare the

arrester protective characteristics with

the insulation to determine if the arrester

selected is suitable for the application.

If the protective level of the arrester is

too high to protect the equipment, then

the user has the option of selecting a

different class of arrester. This includes

Technician monitors discharge voltage test, analyzing oscilloscope results.

the option of using a riser-pole type

arrester, intermediate or station class

arrester.

In the next installment, we will

complete our examination of the design

tests which are appropriate for distribu-

tion class surge arresters.

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Chapter Five

How Does a DistributionClass Arrester Work?One in a Series

ANSI/IEEE StandardC62.11-1987 DescribesRelevant LaboratoryTests For DistributionClass Surge Arresters

In this installment, we will completeour look at design testing of distribution

class surge arresters.

The tests that will be covered are:

1. Radio influence and internal

ionization voltage test

2. Disconnector tests

3. Contamination test

4. Fault current withstand test.

RadioInfluence andInternallonizationVoltage TestThe surge arrester

is continuously

energized with a

60 Hz voltage. If asolid electrical

contact is not

maintained

throughout the surge arrester, it will have

internal ionization which may result in

degradation of the internal elements.

Also, this internal ionization can result in

radio and television interference. Loose

external hardware connections can also

result in radio influence voltage (RIV).

ANSI C62.11 requires testing of the

arrester design with a circuit in accor-

dance with NEMA Standard LA- I . The

arrester must have an RIV/IIV level of

250 microvolts or less. This voltage is

measured at 1000k Hz with the arrester

energized at 1.05 x MCOV.

All Ohio Brass PDV surge arresters

are factory tested at 1.176 x MCOV. The

arrester must exhibit an RIV/IIV of 10

microvolts or less.

Disconnector TestsToday's polymer-housed MOV surge

arresters have a very low failure rate.

They are still subject to system-gener-

ated failures. The majority of arrester

failures occur with the arrester becoming

a short circuit to ground. If a shorted

arrester remains connected to the line, it

is not possible to reenergize the line.

The disconnector serves to disconnect

a failed arrester from the line. This

serves two purposes. It allows the line to

be put back in service and allows the

failed arrester to be identified for future

replacement.

Figure 1

Cross-section of a ground lead disconnector

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The disconnector must not operate

under any normal service condition. All

design tests must be performed with the

disconnector installed on the sample.

The disconnector operation character-

istic must also be verified. This is done

by subjecting samples to rms currents of

20 through 800 amps. The time for the

disconnector to operate is plotted as a

function of current.

The detonation time-current curve for

the Ohio Brass PDV arrester is included

in this article.

Contamination TestsGapless MOV arresters are resistant tocontamination failures. ANSI/IEEE

Standard C62.11 requires an external

contamination test be performed on the

arrester to verify contamination resis-

tance.

The test program consists of three

separate tests. The first test is a voltage

excursion test with a total of 32 test

cycles at voltages from MCOV to duty

cycle voltage.

The second test is a five hour contami-

nation test. This test is performed by 20

separate applications of contaminant

solution. Between contaminant applica-

tions, the arrester is energized at MCOV.

At the conclusion of the test, thermal

stability is verified.

The final contamination test is the

partial wetting test. This is performed by

contaminating the bottom units of amultiple unit arrester. Thermal stability is

verified at the end of the test series.

Ohio Brass PDV arresters comply

with all Contamination Test Require-

ments.

Fault Current Withstand TestSince surge arresters fail as line-to-

ground short circuits, they will conduct

system fault current after failure.

The fault current withstand test is

performed to verify the surge arrester

will not fail in a manner that will cause

large internal parts to be violently

expelled.

The test sample is preshorted by one

of the two methods prescribed bystandards. The shorted arrester is then

energized on a circuit with a given

available fault current. (The standard

does not specify currents and durations.)

Additional test samples are tested at

higher currents until the maximum value

claimed by the design is verified. The

values achieved by Ohio Brass PDV

arresters are summarized in the table

below:

This concludes our discussion of

design tests required by ANSI/IEEEStandard C62.11-1987.

In the next issue, we will look at the

various factory tests used to verify the

quality of metal oxide varistors and

assembled polymer arresters.

Table I

Fault Allowable Duration

Current (cycles)

PDV-65 PDV-100

500A 120 120

2500A 60 60

5000A 30 30

10,000A 10 10

20,000A N/A 10

Figure 2

Curve of Detonation

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Chapter Six

How Does a DistributionClass Arrester Work?

In the last several issues we have

looked at design tests required on

distribution arresters. Design tests

provide a measure of the arrester's

capability, but they cannot verify thequality of the finished arrester as

manufactured.

To ensure the quality of the arrester, a

series of factory tests are performed on

the metal oxide varistors and the arrester

itself. These tests are in excess of any

required by today's industry standards.

The metal oxide varistor blocks used

in all Ohio Brass PDV arresters are made

in a dedicated plant in Wadsworth, Ohio.

The varistors are used in arresters that

are subject to direct lightning strokes. It

is important to verify they will withstand

the type of duty they will see in the field.

Each varistor receives an 8120 current

surge that ~subjects the varistor to its

rated energy.

ANSI Standards do not require this

type of testing. However, the Ohio Brass

100% energy test recognizes the unique

environment in which the PDV arrester

operates.

One of the most important characteris-

tics of a distribution arrester is the

discharge voltage. To assemble anarrester with the proper total discharge

voltage, the discharge voltage of each

varistor must be measured.

Every PDV-100 (heavy duty arrester)

varistor has the 10 kA discharge voltage

measured. The PDV-65 (normal duty

arrester) varistor has a 5 kA discharge

measured. The discharge voltage of each

block is stamped on the metallized face.

RIV and starting voltage tests

performed on all PDV arresters.

An blocks receive an 8/20 classifying current shot.

The batch and m data is printed on each block.

Life tests performed at elevated temperatures

on sample blocks from each batch.

In addition to the above tests which

are performed on every varistor block, a

number of tests are performed on a

sample of blocks from each batch. These

are briefly described below:

1. Square Wave EnergyA sample of

varistors are tested using a switching

surge type waves of successively higher

current. These blocks are taken to thepoint of failure. This test is used to verify

the energy rating of the varistors.

2. High Current TestPDV-100

varistors are tested at 100 kA and PDV-

65 varistors at 65 kA. This testing

verifies the high current strength of the

varistors.

3. AC TestThe watts loss and

capacitive currents of a sample are

measured. These are measured to ensure

the batch is within the design limits for

the arrester.

4. Accelerated Aging TestA sampleof each batch is energized at MCOV at 1

30C for 250 hours. This test is equiva-

lent to energizing the arrester in service

for over 100 years at 40C, per IEEE/

ANSI C62.11-1987.

After all testing is completed on the

blocks, they are shipped to Aiken, South

Carolina, for assembly into arresters.

All finished arresters receive two

electrical tests. Each arrester is tested for

RIV at a voltage equal to 1.176 x

MCOV. The arrester must test at ten

microvolts or less.

A starting voltage test is performed.

This is a measure of the voltage at which

the arrester begins to conduct. This test is

a final check on the assembly. It assuresthe arrester has been energized at least at

MCOV before it is shipped.

For more information on these tests,

please request OB publication EU1150-

HR1 for PDV-100 and EU1281-H for

PDV-65 arresters from your Ohio Brass

customer service representative.

In the next issue, we will look at lead

length effects.

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Chapter Seven

How Does a DistributionArrester Work?

The selection of the best arrester for a

given application can be negated by poor

installation practices. The length and

configuration of the line and ground

leads is critical in determining theamount of equipment protection avail-

able. This chapter will examine the

effects of voltage drop in the leads on

protective margins.

Surge current flowing through the

leads causes an inductive voltage drop.

The voltage in the lead is calculated by

the formula:

V=LYou really do not have to do calculus

to calculate this voltage.For a straight lead wire, the inductance

is .4H/foot. If the lead wire is coiled,

the inductance can be much higher. This

can really hurt the protective margins.

There is always a voltage drop in the

lead wires. This voltage does not always

add to the arrester discharge voltage. For

lead wire voltage to count in protection,

it must carry surge current and beelectrically in parallel with the equip-

ment the arrester is protecting. Ohio

Brass publication EU1202-H covers

various connection methods in much

greater detail. Request a copy of it from

your OB representative.

We will look at the protective margins

achieved by an 8.4kV MCOV PDV-100

arrester protecting a 95kV BIL trans-

former. Figure 1 shows the insulation

coordination curve for this application.

The insulation coordination curve gives a

graphical method of showing the

relationship between the transformer

insulation strength and the arrester

protective level. Both of these are a

function of the time it takes for the

voltage to crest.

The transformer can withstand a higher

voltage for waves that have a voltage

that crests in a short time. You can alsosee that the arrester allows a higher

voltage to be developed for fast rising

waves.

The protection level is the sum of the

arrester discharge voltage and the

voltage drop in the lead wire.

We need to determine the protective

level of the arrester/lead wire combina-

tion. The arrester discharge voltage

comes from the catalog. In this example,

the coordination current will be 10kA.

The 10kA-8/20 discharge voltage of the

arrester is 32kV. Now add the voltage

drop from the lead wire. The voltage is:

.4x10-6 H/ft x =500V/ft

didt

10x103A8x10-6 Sec

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The total protective level at the 8/20

current level (transformer BIL) is:

32+(.5kV/ft) (6ft)=35kV

The margin is:

95kV -1 x 100 = 171%35kV

Next we need to consider the fast front

characteristics. The arrester 10kA .5sec

IR is 36.5kV. For MOV arresters the

current crests in about 70% of the time tovoltage crest. Therefore the current crest

is .35sec. This value is coordinated

with the transformer chopped wave

strength. The chopped wave strength is

approximately 15% higher than the

transformer BIL.

The voltage drop in the lead is:

.4x10-6 H/ft x 10x103A =11.4kV/ft.35x10-6Sec

The total protective level is:

36.5kV + 11.4kV/ft) (6ft) = 104.9

(arrester) (lead wire) (total)The voltage drop in the lead is

significantly higher than for the 8/20

wave. This is a result of the much faster

time for the current to crest.

The protective margin (Figure 2) is:

110.00kV -1 x 100 = 4.9%104.9kV

This is a small margin.

It is easy to see how important it is to

keep the leads as short and straight as

possible!

Utilities will often look at insulation

coordination when selecting arresters

and use the full insulation strength of the

transformer. As the transformer ages

however, the BIL and other insulation

levels will likely reduce. A reduction of

at least 20% is considered typical.

The protective levels of MOV

arresters do not increase with duty. This

also was not true for silicon carbide

designs.

We will look at the protection if the

insulation levels have reduced by 20%.

Figure 2 shows the resulting protective

margins. At the BIL the margin is:

76kV -1 x 100 = 117%35kV

and the fast wave margin is:

88kV -1 x 100 = 16.1%104.9kV

Therefore, for the fast front the

insulation strength is well below the

protective level. The leads must be

shortened or eliminated.

The effect of the line and ground leads

can be reduced if the connections are

made properly. The lead wire from the

phase conductor should go to the arrester

before going to the high voltage bushing

of the transformer. The voltage in the

lead coming to the arrester does not

contribute to the voltage stress on the

insulation. The ground connection

should be made to the tank of the

transformer to minimize the lead effect.

This article shows dramatically how

much the improper lead wire connection

can affect performance of a complete

system.

In the next issue we will look at theeffects of the continuous power loss of

the surge arrester.

[ ]

[ ] [ ]

[ ]

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Chapter Eight

How Does a DistributionArrester Work?Watts Loss and You

In this series we have studied how a

distribution arrester works. We have

reviewed the important tests and product

characteristics.

Another important trait of all MOV

surge arresters is the continuous power

loss resulting from leakage currents. All

MOV arresters, even todays gapped

units, conduct a leakage current. In fact

gapped units can be expected to conduct

higher leakage currents and generate

higher power losses than gapless

arresters.

The power loss in MOV arresters

results from continuous leakage current

at MCOV. This power loss has several

implications for utility engineers. Powerlosses affect:

1. The operating and total ownership

cost of the arrester.

2. The thermal recovery of the arrester

after experiencing high energy duty.

The power loss of the arrester can be

affected by the processing methods and

the ingredients in the MOV and by the

amount of the MOV material used. The

MOV block can be developed to have a

low power loss which will affect the

shape of the MOV volt-amp curve. Also

the amount of MOV used in the arrestercan be reduced to improve the discharge

voltage but this may result in higher

power losses.

Operating and TotalOwnership Costs

The power loss of the MOV surge

arrester results in a cost to the utility. The

utility industry has evaluated the power

loss of distribution transformers for

many years. The same type of economic

analysis can be applied to arresters.

There are two types of losses that areevaluated for transformers. These are the

no-load (or core) losses and the

load losses. The no-load losses are

always present and are independent of

the system loading conditions. The load

losses vary as the system load fluctuates.

The continuous surge arrester losses are

comparable to the no-load loss of the

transformer.

A utility that evaluates losses of

transformers will determine two cost

factors associated with these types of

losses. These include such factors as fuel

costs, time value of money, operating

life, etcetera. The usual method of

determining these factors is based on the

EEI method. These factors are expressed

in $/Watt of power loss. The no-load

factor is commonly known as the A

factor and the load losses are the B

factor.

The distribution arrester engineer will

not need to calculate these factors if the

utility already evaluates transformer

losses since these factors are already

available from the transformer standards

group. The A factor is applied to the

losses of the surge arrester.

To evaluate the long term operating

costs of the surge arrester, the engineerwill need to determine the average watts

loss of the designs under consideration.

This information should be supplied

readily by the manufacturer of the

arrester.

For reference, the average watts loss

of the Ohio Brass PDV-100 arrester is

.018 watts/kV-MCOV. Therefore, for an

8.4kV MCOV PDV-100 heavy duty

arrester the average loss is .151 watts

[(.018 watts/kV-MCOV)*(8.4kV-

MCOV)].

The manufacturer of the arrestershould be contacted for information on

the average power losses. We have

performed tests on some other designs

and the results of the random samples

tested are summarized in Table 1.

TABLE 1

Arrester Type Average

(Watts/kV-MCOV)

Ohio Brass PDV-100 .018

Type Y (gapless) .058Type X (gapless) .061

Type Z (gapped) .200

This table shows that not all power

losses are the same. It also shows the

extremely high power losses of the

gapped type arrester. This high loss may

come as a surprise to some so an

explanation may be in order.

The high losses of the gapped arrester

result from replacing some of the high

resistance MOV elements in the gapless

arrester with lower impedance silicon

carbide grading elements in parallel with

the gap assembly. The silicon carbide

grading circuit in this hybrid arrester

allows for a higher continuous current

flow than is found in similarly rated

gapless designs.

The power losses combined with the

A factor allow the utility to calculate

the effect of the losses on the ownership

cost of the arrester.

Power loss A factors at most

utilities are below $10.00/Watt. If we use

a conservative value of $2.50/Watt

applied to an 8.4kV MCOV arrester the

above losses translate to the operating

costs over the life of the arrester shown

in Table 2.

TABLE 2

Arrester Type Operating Cost

($)

Ohio Brass PDV-100 .38

Type Y (gapless) 1.22

Type X (gapless) 1.28

Type Z (gapped) 4.20

Depending on the first cost of the

surge arrester, these additional operating

costs can have a significant impact onthe total ownership cost of the arrester.

Thermal Recovery andLong Term Aging

When a surge arrester experiences

high energy duty such as a high current

lightning stroke, the MOV blocks absorb

energy. The temperature of the blocks

can rise significantly as a result of the

discharge duty.

MOV blocks exhibit a negative

temperature coefficient in the operating

voltage region of the volt-amp curve.Figure 1 is a typical volt-amp curve for a

gapless MOV arrester. The negative

temperature coefficient is apparent from

the fact that at higher block temperatures

the MOV elements conduct more

current. Since the blocks conduct more

current they become hotter and then will

conduct even more current. If this

condition continues without the excess

heat being removed the arrester will

experience thermal runaway.

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A well designed surge arrester will

never experience thermal runaway for

any reasonable set of circumstances.

The excess heat being generated by

the blocks is dissipated by conduction

and convection through the housing andend hardware. Figure 2 is a typical curve

showing the rate of power generation (of

the MOV blocks) versus the rate of heat

dissipation (of the housing). A polymer

arrester with blocks in contact with the

housing will provide better heat transfer

than a design relying on convection.

In service the arrester will reach

equilibrium at a temperature slightly

above ambient. When it discharges high

energy duty the block temperature rises.

As long as the temperature remains

below the upper equilibrium point thearrester will slowly cool back to equilib-

rium. The Ohio Brass PDV arresters are

designed to remain thermally stable after

being subjected to two rated energy

discharges within one minute. The

energy rating of the PDV-100 is 2.2 kJ/

kV-MCOV and 1.4 kJ/kV-MCOV for thePDV-65.

The curve in Figure 2 is based on the

assumption that the arrester has the

highest watts loss blocks that would ever

be used in that design. The lower the

initial watts loss, the more likely the

arrester will be thermally stable. Designs

that have higher losses can still be

thermally stable, but depending on the

magnitude of the losses special heat

transfer methods may be required.

Also as the MOV blocks age, there is

long term effect. This is why the designtests call for an accelerated aging test.

(However, the standard does not require

one for production. Ohio Brass does such

a test on each batch of MOV blocks.)

If the watts loss of the arrester

increases with time, the arrester watts

generated curve can shift up as shown in

Figure 3. This will result in a reduced

upper thermal equilibrium temperature.

(The Ohio Brass surge arrester actually

has decreasing watts over time which

helps to make it more stable.) Thearrester design test requires that any

thermal recovery tests after high energy

duty simulate any increase in watts loss

resulting from aging effects.

An increasing watt loss will also affect

the economic analysis of the cost of the

losses. The economic calculations are

based on the assumption that the losses

are constant. In the specific case of the

Ohio Brass arresters this is very conser-

vative since the losses of these arresters

decrease. This means that the loss costs

will be even lower than calculated.

Summary

The power losses of MOV arresters

have economic and performance effects

that need to be considered by the utility

engineer. To do an effective job of

evaluating the losses, data must be

gathered on the average watts loss and

the long term aging performance of the

arrester. This information should be

available from the supplier of the surge

arrester.

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Chapter Nine

How Does a DistributionArrester Work?

Hardware Attachments

For PDV Surge Arresters

To optimize surge protection, it isimportant to properly position the surge

arrester near the protected equipment.

System reliability may also be compro-

mised by animal contact to the energized

terminal of the arrester, so proper

selection of hardware options is critical.

This article explains the mounting and

hardware accessories that are available

for Ohio Brass arresters to obtain

maximum benefit.

The table below describes the standard

hardware items which are available on

Ohio Brass Type PDV arresters. The

hardware described in this table is

suitable for the PDV-100 (heavy duty)

and PDV-65 (normal duty) surge

arresters.

Ohio Brass PDV arresters are speci-

fied by the six digit catalog numberwhich describes the MCOV (maximum

continuous operating voltage) rating of

the arrester. For example the PDV-65

arrester 8.4 kV MCOV is Catalog

Number 217259. However, this six digit

catalog number is not sufficient to

completely specify the surge arrester. In

addition to the basic arrester the optional

hardware attachments which are required

must also be specified. The optional

hardware attachments are specified by

the use of a four digit suffix code

beginning with the Number 7.

The three digits following 7 specify

the top end hardware, the mounting

hardware, and the bottom end hardware.

For example, if you need an 8.4 kV

MCOV PDV 65 arrester with a nut, wire

clamp and protective cover on the topend, the insulating base bracket and

NEMA crossarm bracket as the mounting

attachment and the isolator, washer,

terminal nut and nut as the lower end

hardware, then this arrester would be

specified by code 217259-7324.

In response to market requests, Ohio

Brass is now offering a flipper fuse

holder accessory kit. This is available

by specifying 76XX code series. A

drawing of the components is shown in

Figure 1.

This concludes our series on how

MOV distribution arresters work. The

entire series is being reprinted and bound

into Ohio Brass publication number

EU1377-H. If you would like a copy,

please contact your Ohio Brass represen-

tative.

*Must be ordered in conjunction with codes 7060 and 7070.

**Transformer Bracket 11 " for 8.4 kV MCOV and below and 7-1/2" for 10.2 kV MCOV and above.

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EU1377 H

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