How Does A
Metal Oxide
Distribution
Arrester Work?
®
®
POWER SYSTEMS, INC.
Reprint of a Series of Articles from Ohio Brass®
Chapter One
How Does a Metal-Oxide
Distribution Arrester Work?
One in a Series
The comments in this series apply to
Ohio Brass Type PDV and PVR
arresters.
T
he distribution class surge
arrester 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 cases
of 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 improvements in reliability and safety. The
polymer-housed MOV distribution class
surge arrester is an excellent example of
this 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 Function
The 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 distribution 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 to
ground in parallel with this equipment.
Therefore, it has a high resistance at the
arrester’s 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 resistance
of 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
MOV arresters depend on the nonlinear resistance characteristics of their
blocks for suitable discharge and continuous operating capabilities.
Figure 1
3
called the discharge voltage. Since the
arrester 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 to
the mode where it essentially behaves as
an insulator.
Arrester Design
and 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 all porcelain-housed distribution 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
4
primarily of zinc-oxide, with approximately 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 temperatures over 2000°F. 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 (PDV65) and heavy duty distribution polymerhoused 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 application of distribution class surge arresters.
5
Chapter Two
How Does a Metal-Oxide
Distribution Arrester Work?
One in a Series
Design distribution arresters
for 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 thunderstorms. Generated by massive thermal
instability of the atmosphere, thunderstorms 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 which
thunder 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 thunderstorm 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 determining 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,
6
PDV-100 Arrester, 9 kV Unit.
towers and poletop equipment all have
the attributes which make them attractive
targets for lightning. Lightning invariably 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: voltage
and 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 typical
lightning 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 microsecond. 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 microseconds.
The unpredictability of lightning
reasserts itself in attempts to classify
stroke dimensions, however, since it is
Isokeraunic map prepared by the National Weather Service with hatched areas indicating thunderstorm days per year.
7
Rural transmission lines in areas with 30
thunderstorms days per year can expect to
experience approximately one lightning stroke
per mile per year.
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 microseconds. 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
8
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 1000
amperes 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 consolidated 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 a
higher power quality level to the utility
customer.
The surge arresters are not only used
to protect equipment such as transformers 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.
Insulation flashover
and traveling wave
on a power line.
Traveling wave
voltage is equal to
the current magnitude multiplied by
the surge impedance of the line.
Statistical data
compare tower
stroke currents
with station and
distribution arrester currents.
9
Chapter Three
How Does a Distribution
Class Surge Arrester Work?
One in a Series
ANSI/IEEE Standard
C62.11 describes the
relevant laboratory tests
for distribution class
surge arresters.
ANSI/IEEE Standard C62.11, developed
by IEEE, with input from users, producers, 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 standards
can 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,
10
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. Conditions are described as standard where the
ambient temperature does not exceed
40°C and the altitude is not above 6,000
feet and the power system frequency is
limited to 50/60 Hz. Conditions exceeding 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 (Maximum Continuous Operating Voltage)
ratings of the arresters as well as dutycycle voltage ratings. The standard also
defines 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.
Table 1
Arrester Ratings in (kV) rms
Duty-Cycle Voltage
MCOV
3
6
9
10
12
15
18
21
24
27
30
36
2.55
5.1
7.65
8.4
10.2
12.7
15.3
17.0
19.5
22.0
24.4
29.0
Insulation Withstand Test
Requirement
A surge arrester is known as a "selfprotecting" device. During the discharge
of a surge the arrester limits the voltage
to 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
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 requirements 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 Current
Withstand 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 a
surge 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 as
follows:
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 4-6/1015 wave.
Table 2
Insulation Withstand Test Voltages
rms
Duty-Cycle
Voltage Rating
of Arrester
(kV)
Impulse Test
1.2/50 Full Wave
(kV) Crest*
(BIL)
60 Hz rms
Test Voltage
(kV)
1 min Dry Test 1 min Dry Test
3
6
9
10
12
15
18
21
24*
25
27
30
36*
45
60
75
75
85
95
125
125
—
150
150
150
—
15
21
27
27
31
35
42
42
—
70
70
70
—
13
20
24
24
27
30
36
36
—
60
60
60
—
The test is performed on complete
arresters 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 demonstrate 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.
* Insulation values are not covered in standards.
11
The low current long duration portion
of the discharge current withstand tests
are also performed on complete arrester
or on thermally prorated sections of the
surge arrester.
The parameters for the normal duty
The oscillogram below shows the 60 Hz voltage and current wave shape at
the end of the thermal stability test.
End of Thermal Stability (100 kA)
2.09 kV
5.82 kVp
0.25 mA
0.38 kAp
(resistive)
5 ms/major
division
0.49 mAp
(total)
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.
28.3 kV
104 kA
6/14
12
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 demonstrate 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 functional.
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 not
increase 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 distribution 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 which
apply to distribution class surge arresters
and will take a look at how these relate
to the arresters ability to perform its
function.
Chapter Four
How Does a Distribution
Class Arrester Work?
One in a Series
ANSI/IEEE Standard
C62.11-1987 Describes
Relevant Laboratory
Tests For Distribution
Class Surge Arresters
In this segment, we will continue our
look 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 time
characteristic 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 by
energizing 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 distribution 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 60°C. 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 cycle
series, 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 is
critical.
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
13
measured values to determine the
discharge voltage of arresters made from
this same type of varistor. This test helps
to assure the manufacturer’s 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 characteristic of the arrester is also measured.
This is done by using a current wave that
causes the arrester discharge voltage to
crest in .5 ,µsec. 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 performed to ensure the gapped distribution
class arrester's protective level is
adequately defined for the end user.
The voltage impressed on the protected equipment is the higher of
the metal-oxide varistor element discharge voltage or the gap sparkover.
14
Technician monitors discharge voltage test, analyzing oscilloscope results.
The impulse sparkover test is performed by taking a prorated sample of
arrester elements including the metaloxide 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
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 distribution class surge arresters.
Chapter Five
How Does a Distribution
Class Arrester Work?
One in a Series
ANSI/IEEE Standard
C62.11-1987 Describes
Relevant Laboratory
Tests For Distribution
Class Surge Arresters
Radio
Influence and
Internal
lonization
Voltage Test
In this installment, we will complete
The surge arrester
is continuously
energized with a
60 Hz voltage. If a
solid electrical
Figure 1
contact is not
Cross-section of a ground lead disconnector
maintained
throughout the surge arrester, it will have result in radio influence voltage (RIV).
ANSI C62.11 requires testing of the
internal ionization which may result in
arrester
design with a circuit in accordegradation of the internal elements.
Also, this internal ionization can result in dance with NEMA Standard LA- I . The
arrester must have an RIV/IIV level of
radio and television interference. Loose
250 microvolts or less. This voltage is
external hardware connections can also
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.
our look at design testing of distribution
class surge arresters.
The tests that will be covered are:
1.
2.
3.
4.
Radio influence and internal
ionization voltage test
Disconnector tests
Contamination test
Fault current withstand test.
Disconnector Tests
Today's polymer-housed MOV surge
arresters have a very low failure rate.
They are still subject to system-generated 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.
15
Fault Current Withstand Test
Since surge arresters fail as line-toground 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 by
standards. 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:
Figure 2
Curve of Detonation
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 characteristic 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 Tests
Gapless MOV arresters are resistant to
contamination failures. ANSI/IEEE
Standard C62.11 requires an external
contamination test be performed on the
arrester to verify contamination resistance.
16
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 contamination test. This test is performed by 20
separate applications of contaminant
solution. Between contaminant applications, 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 a
multiple unit arrester. Thermal stability is
verified at the end of the test series.
Ohio Brass PDV arresters comply
with all Contamination Test Requirements.
Table I
Fault
Current
500A
2500A
5000A
10,000A
20,000A
Allowable Duration
(cycles)
PDV-65 PDV-100
120
120
60
60
30
30
10
10
N/A
10
This concludes our discussion of
design tests required by ANSI/IEEE
Standard 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.
Chapter Six
How Does a Distribution
Class 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 the
quality 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 characteristics of a distribution arrester is the
discharge voltage. To assemble an
arrester 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.
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 Energy—A sample of
varistors are tested using a switching
surge type waves of successively higher
current. These blocks are taken to the
point of failure. This test is used to verify
the energy rating of the varistors.
2. High Current Test—PDV-100
varistors are tested at 100 kA and PDV65 varistors at 65 kA. This testing
verifies the high current strength of the
varistors.
3. AC Test—The 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 Test—A sample
of each batch is energized at MCOV at 1
30°C for 250 hours. This test is equivalent to energizing the arrester in service
for over 100 years at 40°C, 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 assures
the arrester has been energized at least at
MCOV before it is shipped.
For more information on these tests,
please request OB publication EU1150HR1 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.
Life tests performed at elevated temperatures
on sample blocks from each batch.
17
Chapter Seven
How Does a Distribution
Arrester 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 the
amount of equipment protection available. 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:
di
dt
V=L
You really do not have to do calculus
to calculate this voltage.
For a straight lead wire, the inductance
is .4µH/foot. If the lead wire is coiled,
the inductance can be much higher. This
can really hurt the protective margins.
18
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 be
electrically in parallel with the equipment 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 transformer. 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 also
see 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 combination. 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
10x103A
=500V/ft
8x10-6 Sec
The total protective level at the 8/20
current level (transformer BIL) is:
32+(.5kV/ft) (6ft)=35kV
The margin is:
[
]
95kV -1
35kV
x 100 = 171%
Next we need to consider the fast front
characteristics. The arrester 10kA .5µsec
IR is 36.5kV. For MOV arresters the
current crests in about 70% of the time to
voltage crest. Therefore the current crest
is .35µsec. 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:
and the fast wave margin is:
-1 x 100 = 4.9%
[ 110.00kV
]
104.9kV
88kV -1 x 100 = 16.1%
[ 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:
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 the
effects of the continuous power loss of
the surge arrester.
-1 x 100 = 117%
[ 76kV
35kV ]
19
Chapter Eight
How Does a Distribution
Arrester 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 today’s 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. Power
losses 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 arrester
can be reduced to improve the discharge
voltage but this may result in higher
power losses.
Operating and Total
Ownership 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 are
evaluated 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
20
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 engineer
will 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)*(8.4kVMCOV)].
The manufacturer of the arrester
should 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
Type Y (gapless)
Type X (gapless)
Type Z (gapped)
.018
.058
.061
.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 on
the total ownership cost of the arrester.
Thermal Recovery and
Long 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.
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 and
end 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 the
arrester 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 the
PDV-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 design
tests 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.) The
arrester 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 conservative 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.
21
Chapter Nine
How Does a Distribution
Arrester Work?
Hardware Attachments
For PDV Surge Arresters
To optimize surge protection, it is
important to properly position the surge
arrester near the protected equipment.
System reliability may also be compromised 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 number
which 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 top
end, 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 representative.
*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.
22
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