Publ. SESWG/A2300E
Edition 3, 1995-02
Voltage in p.u. of rated voltage (crest value)
2.5
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~~t~h.!.n~
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er~t~c~o-n
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Current (A)
CONTENTS
Page No
Chapter 1 General intraductian and definitians
1.
2.
Chapter 2
lntroduction
Definition
Matching arresters to system parameters
(Selection of Uc and Ur)
1.
2.
3.
4.
5.
6.
7.
General
Step 1: Obtain system parameters
Step 2: Check for abnormal service conditions
Step 3: Check for other reasons for TOV occurences
Step 4: Select the continuous operating voltage
Step 5: Select suitable TOV capability
Selection tables
Chapter 3 Matching arrestersto system parameters
(Energy withstand capability)
1.
2.
3.
4.
5.
Chapter 4
Chapter 5
General
Step 1: Detertnine the switching parameters
Step 2: Calculate the arrester energy
Step 3: Select an arrester with adequateenergy capability
Step 4: Check capability with respect to lightning energy
Matching the mechanical characteristics
1
1
2
5
5
5
6
6
7
7
9
11
11
12
13
13
16
17
1. General
2. Temperature
3. Terminal connectors
4. Ice
5. Wind
6. Determine porcelain strength
7. Earthquake
8. Determine pressurerelief capability
9. Creepagedistances
10. External insulation of surge arresters
20
Protection philosophy -Selection of protective characteristics
22
I.
2.
3.
4.
5.
6.
lntroduction
Consideration of the protective characteristics
Co-ordinating currents
Protective rnargins
Protective distances
Extremmelyclose lightning strokes
17
17
17
17
17
18
18
19
19
22
22
24
25
25
28
~
Chapter
1
I: GENERAL
D EFINITI
INTRODUCTION
O NS
AND
Introduction
Surge arrestersare the primary protection against different types of overvoltages
(atmospheric or switching). They are generally connected in parallel with the
equipment to be protected to divert the surge currents.
The active elements (blocks) ofABB surge arrestersare manufactured using a
highly non-linear ceramic resistor material composed for the most part of Z nO
mixed with other metal oxides and sintered together. Hence the arrestersare also
commonly called Z nO arrestersor metaIoxide (MO) arresters.A typical
characteristic is shown in Figure 1.1. Further information is available in the
general product literature.
Typical voltage-curreot
characteristics
for ZoO arresters
2.5
"'
Q)
=
~
> 2.0
~
.~i~
-~~i~c-~~~~~
~~ -~-~~~:~~~~~
P!?!~~~?~--~e-v-e}
!~~~~
~
~
g
ö
>
~
~
~
"""'
o
d
ci.
=
~~-- ---~
1.5
I Rated
valtage
1.0
I"-:~
1"
I'J..\)
~tfnuöus-öperätlng-voriåge-lf~
-
~
~---~---~~
0.5
0.0
~
-g
~
o
~
o
q
o
~
ci
o
o
o
o
Current (A)
Figure
.1 TypicaJ voltage-current characteristics for ZnO arresters.
Note! Only the resistive component of the total current is shown.
A major difference from the earlier gapped SiC arrestersis the fact that the blocks
are now stressedcontilluously under service voltage as well as under any
abnormal service conditions. It is, therefore, very importallt to dimension the
arrestersso that they can withstand these stressesfor their lifetime. Satisfactory
(stable), thermal behaviour of the arrestersis of vital importance too.
It also makes it necessaryto define the terrninology used and the following
chapter attempts to deal with this in a rational and simpie-to-understand manner.
(For strict definitions refer to Standards.)
2
Definitions
UnIess otherwise stated hereunder,reference is made to normal A.C. systerns
(15 to 62 Hz).
2.
Max. system voltage (Vm) is the highest r.m.s. phase-to-phasevoltage which occurs
under normaloperation conditions at any time and at any point in the system.
2.2
Actual continuous operating voltage (Uca) is the maximum r .m.s.
pawer frequency valtage which is applied cantinuausly (~ 2 haurs)
between the arrester terminals.
2.3
Continuous operating voltage (Uc), of ten abbreviated as COV or MCOV, is the
designated permissible r.m.s. power frequency voltage that may be applied
continuously between the arrester terminals; thus, Uc ~ Uca. Observe that Uc for
the arrester may be less than the sum of the Uc for its blocks when voltage
distribution along the arrester is not strictly uniform.
2.4
Temporaryovervoltages
(TOV), as differentiated from surge overvoltages, are
oscillatory overvoltages of relatively long duration and which are undamped or
only weakly damped.
TOV frequencies range from a few H z to sofie hundreds of H z and durations
from sofie milliseconds to many hours (depending on fault -clearance time, for
example).
The most common form of TOV occurs on the healthy phasesof a system during
an earth-fault involving one or two phases.Other sourcesof TOV are
ferro-resonance,load-rejection etc.
2.5
Rated voltage (Ur) as per IEC means that an arrester fulfilling the IEC standard must
withstand its rated voltage for at least 10 secondsafter being both preheated to
60OCand subjected to a high energy injection as defined in the standard. Hence,
the TOY capability for 10 seconds,according to IEC, has to be minimum Ur. The
TOY capacity for EXLIM HY surge arrestersexceedsthe IEC requirements.
Additionally, the rated voltage is used as a reference parameter.
2.6
Temporary overvoltage withstand strength factor (T r or T c) is the TOV
capability of the arrester expressedin multiples of Ur or Uc respectively.
2.7
Impulse (of current or voltage) is an unidirectional w ave which rises rapidly to a
maximum and falls, a little less rapidly, to zero. Its waveshape is expressed by
t wo numbers (T1!T2). Tt refers to the virtual front-time and T2 to the virtual time
to half-value of the tail; both expressed in microseconds.
Some importallt currellt impulses are described below
2
Impulse
Waveshape (T1/T2)
Steep current impulse
Lightning current impulse
Switching current impulse
High current impulse
Tt
= 1 J.ls
Tt
= 8 J.ls
Tt ~ 30 J.ls
Tt = 4 J.ls
T2 ~ 20 J.ls
T2 = 20 J.lS
T2 ~ 60 J.ls
T2 = 10 J.ls
A special impulse is the rectangular current impulse which is in the shape of a
rectangle. A common duration is 2000 ~s.
2.8
Eqoipment insolation withstand characteristic
insulation withstand voltages and comprises:
Withstand level
is a general tenn for the equipment
Voltage waveshape
Chopped-wave withstand level
Lightning impulse withstand level
Switching impulse withstand level
Power-frequency withstand
(CWWL:
(LIWL)
(SIWL)
1.2/50
250/2500
50 H z or 60 H z
sinusoidal
Residual voltage (Ures) is the voltage that appears between the tenninals of an
arrester during passage of discharge current through it. It depends on the
magnitude as well as the waveshape of the discharge current and is expressed as a
peak value. For current amplitudes and waveshapes which differ from the
nominal discharge current, Ures is usually expressed in per cent of the residual
voltage for the nominal current.
.
.
')
10
Arrester protective characteristic is the combination of its residual voltages for
different current impulses. For good protection, the arrester characteristic should
lie weIl below the equipment insulation withstand characteristic at all points.
Lightning impulse protection level (LIPL)
for the nominal discharge current.
of the arrester is the residual voltage
Switching impulse protection level (SIPL) of the arrester is the residual voltage
for a specified switching impulse current.
Protective ratio is the ratio of the equipment insulation withstand level to the
corresponding protection level of its arrester .
.
.
2.14 Protective margin is the protective ratio minus 1 and expressedas a percelltage. As
an absolute minimum, the margin should cover the voltage increase due to the
connections between the arrester and the protected equipment as well as the
increase in the residual voltage due to the discharge amplitude and front-time
being different from the nominal discharge current of the arrester.
2.15
Continuous current (Ic) is the clirrent that flows throligh the arrester at Uc. This
clirrent is predominantly capacitive and is generally expressed as a peak vallie.
Reference current (Irer) is the peak value of the power frequency resistive current
at which the reference voltage is measured.For ABB arresters,the reference
current is in the range 0.4 to 10 mA peak for standard arrester types.
Reference voltage (Uref) is the peak value divided by -.J2af the valtage measured
acrass the arrester at reference current (Iref).
Nominal discharge current
used to classify an arrester .
(In) is the peak value of the 8/20 ~s impulse current
3
\
2.19 Single impulse energy capability is the maximum permissible amount of energy,
expressedin kJ, which the arrester is able to absorb in one single impulse with a
specific duration. When expressed~nkJ/kV(Ur), it is called specific single
impulse energy capability.
Earth-fault factor (ke) is the ratio of the voltages in the healthy phases during and
prior to earth-fault conditions. If the system neutral is directly earthed then
ke ~ 1.4 while a resonant earthed or isolated system gives ke as 1.73
approximately.
Pressure relief capability is the ability of the arrester, in the event of its
overloading due to any reason, to conduct the resulting system short-circuit
current through it without a violent explosion which may dammagenearby
equipment or injure personnel. Af ter the operation of the pressure relief, the
arrester must be replaced.
The short-circuit current may be high or low depending on the system impedance
and/or earthing conditions. Hence the-fJfessurerelief capability is verified by both
high and low current tests.
4
Chapter
1
2: MA TCHING
ARRESTERS
SYSTEM
p ARAMETERS
(Selection
of Uc and Ur}
TO
General
An arresteris a vital piece of equipment and an insurance against dammage
to the
other equipment in the station. Hence, it is essential that the arrester itself is stable
under all system operating conditions. This, in tum, requires that the system
behaviour, specially under TOV conditions, must be known. (Unfortunately this
is true usually for EHV systernsonly.) Whenever such is not the case, the arrester
must be selected with a sufficient safety margin.
Selecting the correct Vc and TOV capability is thus of primary importance. In
every caseVc must be higher than or equal to v cawhich is the actual continuous
power frequency voltage between the arrester terminals. The V ca depends on
system voltage as weIl as the method of connection of the arresters.
For example, the V ca for an arresterconnected between phasesand earth in a
3-phasesystem is Vm/"3 while the V cafor an arrester connected between phases
is Vm. Both amplitude and duration ofTOV are important as together they
determine the stressupon the arrester.
Selecting an arrester for a specific application is a compromise between protective
level, TOV capabilityand energy capability. Increasing the TOV capability (by
addition of blocks in series) increasesthe possibility of surviva! of the arrester
under system voltage stressesbut reducesthe margin of protection provided by
the arrester for a given insulation level. An arrester with a higher energy
capability reduces the risk of failure. Optimisation dependson how weIl the actual
arrester stressesare known or can be estirnated.
The stepsoutlined in the following chapters will help simplify the selection of
theseparameters.
2
Step 1: Obtain system parameters
2.
Obtain or estimate the highest system voltage (Um)
.
If only the nominal system voltage is given, Vm can be estirnated as 5 to 10
percent higher. The stated or desired voltage rating of other equipment is also a
clue.
2.2
Earth-fault canditians.
The most commonly known TOV is that at single line-earth fault. The amplitude
is given by multiplying Um/--j3by the earth-fault factor ke which in tum is
determined from the earthing conditions. Figure 2.1 gives the vallie ofke
depending on the system sequencereactancesand resistancesfor the most
unfavourable fault resistance.Should these system parametersbe unknown, ke is
estirnatedas 1.4 for directely earthed systernsand 1.73 for resonant earthed or
isolated neutral systerns.
5
Xo/X
Figure 2.1 Curves showing re1ationshipbetween Ro!Xl and Xorxl for constant va1uesof earth fault
factor ke. (Source: IEC)
Ro = zero sequence
resistance
Xo = zero sequence
reactance
Xl
= positive
sequence
reactance
The duration dependson the fault-clearance time and if this is not known, it is
estirnated as 1 to 3 secondsfor directly earthed HY systernsand 3 to 10 seconds
for directly earthed distribution systerns.For isolated neutral or resonant earthed
systerns,the duration is important as it may vary from a few secondsto some
hours depending on whether fault-clearing is used or not. For anticipated fault
durations over 2 hours, the TOY should generally be considered as continuous (in
most cases)and the arrester chosen accordingly.
3
Step 2: Check for abnormal service conditions
Abnorrnal service conditions such as ambient temperaturesbelow -40°C or above
+45°C, frequencies under 15 H z or above 62 Hz, presence of heat sources
(e.g. fumaces) near the arrestersmay lead to selection of higher Uc and/or Ur and
hence the need to clarify such conditions. If such conditions do exist, refer the
matter to ABB.
4
Step 3: Check for other reasons for TOV occurrences
Generally, only the TOV arising at earth-faults and at load rejection are of
interest. Certain network configurations, however, can give resonance
overvoltages. These may also arise during non-simultaneous operation of breaker
poles. Resonanceovervoltages should be avoided in general by proper system
design (especially for normal A.C. transmission and distribution systerns)and
should not be the basis for the selection of the arrester TOV capability.
In some cases,efforts are made to reduce the earth-fault current by selectively
earthing the neutrals of only a few transformers yet maintaining an
effectively-earthed system overall. In such cases,there is a possibility that some
parts of the system may become non-effectively earthed (i.e. increase in value of
ke) for some periods when olle or more of the earthed-neutral transformers are
taken out of service. An earth fault during this period may lead to higher TOV
and arrester failure if this contingency is not taken into account. Since such
6
occurrence are rare, it may be justified to accept a risk of arrester failure instead
of selecting an arrester with a higher TOV capabilityand thus a higher protective
level.
If dliring a load rejection, an earth falilt is encolintered, the TOV on the healthy
phasestends to rise flirther than it wolild if the events occlirred singly.
5
In a 3-phase system with the arresters connected phase-ground, Uca=Um!-.J3. If the
system does not have abnormal service conditions outlined in step 2 above, Uc
therefore should be equal to or higher than Um!-.J3.To make the selection easier,
the tables in the relevant catalogues contain all the system voltages as per IEC
(and some other commonly found system voltage) and corresponding
IIcommon-choice arresters ".
It should be noted that if the actual system voltage is not above the voltage given
in the tables, all arresters listed for that specific system voltage are equally
suitable concerning the continuous operating voltage. Only if the actual voltage is
higher, then the column which shows Uc for each arrester must be used to make a
proper selection. The required Uc is then calculated as the actual system voltage
divided by -.J3,and an arrester with higher or equal Uc should be selected from the
table.
6
Step 5: Select suitable
6.1
In general, surge arresters are not used to protect equipment against TOV as this
would require an enormous number of parallel columns of blocks. Such
applications may be considered on ly in cases of limitation or elimination of
resonance TOV. Careful detailed studies are required in such cases to select
arresters with suitable energy capability.
TOV
capability
The short-circuit impedance seenfrom the arrester during TOV conditions plays
an important role in determining the energy requirement for the arrester. Figure
2.2 exemplifies this for an arrester type EXLIM P-A.
Figure 2.2
7
As seenfrom the curves in Figure 2.2, the arrester could withstand 10 times
longer prospective TOV at 1.3 p.u. if the short-circuit current is reduced from
40 kA to 0.5 kA r.m.s. On the other hand, the arrester current is too low at 1.1 p.u.
to affect the prospective over-voltage even at only 0.5 kA r.m.s. short-circuit
current.
Other factors affecting TOV capabilityare energy absorbed (i.e. the initial
temperature of the blocks) prior to the application of TOV and the applied voltage
following the TOV.
For a given arrester type the rated voltage (Ur) is a measureof its over-voltage
capability. Hence, the additional TOV capability of the arrester can be specifled
as a multiple of Ur as shown in Figure 2.3. This approach is used for all EXLIM
arresters.(A different philosophy adopted by some manufacturers is to give the
TOV capability in multiples of Uc.)
TOV capability for surge arrester type EXLIM
in multiples of U r (T r )
p expressed
--::. 1.3
E-o
'-"
...
2
1.2
-5
1.1
u
~
00
~
~
-1.0
"'
>
~
0.9
0.8-
0.7
~~
.II
,.
A: Prior energy = 7 kJ/kV
Prior energy = 13 kJ/kV
B: Prior energy = O kJ/kV
Figure
()'
~.
,().
11111
'...'
.().~
...~
(Ur) for EXLIM
(Ur) for EXLIM
...()~
P-A & P-B
P-C
.().-
.0.-
,0
.00
Time (s)
(Ur)
2.3
The curves in Figure 2.3 are established by tests with power frequency voltage.
For TOV with lower frequencies, the same curves may be used.
For frequencies higher than a power frequency of 50 or 60 Hz, it can be assumed
that, for the samevoltage amplitude, the arrester withstands the same time in
secondsas for power frequency if the duration is less than lo s. In other cases
ABB has to be consulted.
8
6.2
The following procedures should be adopted for selecting an arrester with sufficient
TOV capability:
.
'.
Select a preliminary rated voltage (Vro) basedon Vc as Vro = VC/O.8
(Vc taken from step 4 in clause 5, usually equal to Vm/-..J3).
Determine the TOV amplitude and duration at earth fault as
TOVe
= ke X Um!-V 3, where
ke
ke
For other
..
It
~ 1.4 for directly earthed neutral systerns
= 1.73 normally for resonant earthed and isolated neutral systerns
cases,
see Figure
Detennine other ternporary overvoltages TOV1, TOV2, ...TOVn with
amplitude and durations as calculated or estirnated.
Consider the possible energy absorption W, in kl, prior to the TOV and
calculate W /Uro. For each TOV detennine the minimum required rated
voltages Vre,Vrl, Vr2 ...Vr n by dividing the detennined TOV amplitude by
the temporary overvoltage strength factor Tf given in Figure 2.3 for EXLIM
p (or relevant catalogues for others) for the actual duration of the TO Vand
the calculated energy absorption W /Uro. If the calculated specific energy
absorption W /Uro is higher than the values given in Figure 2.3 (or
corresponding Figures) increase Vro or select an arrester type with a higher
energy capability.
Thus Ure = TOVeITre.
ti
7
2.1.
UrI = TOVI!TrI.
Ur2 = TOV2!Tr2
etc.
Select a final rated voltage, Ur, which is the highest of the vallles uro, Ure,
UrI, Ur2
Ur n. If this is a non-standard rating, choose the next higher rating
from the cataloglles.
Selection tables
For the most common 3-phase systerns,ABB recommends arrester ratings as per
table 2.2, basedon the following assumptions in table 2.1.
Assumptions
Directly earthed neutral systerns
Um ~ 123 kV
Um > 123 kV
Resonant earthed and
isolated neutral systerns
TOV in p.u. of
Um!--j3
Fault duration
Prior energy
1.55
1.5
1.73
I s
rated
1s
rated
lOs and 2 h
rated
Table
2.1
The assurnptionsmade in table 2.1 for directly earthed neutral systernsinclude
sorne cornbined effects of earth faults and load rejection, as rnentioned in clause 4,
9
Directlvearthed
l System
Fault-clearance
time
Arrester type
Single impulse
energy
capability
kJ/kV Ur
Vm
kV
4
7
12
18
24
36
45
52
60
72
84
100
123
145
170
245
300
362
420
550
R~sonant earthed or isolatcd neutr'!l
~ Is
EXLIM
P&T
EXLIM
Q
4.5
71)
~
EXLIM
R
2.5
EXLIM
P&T
71)
10
EXLIM
s
:s;
EXLIM
Q
4.5
R
2.5
EXLIM
P&T
102)
102)
133)
133)
133)
Ur
kV
Ur
kV
EXLIM
71)
102)
Ur
kV
4
4
4
6
6
9
12
12
12
15
15
18
21
21
24
30
30
33
36
36
42
42
42
48
48
48
60
60
60
66
66
66
84
Q
4.5
Ur
kV
6*)
9
15*)
21
27
42*)
54
60
66
84
96
120 *)
84
84
96
96
96
120
120
120
132
144*)
162 *)
132
132
162
192*)
192
192
228
276
330
420
Table 2.2 Recommended rated voltages for ABB arresters type EXLIM T, P, Q and R. Selection
based on assumptions as per table 2.1.
l) Energy capability for EXLIM P-A and EXLIM P-B
2) Energy capability for EXLIM T
3) Energy capability for EXLIM P-C
*) Note! For these rated voltages refer to catalogue data for the next higher system voltage.
10
2h
EXLIM
R
2.5
Chapter
3: MA TCHING
SYSTEM
ARRESTERS
p ARAMETERS
(~:nergy withstand
1
TO
capability)
General
This chapter is devoted mainly to the consideration of stressesdue to switching-in
against a trapped charge on a transmission line on arrestersinstalled at the open
far end of the line as shown in Figure 3.1. This case is generally considered as the
decisive case.Other caseswill only be narned.
Figure 3.1 Simple single-phase model.
If the surge travel time of the lille is short compared with olle cycle of
power-frequency and Zl representsa low impedance, the current through the
arrester will have a rectangular shapewith a duration equal to twice the travel
time T of the w ave on the lille. The current through the arrester and its residual
voltage at this current are given by the intersection of the relevant switching surge
characteristic and the load lille and can be determined by plotting a load diagram
as in Figure 3.2.
Figure 3.2 Load diagram.
UL
Z
la
Ures
=
=
=
=
Prospective overvoltage
Line surge impedance
Surge arrester current
Surge arrester residual voltage
I1
In a real case, the arrester current does not have a pure rectangular wavefoml. The
-source irnpedance,Zl, will affect the voltage w ave irnposed on the line at
breaker-closing, the voltage w ave will be distorted during its travelon the line,
return waves will causereflections at the sending end and, for rnulti-phase
systerns,the phaseswill interact.
However, this simple single-phasemodel is useful in many cases.To avoid
expensive computer and/or transient network analyser (TNA) studies, the
simplified method can be applied as a first attempt to estimate the arrester stresses
causedby switching. If these calculations reveal high energies and the need for
more qualified studies than had been considered initially, a more accurate study
would be justified.
Different types and makes of arresterscould also be easily compared when high
absolute accuracy in calculated stressesis not required.
2
Step I: Determine
the switching
parameters
In order to use the simplified method, the parameters in Figure 3.2 must be
determined in some way. Typical vallles for different system voltages are given in
table 3.1.
Under 145
145 to 345
362 to 525
765
450
400
350
300
3.0
3.0
2.6
2.2
Table 3.1
The prospective overvoltage UL dependson parameters such as location of
arresters,type of switching operation, presenceor absenceof preinsertion
resistors, the feeding network and the parallel compensation. In addition to the
typical values given in the above table 3.1, the diagram (published by CIGRE)
shown in Figure 3.3 can give guidance for the estimation of UL for a specific case.
In Figure 3.3 higher values than suggestedin table 3.1 can be found in some
cases.However, such overvoltage factors should be critically analysed before
being used in estirnating arrester energy since usually the highest values found in
Figure 3.3 belong to rare situations, for example in the first stage of a high
voltage system with long radial uncompensatedlines energised from pure
inductive sources.
The w ave propagation time (T) dependson the lille length and the velocity of
w ave propagation. For aeriallines and GIS bus ducts the propagation velocity is
approximately equal to the velocity of light (0.3 km/~s). For cables, the velocity
is much lower (around 0.15 km/~s).
12
Location of measurements
End of the line O
Kind of operation
Closing .
3-phase reclosingo
~mgle-step closrng
Resistors
Yes. NoO
Feeding network
Complex.
InductiveO
Parallel cornpensation
~o%.
<50%0
I Nr.
of evaJualed
varianIs
17 I 8
5
Max
M~j
1.11 11.27' 1.31
1.52 I 1.46 I 1.89 I 2.14
i
~
1.
Figure 3.3 Evaluation of overvoltage factors (2% values) dependent on kind of operation, closing
resistors, feeding network and parallel compensation. (Source: CIGRPlELECTRA)
3
Step 2: Calculate the arrester energy
The energy (W), given in J, absorbedby the arrester is given by the equation as:
w
UL
=
Ures
=
Z
=
T
=
n
=
{(UL-Ures)/Z} x Ures X 2T x n, where
prospective overvoltage or line-charging voltage (kV)
the residua! voltage Qf the arrester (kV)
surge impedance (ohm)
w ave propagation time (Jls) = Uv, where
1 = length of lille (km)
v = velocity of propagation = 0.3 km/Jls
number of consecutive discharges
It is seenthat the energy absorbedalso dependson the protective level. Thus a
higher protective level reducesthe energy demands in kJ/kV.
4
Step 3: Select an arrester
with adequate
energy capability
In the standard for Z nO surge arresters,IEC 99-4, the energy absorption
capability for switching surgeshas to be proven in the so-called line discharge
tests. For ABB surge arresters,the relevant "classes" as per IEC standard are
given in table 3.2.
13
ARRESTER TYPES
LINE DISCHARGE CLASS
EXLIM
EXLIM
EXLIM
EXLIM
EXLIM
2
3
4
5
5
Table
R
Q
P-A and EXLIM
T
P-C
P-B
3.2
Note! The parameters of the lille discharge classesin IEC 99-4 standard result in higher energy
stressesfor the same residual voltages whell compared with the classesin the standard for
non-linear gapped surge arrestersIEC 99-1/1991.
The energy absorbedby an arrester in a lille discharge test is a function of both
the lille discharge class and the switching impulse protection level of the arresterThis is shown in Figure 3.4.
Absorbed specific energy kJ/k V for IEC line discharge classes
~7
~
'-"
>
6
~
~
>.. 5
u
.0
"u
&
~
4-
\'
,\,
'. '\
.'\~~
5'-,
-~
3-
2
~
.
-\ \ ,',, , ,
..,
1
\
o
1
2
I\\~
l
3
4
Ures / Ur
Figure 3.4 Specific energy in kJ per kV rated voltage, Ur, dependent on the ratio of switching
imptilse residual voltage, Ures,to the rms valtie of rated voltage, Ur, of the arrester.
Parameter: Line discharge class. (Source: IEC)
Nate! Due ta required test ta1erances, the actua1 test energies are usua11y 10-20% higher.
14
For a specific ABB arrester, an estimate of the energy absorbedin the relevant
lille discharge class test could be obtained by using the arrester switching surge
protective level from the catalogues and checking for absorbedenergy in Figure
3.4. This value is then compared with the required discharge energy from the
equation 3.1.
The IEC line discharge test coffiprises repeated discharges and the thermal
stability of the arrester has to be verified for t wo consecutive discharges with 50
to 60 seconds between theffi. For single operations, the ABB arresters could be
stressed with a higher energy equal to the single iffipulse energy capability.
tl
u sually , the design case has a very low probability of occurrence and it is
sufficient, therefore, to design for olle single operation and not for t wo
consecutive discharges.The single impulse energy capability is given in table 3.3.
EXLIM p-C is a double column arrester for which both good current and hence
energy sharing are ensured in routine tests.
Arrester Type
Specific single
impulse energy
capability
kJ/kV(Ur)
Approx crest.
current for
4 ms rectangular
w ave
A
EXLIM
R
EXLIM
EXLIM
EXLIM
Q
P-A and EXLIM
T
EXLIM
P-C
P-B
2.5
4.5
7.0
10.0
13.0
300
600
1000
1400
1800
Table 3.3 For all arrester types the full energy can be taken for any impulse with equivalent
rectangular shape and duration 4 ms or longer.
If the chosen energy capability is not sufficient, the most economical solution is
to increasethe arrester rated voltage. If this leads to ullacceptable protection level,
then select another type with a higher energy capability. Arresters with energy
capability in excessof the values in table 3.3 are quoted on request.
In some casesparallel arrestersmust be used to meet high energy requirements.
The arrestersmust then be properly matched together to ensure a sufficiently
good current sharing. This matching of the arrestersmust be specially requested.
With standard arrestersfull current sharing is not necessarily ensured.
15
5
Step 4: Check capability
with respect to lightning
energy
At lower system voltages (below 200 kV), the energy due to switching will
generally be low. At the same time, less attention is paid to effective grounding
and shielding of such systerns.Hence, the design capability will be determined by
lightning stresses.
A conservative estimate for the arrester energy capability for lightning surgesis
obtained in the high current test using 4/10 impulse. This w ave subjects the
arrester to a high energy during a very short time and hence to a thermal shock as
weIl. Note that dischargesof the amplitudes stipulated in the tests occur with a
very low probability in real situations. Furthermore, the real impulse duration
may be longer than the stipulated test impulse duration.
The standard peak current amplitudes are 65 kA and 100 kA as per ANSI/IEC,
ABB arresters withstand even higher currents as seen from table 3.4.
High current withstand
(2 shots, 4/10 impulse)
kA crest
Arrester
Type
EXLIM
EXLIM
EXLIM
EXLIM
EXLIM
R
Q
P-A and EXLIM
T
P-C
P-B
100
100
100
150
200
Table 3.4
An arrester with blocks of larger diameter will withstand the lightning stresses
better for t wo reasons:
l. The current density will be lawer .
2. The residual valtage will be lawer and cansequently alsa the energy discharged.
For this reason, it is advantageousto choose an arrester with larger diameter
blocks (and consequently a higher discharge capability) for:
.
Areas with high lightning intensity.
.
Important installations.
.
Lines and stations where grounding or shielding conditions are inadequate.
It means that in such cases,it may be preferable to use ABB arresterstypes in
ascending order of energy capability as per table 3.3.
16
Chapter
4: MA TCHING
THE
MECHANICAL
CHARACTERISTICS
1
General
It is equally important that the arrestersare mechanically so designed that they
can withstand normal as weIl as specifled abnormal service conditions.
Some of these conditions are detailed here.
2
Temperature
ABB arrestersare designed for mean alnbient temperaturesin the range -40°C to
+45°C.
While temporary increasesup to +60OCmay be permissible, in general
temperatureshigher than +45°C need de-rated special designs and ABB should be
consulted.
Similafly, for alnbient temperatureslower than above, the arrester sealing system
has to be specially designed and ABB should be consulted.
3
Terminal
connectors
The lille conductors, when connected to the arrester lille terminals, impose a load
on the terminals as weIl as a bending moment at the arresterbase. Both these
values can usually be reduced to acceptablevalues by use of lighter tee-offs and
clarnps. Note that arrestersdo not carry current continuously and hence should
not be connected with conductors of heavy cross-section.
4
Ice
Ice on the leads to the arresters will contribute to an increase in the terminal
loading and must be accounted for .
5
Wind
Heavy wind increasesthe horizontalloading on the arrester.Wind velocities up to
100 km/h are generally not a problem. The wind velocity (in mls) can be
expressedas a pressure, Pw = 1/2 x p x v2 (in N/m2). The force on the arrester
surface, according to Htitte, is then expressedas:
Fw
p
v
d
kw
-
Pw x 1 x d x kw, where
air density at 20° C = 1.293 kg/m3
velocity (mls )
arrester height (m)
mean arrester diameter (m)
correction factor for cylindrical
objects as per Fig 4.1
The bending moment at the arrester base (Mw) is then:
Mw = 1/2 x Fw, assuming that the centre-of -gravity is at the arrester middle.
7
4.1
Fig. 4.1
6
Determine porcelain strength
The cantilever strength (bending moment) of the arresterporcelain must be
sufficient for all the loads menrioned above. Table 4.1 below gives minimum
breaking moments for ABB arrestersdefined according to the standard DIN
48113.
Arrester types
For system
voltages
(kVffilS)
EXLIM R-A
EXLIM R-A
EXLIM Q-B
EXLIM Q-A
EXLIM Q-A
EXLIM P-A
EXLIM P-B
EXLIMT-A
EXLIM P-C
~24
~36
~24
;?:36
Minimum breaking
moment (Nm) acc. to
DIN 48113
1600
2500
2500
1600
6000
6000
12500
12500
16600
Table4.1
7
Earthquake
For areaswith high seismic risks, the arresterswithstand capability against
seismic forces should be verified. Different specifications exist and in general
ABB should be consulted. Reference is also made to Technical Information
SESWG/A 2382 E: Earthquake Design ofABB Arresters.
18
8
Determine pressure relief capability
As a safety measurefor other equipment and personnel near the location of an
arrester in the event of its failure, the standardsspecify that the arresters shall be
provided with a so-called "pressurerelief' device which acts rapidly enough to
prevent a violent shattering (explosion) of the porcelain. The device is chosen on
the basis of the prospective symmetrical short-circuit current in the system at the
arrester location or calculated from the formula:
I
Pk /(--13x Vm), where
prospective symmetrical short-circuit current (kA)
3-phase short-circuit power in MY A at the point where the arrester
is to be installed and
the maximum system voltage (kY)
-
I
Pk
Vm =
If Pk is not known, the breaking capacity of the associatedbreaker can be used as
a guide.
The pressurerelief capability of ABB arrestersis shown in table 4.2.
Arrester type
Pressurerelief capability (according
to ANSI or IEC) in kA
EXLIM
EXLIM
EXLIM
EXLIM
EXLIM
EXLIM
50/65
65
R
Q
P-A
P-B
T
P-C
*)
65
80
80
65
*) ActuaI current class depends on arrester rated voltage. For a specific rated voltage consult
catalogues.
Table
9
4.2
Pollution performance
For a given pollution level, the perfonnance of gaplessarresterscan be improved
in general by employing any or all of the following measures:
.Increasing
current.
.Using
the creepagedistancesof the housings to reduce extemalleakage
Z nO blocks of larger volume to improve the energy absorption
capability.
.Improving
the TOV capability, that is by increasing the rated voltage (Ur) for
the same arrester type.
.Improving
the heat transfer mechanism.
.Using
blocks with lower losses at Uc
ABB arrestersalready employ blocks with very low losses and the heat transfer
mechanism is optimised for these blocks. Thus, pollution performance is
improved by using one or more of the first three methods. In this connection it is
important to note that an increasedUc (without corresponding improvement in
TOV capability) is not effective.
19
ABB arrestersare designed to meet different pollution levels as per table 4.3
Creepage distance (I)
Pollution level
between phase and
ground mm/phase-tophase kV (2)
--
il
m
IV
-Light
-Medium
-Heavy
-Very heavy
16
20
25
31
(I) For the actual creepage distance, the specifled manufacturing tolerances are applicable
(see IEC).
(2) Voltage corresponding to the highest voltage for equipment (phase-to-phase).
Table
4.3
For ABB EXLIM arrestersthe letters L, M, H and V in the type designation
(e.g EXLIM P120-AM145) corresponds to IEC pollution levels I, II, III or IV
respectively as per table 4.3.
Note! Nominal creepagedistanceshigher than 31 mm/kV can be supplied on
request.
Usually the same creepagedistance for the arrestersis employed as for other
equipment in the station.
10
External
insolation
of sorge arresters
The primary function of an arrester is to limit, and thus render barmless,
overvoltages to wbicb the protected equipment is exposed. It is obvious,
therefore, that its own insulation (both external and internal) is tbe best-protected
of all.
The voltage across an arrestercan never be higher than that determined by the
arrester's protective characteristics. Only the need for an additional (statistical)
safety factor (margin) including correction for installation altitude can technically
justify a higher external insulation strength. Generally, the risk of an external
flashover less than or equal to 10-3is considered as acceptable;which leads to a
factor of approximately 1.10 to 1.15 (excluding altitude correction) between the
arrester protective levels and the LIWL and SIWL of the housing.
Both the international standards(IEC and ANSI) clearly stipulate that such a
margin is sufficient. ANSI (C62.11/1993) stipulates that the external LIWL of the
housing shall be 20% above the discharge voltage at 20 kA, 8/20 ~s impulse plus
an altitude factor of 9% per every 3000 feet (roughly equal to 10% per every
1000 m). IEC will stipulate a LIWL margin of 15% above the discharge voltage
at nominal current plus an altitude factor of 13% for up to 1000 m.
A longer arrester may, in fact, lead to less effective protection for steepersurges
for which the inductance of the arrester becomes more important. Thus, the
stipulation of high external insulation withstand values (e.g. equal to that for the
protected equipment) may thus be disadvantageousfor the protected equipment
apart form the fact that the cost is higher.
20
The EXLIM surge arresterscornply fully with both the IEC and ANSI and thus
the probability of external fiashovers due to overvoltages in service is extremmely
low.
A camparisall
af the bath
standards
tabulated:
IEC *)
>,
u
=
~
5~
4-;
ANSI
Ur < 200 kV
Ur ~ 200 kV
1.06/"2 x SIPL
for arresters with
In = 10 kA and 20 kA
Not applicable
0.82 x SIPL
1.25
Not defined
&
or
0.88/"2 x LIPL
for arresters with
In = 1.5 kA, 2.5 kA and 5 kA
SIWL
Not applicable
LIWL
1.3 x LIPL for all arresters
~
~
*) All
**)
All
**)
installations
installations
I
x SIPL
1.42 x LIPL at 20 kA
for all arresters
~ 1000 m
~ 1800 m
21
-Chapter
5. PROTECTION
SELECTION
PHILOSOPHY
OF PROTECTIVE
CHARACTERISTICS
1
Introduction
The overvoltage protection of a substation is not only a question of which
arrestersto choose. For protection against close lightning surges it is even more
important to install the arrestersin the most efficient way. By means of
oscillograms from different calculations the importance of short leads and short
distances between the arrestersand the object being protected can be seen.
Further, they show the great benefit of additional arresterson the incoming lines
outside the line breaker.
The possible reduction in insulation levels using improved surge arrestershas
become even more obvious with the introduction of Z nO arresters.However, this
will require a closer look at the actual installations and the arrester locations in the
substations. Arresters on the incoming lilles as weIl as at the transformer may be
necessary.Complex and especially important installations are best analysed using
computer analysis. The following chapter, however, presentsgeneral guidance
basedupon simplified considerations.
2
Consideration
of the protective characteristics
In the catalogues, guaranteed maximum residual voltages for commonly used
co-ordinating currents are found for each type and size of arrester .
Usually residual voltages for 8/20 as weIl as 30/60 currents impulses are given.
For co-ordination against even steepersurges,a voltage level for a current
impulse with virtual front time 1 J.ls,the steepcurrent, is presentedexplicitly or in
per cent of the 8/20 impulses.
According to IEC, actual test current w ave may have a virtual time to half value
of less than 20 Jls. The current w ave, therefore, is designated 1/(2 -20) here.
22
Diagraffis could also be used to show the protective characteristic as in Figure 5.1
for arrester type EXLIM P-A.
Protective characteristics for arresters type EXLIM
Max residual valtage in per cent af residual valtage at lOkA
~
'-"
P-A
8/20
160
I=40kA
<
... 150
o
I
I = 20 kA
-..I
o;:;
140
= 15 kA
-I=10kA
~e
I=
130
--I=
5kA
3kA
120110
100
90
80
0.1
0.5
1.0
10.0
Front time of voltage
(~s)
Figure 5.1
In this figure, the residual voltage is given in per cent of the residual voltage at the
nominal current for different times to voltage crest with the current amplitude as
parameter. This approach based on time to crest of voltage w ave is mainly
adopted by countries following ANSI standard.
The residual voltages are detennined with three current waves having virtual front
times of 8, 2 and 1 ~s. The actual time to voltage crest, which will be somewhat
shorter than for the current, is measuredand then the curves are extrapolated
towards shorter times.
The residual voltage which is obtained at a time to voltage crest of 0.5 ~s is
referred to as the "equivalent front-of-wave discharge voltage". Withstand curves
for equipment which shall be protected by the arrester could then be plotted in the
samediagram to check if a sufficient safety margin exists.
It must however be pointed out that for this type of very steep voltage w ave the
effect of connection leads as well as the distance between the arrester and the
protected equipment must be considered in order to determine an exact voltage
stresson the equipment.
23
A different presentation of the protective characteristics is illustrated in Figure 5.2
for arresterstype EXLIM R.
Protective characteristics for arresters type EXLIM
Max regidual valtage in per cent af regidual valtage at lOkA
Figure
R
8/20
5.2
It should be noted that the characteristic for switching surges with longer front
times is below the 30/60 curve.
In figure 5.2 the residual voltages are plotted in per cent of the guaranteed
residua! voltage at 10 kA 8/20 cuITentimpulse. This is in line with the
IEC standard where the protective characteristics shall be determined in multiples
of a residual voltage determined in routine tests. For ABB arrestersthis
coITespondsto the residual voltage at lOkA 8/20 current impulse except for
double column arresterstype EXLIM P-C for which the 20 kA residua! voltage is
used. The approach based on per cent will be the preferred ABB way of
presenting the protective characteristics.
3
Co-ordinating
3.1
LIWL co-ordination.
currents
It is often difficult to calculate the arrester current, especially those causedby
lightning. Therefore, rough estimations are mostly used. The fairly small variation
in discharge voltage with current waveshapeand amplitude makes this estimation
less critical with Z nO arresters.
For systerns with Vm less than or equal to 420 kV, lOkA is proposed as the
co-ordinating current; for Vm = 550 kV 15 kA and for 800 kV systerns 20 kA.
24
The waveshapeof the current is usually taken as 8/20. For shorter front time of
the current w ave, the residua! voltage shows an increase of less than 10% with a
reduction in front time from 8 to 1 ~s. However, much more important than this
small increase is to consider circuit inductances and distance effects when arrester
protection is wanted for shielding failures or for back fiashovers close to the
station.
3.2
SIWL
co-ordination.
Until now, the switching surgeshave deterrnined the insulation dimensioning at
EHV voltages. With Z nO arrestersthe switching surge protective level becomes
so low that lightning (or pollution perforrnance of the equipment) will be the
limiting factor for the insulation co-ordination. However, for 400 k Vand above
switching surges still playan important role.
The switching surgesproduce low current alnplitudes (with maximum values
ranging from 0.5 kA at medium voltages to approximately 2 kA at EHV) through
the surge arresters.Currents for co-ordination as per table 5.1 are proposed. This
is also in accordancewith the new IEC stipulations.
-Maximum current
System Voltage
kV
kA
< 145
0.5
145 -362
1
420
2
Table
-800
5.
The w ave shapeof the cuuent is assumedto be 30/60. For even longer front times
the deviation in the residual voltage from the 30/60 w ave (for the same cuuent
amplitude) is very small, within a few percent.
4
Protective
margios
The protective level and the electrical withstand of the insulation changes
depending on the steepness,duration and amplitude of the applied impulse. The
protective margin has to be fairly large, therefore, especially if long separation
distancesexist, or if protection against close lightning strokes is necessaryand
only olle set of arrestersis used inside the substation. A better solution then may
be to install arrestersalso on the incoming lille or lilles.
5
Protective distances
One can as well ask w hy a high protective margin is required. The answer is that
the calculated protective margins are only valid if the arrester is directly mounted
on the protected object. When there are connection leads and a distance between
arrester and object, then the protected object will be subjected to a higher
overvoltage.
The situation is illustrated in Figure 5.3
25
Figure 5.3 Voltage increase due to distance effect. (Simplified method)
The old generally-used fonnula to estimate the voltage increase due to distance
effects gives:
u =
U
Ures
S
L
v
Ures + (2 x S x L)/v, where
=
voltage at protected object (kV)
=
residual voltage of the arrester (kV)
=
steepness of the incoming voltage w ave (kV/J.ls)
=
distance between arrester and protected object including connection
=
leads and arrester height (m)
velocity of w ave propagation (m/J.ls)
(approximately equal to velocity of light 300 m/J.ls except for cables for
which 150 m/J.ls may be used)
The available margin, therefore, will drastically decreasewith increased
separation distances as weIl as with increased steepnessof the incoming w ave.
The latter is a function of how close to the substation the lightning strikes the
transmission line.
Example:
The simplified method for rough calculations should be used with sofie caution.
It does not take into account any capacitanceof the protected object, nor inductive
effects nor the initial voltage at the instant of surge.
A comparison between the simple formula and a computer calculation is shown in
Figure 5.4 for a case with a 145 kV transformer with LIWL 650 kV protected by
an arrester type EXLIM P-A with rated voltage 120 kV using the following
assumptions:
Length of the arrester
and connection leads:
Incoming surge:
Arrester LIPL:
26
6m
800 kV with steepness 1200 kV/~s
276 kV
Arrester
protective
Comparison
distance
-simplified
method
and
computer
calculations
System voltage: 145 kV. Arrester type and rated voltage: EXLIM p 120 A
Ampl. ofincoming surge: 800kV. Steepness: 1200kV/fls.
1nitial voltage at instant of surge: -145/..J3 x ..J2kV peak.
800
>
~
'"-"
...
~ 700
~
c2
"'
§ 600
.:::
~
~500
;:::.
ö
;:>c400
Simplified
300
200
o
5
method
...Computer
calculations
for transf. cap. = O nF.
--Computer
calculations
for transf.
15
20
Ii.
25
30
I
3511~~-~-~--r~I
40
45
cap. = 2 nF
50
55
60
10
Distance
in m from arrester connection
point
Figure 5.4
As shown, the simplified method gives a result on the safe side if no capacitance
is assumedfor the transformer. This is mainly due to the fact that the Z nO arrester
starts to conduct before the protective leve1276 kV is reached. On the other hand
if the transformer capacitance is say 2 nF, the simplified method shows a
protective margin which does not really exist for distances less than 40 m.
Rough estirnation, therefore, could be troublesorne if srnall rnargins between
arresterprotective level and transformer LIWL are used. Furthermore, the
irnportance of short distances and connection leads cannot be over-ernphasizedin
suchcases.
In table 5.2 protective distances based on computer calculation are given for some
"common choice arresters" as per table 2.2 in chapter 2.
The computer calculations take into account
.
Power
..
frequency
voltage
crest
value
with
opposite
polarity
at instant
of
mcommg surge.
.
Increase in arrester residual voltage for current impulses with shorter front
times than the standard 8 ~s for the nominal current.
.
Capacitanceof protected object. Worst casefor a capacitanceof 0-4 nF is
given in the table.
27
System Incaming
valtage w ave
LIWL af Surge
pratected arrester
abject
kV
245
kV peak
kV peak
2000
1300
type
EXLIM
Length of Protective distance
connection for incoming w ave
leads plus with steepness
arrester
height
1200kV/~s
2000kV/~s
ffi
m
ffi
P330-B
P360-B
7
24
16
6
10
1500
1175
P276
Q276-A
9
30
27
14
12
1300
1050
P192-A
Q192-A
9
55
50
27
25
800
650
P120-A
Q120-A
R120-A
6
37
35
24
16
15
10
Table 5.2 Protective distances for some "common choice" arresters based on computer
calculations.
Note! For longer length of connection leads plus arrester height the protective distance decreases;
for shorter the protective distance will be longer.
For further infonnation
protection" .
6
Extremely
consult Publ. SESWG/ A 2310 E" Application
Guidelines
for station
close lightning strokes
Regarding very close lightning strokes resulting in shielding failures or
back-fiashovers it is even more important to consider the exact arrester
installation. Additional arresterson the incoming line may also be necessaryin
order to obtain a sufficient protection of important equipment.
The surge arrester protection may also be combined with other measuressuch as
low footing impedance of the transmission line towers and reduced insulation to
ground a few spansout from the station in order to reduce possible current surges
entering the station.
28
9
~
~
..
~
~
i
j
.1!
~
g
'5
o:
~
<
~
~
.
u
~
u
.
c
"-
ABB 8witchgear AB
8urge Ar~ester Division
Dept. 8E~WG/AF
8-771 80 LUDVIKA,8weden
Tel. +4624082000
Telex 74507 abblu s
Telefax +46240 17983
~
i.
~