Calculating Separation Distance
and Surge Current
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Is
Is/2
When lightning strikes the line it can have a very fast rate of rise,
as it moves down the line because of the inductance and
resistance of the line, the front of the wave starts to taper back
along with the tail, and the amplitude starts to decrease.
This works to our advantage, because this tapering back of the
front allows for a greater separation distance of the arresters
from the piece of equipment it is trying to protect
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Arrester clamps
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Arrester clamps
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So how do you calculate the Surge current for
an Arrester, IE which value do you use?
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So how do you calculate the Surge current for
an Arrester, IE which value do you use?
From Shielding you can see that the surge current can go as high as
100kA, but the most probability is 20kA. Since the
12kV,23kV,25kV,34.5kV, and 46kV are unshielded, lightning can have
a direct stroke to the line. This could cause the line insulation to flash,
but since it is self restoring this is not equipment damaging. But if it
strikes equipment in the substation it could be, so in that case we use
20kA as the surge current to use in those station to determine the
protective margins of the arresters. We also assume that there is no
separation distance as the stroke has a very fast rate of rise. But in
stations with shielding and with shielding on the incoming lines the
stroke current can be reduced.
The stroke current will be a function of the shielding failure of the
lines or backflash. The standard is Ia= (2*1.2*Eo)/Z 2 in the
numerator is because of voltage doubling at an open, Z in the
denominator is the surge impedance(usually between 300 to 400
ohms). Eo is the CFO of the insulator and 1.2 multiplier is the voltage
that will definitely cause flash over which will result in the highest
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surge current.
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Separation distance



It makes a difference how far away the equipment is from the arrester
that is protecting it.
Transformers are the heart of the substation and they also cost the most
and take the longest to replace so we put arresters right at the
transformer on both the high and low side.
Surge arresters protect equipment up to a certain distance from the
arrester. Lightning transients are the main concern because of the fast
speed. Their rate of rise is so fast that the voltage can build up at the
line terminal equipment before the arrester can have an effect via the
reflected wave. Switching surges reflect more slow and the distance to
the arrester is normally not a concern.
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Can the line terminal equipment
be protected with the transformer
arrester?
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The incoming lightning wave
is reflected at +1 per unit by the
transformer assuming it is a high
impedance but is clamped at the
arrester protective level.
LPL
Separation distance
The arrester operates and begins
reflecting a negative wave.
The further equipment is from
the arrester the longer it takes for
the reflective wave to reach it and
the higher the equipment voltage
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EBKR = EARR + 2S (L + lead length)
v
L = (EBKR – EARR) v
EARR = Arrester Voltage
EBKR = Breaker BIL = 900kv
S = surge rate of rise kv/u-sec
L = lead length
v = wave speed ft/sec
= 985 x 106 ft/sec
- lead length
2S
Are line arresters needed?
Zsurge
L
EBKR
Earrester
Elead
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Separation distance – cont.
estimate the magnitude & speed of the
incoming surge (line flashover)
 Calculate the arrester discharge current
 Calculate the maximum voltage at the
arrester
 Estimate the arrester lead drop voltage
 Calculate the allowable separation distance
based on the equipment BIL to be protected
(usually the line breaker)

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Separation distance – cont.

Estimate the magnitude & rate of rise of the incoming surge (line flashover)
– Calculate the line flashover level:
» Front of Wave spark-over = 1.2 x 5.75/12 x 170kv/ft x N

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

Assume air gap withstand is 170kv per foot
Line flashover (CFO) is 1.2 times the line insulation level
Line insulators are 5.75 inches air gap (10 inch disc insulators)
N = number of insulators (14 for 230kv steel structures)
» FOWSO = 1368.5kv at 230kv
– Rate of rise can be estimated from Westinghouse data for one flashover in
100 years and a surge originating 5000ft from the substation:

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
138kv line
230kv line
345kv line
500kv line
765kv line
875kv per u-sec
700kv per u-sec
750kv per u-sec
1100kv per u-sec
1300kv per u-sec
Estimate the discharge current:
– ID in KA = 2 x FOWSO/Zsurge
– ID = 2 x 1368.5/350 = 7.82 ka
Zsurge = 350 ohms for 230kv line
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Separation distance – cont.

Estimate the discharge current:
– ID in KA = 2 x FOWSO/Zsurge
– ID = 2 x 1368.5/350 = 7.82 ka

Zsurge = 350 ohms for 230kv line
Estimate the arrester resistance:
– RARR = (E10KA- E5KA) / (I10KA – I5KA) = (510-482) / (10-5) = 5.6 ohms
– EARR = E5KA + (E10KA- E5KA) [(I7.82KA- I5KA) / (I10KA – I5KA)]
EARR = 482 + (510-482)[(7.82 – 5) / (10-5)] = 497.7kv

Calculate the discharge current:
– ID in KA = (2 x FOWSO – EARR) /(Zsurge + RARR)
Zsurge = 350 ohms for 230kv line
– ID = (2 x 1368.5 – 497.7) / (350-5.6) = 6.5 ka
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Separation distance – cont.

Estimate the lead drop in the arrester:
– EL = L di/dt = .4 mh/ft L 2S
L = lead length
Zsurge
the lead length must also include the
length of the ground lead connection
S = rate of rise
– EL = .4mh/ft x 10ft x 2 x 700kv/u-sec
EL= 16kv
350
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Separation distance – cont.

Calculate the maximum voltage at the arrester:
ETOT = EARR +k IDRARR + EL
k=3
ETOT = 497.7 + 3 x 6.5 x 5.6 + 16 = 622.9 kv

Calculate the maximum separation distance:
L = (EBKR – EARR) v
2S
- lead length
L = (900 – 622.9) 985x106
- 10 = 184.9 ft
2 x 700x106
The maximum separation distance for a 228kv
arrester is 184.9 electrical bus feet!
If the breaker is beyond this then line arresters
should be used!
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SELECTION OF TRANSMISSION SUBSTATION SURGE ARRESTERS
The following station class surge arresters have been selected for use in transmission substations:
Nominal System Voltage
(kV)
Arrester Rating (kV)
Type
Stock No.
Bolt Circles*
138 & 115
120
120
SiC
M.O.
---530470
16 ½”
10”
230
228
180
530001
530081
10”
10”
345
264
264
SiC
M.O.
530579
----
16 ½”
16 ½”
500
396
396
SiC
M.O.
---530580
16 ½”
16 ½”
765
588
588
SiC
M.O.
None
None
16 ½”
16 ½”
SiC -
M.O.
M.O.**
Silicon Carbide (These arresters should be scrapped and NOT returned to stock when removed from the field.)
M.O. - Metal Oxide
*A bolt circle adapter is required when replacing a silicon carbide arrester with a metal oxide (Stock #530002) on a 138 or 230kV system.
**For use in Doubs Substation or other stations where the low voltage side of an EHV substation is 230 kV.
Surge arresters are normally required only at the transformer on the 138kV system. However, if substation equipment is located too far from the transformer
arresters, it may not be protected from surges coming in on a line. Therefore, the maximum safe separation distance (in bus feet) is shown below. This is the
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maximum distance allowable between the transformer arresters and substation equipment (breakers, CVT's, PT's, etc.). If equipment is located
than the allowable distance, install additional surge arresters or consult Standards.
Nominal System Voltage
(kV)
Equipment BIL (kV)
Allowable Separation
Distance (ft.)
138 & 115
650
550
160’
110’
230
900
200’
345
1300
340’
500
1800
300’
765
2050
200’
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