1. Calculating Separation Distance and Surge Current

2. 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

8. Arrester clamps

9. Arrester clamps

10. So how do you calculate the Surge current for an Arrester, IE which value do you use?

11. 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 surge current.

14. 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.

15. Can the line terminal equipment
be protected with the transformer
arrester?

16. LPL 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
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
Separation distance

17. EBKR = EARR + 2S (L + lead length) 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 v
L = (EBKR – EARR) v
- lead length 2S
Are line arresters needed?
Zsurge
EBKR L
Earrester
Elead

18. 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)

19. 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
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 = kv 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:
138kv line 875kv per u-sec
230kv line 700kv per u-sec
345kv line 750kv per u-sec
500kv line 1100kv per u-sec
765kv line 1300kv per u-sec
Estimate the discharge current:
ID in KA = 2 x FOWSO/Zsurge Zsurge = 350 ohms for 230kv line
ID = 2 x /350 = 7.82 ka

20. Separation distance – cont.
Estimate the discharge current:
ID in KA = 2 x FOWSO/Zsurge Zsurge = 350 ohms for 230kv line
ID = 2 x /350 = 7.82 ka
Estimate the arrester resistance:
RARR = (E10KA- E5KA) / (I10KA – I5KA) = ( ) / (10-5) = 5.6 ohms
EARR = E5KA + (E10KA- E5KA) [(I7.82KA- I5KA) / (I10KA – I5KA)]
EARR = ( )[(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 – 497.7) / ( ) = 6.5 ka

21. 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

22. Separation distance – cont.
Calculate the maximum voltage at the arrester:
ETOT = EARR +k IDRARR + EL k = 3
ETOT = x 6.5 x = kv
Calculate the maximum separation distance:
L = (EBKR – EARR) v
- lead length 2S
L = (900 – 622.9) 985x106
- 10 = ft
2 x 700x106
The maximum separation distance for a 228kv
arrester is electrical bus feet!
If the breaker is beyond this then line arresters
should be used!

23. 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
SiC
M.O.
----
530470
16 ½”
10”
230
228
180
M.O.**
530001
530081
345
264
530579
500
396
530580
765
588
None
SiC ‑ 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 maximum distance allowable between the transformer arresters and substation equipment (breakers, CVT's, PT's, etc.). If equipment is located further away than the allowable distance, install additional surge arresters or consult Standards.
LIGHTING PROTECTION

24. 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