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Calculating Separation Distance Calculating Separation Distance and Surge Currentand Surge Current

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

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

From Shielding you can see that the surge current can go as high as 100kA, 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 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 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 could cause the line insulation to flash, but since it is self restoring this is

not equipment damagingnot equipment damaging. . But if it strikes equipment in the substation it 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 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 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. 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 But in stations with shielding and with shielding on the incoming lines the stroke current can be reduced. stroke current can be reduced. The stroke current will be a function of the shielding failure of the lines or 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 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 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 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 insulator and 1.2 multiplier is the voltage that will definitely cause flash over which will result in the highest surge current.over which will result in the highest surge current.

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Separation distanceSeparation distance

It makes a difference how far away the equipment is from the arrester It makes a difference how far away the equipment is from the arrester that is protecting it.that is protecting it.

Transformers are the heart of the substation and they also cost the 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 most and take the longest to replace so we put arresters right at the transformer on both the high and low side.transformer on both the high and low side.

Surge arresters protect equipment up to a certain distance from the Surge arresters protect equipment up to a certain distance from the arrester. Lightning transients are the main concern because of the fast 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 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 line terminal equipment before the arrester can have an effect via the reflected wave. Switching surges reflect more slow and the distance to reflected wave. Switching surges reflect more slow and the distance to the arrester is normally not a concern.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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LPL

Separation distance

The arrester operates and begins reflecting a negative wave.The further equipment is fromthe arrester the longer it takes forthe reflective wave to reach it andthe higher the equipment voltage

The incoming lightning waveis reflected at +1 per unit by the transformer assuming it is a highimpedance but is clamped at thearrester protective level.

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Zsurge

Earrester

Elead

L = (EBKR – EARR) v 2S

- lead length

EBKR = EARR + 2S (L + lead length)

L

v

EBKR

EARR = Arrester VoltageEBKR = Breaker BIL = 900kvS = surge rate of rise kv/u-secL = lead lengthv = wave speed ft/sec = 985 x 106 ft/sec

Are line arresters needed?

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Separation distance – cont.Separation distance – cont.

estimate the magnitude & speed of the estimate the magnitude & speed of the incoming surge (line flashover)incoming surge (line flashover)

Calculate the arrester discharge currentCalculate the arrester discharge current Calculate the maximum voltage at the Calculate the maximum voltage at the

arresterarrester Estimate the arrester lead drop voltageEstimate the arrester lead drop voltage Calculate the allowable separation distance Calculate the allowable separation distance

based on the equipment BIL to be protected based on the equipment BIL to be protected (usually the line breaker)(usually the line breaker)

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Separation distance – cont.Separation distance – cont. Estimate the magnitude & rate of rise of the incoming surge (line flashover)Estimate the magnitude & rate of rise of the incoming surge (line flashover)

– Calculate the line flashover level:Calculate the line flashover level:» Front of Wave spark-over = 1.2 x 5.75/12 x 170kv/ft x N Front of Wave spark-over = 1.2 x 5.75/12 x 170kv/ft x N

Assume air gap withstand is 170kv per footAssume air gap withstand is 170kv per foot Line flashover (CFO) is 1.2 times the line insulation levelLine flashover (CFO) is 1.2 times the line insulation level Line insulators are 5.75 inches air gap (10 inch disc insulators)Line insulators are 5.75 inches air gap (10 inch disc insulators) N = number of insulators (14 for 230kv steel structures)N = number of insulators (14 for 230kv steel structures)

» FOWSO = FOWSO = 1368.5kv1368.5kv at 230kv at 230kv

– Rate of rise can be estimated from Westinghouse data for one flashover in Rate of rise can be estimated from Westinghouse data for one flashover in 100 years and a surge originating 5000100 years and a surge originating 5000ft ft from the substation:from the substation:

138kv line138kv line 875kv per u-sec875kv per u-sec 230kv line230kv line 700kv per u-sec700kv per u-sec 345kv line345kv line 750kv per u-sec750kv per u-sec 500kv line500kv line 1100kv per u-sec1100kv per u-sec 765kv line765kv line 1300kv per u-sec1300kv per u-sec

Estimate the discharge current:Estimate the discharge current:– IID D in KA in KA = 2 x FOWSO/Z= 2 x FOWSO/Zsurge surge ZZsurge surge = 350 ohms for 230kv line= 350 ohms for 230kv line

– IID D = 2 x 1368.5/350 = 7.82 ka= 2 x 1368.5/350 = 7.82 ka

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Separation distance – cont.Separation distance – cont. Estimate the discharge current:Estimate the discharge current:

– IID D in KA in KA = 2 x FOWSO/Z= 2 x FOWSO/Zsurge surge ZZsurge surge = 350 ohms for 230kv line= 350 ohms for 230kv line

– IID D = 2 x 1368.5/350 = 7.82 ka= 2 x 1368.5/350 = 7.82 ka

Estimate the arrester resistance:Estimate the arrester resistance:– RRARRARR = (E = (E10KA10KA- E- E5KA5KA) / (I) / (I10KA10KA – I – I5KA5KA) = (510-482) / (10-5) = 5.6 ohms) = (510-482) / (10-5) = 5.6 ohms

– EEARRARR = E = E5KA 5KA + (E+ (E10KA10KA- E- E5KA5KA) [(I) [(I7.82KA7.82KA- I- I5KA5KA) / (I) / (I10KA10KA – I – I5KA5KA)] )]

EEARR ARR = 482 + (510-482)[(7.82 – 5) / (10-5)] = 497.7kv= 482 + (510-482)[(7.82 – 5) / (10-5)] = 497.7kv

Calculate the discharge current:Calculate the discharge current:– IID D in KA in KA = (2 x FOWSO – E= (2 x FOWSO – EARRARR) /(Z) /(Zsurgesurge + R + RARRARR) )

ZZsurge surge = 350 ohms for 230kv line= 350 ohms for 230kv line

– IID D = (2 x 1368.5 – 497.7) / (350-5.6) = = (2 x 1368.5 – 497.7) / (350-5.6) = 6.5 ka6.5 ka

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Separation distance – cont.Separation distance – cont.

Estimate the lead drop in the arrester:Estimate the lead drop in the arrester:

– EELL = L = L di/dt = di/dt = .4 mh/ft L 2S L = lead length .4 mh/ft L 2S L = lead length

ZZsurge surge the lead length must also include the lead length must also include

the the length of the length of the ground lead connectionground lead connection

S = rate of riseS = rate of rise

– EELL = .4mh/ft x 10ft x 2 x 700kv/u-sec = .4mh/ft x 10ft x 2 x 700kv/u-sec 350 350

EELL= 16kv= 16kv

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Separation distance – cont.Separation distance – cont.

Calculate the maximum Calculate the maximum voltagevoltage at the arrester: at the arrester:EETOTTOT = = EEARRARR +k I +k IDDRRARRARR + E + EL L k = 3k = 3

EETOTTOT = = 497.7 + 3 x 6.5 x 5.6 + 16 = 622.9 kv497.7 + 3 x 6.5 x 5.6 + 16 = 622.9 kv

Calculate the maximum separation distance:Calculate the maximum separation distance:L = (EBKR – EARR) v

2S- lead length

L = (900 – 622.9) 985x106

2 x 700x106- 10 = 184.9 ft

The maximum separation distance for a 228kvarrester is 184.9 electrical bus feet! If the breaker is beyond this then line arrestersshould 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 120120

SiCM.O.

----530470

16 ½”10”

230 228180

M.O. M.O.**

530001530081

10”10”

345 264264

SiCM.O.

530579----

16 ½”16 ½”

500 396396

SiCM.O.

----530580

16 ½”16 ½”

765 588588

SiCM.O.

NoneNone

16 ½”16 ½”

 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.

 

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Nominal System Voltage (kV)

Equipment BIL (kV) Allowable Separation Distance (ft.)

138 & 115 650550

160’110’

230 900 200’

345 1300 340’

500 1800 300’

765 2050 200’