Calculating Separation Distance and Surge Current

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

Arrester clamps

Arrester clamps

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

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.

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.

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

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

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

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)

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 = 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: 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 1368.5/350 = 7.82 ka

Separation distance – cont. Estimate the discharge current: ID in KA = 2 x FOWSO/Zsurge Zsurge = 350 ohms for 230kv line ID = 2 x 1368.5/350 = 7.82 ka 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

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 350 EL= 16kv

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 - lead length 2S 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!

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

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