C.The Network with the Lightning Transient The lightning strikes the transmission line of the case study network, as shown in Figure 7. The symbol of the lightning in ATPDraw is shown in figure 6. Figure 6. The symbol of the lightning in ATPDrawIn ATPDraw the lightning is a surge function and its parameters are shown in Figure 8. Data: Amp = in [A], represents the peak value of surge. n= Factor influencing the rate of rise of the function. Increased n increases the maximum steepness. Tsta & Tsto = Starting and ending time in [sec.]. The waveshape of the lightning current in ATPDraw is shown in Figure 9. Figure 7. The network with lightningFigure 8. The lightning parametersProceedings of PMAPS 2012, Istanbul, Turkey, June 10-14, 2012400
Figure 9. The waveshape of the lightning current The voltage wave travelling along the line hit by lightning, for instance, phase A, induces voltage waves on the neighboring phases also. When the coupling factor between phase A and phase B is Kab,then the voltage to the ground induced on phase B is: Vb = KabVa The voltage on phase B and phase A in the case study is shown in Figure 10 at X005. Figure 10. The overvoltage on the node X005 Kab = Vb/Va from that Kab = 0.429 At 0.06 ms, the overvoltage is 17.313MV in phase A and 7.4275MV in phase B at same time can be calculated. D.Shielding Designs There are two methods of shielding designs. One method is based upon the work of Whitehead (Fig.11) and the concepts discussed above. This figure gives the critical mean shielding angle based upon the conductor height hp , (hp = y in Fig 11) and conductor to ground wire spacing c. These quantities are normalized in per unit of striking distance rs . The field results of Pathfinder Project correlate well with this method. The second method is a simple geometric construction for placing the shield wires based upon the electromagnetic model of shielding. The procedure can be discussed with reference to figure. 12. First, the critical shielding current should be found. It is generally increased by 10 percent. Based upon this critical current, the striking distance rs is known. To locate the center Cfor describing an arc with radius equal to the striking distance rs, the ordinate is given by rsg and the abscissa is: = -+ Figure 11. Critical shielding angle in terms of normalized geometryNote that for factor Ksg= 1 Rsg= rs. Draw an arc with center C and radius rs passing through the outermost conductor. It may cut the vertical line through the middle conductor at same point S. A ground shield wire placed at this point will protect all three-phase conductors. Analytically the height of the shield wire is = - A qualification applies that the minimum clearance required between the phase conductor and shield wire should be observed. If this clearance is Sps and hs is less than Sps , then make Sps = hs . The arc thus drawn in Fig 13 may not cross the vertical through the middle conductor. Two shield wires are then required. Draw another arc with P3 as center and Sps as the radius (Fig 13). The intersection of these two arcs gives the location of the shield wire.