The most recent edition of IEEE Technical Guide 1410 on how to improve lightning performance of distribution lines was issued in 2010 when many improvements were added. Because the experts involved had deep roots in the world of lightning and system transient modeling, 1410 contains equations and estimates not usually found in similar IEEE or CIGRE documents. The Guide has remained popular and those who use it discover valuable help in evaluating lightning performance data. For example, engineers can estimate the degree of improvement in lightning performance of distribution lines without having to run complex transient modeling software. Below is an outline of some main details:
Direct Strike Flashover Rate for Distribution Lines
Since distribution lines are seldom shielded with OHGW, direct strikes occur in high lightning areas. To determine flashover rate of a line equipped with arresters, one only need know height, width, location of arresters and ground flash density (GFD). Fortunately, there are numerous sources that offer lightning GFD for areas of interest worldwide. For example, 1410 contains a map of the continental U.S. and, from that, one can see that in the case of Florida as shown in Figure 1, the GFD ranges from 5 to +14 flashes/km2. For a typical 10 m high line of 1 m width in the south-central region where there are 5 m trees to the left and a highway to the right, the number of strikes could be calculated using equation (7) from 1410 where the number of direct strikes per 100km/ year are:
The calculation yields a hit rate of 15.75 times/yr for every 10 km of line. But since objects on either side of the line provide some shielding, they must also be considered. Again, 1410 provides a set of equations to assess this. Allowing for the shielding provided by 5 m trees located 10 m from poles on one side and 5 m trees on the other side of the highway, about 60 m to the right, one can use equation (9) where the shielded hit rate is:
Now, annual hit rate drops to 14.5 direct strikes/yr. If the trees to the left were 10 m tall, strike rate would decrease to 8.03/yr while if the trees were taller than the line (e.g. 15 m), it would effectively be shielded from direct strikes, with a hit rate of only 0.31/yr. Clearly, environmental shielding has significant impact on lightning hit rate. If trying to reduce outages due to lightning, equipping the line with arresters would be a good strategy.