Lightning Protection Module

Purpose

The main purpose of the Lightning Protection Module is to help the user to design a Lightning Protection System (LPS) for an installation by helping to optimise the heights and locations of lightning masts and overhead earth wires to establish zones of protection and shield vulnerable equipment or structures from direct lightning strikes. The Lightning Protection Module uses the Rolling Sphere Method (RSM) along with an automated calculation algorithm which allows arbitrary LPS design configurations to be considered.

A risk assessment to standard IEC 62305-2 or AS/NZS 1768 is usually the first step in any lightning design process.

Protection of Structures

In the protection of structures from direct lightning strikes, an LPS will be comprised of air terminals, overhead earth wires, downconductors, equipotential bonding, and earthing terminations. The vulnerability of structures is caused mainly by the electric field intensification associated with exposed points and corners on the upper surfaces of structures, conductors, or busbars so air terminals and earth wires are installed to provide interception points for the lightning strikes.

Rolling Sphere Method

The Rolling Sphere Method (RSM) is a technique for determining the zones of protection whereby a sphere is theoretically brought up to and rolled over the total structure.
The geometrical boundary of areas that are protected against direct lightning flashes can be determined using the RSM. All points of the structure that the sphere touches are exposed to direct lightning strikes and would need to be protected by air terminals. Another approach is to use the RSM to position and define the heights of lightning masts supporting air terminals connected to earth or earth wires and to use these structures such that they shield nearby structures or equipment in need of protection from direct lightning strikes. The RSM is conservative for flat surfaces because it assumes that these are as susceptible to lightning strikes as are exposed corners or edges of structures.

Protection Level

There are four LPL’s (I to IV) advised in the Standards and each protection level corresponds with a set of fixed parameters (refer to the table below). The interception efficiency of an LPS depends on the minimum lightning current parameters and on the related rolling sphere radius.

The selected LPL is achieved by installing a lightning protection system consisting of air terminals which establishes zones of protection enclosing the whole structure. For the calculation of these zones of protection, the rolling sphere method, with a modification for flat surfaces, is used. The maximum values of lightning current parameters for the different lightning protection levels (not given here) are used to design lightning protection components. The lightning sphere radius (r) is assumed to have a fixed relationship with the peak minimum value of the interception (or discharge) current.

It is common to consider that PL III using a sphere radius of 45 m provides “standard” protection. PL I and II with sphere radius of 20 m and 30 m provide higher degrees of protection and therefore these protection levels will require a considerably greater number of air terminals. Protection level III ensures that for striking distances of 45 m or more, the shortest distance to the structure is an air terminal. Such striking distances correspond with peak currents of 10.1 kA or greater, which has an interception efficiency of 91 %, meaning about 9 % of all possible strikes will have a lower current which would potentially not be intercepted.

Calculating protective sphere radius

The following is obtained from Annex E of IEC 61936-1 [3] and is understood to apply to providing lightning protection within a substation.

A number of formulas have been proposed for determining striking distance (sphere radius). The most common are as follows:

[(IEC 62305-1:2010)]

[(IEEE 998)]

Where the units of I are kA and k is a coefficient to account for different striking distances to a rod or a shield wire (k = 1 for shield wires and k = 1.2 for rods).

Where Z_c (Ω) is the conductor surge impedance and LIWV (kV) is the rated lightning impulse withstand.

For substations with arresters, I can be obtained from the arrester discharge current.

References

[1] AS/NZS 1768 Lightning Protection
[2] IEC 62305-1 Protection Against Lightning

[3] IEC 61936-1 Power installations exceeding 1 kV AC and 1.5 kV DC

[4] IEEE 998 Guide for Direct Lightning Stroke Shielding of Substations