Lightning overvoltages reach a substation through the connected overhead lines, by a direct stroke to a phase or by back flashover. Their severity depends on the lightning performance of the lines, the size and number of lines of the substation and the instantaneous value of the operating voltage. Without arresters, the amplitudes are usually too high to base the co-ordination on them.
Short answer
A substation receives lightning overvoltages through the overhead lines that reach it. Lightning can strike a phase conductor directly, when shielding fails, or strike the tower or the shield wire and cause a back flashover to the phase. The severity depends on three factors: how the connected lines perform under lightning, the size of the substation and the number of lines, and the instantaneous value of the operating voltage at the moment of the stroke. Without arresters, the amplitudes are generally too high to base insulation co-ordination on them.
How lightning reaches the line
IEC 60071-2 distinguishes three mechanisms for lightning overvoltages on overhead lines:
- Direct stroke to a phase conductor. It happens when the shielding does not intercept the lightning.
- Back flashover. The lightning strikes the tower or the shield wire and the insulation flashes over from the tower to the conductor.
- Induced voltage from a lightning stroke to earth near the line. It generally produces overvoltages below 400 kV, so it matters only in lower-voltage systems.
Because of its high insulation level, back flashover is less likely in range II than in range I and is rare in systems of 500 kV or more. The measures that reduce its probability are lowering the tower footing impedance or increasing the insulation. Against direct strokes, the design of the shield wire. The shielding of the substation itself is covered in lightning shielding of substations and its earth grid in earthing grid. To estimate the earthing resistance of rods and grids you can use our earthing resistance tool.
The representative shape is the standard 1.2/50 µs lightning impulse. The amplitude is given as an assumed maximum value or as a distribution of peak values, normally according to the return rate of the overvoltage.
What the severity at the substation depends on
The three factors combine to give the rate of occurrence and the amplitude:
- The lightning performance of the connected lines.
- The size of the substation and, above all, the number of connected lines.
- The instantaneous value of the operating voltage at the moment of the stroke.
Several steps are needed for good protection. The standard says it plainly: without limitation by arresters, the amplitudes are too high to base insulation co-ordination on them. A separate case is substations connected by cable, where the low surge impedance of the cable can reduce the amplitude to sufficiently low values, as self-protection. The connections of a GIS with cables and transformers are covered in GIS interfaces with cables and transformers.
Annex F of the standard explains that, in substations without arresters, the main parameter is the amplitude of the incoming overvoltage. In substations with arresters, the steepness and the distance between the arrester and the equipment matter more. The steepness drops through corona as the wave travels along the line, so only strokes within a certain distance of the substation produce sufficient steepness.
Phase-to-phase and longitudinal insulation
For phase-to-phase and longitudinal insulation, the instantaneous value of the operating voltage on the opposite terminal has to be taken into account.
- Phase-to-phase. The effect of the operating voltage and that of the coupling between the line conductors are assumed to cancel, so the opposite terminal is treated as earthed.
- Longitudinal. That cancellation does not exist and the operating voltage must be included.
The standard gives two rules depending on the mechanism:
| Mechanism | Representative longitudinal overvoltage |
|---|---|
| Shielding failure (direct stroke) | Phase-to-earth lightning overvoltage on one terminal and 0.7 times the peak of the phase-to-earth operating voltage, with opposite polarity, on the other |
| Back flashover | Phase-to-earth lightning overvoltage on one terminal plus the full peak of the operating voltage, with opposite polarity, on the other |
The reason is that shielding failures occur at any point of the operating voltage wave, so the factor 0.7 is appropriate. Back flashover, in contrast, is more likely to occur on the phase that has the highest instantaneous value with opposite polarity.
How the equipment is protected
Beyond the design of the line, the central measure is the surge arrester. The standard mentions that earthed crossarms or spark gaps have been used near substations to limit incoming waves, but those measures tend to increase the probability of flashover near the substation and to generate fast-front waves. For that reason the shielding and the footing earthing of nearby towers should be taken care of.
For the arrester, nominal discharge currents of 5 or 10 kA apply in range I and of 10 or 20 kA in range II. As for energy, it is normally enough to assume that the incoming lightning overvoltage has the amplitude of the 50 % negative lightning impulse level of the overhead line, and a lightning flash is also considered to have several strokes.
Two data points set the real protection of each piece of equipment: the arrester protection level and the distance between the arrester and the equipment. The arrester technology is explained in surge arrester and in Siemens and ABB surge arresters.
To go further, see Arrester protective distance to equipment.
When to contact us
If your substation receives long lines, is in an area of high lightning activity or has had insulation failures from lightning, the co-ordination is solved in the insulation study: accepted failure rates, arresters and location. We carry out your insulation co-ordination study and quote the scope, remotely and worldwide.
Related guides
- Insulation co-ordination under IEC 60071: the steps
- Arrester protective distance to equipment
- Arrester protection level: Upl and Ups
- Very fast front overvoltages in GIS
- Substation shielding: shield wires and masts
References
- IEC 60071-2 (1996 edition), Insulation co-ordination — Part 2: Application guide, clauses 2.3.4.1, 2.3.4.2, 2.3.4.4, 2.3.4.5 and annex F.
Frequently asked questions
What is the difference between shielding failure and back flashover?
In a shielding failure the lightning strikes the phase conductor directly. In a back flashover the lightning strikes the tower or the shield wire, the tower voltage rises and the insulation flashes over from the tower to the phase. Back flashover is less likely in range II because of the higher insulation level of those lines.
How is longitudinal insulation calculated for lightning?
With the phase-to-earth lightning overvoltage on one terminal and the operating voltage of opposite polarity on the other. For shielding failures 0.7 times the peak of the phase-to-earth operating voltage is used. For back flashover the full peak is used.
Is a substation fed by cables better protected against lightning?
Yes, when the substation is fed by cable. The low surge impedance of cables can reduce the overvoltage amplitudes to low values, so the cable itself offers self-protection.