Insulation Coordination of Overhead Lines (IEC 60071-2)

Short answer

On an overhead line, insulation coordination ends at the required withstand voltage, without selecting a value from the IEC 60071-1 tables. Operating voltage and temporary overvoltages define the insulator string length; slow-front overvoltages, the air clearances of lines of 123 kV or more; and lightning, the performance of the line. Line performance determines how often it is re-energized and how many overvoltages reach the substation.

Short answer

Insulation coordination of an overhead line follows the same philosophy as that of a substation, with three differences. It ends at the required withstand voltage: there is no need to choose a level from the IEC 60071-1 tables. The air clearances must account for the movement of the conductors when suspension insulators are used. And line performance conditions the substation: its lightning outage rate defines how often it is re-energized, and lightning near the substation defines how many overvoltages reach it. This guide summarizes clause 6 of IEC 60071-2, 1996 edition.

What changes compared with a substation

The standard lists three considerations specific to lines:

With suspension insulators, the dielectric strength of the air clearances must take the movement of the conductors into account.
Insulator standards give the dimensions of each unit without reference to a highest voltage for equipment or a highest system voltage. The procedure therefore ends when the required withstand voltage is determined. No value from the IEC 60071-1 series is chosen and its tables 2 and 3 do not apply.
Line performance strongly influences that of the substation. The lightning outage rate of a transmission line defines the frequency of re-energization operations. Lightning performance near the substation defines the frequency of fast-front overvoltages reaching it.

Operating voltage and temporary overvoltages

The operating voltage and the temporary overvoltages determine the length of the insulator string and the shape of each unit, according to the severity of the site pollution. That severity is classified in four levels, from light to very heavy.

If the neutral is solidly earthed and the earth fault factor is 1.3 or less, one condition is usually enough. The insulators must withstand the highest system voltage between phase and earth. With higher factors, and especially in isolated or resonant-earthed systems, it may be necessary to consider temporary overvoltages. For the neutral, see isolated versus earthed neutral. Where suspension insulators are used, clearances are determined under extreme swing conditions.

Slow-front overvoltages

Those of interest on lines are earth-fault, energization and re-energization overvoltages. When setting the acceptable failure rate, remember what each one causes:

An insulation failure from an earth-fault overvoltage causes a double phase-to-earth fault.
A failure from a re-energization overvoltage causes a failed reclosing.

Overvoltage	When it matters	Indicative failure rate
Earth fault	Systems with a high earth fault factor: distribution lines or transmission lines in resonant-earthed systems	0.1 to 1.0 flashovers per year, of the order of the two-phase lightning outage rate
Energization	All lines, especially in range II	Of the order of 0.005 to 0.05 flashovers per year
Re-energization	Transmission lines with fast three-phase reclosing, because of trapped charges	Of the order of 0.005 to 0.05 flashovers per year

There are two nuances. On range II lines, energization and re-energization overvoltages are normally controlled to low values. For that reason earth-fault overvoltages may turn out to be more severe and call for special consideration. And re-energization can be ignored with single-phase reclosing on transmission lines, or on distribution lines whose transformers remain connected during the operation.

Slow-front overvoltages are one of the factors that determine air clearances and, on certain insulators, the fittings. Their importance is usually limited to transmission lines of 123 kV or more. On distribution lines, the clearances are set by the insulator and they need not be considered.

Lightning

The lightning performance of a line depends on:

the ground flash density,
the height of the line,
the arrangement of the conductors,
shield wire protection,
tower footing resistance,
the strength of the insulation.

Distribution lines. Every direct hit is assumed to cause a flashover between phases, with or without a flashover to earth. The shield wire does not help, because footing resistance and insulation strength cannot be improved economically enough to avoid back-flashovers. Performance depends mainly on flash density and height. On lines with ungrounded crossarms, such as wood-pole lines, overvoltages induced by nearby strikes do not matter. But the high insulation strength to earth lets large-amplitude overvoltages reach the substation, and that is taken into account when selecting its surge arresters for energy capability. On lines with grounded crossarms, induced overvoltages can affect the required impulse strength.

Transmission lines. Above 72.5 kV, induced voltages are neglected and only direct strikes count. The standard gives no general target rate, because it depends on the consequences of a lightning outage and on the cost of improving shielding, footing resistance and insulation. The section in front of the substation can, however, be designed with a lower outage rate than the rest of the line. This reduces the amplitude and frequency of the overvoltages reaching the substation, and the probability of short-line faults. For shielding, see lightning shielding.

What not to do

Do not choose an insulation level from the IEC 60071-1 tables for an insulator string: the insulator standard and the required withstand voltage are enough.
Do not protect a distribution line with a shield wire expecting to avoid flashovers between phases: the standard considers it ineffective.
Do not ignore temporary overvoltages in isolated or resonant-earthed systems.
Do not design the line without looking at its section approaching the substation, which conditions the surge arresters and the re-energization frequency.

To go further, see Insulation coordination in substations from 52 to 245 kV.

When to call us

The insulation of a connection line and its effect on the substation are defined together. At Tiza Group we include it in the engineering design and studies for your project’s connections, remotely and worldwide. Write to us and we will send you a quote.

References

IEC 60071-2:1996, Insulation co-ordination, Part 2: Application guide, clause 6 (clauses 6.1 to 6.4).
IEC 60071-1:2006, Insulation co-ordination, Part 1: Definitions, principles and rules.

Frequently asked questions

Why are the IEC 60071-1 tables not used for line insulation?

Because insulator standards set the dimensions of each unit without reference to a highest voltage for equipment. According to IEC 60071-2:1996 (clause 6.1), the procedure ends at the required withstand voltage and there is no need to choose a level from tables 2 and 3 of IEC 60071-1.

Which slow-front overvoltages matter on an overhead line?

Earth-fault, energization and re-energization overvoltages. Their importance is usually limited to transmission lines of 123 kV or more, where they influence air clearances. On distribution lines the clearances are set by the insulator, and they need not be considered.

Does a shield wire protect a distribution line against lightning?

Not usefully. The standard indicates that, on distribution lines, every direct hit to the line is assumed to cause a flashover between phases, and that tower footing resistance and insulation strength cannot be improved economically enough to avoid back-flashovers. Performance depends mainly on the ground flash density and the height of the line.