Insulation Coordination in Substations up to 36 kV

Short answer

In distribution substations up to a highest voltage for equipment (Um) of 36 kV, IEC 60071-1 specifies only power-frequency and lightning impulse withstand voltages. Phase-to-earth switching voltages are covered by the power-frequency one; phase-to-phase switching voltages must be considered when choosing one of the two. The lower lightning impulse values are suitable for three types of installation, and in all other cases the higher ones are used. When there is an overhead line in between, the transformer or cable that separates it from the equipment matters. This guide follows clause 7.1 and clauses 7.2.1 to 7.2.1.2 of IEC 60071-2, 1996 edition.

What is verified in this range

Range I of IEC 60071-1 covers from above 1 kV up to 245 kV, in distribution and transmission. Within it, up to 36 kV, coordination is resolved with two voltages: short-duration power frequency and lightning impulse. The standard voltages are in the rated voltage of switchgear and their verification in the rated insulation level.

As a general guide, the required phase-to-earth switching voltages can be assumed to be covered by the standard power-frequency withstand voltage. The phase-to-phase ones must be considered when choosing the lightning impulse or the power-frequency voltage.

When the lower lightning impulse values are suitable

As long as phase-to-phase slow-front overvoltages are covered, equipment designed for the lower values of table 2 of IEC 60071-1 may be suitable in three cases:

In case b, the cables may not reach that capacitance. Capacitors are then added next to the transformer terminals, on the switchgear side, until the total reaches 0.05 µF per phase. In any other case, or when a very high degree of security is needed, the higher lightning impulse value is used.

If high security is required, the standard calls for the higher value in all cases.

Equipment connected to an overhead line through a transformer

Equipment connected to the low-voltage side of a transformer fed from an overhead line does not directly receive the line’s lightning or switching overvoltages. However, the transformer transfers them from the high-voltage winding to the low-voltage winding electrostatically and electromagnetically, and those overvoltages must enter the procedure, with protection if needed.

Annex E of IEC 60071-2 explains the transfer modes. Temporary overvoltage can appear on a secondary with isolated neutral when an earth fault unbalances the primary voltages. Capacitive transfer is usually critical only from high to low voltage, and the winding capacitance is supplied by the manufacturer. Inductive transfer is normally the most important, and to estimate its magnitude direct measurements give more reliable information than simplified equations. The standard itself indicates that its equations serve for a rough estimate.

Equipment connected to an overhead line through a cable

Here the coordination must protect the substation and the cable. When a lightning wave travelling along the line reaches the cable, it splits into a reflected and a transmitted wave, and the transmitted amplitude is much smaller than that of the incident wave. But successive reflections at each end of the cable usually raise the voltage along the cable above that initial value.

For that reason, in general:

• the higher values of lightning impulse from table 2 of IEC 60071-1 are chosen;
• surge arresters are installed at the line-cable junction;
• with wood poles and a single line connected to the substation, additional surge arresters may be needed where the cable enters the substation.

Wood poles raise the amplitude of the incoming waves, because the line’s insulation to earth is high. For the equipment, see the surge arrester and power cables.

What stresses the substation may see

IEC 60071-2 describes the voltage stresses in a substation in its clause 7.1:

• Operating voltage: equal to the highest system voltage, on all parts.
• Temporary overvoltage: an earth fault on the load side affects an entire phase equally. Load rejection may affect only the parts between the breaker and the transformer.
• Slow front: from energization or re-energization of lines and from faults and their clearing.
• Fast front: lightning can appear throughout the substation, with different amplitudes depending on the distance to the surge arrester. Fast-front switching overvoltages appear only in the section being switched.

What not to do

• Do not choose the lower lightning impulse value just because the voltage level is medium voltage: it depends on how the installation is connected to the network.
• Do not forget the low-voltage cables when evaluating the transformer connection: their capacitance to earth is a condition of the standard.
• Do not ignore reflections at the cable ends: they raise the voltage inside it above the initial transmitted wave.
• Do not mix this case with the low-voltage surge protective devices of a plant, which follow other rules.

To go further, see Insulation coordination in substations from 52 to 245 kV and Overvoltages transferred through a transformer.

When to call us

Choosing the lightning impulse, locating the surge arresters and reviewing the cable connection are substation engineering decisions. At Tiza Group we make them within engineering design and studies, remotely and worldwide, and we verify the result with testing and commissioning in Colombia, or elsewhere on request. Write to us and we will send you a quote.

Related guides

• Insulation coordination under IEC 60071: the steps
• Insulation coordination in substations from 52 to 245 kV
• Overvoltages transferred through a transformer
• Protective distance of a surge arrester to the equipment
• Rated voltage of switchgear: series and values

References

• IEC 60071-2:1996, Insulation co-ordination, Part 2: Application guide, clauses 7.1, 7.2.1, 7.2.1.1 and 7.2.1.2, and annex E.
• IEC 60071-1:2006, Insulation co-ordination, Part 1: Definitions, principles and rules, table 2.

Frequently asked questions