Insulation Coordination in Substations above 245 kV

Short answer

Above 245 kV, known as range II, IEC 60071-1 specifies switching impulse and lightning impulse withstand voltages. IEC 60071-2 states that the statistical method should generally be applied. The frequency of switching, fault and lightning overvoltages must be examined. The location of the equipment in the substation also counts, for example at the sending or receiving end of an energized line. The deterministic method based on temporary overvoltages can give standard voltages that are too conservative, and the standard calls for more precise procedures. This guide follows clause 7.2.3 and related parts of IEC 60071-2, 1996 edition.

What changes above 245 kV

In range I, phase-to-earth switching overvoltage is covered by the power-frequency one. In range II it is not. The standard switching impulse voltage must cover two values. One is the required short-duration power-frequency voltage. The other is the continuous power-frequency voltage, if the product committee specifies no other. To compare them, those voltages are converted to switching impulse with the standard’s test conversion factors.

That is why it is necessary to study how many overvoltages are expected, from what source and with what amplitude, at each point of the substation.

Sending end and receiving end

The amplitudes of energization slow-front overvoltages depend on several factors:

• the type of circuit breaker, with or without closing resistor;
• the nature and short-circuit power of the busbar from which energization takes place;
• the compensation used and the length of the energized line;
• the type of line termination: open, transformer or surge arrester.

The high values appear at the open or receiving end of the line. At the sending end they can be substantially lower. Within a substation, only the section between the line entrance and the breaker sees the receiving-end amplitudes, and the rest of the substation sees those of the sending end. That is why the standard asks that the location of each piece of equipment be distinguished.

Three-phase re-energization can generate high overvoltages from trapped charges on the line. At the moment of reclosing, the residual voltage can be as high as the peak of the temporary overvoltage. Its decay depends on three things: the equipment that stays connected to the line, the surface conductivity of the insulators or the corona effect, and the reclosing time. In normal systems, single-phase reclosing does not generate overvoltages greater than energization. But if resonance or the Ferranti effect can be important on the line, single-phase reclosing can give more than three-phase energization.

How the overvoltage distribution is built

IEC 60071-2 (clause 2.3.3.1) defines two methods:

The correct distribution is obtained only by carefully simulating the operations, with digital calculation or transient network analysers. The typical values in the standard serve only as a guide to whether the problem may be serious.

Phase-to-earth, phase-to-phase and longitudinal

Without surge arrester protection, the assumed maximum overvoltage is the truncation value of the distribution. For phase-to-phase, the standard explains that there are two critical instants: that of the peak of the phase-to-phase voltage, and that of the phase-to-phase voltage when the phase-to-earth peak occurs. For all insulations except range II air clearances, the representative phase-to-phase overvoltage is the phase-to-phase peak. For range II air clearances, and even more from 500 kV, it is determined with the phase-to-earth and phase-to-phase peaks according to annex D.

Longitudinal insulation between terminals combines the operating voltage on one terminal and the switching overvoltage on the other. In synchronized systems, both have the same polarity and the longitudinal insulation withstands less than the phase-to-earth insulation. Between non-synchronous systems, it can have the overvoltage at one end and the operating voltage of opposite polarity at the other.

Temporary overvoltage: duration and voltage-time curve

The deterministic method based on temporary overvoltages can give standard voltages that are too conservative in this range. The standard asks for more precise procedures that use the actual duration of the overvoltage and the power-frequency voltage-time characteristic of the insulation.

What not to do

• Do not apply the range I criterion to range II: phase-to-earth switching is no longer covered by power frequency.
• Do not use a single overvoltage value for all equipment in the substation: the receiving end and the sending end see different amplitudes.
• Do not calculate phase-to-phase air clearances with the phase-to-phase peak without checking annex D from 500 kV.
• Do not use the typical values in the standard’s figures as a project result: they are a guide to whether there is a problem.

To go further, see Overvoltage when energizing and reclosing a line, IEC 60071 range I and range II: what is specified and High and extra-high voltage substations: the essentials.

When to call us

An extra-high-voltage coordination study combines network modelling, surge arresters and the insulation of each piece of equipment. At Tiza Group we do it as part of the engineering design and studies for substations, remotely and worldwide, and the conclusions are carried into the switchgear specification. Write to us and we will send you a quote.

Related guides

• Insulation coordination under IEC 60071: the steps
• Safety and test conversion factors
• Minimum air clearances by insulation level
• Statistical method and risk of insulation failure
• Overvoltage when energizing and reclosing a line

References

• IEC 60071-2:1996, Insulation co-ordination, Part 2: Application guide, clauses 2.3.3.1, 5.1.2, 7.1 and 7.2.3, and annex D.
• IEC 60071-1:2006, Insulation co-ordination, Part 1: Definitions, principles and rules, table 3.

Frequently asked questions