Insulation Coordination Under IEC 60071: The Steps

Short answer

IEC 60071-1 describes insulation coordination in four steps: determine the representative overvoltages (Urp) with a system analysis, calculate the co-ordination withstand voltages (Ucw), convert them into required withstand voltages (Urw) with the atmospheric correction and safety factors, and choose the rated insulation level from the standard values.

Short answer

The IEC 60071-1 procedure turns the overvoltages that reach the equipment into a set of standard test values. It starts from a system analysis (Urp), applies a co-ordination factor (Ucw), compensates for the differences between real service and the test (Urw) and ends by choosing the values from the standard’s lists (Uw). Clauses 5.1 to 5.5 describe it and figure 1 summarizes it in a flow chart.

Insulation coordination procedure (IEC 60071-1, figure 1)

System inputs. Origin and classification of voltages, protective level of limiting devices and insulation characteristics
System analysis. Clause 5.2
Representative overvoltages Urp. One for each overvoltage class
Selection of the insulation that meets the performance criterion. Clause 5.3, with the co-ordination factor Kc
Co-ordination withstand voltages Ucw. Result of step 2
Factors for the differences between test and service. Clause 5.4: altitude factor Ka (or atmospheric Kt) and safety factor Ks
Required withstand voltages Urw. Result of step 3
Selection of the rated withstand voltages Uw. Lists in clauses 5.6 and 5.7, with the test conversion factor Ktc and the Um ranges
Rated or standard insulation level. Set of Uw values of the equipment

Step 1: representative voltages and overvoltages (Urp)

The amplitude, shape and duration of the voltages stressing the insulation are determined. The standard asks to do this with a system analysis that includes the selection and location of the devices that prevent or limit overvoltages.

The result is one representative voltage for each class: continuous, temporary, slow-front, fast-front and very-fast-front. Each has the standard shape of its class and can be described with an assumed maximum, a set of peak values or a complete statistical distribution.

When an assumed maximum is adopted, the standard sets how it is translated into a test shape. For continuous voltage, the highest voltage of the system during the life of the equipment is used. For slow-front, a standard switching impulse with the same peak, and for fast-front, a standard lightning impulse with the same peak.

Step 2: co-ordination withstand voltages (Ucw)

This is the lowest withstand voltage of the insulation that meets the performance criterion under the representative overvoltages in service conditions. It has the shape of the overvoltage of its class and is obtained by multiplying Urp by a co-ordination factor Kc.

Kc depends on how accurate the evaluation of the overvoltages is and on an empirical or statistical appraisal of their distribution and of the insulation characteristics. You can work with conventional or statistical withstand voltages, and that changes the procedure and the value of Kc. The standard also allows simulating the events and evaluating the risk of failure directly, without going through the representative overvoltage.

Step 3: required withstand voltages (Urw)

It converts Ucw to the conditions of the standard test, multiplying it by factors that compensate for what distinguishes real service from the laboratory:

Atmospheric correction Kt, for external insulation only. Pressure, temperature and humidity are considered; for coordination, pressure is usually enough. The altitude factor Ka is always applied.
Safety factor Ks, which gathers the differences in equipment assembly, quality dispersion, installation quality, insulation ageing over its life and other unknown influences. If those effects cannot be evaluated one by one, a global factor based on experience is adopted, which IEC 60071-2 develops.

Step 4: selection of the rated insulation level

The most economical set of standard rated withstand voltages (Uw) that demonstrates compliance with all the required voltages is chosen.

Um. The next standard value equal to or above the highest voltage of the system is taken.
Uw values. From the lists in clauses 5.6 and 5.7, the next value equal to or above the required voltage is taken. If the test uses a different waveshape from the overvoltage, it is first multiplied by the test conversion factor Ktc.
One value for several requirements. One Uw value can cover more than one required voltage, reducing the number of values that define the level.
Preference for the tables. In a normal environment, the standard prefers to choose the level of Tables 2 and 3 that corresponds to Um.

If the chosen set matches an association in those tables, it is a standard insulation level. If it was chosen with another technically and economically justified combination, it is a rated insulation level.

A different waveshape in the test

The required temporary, slow-front and fast-front withstand voltages can be demonstrated with the same waveshape or with another, taking advantage of the insulation’s own characteristics. If it is another, the required value is multiplied by the test conversion factor Ktc before choosing the standard value. This allows a single test, for example a lightning impulse test, to cover several requirements at once and reduce the number of values that define the insulation level. In range I, the standard explains that the power-frequency or lightning test must cover the switching requirements; in range II, the switching test covers the power-frequency one.

When the procedure is repeated

The standard warns that optimizing the set of Uw may require reconsidering some input data and repeating part of the procedure (clause 5.1). The input data are those of figure 1: the origin and classification of voltages, the protective level of the limiting devices and the insulation characteristics. The figure draws the sequence in one direction only; the repetition is described in the text of clause 5.1.

What the procedure does not cover

Pollution, partial discharge and long-term voltage tests. These are defined by the product committee of each piece of equipment.
Very-fast-front overvoltages. The standard does not standardize them: product committees specify them when needed.
Surge arresters. The withstand voltage of an arrester’s insulating housing is based on its protective levels and is defined in the arrester standard, not in the IEC 60071-1 lists.
Safety clearances for people. These are dealt with in other standards.

To go deeper, see Insulation coordination: what it is and what it is for, Overvoltages in the system: types and origin, Deterministic method of insulation coordination, Statistical method and insulation risk of failure and Safety factors and test conversion factors.

When to call us

The procedure depends on network data, the type of neutral, the location and the surge arresters. At Tiza Group we model those overvoltages and deliver the insulation level and the protection that backs it up as part of your substation engineering. This is remote work, available worldwide. Learn about our engineering design and studies service or see how we do the primary engineering. Get in touch and we will send you a quote.

References

IEC 60071-1:2006, Insulation co-ordination - Part 1: Definitions, principles and rules: clauses 5.1 to 5.5 and figure 1.
IEC 60071-2:1996, Insulation co-ordination - Part 2: Application guide: co-ordination and safety factors.

Frequently asked questions

What are the insulation coordination steps in IEC 60071-1?

Four: determine the representative voltages and overvoltages Urp (clause 5.2), determine the co-ordination withstand voltages Ucw (5.3), determine the required withstand voltages Urw (5.4) and select the rated insulation level from the standard values (5.5).

Can the procedure be repeated if the result is too expensive?

Yes. The standard states that optimizing the set of withstand voltages may require reconsidering some input data and repeating part of the procedure. For example, a surge arrester with a lower protective level may allow lower insulation.

Must altitude correction always be applied?

Yes, for external insulation. IEC 60071-1 (clause 5.4) says the altitude correction factor Ka must be applied whatever the altitude. The atmospheric correction factor Kt applies only to external insulation.