Deterministic Insulation Coordination Method (IEC 60071-2)

Short answer

The deterministic insulation coordination method starts from a maximum overvoltage assumed at the equipment and multiplies it by a deterministic coordination factor (Kcd). The result is the coordination withstand voltage. With surge arresters and slow-front overvoltages, Kcd is between 1.10 and 1.00 depending on the arrester protective level. Without arresters, Kcd is 1 and the starting point is the truncation value. For lightning, Kcd is 1. This guide follows IEC 60071-2, 1996 edition, clauses 3.3 and 3.3.1 to 3.3.3.

What it is and when it is used

Insulation coordination selects the insulation level of each piece of equipment so that the expected overvoltages do not damage it, without oversizing it. IEC 60071-2 offers two routes: the deterministic and the statistical method. Many real procedures mix both, because some deterministic factors originated from statistical considerations.

The deterministic method is used when there is no test data on the failure rate of the equipment in service. It does not calculate how many failures to expect per year. It compares two maximum values and leaves a margin between them. The standard gives two typical examples:

• Internal insulation against slow-front overvoltages, when a surge arrester protects it.
• Surge arrester protection against lightning, for equipment connected to overhead lines, where experience with similar equipment already exists.

If the goal is to calculate the probability of failure, the route is the statistical method, which has its own guide.

How the coordination voltage is obtained

The sequence is short:

When the insulation is described by an assumed conventional withstand voltage (100 % probability of withstand, the case of internal insulation), the withstand value is chosen equal to the coordination voltage. For external insulation, described by a statistical withstand voltage (90 % probability of withstand), Kcd must also cover the difference between that value and the assumed one.

Slow-front overvoltages with surge arresters

At equipment protected by an arrester, the assumed maximum overvoltage is the arrester’s switching impulse protective level (Ups). The standard points out an effect: the distribution of overvoltages is strongly distorted when the protective level is low relative to the overvoltages the system would generate without arresters. With that distortion, a small variation in the insulation withstand or in the protective level changes the risk of failure a great deal.

To cover that effect, Kcd is evaluated from the ratio between Ups and Ue2, the value of the prospective phase-to-earth overvoltage that has a 2 % probability of being exceeded. On the curve in figure 6 of the standard, Kcd goes from 1.10 when Ups is low relative to Ue2 down to 1.00 when it is high. The figure gives two curves:

A low Ups relative to Ue2 means an arrester that clips heavily; there the standard demands more margin, because of the sensitivity described above. The Ups value comes from the arrester selected, not from the coordination study, according to its product standard. For the equipment itself, see the surge arrester.

Slow-front overvoltages without surge arresters

If the equipment has no arrester limiting the slow front, the assumed maximum overvoltage is the truncation value of its distribution: Uet phase-to-earth or Upt phase-to-phase, defined in clause 2.3.3.1. That value is already at the upper end of what the system can generate, so Kcd is 1.

Fast-front overvoltages

For lightning, the standard applies Kcd = 1 to the assumed maximum overvoltage. The reason is that the representative lightning overvoltage is defined with the probability built into it: it is the amplitude expected at a return rate equal to the acceptable failure rate. An additional margin would count the probability twice. For fast-front switching overvoltages, the same rules as for slow-front apply.

For internal insulation, the standard assumes a 100 % probability of withstand up to the assumed voltage and 0 % above it. The coordination voltage is therefore equal to the representative lightning amplitude at the chosen return rate. For external insulation, the scatter of the discharge is usually small compared with that of the overvoltages, and it is neglected as a simplification.

Temporary overvoltages and continuous voltage

For continuous power-frequency voltage, the coordination voltage is the highest system voltage for phase-to-phase insulation. For phase-to-earth insulation it is that voltage divided by the square root of three. Its duration is the service life. For temporary overvoltage, in the deterministic method the short-duration coordination voltage equals the representative temporary overvoltage.

Limits of the method

The method is simple and fast. That simplicity has a price: it gives no failure frequency and does not allow alternatives to be compared by risk. Two cautions for the user:

• The Kcd margin covers the uncertainty of the assumptions, not a model error. If the assumed maximum overvoltage is poorly estimated, the factor does not correct it.
• The protective level Ups must correspond to the arrester that will actually be installed, with its location relative to the equipment. The coordination is redone if the arrester changes.

What not to do

• Do not apply 1.10 as a fixed factor, or 1.00 as if the arrester never mattered: they depend on the ratio between Ups and Ue2.
• Do not add the coordination factor to the safety factor as if they were the same: the first covers the uncertainty of the method, and the second the differences between test and service.
• Do not take the coordination voltage as the test voltage: the atmospheric and safety factors are still missing, and they bring it to the required withstand voltage.

To go further, see Safety and test conversion factors, Temporary overvoltages: causes and usual values and Lightning overvoltages in a substation.

When to call us

Choosing the insulation level of a substation or of a connection means defining representative overvoltages, the arrester and the margin of each piece of equipment. At Tiza Group we carry out that study as part of engineering design and studies, remotely and worldwide. Write to us and we will send you a quote.

Related guides

• Insulation coordination under IEC 60071: the steps
• Statistical method and risk of insulation failure
• Safety and test conversion factors
• Temporary overvoltages: causes and usual values
• Arrester protective level: Upl and Ups

References

• IEC 60071-2:1996, Insulation co-ordination, Part 2: Application guide, clauses 3.3, 3.3.1, 3.3.2.1 and 3.3.3.1, and figure 6.
• IEC 60071-1:2006, Insulation co-ordination, Part 1: Definitions, principles and rules.

Frequently asked questions