Insulation coordination is the selection of the dielectric strength of equipment in relation to the operating voltages and overvoltages of the network where it will work, the site environment and the protective devices available. IEC 60071-1 sums up the procedure in two decisions: the highest voltage for equipment (Um) and the set of standard withstand voltages that goes with it.
Short answer
Insulation coordination is the work of choosing how much voltage the insulation of each piece of equipment must withstand, so that network overvoltages do not damage it and the substation stays in service. IEC 60071-1 defines it as the selection of the dielectric strength of equipment in relation to the operating voltages and overvoltages of the system, the service environment and the characteristics of the available preventing and protective devices.
What is actually decided
The IEC 60071-1 procedure (clause 5.1) ends in two decisions:
- The highest voltage for equipment, Um. This is the r.m.s. phase-to-phase value for which the equipment is designed with respect to its insulation. It is chosen as the next standard value equal to or above the highest voltage of the system where it will be installed.
- A set of standard withstand voltages. These are the test voltage values (power frequency, switching impulse, lightning impulse) that characterize the insulation. The set is called the rated insulation level and, if it matches the associations in the standard’s tables, the standard insulation level.
Between one decision and the other there is a system analysis: which overvoltages reach the equipment, with what shape and duration, and which devices limit them. That analysis is the core of the engineering work and is done with each network’s data.
A balance between risk and cost
No insulation is immune to every overvoltage. The standard frames this as a performance criterion: the basis for selecting insulation so that the probability of damage or loss of continuity stays at an economically and operationally acceptable level. It is expressed as an acceptable failure rate, for example failures per year, years between failures or risk of failure.
Raising the insulation reduces risk, but it makes the equipment more expensive and the clearances larger. Lowering the overvoltages with a surge arrester, or with elements that prevent the overvoltage at its source, such as a pre-insertion resistor in a circuit breaker, allows lower insulation. Coordination looks for the point where the whole system costs the least without exceeding the accepted risk.
| Measure | What it does | Example |
|---|---|---|
| Preventing device | Avoids or reduces the overvoltage at its source | Pre-insertion resistor |
| Protective device | Limits the peak value of the overvoltage at the equipment terminals | Metal-oxide surge arrester |
| Insulation | Withstands the remaining overvoltage | Equipment insulation level |
Which insulation is being coordinated
The standard distinguishes two types of insulation:
- External insulation: air clearances and surfaces of solid insulation in contact with the atmosphere. It suffers from pollution, humidity and the site environment.
- Internal insulation: internal distances of solid, liquid or gaseous insulation, protected from atmospheric conditions. The oil and paper of a transformer are an example.
A further distinction is made between self-restoring insulation, which fully recovers its properties after a discharge during a test, and non-self-restoring insulation, which loses them or does not recover them completely. That difference changes how it is tested.
Who decides what
The result depends on data supplied by each party:
- System design defines the system voltages, neutral earthing, the expected overvoltages and the limiting devices. This is the project’s engineering work.
- The product committee and the manufacturer define how each piece of equipment is tested, which failure criterion applies and how the highest voltage for equipment is verified, for example with pollution or partial discharge tests. IEC 60071-1 leaves those tests to the product committees.
- The user must declare site conditions that fall outside the normal. The standard takes as normal a temperature not above 40 °C, an altitude up to 1,000 m and pollution not exceeding level II (medium). Beyond that, correction factors are applied or a higher Um is chosen. See special service conditions of switchgear.
Where it shows up in a project
The result of coordination is what you see in the substation specifications: the Um and the withstand voltage values of transformers, circuit breakers, disconnectors, instrument transformers and busbars. For switchgear, those values are explained in rated insulation level of switchgear and in rated voltage of switchgear.
The standard divides the highest voltages into two ranges: range I, above 1 kV and up to 245 kV, and range II, above 245 kV. Different overvoltages weigh in each range, which is why different test values are specified.
What coordination does not solve
- It is not a safety clearance for people. Annex A of IEC 60071-1 gives minimum air clearances to ensure an impulse withstand voltage and warns that safety requirements may call for substantially larger clearances.
- It is not a project study. The standard gives the method and the standard levels. The factors and overvoltages for your substation come from a study with your network data, location and arresters.
- It does not replace equipment tests. Rated values are demonstrated with type and routine tests; see type tests of switchgear.
To go deeper, see IEC 60071 range I and range II: what is specified.
When to call us
If you are going to specify a substation, expand it or install equipment at altitude or in polluted areas, at Tiza Group we carry out insulation coordination as part of the engineering: we define the overvoltages, the insulation level and the protection that backs it up. This is remote engineering work, available worldwide. Learn about our engineering design and studies service and get in touch for a quote on your project.
Related guides
- Insulation coordination under IEC 60071: the steps
- Self-restoring and non-self-restoring insulation
- IEC 60071 range I and range II: what is specified
- Standard insulation levels up to 245 kV
- Standard insulation levels above 245 kV
References
- IEC 60071-1:2006, Insulation co-ordination - Part 1: Definitions, principles and rules: definitions 3.1 to 3.7, 3.22 and clause 5.1; annex A.
- IEC 60071-2:1996, Insulation co-ordination - Part 2: Application guide: clause 1.1 (scope).
Frequently asked questions
What is insulation coordination?
It is the selection of the dielectric strength of equipment in relation to the operating voltages and the overvoltages that can appear in the system where it will be installed, taking into account the environment and the characteristics of the protective devices available (IEC 60071-1, definition 3.1).
Why not insulate everything for the worst possible overvoltage?
Because insulation costs money and takes up space. The standard asks to reduce, to an economically and operationally acceptable level, the probability that overvoltages damage the insulation or affect service continuity. That is why insulation is combined with overvoltage limiting, for example with surge arresters.
Does insulation coordination define safety clearances for people?
No. IEC 60071-1 states that its air clearances address insulation coordination only and that safety requirements may call for substantially larger clearances. The application guide IEC 60071-2 does not cover personnel safety either.