Annex A of IEC 60071-1:2006 gives minimum air clearances that ensure an impulse withstand voltage in installations that cannot be tested as a whole, such as substations. There is one table for lightning impulse (rod-structure and conductor-structure), another for phase-to-earth switching impulse and another for phase-to-phase switching impulse. They address insulation coordination only: safety requirements may call for substantially larger clearances.
Short answer
Annex A of IEC 60071-1:2006 gives the minimum air clearances that guarantee a given impulse withstand voltage in complete installations, such as substations, which cannot be tested as a whole. There are three tables: A.1 for lightning impulse, A.2 for phase-to-earth switching impulse and A.3 for phase-to-phase switching impulse. Each depends on the electrode configuration. These clearances solve insulation coordination only: safety requirements may call for substantially larger clearances.
What principle annex A follows
The withstand voltage of air, at the standard reference atmosphere, must be equal to or greater than the rated lightning and switching impulse withstand voltage. With that principle, minimum clearances were calculated for different electrode configurations, using a conservative criterion that takes practical experience into account. The values do not include the altitude correction: the standard clarifies that they apply for the altitudes considered when determining the required withstand voltage.
There are four configurations:
- Rod-structure: a rod-shaped electrode facing a structure.
- Conductor-structure: a conductor facing a structure, covering a wide range of usual configurations.
- Parallel conductor-conductor: covers symmetrical configurations with similarly shaped electrodes in the two phases.
- Rod-conductor: the most unfavourable asymmetrical configuration normally found in service.
Lightning impulse: table A.1
Table A.1 relates the rated lightning impulse withstand voltage to the minimum clearance. It applies to phase-to-earth and phase-to-phase. For phase-to-earth, the conductor-structure or rod-structure clearance is used. For phase-to-phase, the rod-structure clearance is used. Some values from the table:
| Lightning impulse (kV) | Rod-structure (mm) | Conductor-structure (mm) |
|---|---|---|
| 95 | 160 | no value |
| 125 | 220 | no value |
| 145 | 270 | no value |
| 170 | 320 | no value |
| 250 | 480 | no value |
| 325 | 630 | no value |
| 450 | 900 | no value |
| 650 | 1,300 | no value |
| 850 | 1,700 | 1,600 |
| 1,050 | 2,100 | 1,900 |
| 1,425 | 2,850 | 2,600 |
| 2,100 | 4,200 | 3,900 |
The complete table runs from 20 kV to 2,100 kV. The conductor-structure column only has values from 850 kV.
Range I: up to 245 kV
In range I, the phase-to-earth and phase-to-phase clearance comes from table A.1 with the rated lightning impulse. The short-duration power-frequency withstand voltage can be ignored when the ratio of the standard lightning impulse to the standard power-frequency voltage exceeds 1.7. Tables A.2 and A.3 are not consulted in this range.
Range II: above 245 kV
In range II, the clearance depends on two impulses and the larger is taken:
- Phase-to-earth. The larger of A.1 (lightning, rod-structure) and A.2 (phase-to-earth switching)
- Phase-to-phase. The larger of A.1 (lightning, rod-structure) and A.3 (phase-to-phase switching)
- Longitudinal insulation, lightning. 0.7 times the highest system voltage phase-to-earth peak is added to the lightning impulse and the sum is divided by 500 kV/m
- Longitudinal insulation, switching. Smaller than phase-to-phase; the standard gives no minimum values
Table A.2 gives, for phase-to-earth switching impulse, the rod-structure and conductor-structure clearance:
| Switching impulse (kV) | Rod-structure (mm) | Conductor-structure (mm) |
|---|---|---|
| 750 | 1,900 | 1,600 |
| 850 | 2,400 | 1,800 |
| 950 | 2,900 | 2,200 |
| 1,050 | 3,400 | 2,600 |
| 1,175 | 4,100 | 3,100 |
| 1,300 | 4,800 | 3,600 |
| 1,425 | 5,600 | 4,200 |
| 1,550 | 6,400 | 4,900 |
Table A.3 gives the phase-to-phase switching impulse clearances for parallel conductor-conductor and rod-conductor. Each row relates the phase-to-earth voltage to the phase-to-phase over phase-to-earth ratio (between 1.5 and 1.7) and the resulting phase-to-phase voltage. For example, for 1,050 kV phase-to-earth with a ratio of 1.5, the phase-to-phase voltage is 1,575 kV, and the clearances are 3,600 mm (conductor-conductor) and 4,200 mm (rod-conductor). For 1,425 kV with a ratio of 1.7: 2,423 kV, 7,200 mm and 9,000 mm.
When they can be reduced
The standard allows smaller clearances in two cases. First: tests on the actual configuration or a similar one prove that the standard withstand voltages are met. Those tests must include the environmental conditions that make the electrodes irregular, such as rain and pollution. Second: experience confirms that the overvoltages are lower than those assumed, or that the configuration is more favourable than those of the tables. These clearances also do not apply to equipment whose specification requires an impulse test, because a mandatory minimum clearance would limit its design.
What they do not cover
The clearances in the tables are insulation clearances. They are not safety clearances for personnel, nor substation yard design clearances, which follow other standards. In a conventional AIS substation, the safety clearances must also be checked, and they may turn out larger.
What not to do
- Do not use a clearance from table A.1 as a safety clearance: the standard clarifies that safety may require substantially larger clearances.
- Do not take the conductor-structure column for phase-to-phase: for phase-to-phase, rod-structure is used.
- Do not apply the tables without checking the altitude: they apply for the altitudes considered in the required withstand voltage.
- Do not reduce a clearance by intuition: it is justified only by test or by operating experience.
To go further, see High and extra-high voltage substations: the essentials, Substation busbars and insulators: what to define, Insulation coordination in substations from 52 to 245 kV and IEC 60071 range I and range II: what is specified.
When to call us
The physical layout of a substation starts from these clearances and from the safety ones, and is verified against the insulation level chosen. At Tiza Group we do the primary engineering within 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
- Insulation coordination in substations above 245 kV
- Altitude correction of external insulation: Ka and m
- Conventional AIS substation: busbar arrangements and equipment
- High and extra-high voltage substations: the essentials
References
- IEC 60071-1:2006, Insulation co-ordination, Part 1: Definitions, principles and rules, annex A (normative), tables A.1, A.2 and A.3.
- IEC 60071-2:1996, Insulation co-ordination, Part 2: Application guide, annex A.
Frequently asked questions
What minimum air clearance corresponds to a 170 kV lightning impulse?
According to table A.1 of IEC 60071-1:2006, 320 mm in the rod-structure configuration. That clearance applies to phase-to-earth and phase-to-phase. It is only the insulation coordination clearance: safety clearances for personnel may be substantially larger.
When can the power-frequency voltage be ignored when setting the clearance?
In range I, the clearance is taken from table A.1 using the lightning impulse, and the short-duration power-frequency withstand voltage can be ignored when the ratio of the standard lightning impulse to the standard power-frequency voltage is greater than 1.7.
Can clearances smaller than those in the tables be used?
Yes, if tests on the actual configuration or a similar one show that the standard withstand voltages are met, or if operating experience confirms that the overvoltages are lower or that the configuration is more favourable. The tables do not apply to equipment whose specification requires an impulse test.