For a highest voltage for equipment between 52.5 and 245 kV, IEC 60071-1 specifies power-frequency and lightning impulse withstand voltage. Phase-to-earth switching is covered by the power-frequency voltage, but phase-to-phase switching must be considered when choosing the lightning or power-frequency level at the line entrance, or three-phase equipment must be tested with a phase-to-phase switching impulse. IEC 60071-2 recommends running the procedure for several representative substation and line combinations, with at least the simplified methods of its annex F.
Short answer
In substations with a highest voltage for equipment (Um) between 52.5 and 245 kV, IEC 60071-1 specifies two withstand voltages: short-duration power frequency and lightning impulse. Phase-to-earth switching overvoltage is assumed to be covered by the power-frequency one. Phase-to-phase switching must be considered, when choosing the lightning or power-frequency level at the line entrance, or with additional tests on three-phase equipment. Because there is less variety of equipment and locations than in distribution, the standard recommends running the procedure for several representative substation and line combinations, with at least the simplified methods of annex F. This guide follows clause 7.2.2 of IEC 60071-2, 1996 edition.
What each withstand voltage covers
| Overvoltage | How it is covered at 52.5 to 245 kV |
|---|---|
| Power frequency and temporary | Short-duration power-frequency withstand voltage |
| Phase-to-earth switching | Covered by the power-frequency withstand voltage |
| Phase-to-phase switching | Considered when choosing the lightning or power-frequency level of the equipment at the line entrance, or tested with a phase-to-phase switching impulse on three-phase equipment |
| Lightning | Lightning impulse withstand voltage, chosen with the coordination procedure |
The selection of the lightning impulse follows many of the distribution considerations, explained in the guide on insulation coordination in substations up to 36 kV. Two things change here. Equipment and locations are less varied, and transmission lines define the frequency of the incident waves.
Where the stresses concentrate in a substation
IEC 60071-2 (clause 7.1) describes which part of the substation sees each stress:
- Temporary overvoltage from an earth fault: affects all parts of a phase equally.
- Load rejection: depends on the protection scheme. A fault at the remote substation can affect the entire section between the incoming breaker and the transformer. A fault at the substation itself affects only the section between the breaker and the transformer. If the transformer is connected to a generator, the breaker may be subjected to longitudinal stresses during synchronization.
- Line energization and re-energization: the high amplitudes at the receiving end appear only between the line entrance and the breaker. The rest of the substation sees those of the sending end.
- Lightning: can appear anywhere, with different amplitudes depending on the distance to the surge arrester.
- Fast-front switching: only in the section being switched, for example a busbar energized with a disconnector.
The standard adds three notes. At an initial stage there may be only one line in service and temporary overvoltages from load rejection after an earth fault must be considered. If transformers are energized through a long line, slow-front overvoltages can affect the transformer and the busbar. And in a GIS, very-fast-front overvoltages from disconnector operation may require consideration.
What determines the lightning overvoltage
Annex F of IEC 60071-2 explains what it depends on. The overvoltages in the substation depend on the amplitude and shape of the wave arriving along the line and on the travelling-wave behaviour of the substation itself. The frequency with which they arrive is given by the lightning performance of the connected line.
- In parts without surge arresters, the most important parameter is the amplitude of the incident wave.
- In parts protected by surge arresters, their steepness and the separation distance between the arrester and the equipment matter.
The steepness of the wave is reduced mainly by the corona effect on the line. For that reason only a strike within a certain distance of the substation produces a steepness sufficient to cause a given amplitude. Farther away, the steepness is too low, regardless of amplitude. That limiting distance is what must be simulated carefully in a detailed digital transient calculation.
The simplified method (annex F, clause F.4) assumes that any strike within a certain distance causes an overvoltage at the equipment greater than an assumed value, and any strike outside it, a smaller one. The standard admits that neither assumption is strictly valid. Surge arrester protection practice indicates that the two inaccuracies compensate each other sufficiently. For the arrester, see the surge arrester.
How it applies to a 52.5 to 245 kV substation
- Representative cases. Substation and line combinations, with the configuration and number of lines in service
- Representative overvoltages. Temporary, slow-front (phase-to-phase) and lightning, with the simplified method as a minimum
- Required withstand voltages. With the safety and atmospheric factors
- Insulation level. Power frequency and lightning, and a phase-to-phase switching test if required
The result sets the insulation level, with attention to the line entrance, where phase-to-phase switching weighs most. For yard equipment, see the conventional AIS substation and live-tank high-voltage circuit breakers.
What not to do
- Do not assume that phase-to-phase switching impulse is covered at the line entrance just because it is not in the tables for this range.
- Do not set the lightning impulse with a single line and substation combination: the standard recommends several representative combinations.
- Do not place the surge arrester far from the transformer without calculating the separation distance: it influences the overvoltage the equipment sees.
- Do not take a worked case from the standard as project data: it depends on your line, your arrester and your configuration.
To go further, see IEC 60071 range I and range II: what is specified and High and extra-high voltage substations: the essentials.
When to call us
Setting the insulation level of a 52.5 to 245 kV substation requires a study of its line and arrester combinations. At Tiza Group we do it within engineering design and studies, remotely and worldwide, and we deliver the selection of switchgear and surge arresters. Write to us and we will send you a quote.
Related guides
- Insulation coordination under IEC 60071: the steps
- Insulation coordination in substations up to 36 kV
- Protective distance of a surge arrester to the equipment
- IEC 60071 range I and range II: what is specified
- High and extra-high voltage substations: the essentials
References
- IEC 60071-2:1996, Insulation co-ordination, Part 2: Application guide, clauses 7.1 and 7.2.2, and annex F (F.1, F.2 and F.4).
- IEC 60071-1:2006, Insulation co-ordination, Part 1: Definitions, principles and rules.
Frequently asked questions
Which withstand voltages are specified for 52.5 to 245 kV equipment?
IEC 60071-1 specifies the short-duration power-frequency withstand voltage and the lightning impulse withstand voltage. It does not specify a switching impulse in this range, because the phase-to-earth switching overvoltage is assumed to be covered by the power-frequency voltage (IEC 60071-2:1996, clause 7.2.2).
What has to be done about phase-to-phase switching overvoltages?
Consider them when choosing the lightning impulse or power-frequency voltage of the equipment at the line entrance. Another option is additional phase-to-phase switching impulse tests on three-phase equipment.
What does the lightning overvoltage depend on in equipment with surge arresters?
On the steepness of the incoming wave and on the separation distance between the arrester and the equipment. Without arresters, the most important parameter is the amplitude of the incident overvoltage (IEC 60071-2:1996, annex F).