Energizing or reclosing a line produces slow-front overvoltages on all three phases. Three-phase reclosing can be more severe because the line keeps trapped charge. The standard estimates the distribution from the 2 % value, the deviation and the truncation, and proposes limiting it with pre-insertion resistors, controlled closing and arresters.
Short answer
Closing a breaker onto a transmission line produces slow-front overvoltages on all three phases. If the closing is a three-phase reclosing, the line can keep trapped charge and the overvoltage can be higher than in a normal energization. IEC 60071-2 estimates the distribution of amplitudes from the 2 % value, the deviation and the truncation value. It is limited with pre-insertion resistors, controlled closing, inductive voltage transformers and arresters.
What happens when the breaker closes
A three-phase energization or reclosing produces overvoltages on all three phases of the line. Each switching operation generates three phase-to-earth overvoltages and the three corresponding phase-to-phase ones.
For practical use the standard simplifies the counting of values per operation with two methods:
- Phase-peak method. From each operation the highest peak of each phase to earth, or of each phase-to-phase combination, is taken. Each operation contributes three values to the distribution, and that distribution is assumed to apply to each of the three insulations.
- Case-peak method. From each operation the highest peak of the three phases is taken. Each operation contributes a single value, and the distribution is applied to one insulation of each type.
The magnitude depends on several factors:
- the type of breaker, with or without closing resistor;
- the nature and short-circuit power of the busbar that feeds the line;
- the compensation of the line and its length;
- the line termination: open, with a transformer or with arresters.
The overvoltages at the receiving end, the open one, can be considerably higher than at the sending end.
Why reclosing is more demanding
When a line is opened it can be left with trapped charge. At the instant of three-phase reclosing, the voltage left on the line can be as high as the peak of the temporary overvoltage. How that charge decays depends on which equipment stays connected to the line, on the surface conductivity of the insulators or the corona conditions, and on the reclosing time.
According to the standard, in normal networks a single-phase reclosing does not produce higher overvoltages than energization. On lines where resonance or the Ferranti effect are important, it can give more than a three-phase energization. This relates to the action of auto-reclosing (ANSI 79), which decides when and how the line is re-energized.
Inductive voltage transformers connected at the line terminals effectively reduce the trapped charge after opening. In that way the overvoltage of a later three-phase reclosing is limited to the level of a simple energization.
How the phase-to-earth distribution is estimated
The standard does not give a single value. The starting point is the phase-to-earth 2 % value, Ue2, in per unit of the base Us·√2/√3. Its approximate range, without arrester limitation, is given in a figure of the standard according to voltage and type of energization. That figure is only a rough guide, useful to judge whether the overvoltage in a given situation can be a problem and by how much it could be limited. Detailed studies are needed to decide.
The representative distribution is estimated from Ue2. The expressions in annex D are:
| Method | Deviation | Truncation value |
|---|---|---|
| Phase-peak | σe = 0.25 · (Ue2 − 1) | Uet = 1.25 · Ue2 − 0.25 |
| Case-peak | σe = 0.17 · (Ue2 − 1) | Uet = 1.13 · Ue2 − 0.13 |
The truncation values of both methods are equal for the same operation, which is why the 2 % values and the deviations differ. The standard notes that, because of the scatter of the results, the same figure can be used for both methods.
For phase-to-phase insulation one more parameter is added, because the strength depends on how the overvoltage is shared between the two phases at the instant considered. The standard keeps two instants: that of the phase-to-phase peak, and that of the phase-to-phase voltage at the instant of the phase-to-earth peak. For all insulations, except air clearances in range II, the representative phase-to-phase overvoltage is the phase-to-phase peak. For air clearances in range II, especially from 500 kV, it is determined with the procedure in annex D.
Longitudinal insulation
The longitudinal overvoltage during energization is the sum of the operating voltage on one terminal and the switching overvoltage on the other. In synchronized systems, the highest overvoltage peak and the operating voltage have the same polarity, and the longitudinal insulation is stressed less than the phase-to-earth insulation. Between non-synchronized systems there can be an energization on one terminal and the operating voltage peak of opposite polarity on the other.
Assumed maximum value without arresters
If there are no limiting arresters, the standard takes as the assumed maximum for energization or reclosing:
- Phase-to-earth: the truncation value Uet.
- Phase-to-phase: the truncation value, or for external insulation in range II the annex D value, both divided into two equal components of opposite polarity.
- Longitudinal: the phase-to-earth truncation on one terminal and the operating voltage peak, of opposite polarity, on the other.
How they are limited
The most widely used measure is the pre-insertion resistor in the line breaker. Controlled closing at the right instant on the voltage wave, varistors in parallel with the interrupter units and the line’s inductive voltage transformers also help. Gapless metal-oxide arresters limit these overvoltages. The current through them during energization is below about 0.5 to 2 kA. In general they do not limit earth-fault inception and clearing overvoltages, because their expected amplitude is too low.
The first switching operation of a new line is described in energizing a new substation.
To go further, see Models for EMT studies: transformers, lines and equipment, Statistical method and insulation failure risk, Arrester protection level: Upl and Ups and Overvoltages in the system: types and origin.
When to contact us
If you are going to energize a high- or extra-high-voltage line, or change the breaker or the type of switching, the real overvoltage in your network is only known by simulating the case. We carry out your energization transient study and quote the scope, remotely and worldwide.
Related guides
- Insulation co-ordination under IEC 60071: the steps
- Slow-front overvoltages: switching and fault
- Statistical method and insulation failure risk
- Arrester protection level: Upl and Ups
- Models for EMT studies: transformers, lines and equipment
References
- IEC 60071-2 (1996 edition), Insulation co-ordination — Part 2: Application guide, clauses 2.3.3, 2.3.3.1 to 2.3.3.1.4, 2.3.3.6 and annex D.
Frequently asked questions
Why is three-phase reclosing more severe than energization?
Because the line can keep trapped charge when it opens. At the instant of reclosing, the voltage left on the line can be as high as the peak of the temporary overvoltage, and the closing overvoltage adds to it instead of starting from zero.
What is the 2 % value of a switching overvoltage?
It is the amplitude exceeded in only 2 % of the switching operations. The standard uses it as the starting point to estimate the full distribution of amplitudes and its truncation value.
Does single-phase reclosing also exceed energization?
In normal networks it does not produce higher overvoltages than energization. On lines where the Ferranti effect or resonance is significant, single-phase reclosing can give more than three-phase energization.