Interrupting inductive currents (reactors, unloaded transformers, starting motors, arc furnaces) or capacitive currents (cables and capacitor banks) can produce overvoltages. Breaker restrike when interrupting capacitive currents is especially dangerous. The standard limits them with restrike-free breakers, arresters and controlled closing.
Short answer
Switching inductive or capacitive currents can produce overvoltages that need attention. Inductive cases include the starting current of motors, the magnetizing current of a transformer and the current of a shunt reactor. Capacitive cases include unloaded cables and capacitor banks. The most dangerous case is breaker restrike when interrupting capacitive currents, which is why the standard calls for restrike-free breakers. IEC 60071-2 also gives reference values of 2 and 3 p.u. for fast-front switching.
Which switching operations the study must consider
Clause 2.3.3.4 of IEC 60071-2 names these operations explicitly:
- Interruption of motor starting currents.
- Interruption of inductive currents, for example the magnetizing current of a transformer or the disconnection of a shunt reactor.
- Switching and operation of arc furnaces and their transformers, which can lead to current chopping.
- Switching of unloaded cables and capacitor banks.
- Current interruption by high-voltage fuses.
Although they are classified with the slow-front ones, some of these switching operations give fast transients when the equipment is connected or disconnected through short connections inside a substation.
Interrupting inductive currents
When a breaker interrupts a small inductive current, such as the magnetizing current of an unloaded transformer or the current of a reactor, it can chop the current before its natural zero crossing. That current chopping is the origin of the overvoltage. It is the same phenomenon described for the reactor in switching shunt reactors with a circuit breaker and in shunt reactor: rated values and tests.
The standard adds a warning about medium voltage: when switching reactive loads, some types of medium-voltage breakers produce multiple transient interruptions (multiple reignitions). Without suitable protective measures, this can give overvoltages of up to 6 p.u.
Interrupting capacitive currents and restrike
When interrupting the capacitive current of an unloaded line or cable, or of a capacitor bank, the breaker can restrike. A restrike can generate especially dangerous overvoltages, and the standard states that restrike-free breakers must be used. The classification of breakers by this property is in classes C1 and C2 for capacitive current switching.
When energizing capacitor banks, especially those without an earth connection, the phase-to-phase overvoltages must also be evaluated. The consequences for the bank and its protection are described in capacitor bank and in capacitor bank tripping.
Reference values from the standard
When equipment is connected or disconnected through short connections inside the substation, fast-front overvoltages appear. Although they are usually oscillatory, for insulation co-ordination they are represented with the standard 1.2/50 µs lightning impulse. Special attention is needed for equipment with windings, because of the high turn-to-turn stresses.
Their maximum peaks are normally lower than those of lightning, and for that reason their importance is limited to special cases. The standard characterizes their amplitude with these maxima, in p.u. of Us·√2/√3:
| Switching | Reference maximum |
|---|---|
| Breaker without restrike | 2 p.u. |
| Breaker with restrike | 3 p.u. |
| Disconnector | 3 p.u. |
Since it is very unlikely that these overvoltages appear at the same time on more than one phase, it is assumed that there are no phase-to-phase overvoltages higher than the phase-to-earth ones. If these values turn out to be decisive for setting the lightning impulse withstand voltage of the insulation, the standard recommends investigating the case more carefully.
Slow front and fast front in the same switching operation
The standard separates switching operations by the shape of the overvoltage they produce. Slow-front ones have fronts from tens to thousands of microseconds and are oscillatory. Fast-front ones appear when equipment is connected or disconnected through short connections inside the substation, or when external insulation flashes over. Those events can subject nearby internal insulation, such as windings, to especially severe stresses.
For that reason the same switching operation is checked against two criteria. The first is the amplitude of the phase-to-earth and phase-to-phase overvoltage, with the arresters and breakers foreseen. The second is the stress on the insulation of equipment with windings, such as motors and transformers, which depends on the steepness of the front as well as on the amplitude.
The severity of fast-front overvoltages generated by switching is limited by choosing the switching equipment well: restrike-free breakers, breakers with low current chopping, with opening or closing resistors or with controlled closing.
How they are limited
- Restrike-free breakers for capacitive currents.
- Breakers with low current chopping for inductive currents.
- Opening or closing resistors and controlled closing on the voltage wave. The standard also cites these measures for limiting switching overvoltages.
- Metal-oxide surge arresters, which limit the overvoltages from switching inductive and capacitive currents. Their scope and limits are explained in surge arrester.
- Varistors in parallel with the interrupter units.
On arresters, the standard states that metal-oxide arresters limit the phase-to-earth amplitude to approximately twice their rated voltage, with the peak in kV and the rated voltage in r.m.s. kV. That is why they work for switching overvoltages from inductive and capacitive currents. Inside the substation it is not necessary to consider the separation effect between arrester and equipment for these slow-front overvoltages.
When to contact us
The real overvoltage depends on the breaker, on the length of the connections and on the load. If you are going to install a reactor, a capacitor bank or a long cable, or have already had damage after switching them, we carry out your switching transient study and quote the scope, remotely and worldwide. To verify the equipment you already have, we test your reactors, capacitor banks and surge arresters in Colombia, or elsewhere on request.
Related guides
- Insulation co-ordination under IEC 60071: the steps
- Slow-front overvoltages: switching and fault
- Arrester protection level: Upl and Ups
- Switching shunt reactors with a circuit breaker
- Classes C1 and C2: breakers for capacitive currents
References
- IEC 60071-2 (1996 edition), Insulation co-ordination — Part 2: Application guide, clauses 2.3.3.4, 2.3.3.6, 2.3.3.7 and 2.3.4.3.
Frequently asked questions
What is breaker restrike?
It is a new flow of current between the contacts after the breaker has interrupted. When interrupting capacitive currents (unloaded lines or cables, capacitor banks) it can generate especially dangerous overvoltages, which is why the standard calls for restrike-free breakers.
Which overvoltage values does the standard give for fast-front switching?
For a breaker without restrike, 2 p.u.; with restrike, 3 p.u. For a disconnector, 3 p.u. The base is the peak of the highest phase-to-earth voltage, Us·√2/√3. These values are reference maxima to judge whether the switching operation matters.
Why take care when energizing capacitor banks with isolated neutral?
Because the standard asks for a careful evaluation of phase-to-phase overvoltages when energizing capacitor banks, in particular those without an earth connection.