Clause 8 of IEC 60076-6 covers series reactors that limit fault current and those placed between neutral and earth. The purchaser specifies the thermal short-circuit current, its duration (2 s for current-limiting reactors and 10 s for neutral reactors if none is stated) and the impedance. The rated mechanical current is 2.55 times the thermal one if no other value is specified.
Two reactors, one problem
When the short-circuit current of a busbar exceeds what the equipment can withstand, there are two routes. A current-limiting reactor is placed in series with the circuit. Or a neutral reactor is placed between the system neutral and earth, to limit the earth-fault current. To dig deeper into the arc suppression coil between the neutral and earth, read arc suppression coil.
IEC 60076-6 treats both in clause 8. This guide summarizes what the purchaser must specify, what the reactor must withstand and how it is verified. The full overview of types is in the guide on reactor types per IEC 60076-6. For more detail on the reactor types and what each one is for, read reactor types per IEC 60076-6.
Difference in normal service
- The current-limiting reactor carries continuous current in normal operation. That is why it has a rated continuous current and a measured loss.
- The neutral reactor carries very little current or none in normal operation (clause 8.1). It works only during an earth fault.
Clause 8.1 also includes bus-tie, motor starting, power-flow control, arc furnace, insertion, test and converter reactors. For those uses, deviation from the requirements is agreed between manufacturer and purchaser.
Rated quantities
| Quantity | What it indicates | Clause |
|---|---|---|
| Rated continuous current Ir | Permanent r.m.s. value at rated frequency. In a series reactor per phase it is a symmetrical three-phase current. In a neutral reactor, it is specified if it exceeds 5 % of the thermal or short-time current | 8.4.1 |
| Thermal short-circuit current ISCr | Symmetrical steady-state component of the short-circuit current, for the rated duration. Not lower than the highest symmetrical current expected in service | 8.4.2 |
| Thermal duration TSCr | If not stated: 2 s for current-limiting reactors and 10 s for neutral reactors connected to the system neutral | 8.4.3 |
| Mechanical short-circuit current IMSCr | Asymmetrical peak value. Calculated per IEC 60076-5, clause 4.2.3. If not specified: 2.55 · ISCr | 8.4.4 |
| Short-time current ISTr and its duration or duty cycle | For starting reactors, test reactors or neutral reactors of a shunt reactor | 8.4.5, 8.4.6 |
| Coupling factor k | Mutual reactance in per unit of rated reactance | 8.4.7 |
| Short-circuit impedance ZSCr | Set by the purchaser. It can also be derived from the system short-circuit current or power and the desired thermal current | 8.4.8 |
Some points change cost and design.
- Fault duration. It must reflect the accumulated thermal effect of automatic reclosing and the time a fault is allowed to persist. If the short-circuit current exceeds roughly twenty-five times the continuous current, the thermal duration can make the reactor more expensive (clause 8.4.3, notes 1 and 2).
- Frequency of application. Unless otherwise indicated, the interval between faults that add up to the rated duration is at least 6 h. If more than about ten faults per year are expected on average, the purchaser must say so (clause 8.5).
Coupling between phases and impedance
If the coupling factor is below 5 %, it is enough to specify the three-phase short-circuit impedance ZSCr3 (clause 8.4.8.1). If it is 5 % or more, as is normal in three-phase reactors stacked vertically, there are two impedances: three-phase ZSCr3 and single-phase ZSCr1 (clause 8.4.8.2).
The system decides which one governs. With high-impedance earthing, the series reactor limits only symmetrical three-phase faults and ZSCr3 is enough. With effective earthing, single-phase and three-phase faults are evaluated, and both impedances are considered. The impedance during a single-phase fault can differ from that of a three-phase fault when there is stacking and coupling.
If the three currents must be equal in a three-phase fault with stacked reactors, the purchaser specifies it and the mutual coupling is compensated. In that case the self-inductances of each phase are not equal, and the effective impedance in a single-phase fault is lower than in a three-phase one (clause 8.4.8.2). On the reactors that work with capacitors, read filter, damping and discharge reactors.
What it must withstand
A current-limiting or neutral reactor is designed to withstand the thermal and dynamic effects of the rated short-circuit current, with the associated electrical stress, for the rated duration (clause 8.5).
The calculated winding temperature after the short-circuit current cannot exceed the values that IEC 60076-5, clause 4.1.4, sets for transformer windings (clause 8.6.2). In continuous service, the limits of IEC 60076-2 and IEC 60076-11 apply, and loading and overloading follow IEC 60076-7 for liquid-immersed units and IEC 60905 for dry-type ones.
Insulation
In a current-limiting reactor, insulation between phases and to earth generally corresponds to the highest voltage for equipment Um. Insulation between the ends of the winding can be lower, especially if there are surge arresters in parallel with the reactor. The standard recommends choosing the rated voltage of the parallel arrester not lower than 1.2 times the voltage that appears across the reactor at the thermal short-circuit current (clause 8.7.1.1).
In a neutral reactor, insulation is matched to that of the system neutral or the shunt reactor, and the earth terminal admits a lower level (clause 8.7.1.2). For more on where to place the taps of an autotransformer: autotransformer with taps at the neutral or the X terminal. More on the ways of grounding a system neutral in system neutral grounding per IEEE C62.92.
Tests
Clause 8.9 divides them as follows.
| Group | Tests |
|---|---|
| Routine | Winding resistance; impedance at rated continuous current; impedance at short-time current in neutral reactors of a shunt reactor, starting reactors and test reactors; losses at ambient temperature; separate-source AC withstand voltage on liquid-immersed units; winding overvoltage; insulation resistance and dissipation factor (tan δ) on liquid-immersed units |
| Type | Temperature rise at rated continuous current; lightning impulse on current-limiting reactors; power consumption of fans and oil pumps |
| Special (at the purchaser’s request) | Short circuit; winding reactance without the core in gapped or shielded reactors; sound level; vibration on liquid-immersed units; switching impulse; double-ended lightning impulse; coupling factor; wet tests |
Winding overvoltage
The induced voltage test cannot be done on a low-impedance reactor the way it is done on a transformer. Instead, winding overvoltage is tested with lightning impulse at each end of the winding, with the other end earthed. Because of the low impedance, the standard waveform is usually not achieved (clause 8.9.9). For neutral reactors a front time of up to 13 µs is accepted (clause 8.9.10).
Short circuit
When the purchaser requests it, the test is generally made per IEC 60076-5, clauses 4.2.2 to 4.2.7 (clause 8.9.13). The specification must include the test current level, the duration of each shot, the number of shots and the tap. If nothing is specified, the standard sets two shots of 0.25 s per phase, with the first peak equal to the rated mechanical current. The peak achieved cannot deviate more than 5 % from the specified one.
The thermal test, if specified, is one symmetrical shot at the rated thermal current for the rated duration. If that current cannot be reached, the duration is extended up to 6 s at a lower current, to give at least the specified I²t value.
The acceptance criterion is applied as follows (clause 8.9.13.2). Before and after the test, routine tests are repeated, with impedance and losses, and the 100 % winding overvoltage test. The values must be consistent and the oscillograms must not change. In a dry-type reactor, deterioration of the winding clamping or a notable increase in surface cracks counts as a failure. If in doubt, up to three more shots with fully asymmetrical current are applied to see whether the condition stabilizes.
Tolerances
| Quantity | Tolerance | Clause |
|---|---|---|
| Impedance of reactors for other uses, at short-time or continuous current | ±5 % of rated impedance | 8.10.1 |
| Current per phase in three-phase reactors compensated for coupling | Maximum deviation of 5 % from the average | 8.10.2 |
| Single-phase impedance in compensated reactors | Not lower than 85 % of the rated value | 8.10.2 |
| Total losses | Not to exceed the guarantee by more than 10 % | 8.10.3 |
When you need support
The ISCr value comes from a short-circuit study, and the reactor is chosen from that result. At Tiza Group we carry out short-circuit and interconnection studies and transformer and reactor testing, the latter in Colombia or elsewhere on request. You can calculate a first fault current with the short-circuit current tool.
Related guides
- Power reactor
- Short-circuit study
- Isolated, resonant and solidly earthed neutral
- Earthing transformer
- Autotransformer Taps at the Neutral or the X Terminal
- Reactor Types per IEC 60076-6 and What Each Is For
- Arc Suppression Coil (Petersen): Ratings and Tests
- Filter, Damping and Discharge Reactors for Capacitors
- System Neutral Grounding: IEEE C62.92
References
- IEC 60076-6: power transformers, reactors (clause 8).
- IEC 60076-5: ability to withstand short circuit (clauses 4.1.4 and 4.2).
Frequently asked questions
What is the mechanical short-circuit current a current-limiting reactor must withstand?
It depends on the X/R ratio of the system and is calculated per clause 4.2.3 of IEC 60076-5. If the purchaser specifies neither system impedance nor X/R, clause 8.4.4 of IEC 60076-6 takes 1.8 · √2 times the thermal current, that is, 2.55 times.
Why does a magnetically shielded reactor have less reactance at short-circuit current?
Because the shield is designed to saturate at the high short-circuit current, which reduces the reactance. That is why the reactance at short-circuit current is lower than the reactance at rated continuous current (clause 8.2, note 1). The purchaser must take this into account when calculating the limited fault current.
What is checked to say that a reactor passed the short-circuit test?
Routine tests are repeated before and after, including impedance and loss measurement, and the 100 % winding overvoltage test. The values must be consistent within measurement tolerance and the oscillograms must show no changes. For dry-type reactors a visual inspection is added (clause 8.9.13.2).