Current limiting reactors (JBS Ready)

Current-limiting reactors are series reactors with the purpose to reduce the short circuit currents in the power system. The motive to reduce the short circuit currents is to use circuit breakers with lower short circuit current breaking capacity and consequently less expensive circuit breakers.

Sometimes other system components also need protection against too high short circuit currents, like for instance auto- connected transformers that are not self-protecting due to their low impedance.

Another application is limitation of the inrush current when starting large motors.

Current-limiting reactors are sometimes used to limit discharge currents of capacitor banks. In such cases a bifilar wound resistance wire is included and connected in parallel with the inductance.

Dry-type current-limiting reactors

For moderate voltages and power ratings the cheapest type of current limiting reactors is usually the simple, naked dry-type reactor without iron core and any enclosure, cooled by natural air circulation. Numerous reactors of this type are in operation around in the world, most of them probably in medium high voltage power systems.

The magnitude of the inductance of these reactors is normally in the order of millihenry. The inductance remains constant when short circuit current flows through the reactor. There is no decline in the inductance due to saturation in an iron core.

The conductor material in the winding may be either copper or aluminium. The insulation and the supporting materials for the winding are nowadays synthetic, frequently with a high temperature class. When short circuit current due to failure in the power system flows through the reactor high mechanical forces arise in the reactor, within each phase winding as well as between phases. The

D

a

r

reactors must be designed to withstand these forces, which often determines the dimensioning of the reactor.

The absence of an iron core makes the winding capacitance to earth quite small, which gives the advantage that the voltage distribution within the winding

deviates just moderately from linearity during transient voltage conditions.

Figure 2-33 shows a sketch of one phase of such a reactor. The two rectangles illustrate the outer contours of the winding. a is the total height and r is the radial width of the winding. D is the mean winding diameter. The inductance is proportional to the square of the number of turns, D and a factor that depends on the quotients a/D and r/D.

The mutual inductance between the phases comes in addition, depending on the distance between the phases and whether the phases are placed above each other or besides each other, see Figure 2-34

Figure 2-33

Figure 2-34

The air core reactors may require relatively large space because the magnetic field spreads freely in the surroundings and may cause excessive heating in iron reinforcements in concrete walls, floors and ceilings, wire fences and other metal items. Appropriate distance from the reactors must then be kept.

The distance should be sufficiently large to keep the magnetic field below 80 A/m at the floor and at the ceiling. At adjacent wall the magnetic field should not exceed 30 A/m. These field values are referred to continuous rated current through the

Figure 2-35

reactor or to temporary currents lasting more than a few minutes. The thermal time constant of reinforcement iron and wire fences is short. The supplier should inform the purchaser regarding necessary distances from the reactor, at which the magnetic field values have declined to those mentioned above.

Vertical

Transformer handbook. Draft. Rev. 02Q Page 42 of 197

Figure 2-35illustrates the magnetic field attributed to an air-core reactor without any shield. Considerable forces due to the field can draw loose iron items in the vicinity into the reactor and cause damage. So it is recommended to keep the surroundings clean.

Possible disturbing influence of the magnetic field on the functioning of other electrotechnical product in the vicinity must be considered.

Humans should not regularly stay for longer time near the strong magnetic field from such reactors when current is flowing through the windings.

OBSERVE: Persons bearing pacemaker should stay far away from such reactors. Oil-immersed current-limiting reactors

Dry-type reactors for higher voltages may not be suitable in heavy polluted areas because the risk of dielectric failure. In such cases oil-immersed reactors might be more reliable.

To avoid excessive heating in the tank a frame of laminated core steel must enclose the oil- immersed reactor winding. See.Figure 2-36 A core steel limb with gaps in the centre of the winding may not be necessary. The dimensioning of the reactor must be such that the inductance is sufficiently large when short circuit current flows through the reactor and when saturation may occur in the core.

The eddy current losses in the windings of reactors without a gapped centre limb of laminated core steel are quite high because of the strong magnetic field where the winding conductors are situated. A gapped centre limb would reduce these eddy losses, but on the other hand the core would be more complicated and costly.

The magnetic field can also be shielded by means of plates of highly conductive materials like copper or aluminium. However, the losses will be high in the shields due to the high counter- circulating currents, so this shielding method is only applicable for smaller reactor power ratings. The cost of an oil-immersed reactor will be considerably higher than of a dry-type, while the oil- immersed reactor might be less space consuming.

Figure 2-36

When enquiring for a current-limiting reactor, the following information should be given: • System voltage;

• Frequency;

• Short circuit power of the feeding system; • Insulation level;

• Rated continuous current and/or rated short-time current and duration;

• Rated impedance of the reactor or alternatively, the reduced short circuit power after the reactor; Single- phase reactor with frame core 3-phase reactor with frame core

• Dry-type or oil-immersed; • Indoor or outdoor installation.