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(11) | EP 2 088 603 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Shunt reactor |
| (57) The invention is related to a three phase four or five leg iron core type shunt reactor
for power transmission or distribution networks. A range of the ratio Zo : Z1 between about 0,3 and 1 is achieved by arranging an air-gap on the flux return leg.
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BACKGROUND OF THE INVENTION
[0001] The invention is related to three-phase reactors and more particularly to iron-core type shunt reactors. An iron core shunt reactor with an air-gap in each phase leg is one typical reactor type to be used for compensating the three-phase capacitive reactive power and reducing the earth fault current in electric power transmission or distribution networks. A compact structure, light weight and low losses can be achieved by using this iron-core reactor type.
[0002] For a reactor with a three-leg core, depending of the distance from the core to the wall of the tank, the zero impedance Z0 typically is about 30% of the positive sequence impedance Z1, so in other words the ratio Z0 : Z1 ≈ 0,3. Because the leakage flux is strongly heating the wall of the tank, the network shall be disconnected immediately in a case of earth fault.
[0003] By arranging one or two additional unwounded flux return legs to the core we will have a four or five leg core shunt reactor. The zero impedance Z0 for those types is about equal to the positive sequence impedance Z1, so in other words the ratio Z0 : Z1 ≈ 1. The flux return leg is a path for the flux caused by the homopolar flux, enabling to continue running the power network in a case of earth fault. In some cases a problem is the potential over-compensation of the earth fault current.
[0004] In some circumstances the values of the ratio Z0 : Z1 between 0.3 and 1 might be needed to run the network during earth fault but still not overcompensating the network.
BRIEF DESCRIPTION OF INVENTION
[0005] The object of the invention is to provide a shunt reactor such that the above-mentioned problem can be solved. This is achieved by a reactor which is characterized in what is disclosed in the independent claim 1. The preferred embodiments of the invention are disclosed in the dependent claims.
[0006] The invention is a three-phase iron-core type shunt reactor with a four or a five leg core where all three phase legs are equipped with one or plurality of air-gaps but the flux return-leg or flux return-legs are equipped with at least one air-gap, as well.
[0007] The cross-sectional area of the flux return leg in a four leg arrangement is preferably the same as for the three phase legs. In a five leg arrangement, the cross-sectional area of the flux return legs could be half of that. The air-gap should be about equal on both sides to ensure a linear functionality in function of earth fault current.
[0008] A leg consisting of an iron core with an air-gap could be formed by laminating a plurality of block iron cores having at least one pure air-gap or air-gap filled by a non-magnetic material.
[0009] An air-gap of the flux return leg could be fixed, but in another embodiment it could be adjustable. The flux return leg could be equipped with an arrangement to move one part of the leg with reference to another part of the flux return leg to adjust the path for homopolar flux. The ratio Zo : Z1 could even be continuously adjusted by an automatic control arrangement.
[0010] A shunt reactor is typically directly earthed, but the range of ratio Zo : Z1 could be expanded by connecting an additional one-phase zero-point coil, called a neutral earthing reactor between the star point of the shunt reactor windings and the earth. The values for ratio Zo : Z1 up to more than 1 could be achieved. If the reactor has no star point connection to the earth or is delta connected, the Zo has an infinite value.
BRIEF DESCRIPTION OF FIGURES
[0011] In the following the invention will be described in greater details in connection with preferred embodiments, with reference to the attached drawings, wherein
Figure 1 is a view of a three-phase four-leg reactor;
Figure 2 is a view of a three-phase five-leg reactor;
Figure 3 shows a principle circuit diagram for a shunt reactor;
Figure 4 shows a principle circuit diagram with an additional one-phase zero-point coil;
Figure 5 shows a principle drawing of an embodiment of the invention with an adjustable air cap
Figure 6 shows a principle drawing of another embodiment of the invention with an adjustable air cap
DETAILED DESCRIPTION OF THE INVENTION
[0012] Fig. 1 shows a three-phase shunt reactor according to the invention. The core (1) of the reactor is typically oil-immersed in a tank (2). The shunt reactor comprises an upper yoke (3) and a lower yoke (4), one phase leg (5) for each phase, a winding (6) wound surrounding each phase leg and a flux return leg (8). Each phase leg is equipped with an air-gap (7). The flux return leg (8) is equipped with at least one air-gap (11), as well. The length of the air-gaps (7,10) could roughly be calculated by the common formulas for magnetic circuits having air and iron portions. The flux return leg (8) should be constructed to carry the mechanical forces like what is commonly known from the designs of phase legs (5) for typical shunt reactor with air-gaps (7). The air-gap (7, 10) could be filled by non-magnetic material to keep more easily the length fixed against to mechanical forces. The first end of each winding (6) is connected to a bushing insulator (not shown) on the top of the tank (2). Another ends of the windings could be internally star-coupled and the star-point connected to a bushing insulator or a particular earthing terminal to offer one connection point to the earth. Alternatively the another end of each winding could be connected to a corresponding bush insulator to be star-coupled outside of the tank.
[0013] Fig. 2 shows another embodiment of a three-phase shunt reactor with two flux return legs (8). The cross-section area of the flux return legs could be half of that on phase legs. The dimensions, like the length of the air-gap (10) of both flux return legs should be essentially equal to ensure linear functionality of the shunt reactor as a function of earth fault current.
[0014] Fig. 3 shows a principle main circuit drawing of the shunt reactor. The first end of each winding (6) is connected to corresponding phase of the three phase transmission or distribution line by a circuit breaker or disconnector. The shunt reactor could be connected to the substation bus bar, as well. The second ends of each winding are star connected and the star connection point is directly connected to the earth.
[0015] Fig. 4 shows a principle main circuit drawing of another embodiment of the shunt reactor. The first end of each winding is connected to corresponding phase of the three phase transmission or distribution line. The second ends of the winding are star connected and the star connection point is connected by a one-phase zero-point coil (12) to the earth. The zero-point coil (12) could be fixed or adjustable.
[0016] Fig. 5 shows an embodiment of a three-phase shunt reactor. The flux return leg (8) is equipped with an arrangement to adjust the length of the air-gap (10). The lower portion (10) of the flux return leg is fixed and magnetically connected to the lower yoke (4). The upper portion (13) of the flux return leg is moveable arranged and magnetically connected to the upper yoke (3).
[0017] Fig. 6 shows another embodiment of a three-phase shunt reactor. The flux return leg (8) is equipped with an arrangement to adjust the magnetic circuit. A moveable portion (14) is arranged between the fixed upper portion (9) and the fixed lower portion (10) of the flux return leg. The magnetic circuit through the return path is adjustable by moving the moveable portion (14) transversely related to the fixed portions (9,10).
[0019] When the air-gap is open the return flux path goes at least partially through the side wall of the tank (1) making the calculation of the Zo challenging. That's why the final characterising curve for Zo related to the mechanical dimensions of the air-gap (11) should be defined by electrical measurements in a case of new design.
1. A three-phase shunt reactor for power transmission or power distribution networks,
the iron core (1) of the shunt reactor comprising an upper yoke (3), a lower yoke
(4), a phase leg (5) for each phase, at least one air-gap (7) for each phase leg,
a winding (6) wound surrounding each phase leg and at least one flux return leg (8),
said iron core (1) being arranged in a tank (2), the first end of each winding being
connected to a bush insulator and another ends of the windings arranged to be star-coupled
and earthed or reactor earthed characterized in that said flux return leg (8) comprises at least one air-gap (11) to adjust the reluctance
of the flux return path.
2. A shunt reactor as claimed in claim 1, characterized in that at least one portion (13) of the flux return leg (8) is arranged to be moveable in
a linear fashion relative to the fixed portion (10) of the flux return leg (8) to
adjust the length of the air gap (11) for making the reluctance of the flux return
path continuously adjustable.
3. A shunt reactor as claimed in claim 1, characterized in that at least one portion (14) is arranged to be moveable in a linear fashion transversely
relative to the fixed portions (9,10) of the flux return leg (8) to adjust the opposing
cross-section areas over the air gap (11) for making the reluctance of the flux return
path continuously adjustable.