EP0216500A1

EP0216500A1 - Electromagnetic induction apparatus

Info

Publication number: EP0216500A1
Application number: EP86306422A
Authority: EP
Inventors: Kentaro C/O Ako Works Taninouchi; Katsuji C/O Ako Works Sokai
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1985-08-19
Filing date: 1986-08-19
Publication date: 1987-04-01
Anticipated expiration: 2006-08-19
Also published as: EP0216500B1

Abstract

An electromagnetic induction apparatus according to this invention is furnished with a short-circuit winding (1c) which is electromagnetically coupled to at least two windings (1a. 1b), thereby to realise a transformer (1A) which effects the shunt reactor function of supplying "leading" reactive power necessary for transmission or distribution lines, without installing a shunt reactor. In another aspect of performance of this invention, an electromagnetic induction apparatus is furnished with a reactor winding (1c) which is electromagnetically coupled to at least two windings (1a, 1b) including a tap winding (1 b) and which is inserted between one end of the tap winding and a tap (30), and a tap voltage which is lower than the open-circuit voltage of the reactor winding (1c) is forcibly applied from the tap (30) to the reactor winding ( 1c), thereby to generate a desired magnitude of leakage flux (18) between the reactor winding (1 c) and the tap winding (1 b) and to make the capacity of an equivalent shunt reactor variable.

Description

BACKGROUND OF THE INVENTION

This invention relates to an electromagnetic induction apparatus which comprises a transformer and which is utilized for power transmission and distribution systems, etc., and more particularly to an electromagnetic induction apparatus which does not require the installation of a shunt reactor.
Fig. 20 is a circuit diagram showing the single-line connection state of a substation which employs and electromagnetic induction apparatus comprising a transformer as has heretofore been utilized in a power transmission or distribution system, while Fig. 21 is a perspective view showing a practical construction in the case where the circuit in Fig. 20 is three phases.
Referring to the figures, designates a transformer having a primary winding la and a secondary winding lb, and a tertiary winding lc which is electromagnetically coupled to the primary winding la as well as the secondary winding lb. A primary side switching device 2 is connected to the primary winding la, a primary side transmission or distribution line 3 has one end connected to the primary side switching device 2 and the other end connected to an external circuit (not shown), and a capacitance 4 is formed between the primary side transmission or distribution line 3 and ground. A secondary side switching device 5 is connected to the secondary winding lb, a secondary side transmission or distribution line 6 has one end connected to the secondary side switching device 5 and the other end connected to an external circuit (not snown), and a capacitance 7 is formed between the secondary side transmission or distribution line 6 and ground. A tertiary side switching device 8 is connected to the tertiary winding lc, and a shunt reactor 9 has one end connected to the tertiary side switching device 8 and the other end grounded or which is star- or delta-connected in three phases. The flows 10 and 11 of "leading" reactive powers are supplied from the shunt reactor 9 to the respective capacitances 4 and 7 when the switching devices 2, 5 and 8 are closed.
Next, the operation of the substation furnished with the prior-art electromagnetic induction apparatus will be explained.
Usually, the switching devices 2, 5 and 8 are closed, so that the "leading" reactive powers are supplied from the shunt reactor 9 through the transformer 1 to the capacitance 4 of the primary side transmission or' distribution line 3 and the capacitance 7 of the secondary side transmission or distribution line 6 respectively as indicated by the arrows 10 and 11.
In a case where the transformer 1 and tne shunt reactor 9 have become unnecessary for the power system or where an accident has occurred, the switching devices 2, 5 and 8 are opened, and the transformer 1 and the shunt reactor 9 are separated from the power system.
In addition. Fig. 22 is a circuit diagram showing the single-line connection state of another prior-art example, wnile Fig. 23 is a perspective view showing a practical construction in the case where the circuit in Fig. 22 is used in three phases. Symbols 2 - 7 denote the same constituents as those shown in Figs. 20 and 21.
A transformer 20 has a primary winding 20a and a secondary winding 20b, a switching device 12 on the primary side is connected to the primary side transmission or distribution line 3, and a shunt reactor 13 on the primary side has one end connected to the switching device 12 and the other end grounded or which is star- or delta-connected in three phases. A switching device 14 on the secondary side is connected to the secondary side transmission or distribution line 6, and a shunt reactor 15 on the secondary side has one end connected to the switching device 14 and the other end grounded or which is star-or delta-connected in three phases.
The flows 16 and 17 of "leading" reactive powers are respectively supplied from the shunt reactors 13, 15 through the switching devices 12, 14 to tne capacitances 4, 7 wnen the switching devices 12, 14 are closed.
Next, tne operation will be explained. Usually, the switching devices 2, 5, 12 and 14 are closed, so that the "leading" reactive powers are supplied from the shunt reactors 13 and 15 througn the corresponding transmission or distribution lines 3 and 6 to the respective capacitances 4 and 7 as indicated by tne arrows 16 and 17. Besides, in a case wnere the transformer 20 and the snunt reactors 13, 15 have become unnecessary for the operation of the power system or where an accident has occurred, the switching devices 2, 5, 12 and 14 are opened, and the transformer 20 and the shunt reactors 13, 15 are separated from the power system.
In general, in a case where the transmission or distribution lines 3 and 6 are overhead lines of long distances or where they are constructed of cables even when they are of short distances, they have great capacitances 4 and 7, wnich consume "leading" reactive powers. The supply of the reactive powers from a power station (not shown) in a remote place results in inflicting heavy power losses on the power transmission or distribution system and spoiling the stability of this system. Accordingly, the "leading" reactive powers which the transmission or distribution lines 3 and 6 require are supplied from the substation near these lines 3 and 6 by disposing the phase modifying means, namely, the shunt reactor 9 or shunt reactors 13, 15 as shown in Figs. 20 - 23.
Fig. 24 is an equivalent circuit diagram of the transformer 20 having the two windings as shown in Fig. 22, and Fig. 26 is a horizontal partial sectional view of the transformer 20. In these figures, 21 indicates the primary side terminal of the transformer 20, 22 the secondary side terminal of the transformer 20, 23 a magnetic space defined between the primary winding 20a and the secondary winding 20b, 24 a leakage flux generated in the space 23 by the primary winding 20a and the secondary winding 20b, X the leakage reactance of the transformer 20 induced by the leakage flux 24, r the winding resistance of the transformer 20, and Zm the excitation impedance of the transformer 20.
Usually, the following inequalities hold:

Therefore, the winding resistance r can be neglected as it is sufficiently small, and the excitation impedance Zm can be regarded as being infinitely large, so that the equivalent circuit in Fig. 24 is simplified as shown in Fig. 25. Accordingly, the shunt reactors (reactances connected in parallel with the power transmission or distribution system) do not exist equivalently. Thus, the equivalent circuit of the transformer 20 is expressed by only the leakage reactance X as shown in Fig. 25, so that the connection of the respective transmission or distribution lines 3 and 6 to the terminals 21 and 22 corresponds to connecting "a reactor whose reactance is X" in series with the power transmission or distribution system.
Likewise, the equivalent circuit of the transformer 1 with the three windings as shown in Fig. 20 is expressed as shown in Fig. 27, and it involves the following:

X₁₂: leakage reactance caused by a leakage flux flowing through a magnetic space defined between the primary winding la and the secondary winding lb,
X₁₃: leakage reactance caused by a leakage flux flowing through a magnetic space defined between the primary winding la and the tertiary winding lc,
X₂₃: leakage reactance caused by a leakage flux flowing through a magnetic space defined between the secondary winding lb and the tertiary winding lc,
where X₁₂ = X₁ + ^X2 X₁₃ = ^X ₁ ^{+ X} ₃ X₂₃ = ^X ₂ ^{+ X} ₃

In Fig. 27, 25 indicates the primary side terminal of the transformer 1, 26 the secondary side terminal thereof, and 27 the tertiary side terminal thereof. The connection of the primary side transmission or distribution line 3, the secondary side transmission or distribution line 6 and the tertiary side transmission or distribution line to the primary side terminal 25, the secondary side terminal 26 and the tertiary side terminal 27 respectively corresponds to connecting "reactors whose reactsances have magnitudes X₁, X₂ and X₃ respectively" in series with the power transmission or distribution system. Accordingly, as in Fig. 25, no reactance connected in parallel with the power transmission or distribution system exists with the transformer 1 only, and the snunt reactor 9 needs to be separately disposed.
As stated above, the prior-art electromagnetic induction apparatus in the substation is equipped with the shunt reactor 9 or shunt reactors 13, 15 as the phase modifying means for compensating the "leading" reactive powers which are consumed by the capacitances 4, 7 between the respective transmission or distribution lines 3, 6 and ground. Therefore, it has nad several problems to be explained below:

(i) A large installation space for the shunt reactor(s) 9 or 13, 15 is required as shown in Fig. 21 or Fig. 23. Moreover, expenses necessary for subsidiary installations such as foundamental fire-prevention devices for the shunt reactor(s) 9 or 13, 15 become enormous.
(ii) In a case where the shunt reactor(s) 9 or 13, 15 operate(s) as the phase modifying means, current flow through tne shunt reactor(s) 9 or 13, 15. Power loss to be incurred in the winding(s), electromagnetic shield(s) etc. of the shunt reactor(s) 9 or 13, 15 by the current cannot be neglected, either.
(iii) When the shunt reactor 9 is connected to the tertiary winding lc as shown in Fig. 20, not only the shunt reactor 9 but also the tertiary winding lc undergoes power loss. On the other hand, when the shunt reactors 13, 15 are directly connected to the transmission or distribution lines 3, 6 of high voltages as shown in Fig. 22, the shunt reactors 13, 15 must also have high-voltage specifications, so they become large in size and high in cost which entails increases in installation space as well as power loss.

SUMMARY OF THE INVENTION

This invention has been made in crder to solve all the problems as mentioned above, and has for its object the provision of an electromagnetic induction apparatus which is so constructed as to have the function of supplying "leading" reactive powers without the installation of a shunt reactor, thereby to reduce the space and to lower the power loss.
Another ooject of this invention is to make the capacity of an equivalent shunt reactor which functions to supply "leading" reactive powers variable.
An electromagnetic induction apparatus according to this invention comprises a short-circuit winding which is electromagnetically coupled to at least two windings.
An electromagnetic induction apparatus in another aspect of performance of this invention comprises a reactor winding which is electromagnetically coupled to at least two windings including a tap winding and which is inserted between one end of the tap winding and a tap.
In this invention, two windings operate as an ordinary transformer, while a leakage flux appears in the magnetic space between the two windings and a short-circuit winding, and it equivalently functions as a shunt reactor, to supply "leading" reactive powers to transmission or distribution lines and capacitances.
In another aspect of performance of this invention, a tap voltage whicn is lower than tne open-circuit voltage of a reactor winding is forcibly applied from a tap to the reactor winding, to generate a desired magnitude of leakage flux between the reactor winding and a tap winding.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a circuit diagram showing an embod_lment- of this invention,
Fig. 2 is a perspective view showing an embodiment in which a transformer in Fig. 1 is constructed of a shell type transformer,
Fig. 3 is a horizontal partial sectional view corresponding to Fig. 2,
Fig. 4 is a circuit diagram showing in a single-line connection state a case where the transformer in Fig.l is used as the electromagnetic induction apparatus of a substation,
Fig. 5 is a perspective view showing a case where the circuit in Fig. 4 is used in three phases,
Fig. 6 is an equivalent circuit diagram corresponding to Fig. 1,
Fig. 7 is a perspective view showing an embodiment in which the transformer in Fig. 1 is constructed of a core type transformer,
Fig. 8 is a horizontal partial sectional view showing an embodiment in which the magnetic space of the transformer in Fig. 3 is formed of a gapped core,
Fig. 9 is a circuit diagram showing in a single-line connection state an example where in an embodiment in which a switching device is added to a short-circuit winding in Fig. 1 is used as the electromagnetic induction apparatus of a substation,
Fig. 10 is a perspective view showing a case where the circuit in Fig. 9 is used in three phases,
Figs. 11 and 12 are horizontal partial sectional views respectively showing the states in which the switching device of the transformer in Fig. 9 is opened and closed,
Fig. 13 is a circuit diagram showing another aspect of performance of this invention,
Pic. 14 is a perspective view showing an embodiment in which a transformer in Fig. 13 is constructed of a shell type transformer,
Figs. 15 and 16 are horizontal partial sectional, views respectively showing the states in which the tertiary winding of the transformer in Fig. 14 is opened and is connected to a tap,
Fig. 17 is a circuit diagram showing an embodiment in which a changer is added to the taps of the transformer in Fig. 13,
Fig. 18 is a horizontal partial sectional view showing an embodiment in which the magnetic space of a transformer in Fig. 15 is formed of a gapped core,
Fig. 19 is a horizontal partial sectional view showing and embodiment in which the transformer in Fig. 13 is constructed of a core type transformer,
Fig. 20 is a circuit diagram showing the single-line connection state of a substation in a prior art,
_Fig. 21 is-a perspective view showing a case where the circuit in Fig. 20 is used in three phases,
Fig. 22 is a circuit diagram showing another substation in a prior art under a single-line connection state,
Fig. 23 is a perspective view showing a case where the circuit in Fig. 22 is used in three phases,
Fig. 24 is an equivalent circuit diagram corresponding to Fig. 22,
Fig. 25 is an equivalent circuit diagram corresponding to Fig. 24,
Fig. 26 is a horizontal partial sectional view of a transformer in Fig. 22, and
Fig. 27 is an equivalent circuit diagram of a transformer in Fig. 20.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, an embodiment of this invention will be described with reference to the drawings. Fig. 1 is a circuit diagram in a single-line connection state showing the embodiment of this invention as a transformer which has, for example, three windings, Fig. 2 is a perspective view showing a practical construction corresponding to Fig. 1, Fig. 3 is a horizontal partial sectional view corresponding to Fig. 2, Fig. 4 is a circuit diagram showing in a single-line connection state a case where the transformer in Fig. 1 is applied to a substation, Fig. 5 is a perspective view showing a case where tne circuit in Fig. 4 is used in three phases, Fig. 6 is an equivalent circuit diagram corresponding to Fig. 1, Fig. 7 is a perspective view showing an embodiment in which the transformer is constructed of a core type transformer, Fig. 8 is a horizontal partial sectional view showing an embodiment in which a magnetic space is formed of a gapped core, Fig. 9 is a circuit diagram showing a single-line connection state in the case wnere an embodiment having a switching device added to a short-circuit winding is applied to a substation, Fig. 10 is a perspective view showing a case where the circuit in Fig. 9 is used in three phases, and Figs. 11 and 12 are horizontal partial sectional views respectively showing the states in which the switching device in Fig. 9 is opened and closed. In Figs. 1-12, la-lc, 2-7, 10 and 11 indicate the same portions as in the prior art examples stated before, and 1A indicates a transformer corresponding to the transformer 1.
The core of the transformer 1A is denoted by ld and S is a short-circuit line which short-circuits both the ends of the tertiary winding lc and due to which the tertiary winding lc becomes a short-circuit winding.
A leakage flux 18 is induced between the tertiary winding lc and the primary winding la or secondary winding lb by the short-circuit current of the tertiary winding lc, and tnougn not shown, such a leakage flux is also generated between the primary winding la and the secondary winding lb. The leakage flux 18 passes through a magnetic space 19 which is illustrated only between the secondary winding lb and the tertiary winding lc nere.
A switching device 29 added to the short-circuit line S. In actuality, the switching device may well be connected between one end of the tertiary winding lc and ground by leading the terminal of the tertiary winding lc out of the transformer 1A as shown in Fig. 9, or between the lines of the tertiary winding connected in three phases. A power source V₀ such as a power station (not shown) or the like is connected to the primary winding la, and a magnetic flux 18A is generated in the core ld by the power source V₀ when the switching device 29 has been opened. Thus, when the switching device 29 shown in Figs. 9 to 12 is opened as depicted in Fig. 11, the tertiary winding lc falls into an unshorted state, and a snunt reactor function vanishes. When the switcning device 29 is closed as depicted in Fig. 12, the tertiary winding lc serves as the short-circuit winding and gives rise to the shunt reactor function, quite similarly to the state in which the short-circuit line S is provided as illustrated in Figs. 1 and 2. That is, the supply of the "leading" reactive powers 10, 11 can be on-off-controlled as is necessary by means of the switching device 29.
Now, the operation of the embodiment of this invention will be described. First, with the switching device 29 opened as shown in Fig. 11, the power source V₀ of the power station (not shown) is connected to the primary winding la to excite the transformer lA. Then, the magnetic flux 18A is generated in the core ld and interlinks with the primary winding la, secondary winding 1b and tertiary winding lc in common. Under the electromagnetic induction action of the magnetic flux 18A, voltages proportional, to the numbers of turns of the respective windings are generated across the primary winding la, secondary winding lb and tertiary winding lc. That is, in the state of Fig. 11, the output terminals of the secondary winding lb are connected to the external circuit (not shown), thereby to operate as the output terminals of the ordinary transformer. Besides, since the tertiary winding lc is in the open state, it is merely generating the voltage. Accordingly, no current flows through the tertiary winding lc, and the tertiary winding lc is not supplying electric power externally.
On the other hand, in the state in which both the ends of the tertiary winding lc are short-circuited by the short-circuit line S or by the closure of the switching device 29 as shown in Figs. 1 and 2 or in Fig. 12, a short-circuit current flows through the loop of the tertiary winding lc. Since the voltage across the tertiary winding 1c is forcibly rendered zero by the short-circuit current, tne magnetic flux 18A in Fig. 11 flows through the magnetic space 19 between the tertiary winding lc and the secondary winding 1b as well as the primary winding la. That is, the leakage flux 18 appears in the magnetic space 19 as shown in Fig. 3 or Fig. 12.
At this time, magnetic energy Q is generated in the magnetic space 19, and the value thereof is expressed by"
where f denotes the frequency of the power source V₀, _B the flux density of the magnetic space 19 and V the volume of the magnetic space 19. Simultaneously, a short-circuit current 1₃ of a magnitude establishing a magnetic field of the flux density B in the magnetic space 19 flows through the tertiary winding lc. In addition, currents I₁ and I₂ satisfying the following flow through the primary winding la and the secondary winding 2b in accordance with a transformer operation:
where N₁: number of returns of the primary winding la,

N₂: number of turns of the secondary winding lb,
N₃: number of turns of the tertiary winding lc,

The leakage flux 18 functions equally to the shunt reactor(s) 9 or 13, 15 in tne prior-art examples shown in Figs. 20 to 23 and supplies the "leading" reactive powers 10, 11 to the capacitances 4, 7 of the primary side and secondary side transmission or distribution lines 3, 6 as shown in Figs. 4, 5, 9 and 10.
Now, there will be explained the equivalence between the leakage flux 18 shown in Figs. 3 and 12 the shunt reactor(s) 9 or 13, 15 in the prior art.
As elucidated in conjunction with Figs. 24 to 27, the leakage flux 24 appears also in the two-winding transformer 20 or three-winding transformer 1 of the prior-art construction. As illustrated in the equivalent circuit of Fig. 25 or 27, however, the leakage reactance functions as the series reactance which is connected in series with the circuit.
Meanwhile, tne equivalent circuit of the transformer 1A with tne tertiary winding lc short-circuited is expressed as in Fig. 6. In the figure, 25-27 and X₁-X₃ are the same as in Fig. 27. By short-circuiting the tertiary winding lc to ground, the reactance X₃ on the tertiary side functions as a parallel reactance which is connected in parallel with the circuit. That is, connecting the primary side transmission or distribution line 3 and the secondary side transmission or distribution line 6 to the primary side terminal 25 and tne secondary side terminal 26 respectively corresponds to connecting "a shunt reactor whose reactance has the magnitude 1₃" in parallel with the power transmission or distribution system. This signifies nothing but the fact that the magnetic space 19 shown in Figs. 3 and 12 functions physically as the magnetic space of the shunt reactor in the prior art.
Moreover, in the case of the two-winding transformer, when it is further provided with the tertiary winding lc which is electromagnetically coupled to the primary winding la and the secondary winding lb, it becomes identical to the construction of the three-winding transformer 1A and can also be operated similarly to the shunt reactor.
In this manner, the three-winding transformer 1A with both the ends of the tertiary winding lc short-circuited functions also as the shunt reactor whose reactance has the magnitude X₃, without spoiling the original voltage transformation function of the transformer and can supply the "leading" reactive powers to the capacitances 4, 7 of the transmission or distribution lines 3, 6. Moreover, the voltage of the tertiary winding lc can be selected at will irrespective of the voltages of the primary transmission cr distribution line 3 and the secondary transmission or distribution line 6. Therefore, when the voltage across the tertiary winding lc is rendered sufficiently low, the transformer 1A need not be especially enlarged.
Although the above embodiment has been explained as to the case wnen the transformer 1A is constructed as a shell type, the transformer may also be constructed with a core type one as shown in Fig. 7.
Also, while tne air-core structure has been adopted as the magnetic space 19 through which the leakage flux 18 passes, a gapped-core structure may well be adopted by interposing a gapped core 28 as shown in Fig. 8.
Further, while the transformer 1A of the three windings has been explained as an example, it is needless to say that, even when the invention is applied to a transformer of four or more windings not shown, an electromagnetic induction apparatus having the same functional effect as stated above can be realized by short-circuiting the tertiary winding.
Next, another aspect of performance of this invention will be described. Fig. 13 is a circuit diagram showing the embodiment of another aspect of performance of this invention, Fig. 14 is a perspective view showing a practical construction corresponding to Fig. 13, and Figs. 15 and 16 are horizontal partial sectional views respectively showing the states in which a tertiary winding in Fig. 14 is opened and is connected to a tap. In the figures, ia-ld, 18 and 19 denote portions similar to those described before, and 1B denotes a transformer corresponding to the transformer lA.
The secondary winding lb is provided with a plurality of taps 30, and owing to wnich the secondary winding lt becomes a tap winding. In addition, the tertiary winding lc has one end connected tc one end of the secondary winding l_b at a node P and has the other end connected to one of the plurality of taps 30, thereby to become a reactor winding.
Next, the operation of the embodiment of another aspect of performance of this invention will now be described. First, it is assumed that the primary side transmission or distribution line, namely, the power source V₀ of a power station or the like be connected to the primary winding la in the state in which the tertiary winding lc as the reactor winding is open as depicted in _Fig. 15. Then, the magnetic flux 18A of a magnitude φ₀ is generated in the core ld and interlinks with the primary winding la, secondary winding lb and tertiary winding lc in common. This state is the same as in the case of Fig. 11.
At this time, a voltage equal to the supply voltage V₀ is generated across the primary winding la, and voltages according to the numbers of turns N_l-N₃ of the respective windings la-lc are generated across the secondary winding lb and tertiary winding lc. The voltages generated across the windings la-lc, denoted by V₁₀-V₃₀ respectively are expressed as follows:

wnere K: constant. Accordingly, the secondary winding lb can be used for the ordinary transformer when connected to the secondary side transmission or distribution line, namely, the external circuit. Besides, since the tertiary winding lc is in the open state, it is merely generating the voltage V₃₀ and does not execute the reactor function at all.
Subsequently, as depicted in Fig. 16, one end of the tertiary winding lc is connected to one end of the secondary winding lb through the node P, and the other end of the tertiary winding lc is connected to one of the taps 30, whereby a tap voltage V₃ lower than the open-circuit voltage V₃₀ as evaluated with Eqs. (3) and (5) is forcibly applied to the tertiary winding lc. After the application of the tap voltage V₃, the magnitude φ of the magnetic flux 18B interlinking with the tertiary winding lc becomes a value satisfying the equation:
in which V₃ and φ are respectively substituted for V₃₀ and ↕₀ in Eq. (5). Accordingly,
holds.
On the other hand, the magnitude to of the magnetic flux 18A in the open-circuit condition becomes:
in accordance with Eq. (5). From Eqs. (6) and (7), the value ↕ of the magnetic flux 18B upon being connected to the tap is expressed relative to the value φ₀ of the magnetic flux 18A in the open-circuit condition as follows:
In addition, since the relation of the voltages V₃ and V₃₀ is:
the relation of the flux values and φ₀ becomes:
in accordance with Eq. (8). Accordingly, the magnitude Δφ of a leakage flux 18C to flow through the magnetic space 19 is expressed by:
In this manner, upon being connected to the tap illustrated in Fig. 16, the leakage flux 18C of the magnitude Δφ flows through the magnetic space 19, whereby predetermined magnetic energy is generated to effect the shunt reactor function.
Here, it is understood that, by altering the tap 30 of the secondary winding lb to which the other end of the tertiary winding 1c is connected, to change the tap voltage V₃ in Eq. (8), the value : of the magnetic flux 18B interlinking with the tertiary winding lc varies in proportion to the tap voltage V₃. At this time, the value Δφ of the leakage flux 18C flowing through the magnetic space 19 changes simultaneously, so that the magnitude of the magnetic energy of the magnetic space 19 changes to change the capacity cf the shunt reactor. In particular, the state in which the tap voltage V₃ is rendered zero is the same as tne case illustrated in Fig. 12. The value φ₀ of the magnetic flux 18A in the open-circuit condition flows entirely to the magnetic space 19 upon being connected to the tap and the capacity of the shunt reactor becomes the maximum.
while the above embodiment has referred to the three-winding transformer by way of example, it is needless to say that, quite similarly to the case of tne three-winding transformer, an electromagnetic induction apparatus comprising a shunt reactor of variable capacity can be realized even with a transformer of four or more windings by using one of the windings as a reactor winding and another as a tap winding.
Besides, while the other end of the tertiary winding lc has been connected to a proper one of the taps 30, a changer 31 for changing the taps 30 in an on-load condition may well be disposed between the other end of the tertiary winding lc and the taps 30 as shown in a circuit diagram of Fig. 17. In this case, the change of the taps 30, in other words, tne alteration cf the capacity of the shunt reactor can be performed in the energized state. In this way, when the respective tap voltages V₃ at the plurality of taps 30 are properly changed-over and applied to the tertiary winding lc, the capacities of the equivalent shunt reactor can be turned on and off stepwise, and hence, an instantaneous voltage fluctuation is not incurred in the power transmission or distribution system.
Besides, while the magnetic space 19 through which the leakage flux 18C passes has been illustrated as the air-core structure as depicted in Fig. 16, it may well be a gapped-core structure with a gap core 28 interposed therein as depicted in Fig. 18.
Further, while the transformer 1B as the electromagnetic induction apparatus has been illustrated as the shell type transformer, it may well be a core type one as shown in Fig. 19.
As described above, according to this invention, a short-circuit winding which is electromagnetically coupled to at least two windings is disposed. This produces the effect that a transformer having also a shunt reactor function for supplying "leading" reactive powers required by transmission or distribution lines can be realized, and that an electromagnetic induction apparatus which dispenses with facilities for a shunt reactor and which can reduce the construction cost of a substation and power loss is provided.
In another aspect of performance of this invention, a reactor winding whicn is electromagnetically coupled to at least two windings, one being a tap winding, and whicn is connected between one end of the tap winding and a tap is disposed, and the difference flux of respective magnetic fluxes appearing when tne reactor winding is opened and is connected to the tap is generated in a magnetic space. This produces the effect that an electromagnetic induction apparatus in which the capacity of an equivalent shunt reactor is variable is provided.

Claims

1. An electromagnetic induction apparatus comprising a transformer (lA) having at least two windings (la, lb) connected to external circuits. characterised in that a short-circuit winding (lc) is electromagnetically coupled to said windings.

2. An electromagnetic induction apparatus as defined in Claim 1. wherein said transformer is a three-winding fransformer.

3. An electromagnetic induction apparatus as defined in Claim 1, or Claim 2, wherein a magnetic space (19) intervening between said short-circuit winding (lc) and each of said at least two windings (la, lb) forms an air-core structure.

4. An electromagnetic induction apparatus as defined in Claim 1 or Claim 2. wherein a magnetic space (19) intervening between said short-circuit winding (lc) and each of said at least two windings (la, lb) forms a gapped-core structure (28).

5. An electromagnetic induction apparatus as defined in any one of Claims, 1 to 4 wherein a switching device (29) is connected to one end of said short-circuit winding (lc).

6. An electromagnetic induction apparatus comprising a transformer (lA) having at least two windings (la, lb) connected to external circuits, characterised in that a reactor winding (lc) is electromagnetically coupled to said windings, one of said at least two windings (la, lb) is provided with a tap (30) to form this winding into a tap winding, (lb). said reactor winding (lc) has one end thereof connected to one end of said tap winding (lb) and has the other end thereof connected to said tap (30).

7. An electromagnetic induction apparatus as defined in Claim 6. wherein said transformer (lA) is a three-winding transformer.

8. An electromagnetic induction apparatus as defined in Claim 6 or Claim 7, wherein a magnetic space (19) intervening between said reactor winding (lc) and each of said at least two windings (la, lb) forms an air-core structure.

9. An electromagnetic induction apparatus as defined in Claim 6 or Claim 7, wherein a magnetic space (19) intervening between said reactor winding (lc) and each of said at least two windings (la. lb) forms a gapped-core stucture (28).

10. An electromagnetic induction apparatus as defined in any one of Claims 6 to 9, wherein said tap winding (1b) is provided with a plurality of taps (30).

11. An electromagnetic induction apparatus as defined in Claim 10, wherein a changer (31) for changing said taps (30) in an on-load condition is connected between the other end of said reactor winding (lc) and said taps.