FR2695769A1 - Transformer on=load tap-changer using solid state switching - has primary supplied through two thyristor switches, which are bridge-connected with selected diode pair for required tap - Google Patents

Abstract

The circuit includes a secondary transformer (TR), which when on load (Ch), a primary voltage (AL) is applied between one winding end and that tapping (P1, P2,,,,Pp) whose selector switch (C1, C2...Cp) is closed. In the bridge unit (MC), the thyristor arms (T1, T2), conducting alternate half-cycles, are always in the circuit, the bridge being completed by the diode pair (21, 22,..2p) for the time being connected to the selected tap. Thyristor and selector switching (CT1, CT2;CO) is performed by an overall controller, which operates the selectors when both thyristors are non-conducting. Polyphase transformers have each phase thus equipped. Optionally, thyristors replace the diodes in one or more pairs, eliminating associated selectors. Alternatively, each selector controls several taps, with further switching within groups giving coarse and fine regulation. ADVANTAGE - High-speed, low-cost tap-changer with low maintenance costs.

Description

STATIOUS CIRCUIT INVERTER VOLTAGE IN LOAD
The present invention relates to a static alternating voltage circuit on load.

 The AC voltage variator circuits currently used on an industrial scale make it possible to satisfactorily adapt the supply voltage to operating conditions, or conditions of use, which vary in particular with the load consuming the electrical energy thus distributed.

 The circuits currently implemented for carrying out such high-voltage regulation essentially use a transformer with adjustable voltage under load, the adjustment of the delivered voltage being effected by displacement of a movable contact on the multiple contact sockets of the winding. primary of an adjustment transformer. The high-voltage regulated powers, 10 to 100 kV, can reach, using the techniques of the aforementioned circuits, from a few hundred to several thousand kVA or more.

 These systems are satisfactory in their fields of application, such as electric heating, industrial electric ovens or the like, but nevertheless have the drawback of presenting a problem of mechanical wear of the movable contacts.

 In addition, current circuits, by reason of their implementation of an essentially mechanical solution, do not allow, except in exceptional cases, to carry out fine adjustment of the regulated voltage or power levels, provided that they accept expensive equipment both in terms of investment and maintenance.

 Various attempts have been made to reduce the number or eliminate the solicitation of mobile contacts.

 We can, for example, cite the switch described by patent application 2 345 848 in which a switching command is carried out by means of thyristors mounted head to tail, these thyristors being however solicited only during the change of contact by the primary winding of the adjustment transformer.

 Mention may also be made of German patent 2,120,679, which describes a device for assisting the switching of a movable contact-changer on the primary winding of an adjustment transformer. In the device described by the aforementioned patent, thyristor circuits, also mounted head to tail, make it possible to reduce the duration of the short circuit in the connection resistors of the thyristor circuits to the movable contacts.

 More recently, in European patent application 0 124 904, a mechanical switch has been described in which, in addition, an electronic thyristor circuit, comprising two thyristors mounted upside down, makes it possible to move the contact cursor under load. movable in the absence of an electric arc and a short circuit between the studs. In such a type of circuit, the thyristors must necessarily be controlled in perfect synchronism with the mechanical position and the slow movement of the movable cursor.

Finally, we may cite, for the record, the patent published in
USSR under number 675 571 in which an electronic switching circuit is described. However, this circuit is very complex and in particular requires an auxiliary circuit for rectification bias by a DC voltage.

 The object of the present invention is to remedy the drawbacks or limitations of the aforementioned devices of the prior art.

 Another object of the present invention is the implementation of a static AC voltage variator circuit in which the movable contacts by cursor can be at least simplified.

 Another object of the present invention is also the implementation of a static alternating voltage variator circuit in which, although mechanical contactors are used, controlled rectifiers make it possible to ensure not only a switching aid but also a fast fine adjustment of the alternating voltage and of the power finally delivered by the circuit.

 Another object of the present invention is finally the implementation of a static alternating voltage variator circuit in which, in the absence of specific movable slider contactors of conventional type of the primary windings of the adjustment transformers of the prior art , commercially available contactors are used.

 The alternating voltage static circuit object of the present invention comprises an adjustment transformer with multiple tap primary winding. It is remarkable in that it further comprises a mixed bridge circuit comprising a first branch with mid-point formed by two thyristors in cascade, the mid-point receiving an input supply voltage, and a plurality of second branches with point middle. Each second branch is formed by two diodes in cascade and mounted in antiparallel on the first branch and the midpoint of each second branch is connected via a contactor to one of the sockets of the primary winding of the adjustment transformer.

 The alternating voltage static circuit object of the present invention finds application to the electrical supply of glass or other furnaces, or tubes with direct passage of electric current and, more generally, to the supply of industrial users or users high intensity currents requiring the use of a transformer.

It will be better understood on reading the description and observing the drawings below in which - Figure 1 shows a block diagram of the circuit
static alternating voltage drive object of the
present invention, - Figure 2 shows an alternative embodiment in
the case of a circuit object of the invention for a voltage
polyphase alternative, - Figure 3 shows another alternative embodiment
in which a contactor is further associated with a
selector, - Figure 4 shows another alternative embodiment
in which each contactor is associated with a selec
tor, - Figure 5a shows another alternative embodiment
in which one of the primary school’s multiple takes
adjustment transformer is permanently connected to
a second branch, the corresponding contactor being
replaced by a permanent link, - Figure 5b shows an alternative embodiment of the
Figure 5a in which a second branch is equal
ment formed by two thyristors T'1, T'2 mounted in
cascade.

- Figure 6 represents chronograms of forms
remarkable waves of voltages or currents in
remarkable points of alternating voltage according to the invention
tion.

 A more detailed description of the alternating voltage static circuit, object of the present invention, will now be given in conjunction with FIG. 1.

 According to the above-mentioned figure, the alternating voltage static circuit object of the present invention comprises an adjustment transformer TR with primary winding with multiple tap. In the above-mentioned figure, the multiple taps of the primary winding are denoted P1 to Pp and the secondary winding is shown closed on a load Ch to which the corresponding electrical energy is delivered.

 The adjustment transformer TR is a conventional type adjustment transformer, and as such, it will not be described in detail.

 In accordance with a particularly advantageous aspect of the alternating voltage static circuit object of the present invention, the alternating voltage static circuit shown in FIG. 1 further comprises a mixed bridge circuit, denoted MC, playing the role of switching module, and comprising a first branch, denoted 1, at mid-point, formed by two thyristors, T1, T2 connected in cascade. The midpoint of the first branch receives an input supply voltage, denoted AL in FIG. 1.

This supply voltage is for example that delivered by a medium voltage supply line, when the static circuit object of the present invention is intended to supply, for example, installations for glass furnaces or the like.

 In addition, as shown in FIG. 1, the mixed bridge circuit comprises a plurality of second branches, noted 21 to 2p, each second branch having a midpoint, and being formed by two diodes, D1, D2 Cascade. Each second branch is mounted in anti-parallel on the second branch 1, and the midpoint of each branch is connected via a contactor, denoted C1 to Cp, to one of the sockets of the primary winding of the transformer. TR setting.

The general operation of the alternating voltage static static circuit object of the present invention as shown in Figure 1 is as follows the first branch, formed by the thyristors T1 and T2, operates from control signals CT1, CT2, which will be explained later in the description, as a voltage dimmer. The control signals, CT1, CT2, make it possible to vary the conduction angle of the aforementioned thyristors T1 and T2. The bridge circuit then makes it possible to ensure conduction at each alternation of the supply voltage AL via the diodes D1, D2, respectively. The conduction angle of each thyristor T1, T2 is slightly less than 180 degrees, in the absence of overlap of their respective conduction angle. The time interval, or angle at which the two thyristors T1 and T2 are blocked, is used to switch contactors C1 to Cp, which thus makes it possible to vary the transformation ratio of the adjustment transformer TR on command by a switching signal, denoted CO. In addition, it can be seen that during the switching, that is to say the passage from one pad to the other, the contactors C1 and C2 are temporarily and simultaneously closed without however causing a short circuit between the sockets P1 and
P2 corresponding thanks to the diodes and the appropriate thyristor control. See fig. 6.

 As shown in addition in FIG. 2, it will be understood that the alternating voltage static circuit object of the present invention is not limited to a single-phase embodiment. On the contrary, the adjustment transformer TR can be constituted by a polyphase transformer with n phase, and in FIG. 2, a particular non-limiting case has been shown where the adjustment transformer TR is a three-phase transformer. As such, and by way of example only, the secondary of the adjustment transformer can be mounted in a star, the load being subdivided into a three-phase load, denoted Ch1 to Ch3 also mounted in a star. The primary windings are shown in the figure 2, and can be mounted in star with neutral output, denoted N, the device which is the subject of the present invention then comprising three modules, MC, or mixed-bridge circuits substantially identical to that shown in FIG. 1. Each module MC is then supplied by one of the phases, noted Ph1 to Ph3, of the polyphase or three-phase alternating network. The contactors are then noted with reference to the order of the corresponding phases C11 to Clip, C21 to C2p, respectively C31 to C3p, only one or two phases which can be fitted with the circuit according to the invention.

According to an alternative embodiment as shown in FIG. 3, it may be advantageous to interpose between the multiple taps of the primary winding of the adjustment transformer TR and the midpoint of each second branch, more particularly between a given contactor, noted C by way of simplification in FIG. 3, and the aforementioned primary winding, a selector, denoted S, making it possible to play the role or to complete the corresponding contactor C. It is of course understood that the use of a selector S makes it possible to multiply for a given contactor the number of taps on the primary winding of the adjustment transformer TR, this in order to define the value of the transformation ratio of the latter, and finally the level of energy transmitted to the load. We understand of course that in such a case, the operation of the selector S can then be carried out by means of a selector operation motor MS, which makes it possible to modify the position of the selector S of a socket, Pl to P5, as shown in FIG. 3, by means of a switching control signal COS of the selector, this switching being of course carried out while the contactor C is open by means of the signal CO contactor switch, when contactor C has not been removed. Of course, it is also understood that the contactor C can be deleted, the selector S then being able to play the role of both the selector and the contactor
C, and the switching of the selector possibly being carried out during the extinction angle of the thyristors T1 and T2 previously mentioned.

In Figure 4, there is shown a particularly advantageous variant of an embodiment of the alternating voltage static circuit object of the present invention in which each contactor C1 to Cp being maintained, the selector S allows, in conjunction with each contactor , a selector being associated with each contactor and bearing a corresponding reference, S1 for the contactor C1, respectively S2 for the contactor C2, these selectors in fact making it possible to carry out a voltage adjustment delivered by the adjustment transformer TR according to a coarse adjustment through contactors only
C1, C2, fine adjustment via the selectors S or Sz, and very fine adjustment via the contactors C1, C2, and selectors S1, S2.

 According to a particularly advantageous embodiment as shown in FIG. 5a, one of the midpoints of a second branch, the branch 21 in the above-mentioned figure, is in direct electrical connection with one of the multiple taps of the winding of the adjustment transformer TR. It is understood in this case that the contactor C1 has therefore been deleted and replaced by a permanent electrical connection.

 Preferably, the midpoint considered is' in connection with a determined socket, the socket denoted Pl in FIG. 5a, this socket corresponding to a maximum or, if appropriate, minimum power delivered by the adjustment transformer TR.

 In FIG. 5a, we also note the presence of a control unit, noted 3, which makes it possible to generate the various control signals previously mentioned, CTl and CT2 for the control of thyristors T1 and T2, the switching command. CO for switches C1, C2 or C3, this switching control signal being able to consist of a simple voltage step making it possible to control the coil of a control relay of the various aforementioned contactors and, finally, the switching control signal of the selector, COS, via the MS motor. It should be noted that the MS motor can be constituted by a stepping motor, which can then be conventionally controlled digitally with transformation into an analog signal making it possible to control precisely displacement of the motor MS in steps, allowing the passage from one contactor to the next, for example. This type of control will not be described in detail, because it is perfectly known to those skilled in the art.

 Note in addition in FIG. 5a the presence of a capacity resistance circuit, RC, which can be permanently connected for example on the contactor C2 in parallel thereon, so as to reduce any switching transients.

Finally, in the embodiment of FIG. 5b, there is shown an advantageous alternative embodiment in which at least one of the second branches, branch 21 for example, is replaced by a pseudo-first branch with mid point formed by two thyristors in cascade
T'1 and T'2. The second branch is of course mounted in anti-parallel with the first branch to perform both the second branch function of the mixed bridge circuit MC during the conduction of the thyristors T'1, T'2 of the pseudo-first branch and the switching function of the contactor, the contactor C1, which in this case can be eliminated, as shown in FIG. 5b, when the thyristors T'1 and T'2 are extinguished. The above-mentioned switching then corresponds to a quasi-discrete or power wave train switching between two power levels corresponding to the sockets P1 and P2.

The operation of all of the embodiments or variants of the alternating voltage static static circuit object of the present invention represented in FIGS. 1 to 5b can be illustrated by means of the timing diagrams represented in FIG. 6, in which the voltage has been successively represented. power supply or the voltage of each phase Ph1, Ph2, Ph3 as a function of time, A designating the amplitude, then the control signals of the thyristors T1 respectively T2 as a function of time, or, where appropriate, of the thyristors T ' 1, T'2, the time intervals
At represented corresponding to the time intervals during which the thyristors T1 and T2 respectively T'1, T'2 are non-conductive, then finally, the current in the load, IC, or in each three-phase load, IC1, IC2, IC3, the conduction angles are respectively Oc and the extinction angles being denoted 6th and corresponding to the time intervals AT previously mentioned.

 It will of course be understood that the control module 3 in fact makes it possible to vary the effective conduction angle Oc, this conduction angle possibly being arbitrary in transient conditions, and in particular the extinction angle 6e that can be increased during the periods for switching switches or selectors, the extinction angle 6e being on the contrary very small, a few degrees, in steady state, but never zero. It is thus understood that the aforementioned very fine adjustment can be carried out via the thyristors, which operate as a dimmer.

 We have therefore described a particularly efficient alternating voltage alternating voltage circuit under load, which can be used in high power electrical power supply circuits in applications in which the permanent disturbances associated with power electronics are not acceptable, the generations of harmonics and parasitic oscillations being suppressed, the risks of short-circuit and overvoltage are negligible, and in which the regulation of the power transients must be rapid and cover a wide dynamic range, only short-term disturbances being able to be tolerated.

 In addition, it is understood that a permanent fine regulation and of small amplitude of the transformation ratio of the adjustment transformer can be obtained, and correlatively, of the level of electrical energy thus transmitted to the load.

 In addition, slow regulation of large amplitude and without disturbance can also be obtained due to the flexibility of the circuit and in particular of the switching module MC previously defined.

Claims (6)

 1) Static circuit for alternating voltage regulator under load, comprising an adjustment transformer (TR) with primary winding with multiple taps, characterized in that it also comprises a circuit (MC) in mixed bridge comprising - a first branch (1 ) at mid point formed by two  thyristors (Tl, T2) in cascade, the midpoint receiving  an input supply voltage, - a plurality of second branches (21 to 2p) at point  middle, each second branch being formed by two  diodes (D1, D2) in cascade and mounted in anti-parallel on  said first branch, the midpoint of each  second branch being connected via a  contactor (C1 to Cp) at one of the winding sockets  primary of said adjustment transformer.  2) Circuit according to claim 1, characterized in that said adjustment transformer (TR) is a single-phase or polyphase transformer with n phases, said circuit comprising a mixed bridge circuit per phase considered.  3) Circuit according to one of claims 1 or 2, characterized in that it comprises, interposed between said sockets and said midpoints, a selector allowing to play the role or to supplement the contactors.  4) Circuit according to claim 3, characterized in that each contactor (Cl to Cp) being maintained, said selector makes it possible, in connection with each contactor, to carry out an adjustment of voltage delivered by said adjustment transformer (TR) according to a coarse adjustment, via the contactors only, fine, via the selector, and, very fine, via the contactors and the selector, the very fine setting being adjusted via the thyristors.  5) Circuit according to one of claims 3 or 4, characterized in that one of the midpoints is in direct electrical connection with one of the multiple taps of the winding of said adjustment transformer (TR), said corresponding tap at a maximum or minimum power delivered by the regulating transformer (TR).  6) Circuit according to one of claims 1 to 5, characterized in that at least one of said second branches is replaced by a pseudo-first branch at mid point formed by two thyristors in cascade, said pseudo first branch being mounted in anti-parallel with said first branch to carry out both the function of the second branch of the circuit in mixed bridge during the conduction of the thyristors of said pseudo-first branch and the switching function of the contactor associated with said second branch during extinction of said thyristors, this switching being analyzed as a quasi-discrete power switching between two distinct power levels.