ZA200610206B

ZA200610206B - Power converter

Info

Publication number: ZA200610206B
Application number: ZA200610206A
Authority: ZA
Inventors: Coyaud Martin
Original assignee: Faiveley Transport
Priority date: 2004-05-06
Filing date: 2006-12-06
Publication date: 2008-04-30
Also published as: EP1751861B1; PL1751861T3; FR2870057B1; EP1751861A1; ES2601959T3; FR2870057A1; RU2006143062A; WO2005122399A1; WO2005122399A8

Description

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POWER CONVERTER

The area of application of the invention is that of electrical energy conversion.

More precisely, the invention relates to switched-type electric converters using controlled switching resources.

It is used in particular, but not exclusively, in the area of power conversion, for example in converters to supply train-mounted air-conditioning and/or power-supply systems, or indeed in traction-power converters.

More generally, it can be used in all sizes of power converter (typically from 1 W to 100 MW or more).

Patent application EP 1 231 704 from the applicant has already proposed a converter structure that includes several transformers, each with a primary winding and a secondary winding, which can be combined in series and/or in parallel by means of switching resources.

In fact, the primary windings are connected together, as well as to the input terminals of the structure, by means of switching resources that are capable of being controlled to allow connection of the said primary windings according to different configurations in which they are in series and/or in parallel.

The secondary windings are also connected together, as well as to the output terminals of the structure, by means of switching resources that also allow different series or parallel configurations to be achieved.

Such a structure allows the converter to be divided into several converters in series for very high input voltages, or so as to create several converters in parallel for very low input voltages.

It is thus possible to handle ranges of input voltage corresponding to large circuit variations, without the need,

“coo, i 5 for the components of the different primary and secondary circuits, the same rating as in a conventional converter structure.

Nevertheless, such a converter is not totally satisfactory.

In particular, and amongst other drawbacks, the possible range of its input voltage is limited.

In order to overcome this limitation, it has been proposed to over-dimension the voltage that is acceptable by the switching resources. This, however, leads in particular to reduced standards of performance and an increase in the cost of the converter.

The invention particularly has as its objective to propose a solution that allows the range of input voltage to the converter to be extended, and which does not result in the disadvantages mentioned above.

To this end, the invention proposes a power converter that has a multiplicity of transformers, with the primary windings of these transformers being connected together by switching resources, and with their secondary windings also being connected together by switching resources, characterised in that it includes, for at least one of the said switching resources, an electric circuit that includes: - an energy storage circuit connected in parallel to the switching resource, - a discharge circuit for the energy storage circuit, connected to a first terminal connected to the energy storage circuit and to a second terminal connected to at least one converter node with a potential whose fluctuation, arising from the effect of the switching of the converter switching resources, makes it possible -g discharge the storage circuit by means of the discharge circuit.

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Preferred but not limiting aspects of converter according to the invention are as follows: —- the converter includes an electric circuit of the type described above for each of the switching resources, - the energy storage circuit includes at least one energy storage resource and at least one valve in series, —- the first terminal of the discharge circuit is connected to the node of the energy storage circuit composed of the link between the storage resources and the valve, - the discharge circuit includes at least one energy discharge resource, and it can also include a valve in series with the said energy discharge resource, - the storage and discharge circuits are connected so that the energy storage circuit is discharged through the discharge circuit during a converter operating cycle.

Other characteristics, objectives and advantages of the invention will appear on reading the detailed description that follows, and with reference to appended figures 1 to 4, which are provided by way of non-limiting examples and in which different implementation variants of a converter according to the invention have been shown.

The invention concerns a power converter with a multiplicity of transformers, with the primary windings of these transformers being connected together by switching resources, and with their secondary windings also being connected together by switching resources.

The converter «can be powered by direct voltage or alternating voltage.

In particular, the converter is intended to be used in a fixed or mobile railway installation.

The converter operation is based on the switching of electric magnitudes by means of switching resources. The switching resources used can be of unidirectional or bidirectional switches, such as diodes, thyristors, GTO thyristors, 1GCT thyristors, triacs, transistors of the FET, bipolar, IGBT, MOSFET, MBS type or similar, or combinations of these switches. ’

More particularly, the converter according to the invention includes, for at least one of the switching resources, and preferably for each of the switching resources, an electric circuit that includes: © an energy storage circuit connected in parallel with the switching resource, = a discharge circuit for the energy storage circuit, connected to a first terminal corrected to the energy storage circuit and to a second terminal connected to at least one node of the converter with a potential than can be less than that of the Storage circuit, so as to allow the discharge of the storage circuit through the discharge circuit.

Figure 1 shows one possible method of implementation of the primary circuit of a converter according to the invention.

It can be seen that only a part of the primary circuit (composed of three primary windings Py, Py, P,) is illustrated in figure 1, in which the broken lines represent electric conductors connecting the said part of the primary circuit to the other stages constituting the primary circuit.

The primary circuit thus includes two electric conductors

Ci, C,, whose bottom ends firstly, and whose top ends secondly, are connected together so that the two electric conductors C;, C, of the primary circuit are connected in parallel between the input terminals of the converter.

Each electric conductor Ci, Cz includes series switching resources I, I, and I',, I’, respectively for the part of the primary circuit shown in figure 1.

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The primary windings Pg, Pi, P, are connected together by the switching resources In, I'y, I, 1I',, with switching resources

I. and 1’, connecting the stages of the primary circuit composed of windings P, and P1, and switching resources I, and 5S I’, connecting the stages of the primary circuit composed of windings P; and Ps.

The figure also shows the respective leakage inductances of primary windings Py, P;, and P, under the references Lg, I and L,.

The switching resources I,, I'y, I, and I’, shown in this figure are unidirectional switches, hereinafter referred to simply as switches.

In order to give a clearer representation in figure 1, only a single electric circuit is shown, including a storage circuit and a discharge circuit, for switch I,.

It can be seen however that such a circuit is preferably intended to be associated with each of the converter switching resources.

The storage circuit, connected in parallel with switch I, includes a valve in series with an energy storage resource.

The valve shown here by way of an example is more precisely a diode D.. The energy storage resource shown here is a capacitance C.

The discharge circuit is used to connect the node composed of the link between the diode Dc and the capacitance C to at least one converter node chosen so that it has a potential that allows the storage circuit to be discharged via the discharge circuit.

The node N, chosen here is that composed of the link between winding P; and electric conductor C,.

There is thus a connection, via the electric circuit that includes the storage and discharge circuits, between node N, composed of the link between winding P; and conductor Ci, and

. R - Co. . 6 node N, composed of the link between winding P, of the top stage and the other conductor C,.

As will be described in detail later, use is made of the fluctuation of potential at node N, of the converter, determined by the sequence of switch operations of the converter switching resources.

The discharge circuit includes a discharge resource that can be connected in series with a valve (here diode Dp).

The said discharge resource can be a resource that is designed to limit the discharge current and to smooth the energy, such as an inductance for example (as is the case of inductance IL; shown here).

Figure 2 illustrates another possible method of implementation of the invention according to which the discharge circuit has no valve.

Figures 3 and 4 for their part, illustrate variants of figures 1 and 2, according to which the chosen node N, is not at a top stage but at a lower stage.

In the context of figures 3 and 4, a connection 1s thus made via the electric circuit that includes the storage and discharge circuits, betwenn node N; composed of the link between winding ?, and conductor C1 to node N; composed of the link between winding P, of the lower stage and the other conductor C,.

It can be seen that these different variants are particularly useful when one is “equipping” the switching resources of the lower and upper stages respectively of a converter structure. In fact for these stages, there is no lower stage or higher stage respectively to which it jis possible to look for the fluctuating potential that allows for the storage circuit to be discharged.

The variants of figures 3 and 4, according to which the connection is made “downwards”, can thus be used for the

Co ; top stage of a converter structure, while figures 1 and 2 in which the connection is made “upwards” can be used for the lower stage.

It will also be seen that the variants illustrated in figures 2 and 4, according to which the discharge circuit has no valve, allow better discharging of the discharge circuit to be achieved.

It should be understood that the valves mentioned earlier, both at the storage circuit and at the discharge circuit, can be controlled or not in order to command their opening and closure.

It should be understood here, by way of an example, that for a switching frequency of about 20kHz and a primary voltage of about 400V, the values of the various electrical components of the converter are typically as follows: inductance L, is about 5 pH, capacitance C is about 100 nF, inductance Lp is about 1 mH, and diodes D. and Dp are chosen as a function of the converter power.

It will also be seen that the implementation examples illustrated in figures 1 to 4 transpose easily to the secondary circuit of the converter, as well as to the dual configuration of the converter structure discussed above.

A description now follows of one cycle of operation (consisting of the stages referenced 1 to 6 below) of the converter, according to the possible method of implementation of the invention illustrated in figure 1.

This operating cycle is more particularly determined by the control law for switching of the switching resources.

It is considered that the storage resources of the energy storage circuit have been discharged at the start of the cycle. 1) On opening switch 1I;, the current passing through the different leakage inductances (including inductance 1,

Ce q associated to winding P;), and that coming from the converter input, are diverted into capacitance C through diode D.. This causes capacitance C to charge.

The residual current in switch I, is low, which has the effect of limiting the losses suffered on opening the switch.

One then speaks of soft switch-off of the switching resource, in contrast to the hard switching that is experienced conventionally in the absence of a discharge circuit. 2) Capacitance C charges and absorbs the energy of the different leakage inductances (including inductance L;) of the transformers affected by the operation of switch I,, up to the switch-off of diode D., which is a soft switch-off due to the effect of the various leakage inductances.

This effect limits the voltage peaks applied to switch I, and thus eliminated the need to over-dimension the switch in terms of voltage rating. 3) Capacitance C remains charged as long at the discharge circuit, composed of inductance L, and diode Dp, 1s not activated.

The variation in the potential difference between nodes N; and N, 1s of course determined by the control law for switching of the switching resources. 4) Switch I, closes without affecting the electric circuit that includes the storage and discharge circuits.

The closure of switch I, occurs through different leakage inductances (including inductance L;), which limits the losses suffered on closure of the switch.

One then speaks of soft switch-on of the switching resource in contrast to the hard switching that is conventionally observed in the absence of the said electric circuit. 5) When the voltage at the terminals of the discharge circuit composed of inductance L, and diode Dp becomes cL . 9 negative by lowering of the potential at node N,, capacitance

C discharges progressively through the discharge circuit.

The voltage on capacitance C progressively reduces due to inductance L,. The current rises progressively, having the effect of opening diode Dp by soft switch-on. ©) The residual current flowing in the discharge circuit composed of inductance Lp and diode Dp 1s removed through diode Dc into the transformers and closed switches present between nodes N; and N,, until full or partial discharge of the energy contained in inductance Lp. The current reduces progressively, which has the effect closing diode Dp by soft switch-off.

During stage 1 of the operating cycle, capacitance C is able to store the energy of different leakage inductances (including inductance L;) and that coming from the converter input.

This storage of energy has the effect of allowing the use of the energy storage circuit as a temporary and variable voltage source (with this source depending on the current injected into the energy storage circuit).

The mean voltage that can be applied at the converter input can thus be increased, in particular allowing one to limit the number of stages of the switching resources necessary in the converter in order to handle the maximum working voltage.

The converter according to the invention thus has ga higher working range than that of a conventional converter.

In particular, such an improvement in the working voltage range is achieved with no over-dimensioning of the switching resources.

As an example, for a conventional converter rated at 50 kW, the maximum voltage that can be handled is about 2800

V. For a converter according to the invention rated for

. To. . 10 the same power, the maximum voltage is clearly greater, and is typically over 8000 V.

The converter of the invention also has other advantages in relation to the conventional converters that do not have an electric circuit that includes the storage and discharge

Circuits presented above, for all or part of their switching resources.

In fact conventional converters suffer large switching losses, in particular through the use of so-called hard-type switching resources, which has the effect of limiting the rise in frequency and reducing the conversion efficiency.

The use according to the invention of switching resources of the soft-switch type (in particular on switch-off) allows the switching losses to be limited or eliminated.

The conversion efficiency of the converter is thus improved.

The converter switching frequency can also be increased, thus increasing the options for converter integration, and the effectiveness of regulation.

In conventional converters, the leakage inductances of the transformers generate voltage peaks that involve having recourse to special dimensioning of the switching resources.

In the context of the invention, the impact of parasitic components (such as the leakage inductances of the transformers) on dimensioning of the switching resources is limited.

The switching on and off of conventional converters is a problem. In particular, they require and additional device for the pre-loading of their secondary circuit, so that they can be started.

In fact, control of the input current of a conventional converter is possible by means of the voltage present on the transformer windings. The absence of voltage in the secondary circuit results to an absence of voltage on the windings the transformer primaries and therefore the impossibility to control the start-up current of the conventional converter.

Momentary or permanent shut-down during the start-up phase or in normal operation cannot therefore be handled without opening the primary circuit. Opening the primary circuit of the conventional converter, on load or at start- up, causes the appearance of voltages that are liable to destroy the switching resources.

The converter of the invention allow the primary circuit to be opened by diverting the energy of the primary circuit into the energy storage resources. It simultaneously has the advantages of being able to create a voltage at its terminals to compensate for the voltage of the input circuit, and being able to control the current. Pre-loading of the secondary circuit of a converter of the invention can be effected without any external device, simply by controlling the converter switching resources according to the invention.

In the context of the invention, the voltage fronts due to the switching resources are limited, with the effect of improving the performance of the converter in terms of electromagnetic compatibility.

Claims

oC 12 CLAIMS

1. A power converter that includes a plurality of transformers, with the primary windings of these transformers being connected together by switching resources, and with their secondary windings also being connected together by switching resources, characterised in that it includes, for at least one of the said switching resources, an electric circuit that includes: T an energy storage circuit connected in parallel with the switching resource, - a discharge circuit for the energy storage circuit, connected to a first terminal that is connected to the energy storage circuit and to a second terminal connected to at least one node of the converter having a potential whose fluctuation, arising from the effect of the switching of the converter switching resources, makes it possible to discharge the storage circuit by means of the discharge circuit.

2. A converter according to the previous claim characterised in that it includes the said electric circuit for each of switching resources.

3. A converter according to one of the previous claims, characterised in that the energy storage circuit includes at least one energy storage resource and at least one valve in series.

4. A converter according to the previous claim characterised in that the first terminal of the discharge circuit is connected to the node of the energy storage circuit composed of the link between the storage resources and the valve.

5. A converter according to one of the previous claims characterised in that the discharge circuit includes at least one energy discharge resource.

6. A converter according to the previous claim characterised in that the discharge circuit also includes at least one valve in series with the said energy discharge . resource.

7. A converter according to one of the previous claims characterised in that the said storage and discharge circuits are connected so that the energy storage circuit is discharged through the discharge circuit during a converter operating cycle determined by the control law for switching of the switching resources.