GB1571083A - Electric current sharing arrangements - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRIC CURRENT SHARING ARRANGEMENTS (71) We, WESTINGHOUSE BRAKE AND SIGNAL COMPANY LIMITED, a Company incorporated under the Laws of Great Britain, of 3, John Street, London WC1N 2ES, England, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to electric current sharing arrangements.

It is known to rectify polyphase alternating current by means of phase-anglecontrolIed rectifiers or thyristors which are well known semiconductor devices. A common rectifying arrangement is the arrangement by which two banks of transformer secondary windings deliver currents through respective halfwave groups of thyristors in antiphase to provide full-wave rectified current. It is also possible to operate a duplicated such arrangement in parallel to double the direct current that can be delivered. It has been discovered in such an arrangement that where chokes are em- ployed for inducing current sharing and also for providing a certain amount of smooRing, there are certain fault conditions that have undesirable effects out of proportion to the faultiness of the rectifier apparatus. Taking the particular instance of two star connected transformer secondary winding groups in a three-phase system, there being no phase displacements in the open-circuit output voltages of the two windings in each phase, each winding feeding through a respective single rectifying phase-angle-controlled thyristor, the outputs of each group being taken in common through a respective choke and the choke outputs being paralleled to a single d.c. output (returned through the common star point), it has been found that the open-circuit failure of only a single thyristor in one group unexpectedly throws substantially the entire output current on to one group, thereby prac ticalty doubling the current carried by the components particular to that group despite two-thirds of the active (switch able) com- ponents of the other group remaining normal.

The foflowing reasoning sets out the probable cause of this undesirable phenomenon.

Assuming in the first instance that the inductances of the two (separate) chokes re- main equal, and that they are high enough to maintain a continuous flow of current from 7each rectifier group, the. failure of a thyristor in one group to conduct must cause a sub- stantial reduction in the mean output voltage of that group relative to that of the healthy group. This unbalance of mean voltages at the inputs to the relatively low resistance choke windings used results in a very large unbalance in current; in fact with a reasonable choke resistance only a small unbalance in mean voltage would be necessary to cause the load current to flow entirely in one choke, so that in practice the assumption of continuous current in both chokes under tins fault condition is hardly tenable.

If the current is acknowledged to be discontinuous from the faulty group, its magnitude relative to the total load current will depend on various factors including the inductance of the chokes; if the inductance is so high that the ripple current in both chokes is small, the average current in the faulty group can only be small also. If the inductance is small, the conducting thyristors in the faulty group may carry substantial pulses of current, albeit not sufficient to make their average contribution comparable with that of the healthy group.

The magnitude of the pulses of current in the faulty rectifier group may be less than it would otherwise be as a result of the increase in the choke inductance which is to be expected at low values of current. A further unfavourable effect which has been noted, also aggravated by an increase in choke inductance at low currents, is that, especially when the direct voltage on the output side of the choke is maintained by a capacitor, the current in the good thyristors of the faulty group may not rise fast enough to permit latching, particularly if very short firing pulses are used, in that case, the healthy group is left to supply the whole of the load current.

According to the invention there is provided a direct current supply arrangement for providing a d.c. voltage between a pair of supply terminals comprising a source of alternating current, a plurality of thyristors connected between the a.c. source and a first of the supply terminals and a return current path between a second of the terminals and the a.c.

source, and inductance means having a plu rality of windings mutually coupled to means of a common magnetic circuit and each of which windings is connected in the same sense in the load current path of a respective thy ristor whereby, in operation, the close magnetic couping between the inductance windings is effective to produce equal sharing of current between the thyristors.

By close magnetic coupling is meant that the inductance means, in the form, for ex ample, of respective windings, are on a common magnetic core or are otherwise closely magnetically linked. By way of further example it is known that a plural air-gapped iron-cored choke with windings round res pective limbs each separated by air-gaps does not, in normal practice, provide sufficiently close coupling to carry out the invention. The windings must be on a common core in normal circumstances. Preferably the inductance means comprises a poly-filar winding on a magnetic core (the magnetic circuit possibly containing one or more air-gaps), each filament of the poly-filar winding being connected to carry the current of one thyristor or group of thyristors between which current is to be shared. Thus, in the case of a pair of thyristors or a pair of groups of phase displaced thyristors between which pair current is to be shared, the winding is pre ferably bifilar; in the case of three thyristors or groups of thyristors, the winding is trifilar; and so on.

The invention may be applied to a single phase arrangement (one thyristor per sub division of the phase into groups) or be applied to the more common polyphase arrangement (typically three, six, twelve, or twenty-four phases) wherein there is a thyristor for each phase within each group, the groups numbering two, three, or four, or a higher number according to the fraction of the total output current supposed to be carried by a single thyristor at any ume.

The present invention is not to be confused with the system where paralleled outputs of a transformer/rectifier arrangement are commoned through a current-sharing double winding transformer, because the windings in that case are in phase opposition and the inductive effects cancel out.

In order that the invention be more clearly understood and readily put into effect, a preferred embodiment of the same will now be described by way of example and with reference to the accompanying drawing, the sole figure of which is a simplified circuit diagram of the embodiment.

Referring to the accompanying drawing, a three-phase transformer has secondary windings, R, Y, B (one secondary winding per phase, the primary windings and the core being omitted for clarity) connected to have a common star-point S. The transformer is part of a transformer/rectifier unit designed to deliver 3000 amps (nominal) at 50 volts, and because 1500 amps is the current limit of the rectifiers, the rectifiers (about to be described) are doubled-up and run in parallel.

For the purposes of voltage regulation and fault control, phase-angle-contro11ed thyristors are employed. There are two thyristors per phase, designated RA, RB; YA, YB; and BA, BB for the red, yellow, and blue (R, Y and B) phases respectively, divided into two groups A and B. The thyristor cathodes in each phase are common to the respective transformer secondary windings, and the thyristor anodes are commoned to group busbars GA and GB. (Thyristor firing circuits, fault sensing arrangements, fuses and other protective arrangements, and voltage and current monitoring arrangements have all been omitted for the sake of clarity).

It is required that the group bus-bars GA and GB be connected to deliver the respective group outputs in parallel without the aforementioned failure properly to share current that arises in certain circumstances. To this end, the group bus-bars GA and GB are connected to the single negative output terminal N by way of respective windings LA and LB of bifilar choke L. The bifilar windings LA and LB are wound from copper strip, in known manner, round a laminated iron core which may or may not (according to design preference) have one or more air-gaps or magnetically saturable sections in its magnetic loop. A conventional plural-strand-wound choke (wherein the strands are individually electrically insulated and electrically connected together only at their ends) may be modified to the purposes of the present invention by having the strands soldered or otherwise connected together at the output side of the choke, but at the input, the strands are separated into the appropriate number of groups.

In operation, current is shared, in known manner, between the thyristors within each group, RA, YA, BA, and RB, YB, and BB respectively by natural phase advance of the supply voltage vector. Current is caused to be shared between the thyristors in different groups by reason of the very close magnetic coupling of the choke windings (LA and LB respectively) in the two groups, the close magnetic coupling being a consequence of the bifilar nature of the windings on the choke L.

The choke L provides a common inductance for each group and exerts a certain smoothing effect on the currents provided thereby. Unlike previously known double-wound chokes which act as opposed-winding transformers (for the purpose of forcing the firing of the thyristors), the inductance effects of the twinwinding choke L do not cancel out.

The main positive terminal P is a direct connection back to the star-point S. Obviously the arrangement shown in the drawing is only a half-wave rectifying arrangement, and is so shown for the sake of simplicity, but in normal practice, the illustrated arrangement would form part of a full-wave rectifying arrangement by the duplication of the arrangement with the choke outputs commoned and the supply positive still taken from the starpoint S, i.e. the well known hi-phase arrangement. A single-phase supply may be employed in place of the polyphase supply illustrated.

The invention is also applicable to singlephase and polyphase bridge configurations of thyristors, the bridges being duplicated or otherwise multiplied and run in parallel for current-sharing purposes, the d.c. output terminals in each case being commoned through closely mutually magnetically coupled inductance means, preferably in the form of polyfilar windings.

WHAT WE CLAIM IS:- 1. A direct current supply arrangement for providing a d.c. voltage between a pair of supply terminals comprising a source of alternating current, a plurality of thyristors connected in parallel between the a.c. source and a first of the supply terminals and a return current path between a second of the terminals and the a.c. source, and inductance means having a plurality of windings mutually coupled by means of a common magnetic circuit and each of which windings is connected in the same sense in the load current path of a respective thyristor whereby, in operation, the close magnetic coupling between the inductance windings is effective to produce equal sharing of current between the thyristors.

2. A direct current supply arrangement according to Claim 1, wherein the inductance means comprises a poly-filar winding.

3. A direct current supply arrangement according to Claim 1, wherein the inductance means comprises separate windings on a common core.

4. A direct current supply arrangement according to any preceding claim, wherein each parallel thyristor comprises a half-wave rectifier.

5. A direct current supply arrangement according to Claim 4, wherein there are two parallel half-wave rectifiers each in series with an inductance winding.

6. A direct current supply arrangement according to any one of Claims 1 to 3, wherein each parallel thyristor is replaced by a fullwave bridge arrangement of thyristors.

7. A direct current supply arrangement according to any preceding claim, wherein the a.c. source comprises a single phase a.c.

supply and the parallel thyristors are connected to the output winding of a single phase a.c.

transformer.

8. A direct current supply arrangement according to any one of Claims 1 to 6, wherein the a.c. source comprises a multi-phase a.c. supply and there is a plurality of thyristors connected in parallel to each output winding of a star-connected multi-phase a.c. transformer and the a.c. current return path is connected to the star point neutral line.

9. A direct current supply arrangement according to Claim 8, wherein respective thyristors connected to each a.c. phase supply are connected in common to a respective inductance winding.

10. A direct current supply arrangement, substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. bifilar nature of the windings on the choke L. The choke L provides a common inductance for each group and exerts a certain smoothing effect on the currents provided thereby. Unlike previously known double-wound chokes which act as opposed-winding transformers (for the purpose of forcing the firing of the thyristors), the inductance effects of the twinwinding choke L do not cancel out. The main positive terminal P is a direct connection back to the star-point S. Obviously the arrangement shown in the drawing is only a half-wave rectifying arrangement, and is so shown for the sake of simplicity, but in normal practice, the illustrated arrangement would form part of a full-wave rectifying arrangement by the duplication of the arrangement with the choke outputs commoned and the supply positive still taken from the starpoint S, i.e. the well known hi-phase arrangement. A single-phase supply may be employed in place of the polyphase supply illustrated. The invention is also applicable to singlephase and polyphase bridge configurations of thyristors, the bridges being duplicated or otherwise multiplied and run in parallel for current-sharing purposes, the d.c. output terminals in each case being commoned through closely mutually magnetically coupled inductance means, preferably in the form of polyfilar windings. WHAT WE CLAIM IS:-

1. A direct current supply arrangement for providing a d.c. voltage between a pair of supply terminals comprising a source of alternating current, a plurality of thyristors connected in parallel between the a.c. source and a first of the supply terminals and a return current path between a second of the terminals and the a.c. source, and inductance means having a plurality of windings mutually coupled by means of a common magnetic circuit and each of which windings is connected in the same sense in the load current path of a respective thyristor whereby, in operation, the close magnetic coupling between the inductance windings is effective to produce equal sharing of current between the thyristors.

2. A direct current supply arrangement according to Claim 1, wherein the inductance means comprises a poly-filar winding.

3. A direct current supply arrangement according to Claim 1, wherein the inductance means comprises separate windings on a common core.

4. A direct current supply arrangement according to any preceding claim, wherein each parallel thyristor comprises a half-wave rectifier.

5. A direct current supply arrangement according to Claim 4, wherein there are two parallel half-wave rectifiers each in series with an inductance winding.

6. A direct current supply arrangement according to any one of Claims 1 to 3, wherein each parallel thyristor is replaced by a fullwave bridge arrangement of thyristors.

7. A direct current supply arrangement according to any preceding claim, wherein the a.c. source comprises a single phase a.c.

supply and the parallel thyristors are connected to the output winding of a single phase a.c.

transformer.

8. A direct current supply arrangement according to any one of Claims 1 to 6, wherein the a.c. source comprises a multi-phase a.c. supply and there is a plurality of thyristors connected in parallel to each output winding of a star-connected multi-phase a.c. transformer and the a.c. current return path is connected to the star point neutral line.

9. A direct current supply arrangement according to Claim 8, wherein respective thyristors connected to each a.c. phase supply are connected in common to a respective inductance winding.

10. A direct current supply arrangement, substantially as hereinbefore described with reference to the accompanying drawing.