GB2113927A - Polyphase rectifiers - Google Patents

Abstract

A rectifier equipment for conversion from 3-phase a.c. to d.c. having a transformer (4) with two secondary windings, one connected star (1) and the other delta (2), for phase-displaced supplies to two rectifier bridges (5) and (6) to give 12- pulse rectification. To obtain equal currents in the diode or thyristor rectifier bridges the two secondary windings are linked by a compensating transformer (10) which may be a 3-phase transformer or comprise three single phase transformer units. The two secondary windings on each limb of the rectifier transformer (4) are connected in series with two windings on one limb or unit of the compensating transformer where these windings are connected in opposition and act mutually as primary and secondary to one another thus having equal ampere turns, the turns ratio being root three to one for the delta and star winding circuits respectively. The compensating transformer serves also to absorb the instantaneous voltage differences between the two rectifier bridge circuits. <IMAGE>

Description

SPECIFICATION Improvements in rectifiers This invention relates to rectifiers for producing direct current from a 3-phase a.c. source, more particularly for obtaining 12-pulse rectification from two phase-displaced 3-phase bridge circuits, where the transformer secondary windings are connected respectively star and delta to produce the required phase shift. An example of this arrangement, in its known form, is shown in Fig. 1.

In Fig. 2 the d.c. voltage waveforms R1 and R2 from the two phase-displaced rectifier bridge circuits are shown dotted. It will be seen that they alternately have the higher voltage and would tend alternately to carry the total d.c. current. The two d.c. circuits are therefore, in the known arrangement, paralleled through an auto transformer 7 in Fig. 1, known as an interphase transformer. This interphase transformer absorbs the voltage differences between R1 and R2, shown shaded in Fig. 2, and its midpoint provides a d.c. terminal at a voltage midway between that of the two individual rectifier circuits, shown by the full line in the figure. Thus each individual rectifier bridge is enabled to carry a half the total d.c. current continuously.It is important to realise that the interphase transformer does not induce equal current sharing between the two rectifier bridges as the relevant current components in this context are d.c. and produce no transformer effect.

Preferably the transformer secondary windings 1 and 2 are on a common core and are closely coupled in the magnetic field. Thereby for a certain volt/amp d.c. characteristic over the operating current range the short circuit current is approximately a half of what it would be if these windings were in separate magnetic fields, e.g. on separate cores. This effect is implicit in the textbook relationships between percentage d.c.

voltage regulation and percentage reactance.

When the transformer secondary windings are closely coupled on a common core the percentage d.c. voltage regulation due to reactance is 0.259 times the percentage reactance, compared with 0.5 times when the windings are magnetically separate. Thus for the same d.c. volt/amp characteristics the reactance for the closely coupled winding arrangement is designed to be practically double what it would be for no coupling and the short circuit current is thereby approximately halved.

The significance of this close coupling in this context is that it results in most of the reactance between the primary and secondary windings being common to the two secondaries and therefore not helping in current sharing. This can result in one or other of the rectifier bridge circuits carrying much more than its designed share of the current.

To achieve satisfactory natural current sharing it is necessary to obtain close equality of voltage, reactance and resistance in respect of the two transformer secondary windings. As one winding is star connected and one delta there is a root three relationship between the desired secondary winding voltages and conversely for the currents.

Equality of the relevant factors and at the same time compatibility with the normal transformer design proportions for satisfactory cost and efficiency is difficult.

Having achieved satisfactory current balance on a good supply voltage waveform this balance can still be upset by harmonics, particularly the 5th and the 7th, in the supply voltage waveform.

The primary object of the present invention is to make the currents equal in the two parallel bridge circuits irrespective of any natural lack of balance within the rectifier equipment or of external unbalancing effects.

The principle of a compensating transformer is well known. The currents in the primary and secondary windings of a simple transformer have a fixed ratio determined by the number of turns in each winding. Thus by having two windings on a common core and connecting one winding in one circuit and the other in a second circuit, with appropriate polarities to produce opposing magnetic fields, a fixed current relationship between the two circuits is obtained, depending on the ratio of turns, and provided the core section and the numbers of turns are adequate to support the maximum voltage difference that may occur.

The difficulty in the application of this principle in this case is that the currents into the two rectifier bridges are out of phase with one another and must remain so to produce 12-pulse rectification. It is known that overall equal phasing can be obtained by a combination of compensator windings where two windings associated with one bridge are combined to equate in phase with one winding associated with the other bridge.

In the present invention a compensating transformer is applied directly to the rectifier transformer winding circuits before the delta connection has been made and therefore where the winding currents are still in phase with one another.

The compensating transformer is designed to have the a.c. voltage range necessary to absorb the voltage differences between the star and delta windings so permitting the d.c. terminals of the two phase displaced rectifier circuits to be connected directly in parallel without an interphase transformer.

A further effect of the compensating transformer arrangement in accordance with the invention is to cancel the 5th and 7th harmonic currents from the individual rectifier bridge circuits. This reduces the rectifier transformer r.m.s. secondary currents and the corresponding winding kVA, and this reduction applies also to the compensating transformer itself.

This improvement of the circuit waveforms reduces the losses of the rectifier diodes and allows their mean current rating to be increased.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings:- Figure 1 is a circuit diagram of a 12-pulse parallel bridge rectifier in its known and usual form.

Figure 2 shows the d.c. voltage waveforms to illustrate the function of the interphase transformer 7 in Fig. 1.

Figure 3 is a circuit diagram of a rectifier equipment incorporating this invention.

Referring to Fig. 3 the primary winding 3 of the rectifier transformer 4 may be delta connected as shown, or alternatively star connected. The secondary winding 1 is star connected. The other rectifier transformer secondary winding 2, which is to be connected delta, has both ends of each phase winding brought out for completion of the delta connections through a compensating transformer to be described. The two secondary windings 1 and 2 are closely coupled on a common core with a common primary winding.

The degree of coupling, i.e. the sharing of a common magnetic flux, may however be varied by the winding proportions and relative positions to give alternative d.c. output volt/amp characteristics.

Additional to the rectifier transformer is a compensating transformer or transformers 10.

Considering these as three single phase units each unit has two windings, one for each bridge. Each unit is associated with one star winding and a delta winding on the same limb of the rectifier transformer. In the rectifier transformer the two currents are in phase. In the compensating transformer they are in opposition and so act mutually as primary and secondary to one another, maintaining a current ratio determined by the turns ratio. The voltage and turns ratio between the rectifier transformer delta secondary winding 2 and the star winding 1 is root three to one and the current ratio the inverse of this. The same ratios of turns and currents apply to the corresponding compensating transformer windings.To facilitate explanation the compensating transformers have been described as three single phase units, which they can be, but in practice it is generally more economic for them to be combined as a 3-phase transformer.

The compensating transformer 10 according to this invention is designed with a voltage rating sufficient to absorb the voltage differences between the two rectifier bridges indicated in simplified form by the shaded area in Fig. 2. The d.c. terminals of the two rectifier bridges are then connected directly in parallel and there is no interphase transformer as 7 in Fig. 1.

The d.c. currents from the two rectifier bridges are by the compensating transformer according to this invention constrained to be equal. The 5th and 7th current harmonics present in opposite polarity in the a.c. circuits to the two rectifier bridges in the circuit according to Fig. 1 are cancelled out by the compensating transformer in accordance with this invention and the r.m.s. values of the currents thereby reduced by 3.4%.

The invention has been described and drawn for simple rectification by diodes. It will be appreciated that it is also applicable to controlled rectification by thyristors.

Claims (4)

1. A rectifier equipment for producing unidirectional current from a 3-phase a.c. supply, incorporating two 3-phase diode rectifier bridges supplied from transformer secondary windings connected respectively star and delta for the phase displacement necessary for 1 2-pulse operation, the windings preferably being closely coupled on a common core, and a 3-phase compensating transformer where one winding on each limb is connected in series in the star rectifier circuit and a second winding connected within the delta to make the currents in the two rectifier circuits equal and also to absorb the instantaneous voltage differences between the two circuits so that no interphase transformer is necessary.

2. A rectifier equipment according to claim 1 but with three single phase compensating transformer units.

3. A rectifier equipment according to claim 1 but where the rectifier elements are thyristors.

4. A rectifier equipment according to claim 2 but where the rectifier elements are thyristors.