GB2090073A

GB2090073A - Transformers

Info

Publication number: GB2090073A
Application number: GB8040902A
Authority: GB
Original assignee: AEI Semiconductors Ltd
Current assignee: AEI Semiconductors Ltd
Priority date: 1980-12-20
Filing date: 1980-12-20
Publication date: 1982-06-30
Also published as: GB2090073B

Abstract

The output voltage provided by a transformer winding 4 is determined by choosing an appropriate selectable tap point. Each selectable tap point 8, 9, 10 or 11 is connected via one or more switchable thyristors 12, 13, 14 or 15 to an output point 5 in a manner which requires only the minimum number of thyristors. High power diodes 16, 17, 18 prevent winding elements between tapping points being short circuited when the appropriate selectable tap points are chosen. A further set of thyristors 22, 23, 24, 25 and diodes 26, 27, 28 are provided for the other half-cycle. <IMAGE>

Description

SPECIFICATION Transformers This invention relates to transformers and is particularly applicable to transformers having a plurality of tap points. The voltage transfer characteristic of a transformer can be changed by altering the effective ratio of the number of input winding turns to that of the output winding turns and it has been customary to select appropriate tap points by means of mechanical switches. The requirement to change tap points arises most often in connection with very high power transformers in which the voltage or current rating is very large and it is necessary to effect the tap changing in a relatively rapid and lossless manner.

The present invention seeks to provide an improved transformer having selectable tap points.

According to this invention, a transformer includes a winding having a reference tap point and a sequence of selectable tap points arranged along the electrical length of the winding with each tap point being positioned along the winding to define a plurality of winding elements one end of each of which is represented by its associated selectable tap point; a plurality of serially connected switchable semiconductor devices each of which is associated with a respective selectable tap point in the sequence; means for rendering said devices conductive in predetermined combinations and means for preventing a winding element being short circuited when its associated switchable device is rendered conductive.

Preferably each switchable semiconductor device is a thryistor.

Preferably the means for preventing a winding element being short circuited comprises a diode connected between a respective tap point and the junction point of two of the serially connected switchable semiconductor devices.

The arrangement as described so far is suitable for handling one polarity of an alternating voltage and preferably two sets of suitable semiconductor devices and diodes are provided with one set being poled in an opposite sense to the other set, so as to enable the transformer to handle both positive and negative voltages, the output points of the two sets being connected together.

The invention is further described by way of example with reference to the accompanying drawing which illustrates a transformer having selectable tap points.

The drawing shows a transformer arrangement having a primary winding 1 connected between two input terminals 2 and 3 and a secondary winding 4 coupled to two output terminals 5 and 6. The secondary winding 4 is provided with a reference tap point 7 at one end and a sequence of four selectable tap points 8, 9, 10 and 11, tap point 11 being at the far end of the secondary winding. The magnitude of the output voltage present at output terminals 5 and 6 can be adjusted by choosing the appropriate selectable tap point. By changing the selectable tap point the output voltage of the transformer can be adjusted in accordance with the power requirements of an electrical device which is fed or driven by the transformer, or alternatively compensation can be provided for variations in the input voltage present at terminals 2 and 3.Although in the drawing, the selectable tap points are shown as being on the secondary winding, this is not essential, and in some circumstances it may be desirable to position the selectable tap points and their associated control devices on the primary winding.

The invention is also applicable to auto transformers.

Four thyristors 12, 13, 14 and 1 5 are connected in series between the tap point 11 and the output terminal 5. A respective diode 16, 1 7 or 18 is connected between the tap points 10, 9, 8 respectively and the junction point between each pair of adjacent thyristors. The thyristors and diodes described so far control the positive half cycles of an alternating signal generated within the secondary winding 4. A second set of thyristors 22 to 25 and diodes 26 to 28 are provided to handle the negative going half cycles and these are shown interconnected by broken lines merely to distinguish them from the set already described. The connection configuration of the second set is entirely analogous with the first set and it will be noted that the polarity of the thyristors and diodes is reversed.

When it is desired to obtain the maximum output voltage from the transformer, all thyristors are rendered conductive so that a bidirectional current path exists from the tap point 11 to the output terminal 5. It will be seen that, for example, when the thyristor 12 or 22 is rendered conductive, it would short out the winding element iocated between the selectable tap points 10 and 11 were it not for the presence of the reverse biassed diodes 1 6 and 26. It is for this reason that diodes 16, 17, 18 and 26 to 28 are provided.

Thyristors can be seriously damaged by the application of excessive voltages and it will be noted that the voltage appearing across each of the thyristors is determined by the spacing of the selectable tap points along the secondary winding 4 and this completely avoids the need to provide complex voltage sharing circuits to ensure that any one of the serially connected thyristors does not experience a disproportionate voltage drop across it. This aspect greatly enhances the lifetime of the thyristors, and enables them to be selected so that their operating characteristics closely match the demands which are put upon them.

When an output voltage is required which is less than the maximum, the appropriate thyristors are rendered conductive, whilst the remainder are rendered non-conductive. For example, if an output voltage corresponding to tap point 9 is required, the thyristors 14 and 1 5 and the corresponding two thyristors 24 and 25 which handle negative going half cycles are rendered conductive, whereas thyristors 12, 13 and 22, 23 are non-conductive. It will be appreciated that in addition, the thyristor 1 5 can be regarded as an on-off switch, since when it is non-conductive it effectively disconnects all tapping points.

By way of example, the transformer may be required to handle an r.m.s. current of 1 500 amps, and a total secondary winding voltage of 2000 volts - it is assumed that the tap points 8, 9, 10, 11 correspond to 500 volts, 1000 volts, 1 500 volts, 2000 volts In view of the large currents, a number of thyristors can be connected in parallel, and in this example each thyristor shown in the drawing would be three type DCR 1007 thyristors in parallel, and each diode shown would be two type DS 1104 thyristors in parallel - the reference numbers are type numbers of products produced by AEI Semiconductors Limited.

Suitable cooling arrangements would be provided, but this could be conventional.

A suitable drive circuit would be arranged to control the appropriate combination of thyristors when a desired output voltage is selected. It will be noted that the circuit configuration shown in the drawing enables the absolute minimum of thyristors to be used. That is to say, the serially connected thyristors are progressively brought into circuit as the output voltage requirements rise. This avoids the need to provide a separate output circuit connected to each tap point.

Claims

1. A transformer including a winding having a reference tap point and a sequence of selectable tap points arranged along the electrical length of the winding with each tap point being positioned along the winding to define a plurality of winding elements one end of each of which is represented by its associated selectable tap point; a plurality of serially connected switchable semiconductor devices each of which is associated with a respective selectable tap point in the sequence; means for rendering said devices conductive in predetermined combinations and means for preventing a winding element being short circuited when its associated switchable device is rendered conductive.

2. A transformer as claimed in claim 1 and wherein each switchable semiconductor device is a thyristor.

3. A transformer as claimed in claim 1 or 2 and wherein the means for preventing a winding element being short circuited comprises a diode connected between a respective tap point and the junction point of two of the serially connected switchable semiconductor devices.

4. A transformer as claimed in claim 1, 2 or 3 and wherein said winding is the secondary winding of the transformer.

5. A transformer substantially as illustrated in and described with reference to the accompanying drawing.

5. A transformer as claimed in claim 3 and wherein two sets of suitable semiconductor devices and diodes are provided, with the two sets being poled in a mutually opposite sense.

6. A transformer substantially as illustrated in and described with reference to the accompanying drawing.

New claims of amendments to claims filed on 3 DEC.1981.

Superseded claims 1-6 inclusive.

New or amended claims: CLAIMS

1. A transformer including a winding having a reference tap point and a sequence of selectable tap points arranged along the electrical length of the winding with each tap point being positioned along the winding to define a plurality of winding elements one end of each of which is represented by its associated selectable tap point; a plurality of serially connected switchable thyristors each of which is associated with a respective selectable tap point in the sequence; means for rendering said thyristors conductive in predetermined combinations; and diodes connected to prevent a winding element being short circuited when its associated thyristor is rendered conductive, the thyristors and the diodes being connected such that when a thyristor is in a non-conductive state, the voltage appearing across it is determined by the voltage developed by a winding element connected to its associated tap point.

2. A transformer as claimed in claim 1 and wherein the means for preventing a winding element being short circuited comprises a diode connected between a respective tap point and the junction point of two of the serially connected switchable semiconductor devices.

3. A transformer as claimed in claim 1 or 2 and wherein said winding is the secondary winding of the transformer.

4. A transformer as claimed in any of the preceding claims and wherein two sets of thyristors and diodes are provided, with the two sets being poled in a mutually opposite sense.