GB2152764A - Improvements to transformers - Google Patents

Abstract

A magnetically-cored transformer has in each phase and in each winding a plurality of winding sections 9...20 that can be connected by switches 21,22,23 in series or shunt or in a combination of series and shunt to form winding arrangements that give lower combined core and winding power losses the switches being operated either manually or automatically according to the actual or expected loading on the transformer. As shown, with switches 21 and 22 closed and 23 open windings 19,18 and 20,17 are in parallel; with these switches reversed they are in series. A load current sensitive arrangement for automatic control is disclosed (Fig. 3). <IMAGE>

Description

SPECIFICATION Improvements to transformers The invention described relates to magnetically-cored transformers and to means of switching sections of the windings of the said transformers into such different arrangements as will reduce the combined power losses in the magnetic core and in the windings as the loading on the transformer changes between a relatively low value and a relatively high value.

Magnetically-cored transformers have power losses both in the core materials when these carry magnetic flux and in the winding materials when these carry current. Almost all transformers are subjected to fluctuating load currents causing the winding power loss to vary while the core power loss remains substantially constant. Design of transformers is based on predicted present worth costs of core and winding power losses and is a compromise giving the most economic performance at a loading which usually lies between 30% and 60% of full load depending on particular circumstances while at other loadings the performance is not the most economic one.

It is an object of the present invention to enable both the core power losses and the winding power losses to be controlled according to the electrical loading on the transformer. A magnetically-cored transformer is provided with various winding sections in each phase and in each winding that may be interconnected in series or shunt or in a combination of series and shunt in different ways by the action of mechanical switches or by solid-state switches. The connections may be switched manually or automatically either on or off load to provide two or more modes of operation. When two modes are used one is selected for use from no-load to medium load and the other from medium load to full load. When more than two modes are provided the appropriate switching of winding sections allows the full range of loading to be further sub-divided.The dual purpose of switching the windings is to reduce the combined energy losses in the core and windings of the transformer over a typical operating duty cycle and at the same time to help stabilise the secondary voltage during variations in the current loading. In a three-winding transformer, all three windings may be switched or alternatively just one or two windings may be switched. For a two-winding transformer, one or both windings may be switched in each phase. Auto-transformer windings may also be switched. The manner of switching may be the same for all the windings or may be different.

The invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a transformer in a tank filled with oil as used at high voltage with core, windings, transducer and switches identified.

Figure 2 shows details of a switchable winding arrangement.

Figure 3 shows details of a current and voltage transducer.

Figure 4 shows the elements of an analogue control system.

Figure 5 shows the elements of a digital control system.

Figure 6 shows an example of a mechanical switch.

Figure 7 shows the basic layout of a winding section.

A typical distribution transformer embodying the invention is shown in Figure 1. The h.v. bushing, say at 11 kV or 6.6 kV threephase, is 1 and 2 is the 1 .v. bushing operating say at 41 5-V three phase. The unit 3 contains required current and voltage transducers as well as power switches. the h.v.

windings are 4 and the 1 .v. windings 5 placed around a magnetic core, 6. An inspection port 7 may be provided. The oil level is denoted by 8.

In one typical form of winding construction, shown in Figure 2, 6N turns are doublewound in one winding of each phase so that in effect 1 2N turns of single wire are wound numbered 9 to 20 inclusive. Suppose the electrical resistance of each singly-wound unit of N turns is R ohms. Of the 6N doublywound turns 4N doubly-wound turns numbered 9 to 1 6 inclusive comprise a 'fixed' section and through transposition of each unit of N turns an equal division of current is achieved between parallel sections. This fixed section has 4N effective turns and a resistance of 2R ohms per phase. The remaining 2N doubly-wound turns provide 4 units numbered 17, 18, 19,20.These are transposed to provide electrically identical sections made up of units 1 7 and 20 which together form a 'separable' section and units 1 8 and 1 9 which form a 'tapped' section. The separable and tapped sections may be connected in parallel by closing the switches 21 and 22 with switch 23 open. In this parallel connection the total effective number of turns in the whole winding is 6N and the effective resistance is 3R ohms. Alternatively the separable and tapped sections may be connected in series by having the switches 21 and 22 open and switch 23 closed. In this series connection the effective number of turns in the whole winding connected between terminals T, and T2 is 8N and the effective resistance is 6R ohms.

The series connection gives a higher leakage reactance than the parallel connection and this helps to maintain a more constant output voltage over the full range of electrical loading.

When the transformer is operating between medium and full load the parallel connection is used. this gives a lower winding resistance and lower winding losses for a given current but there are fewer effective turns and so there is a higher magnetic flux density and a larger power loss in the core. Conversely when the transformer is operating between no load and medium load the series connection is used. This gives a higher winding resistance and therefore a higher power loss in the winding for a given current but there is a larger number of effective turns and so a lower magnetic flux density and a lower power loss in the core. There is a critical level of loading, expressed either in VA or in the current carried by one of the windings, at which the power loss is the same for the parallel connection as for the series connection.An automatic controller used to operate switches 21,22 and 23 and also to operate the corresponding switches on the other phases and on the other winding will alter the switch conditions at or close to this critical loading. If the load increases and passes the critical value the winding connection is changed from series to parallel by the automatic controller, if a manual controller is used the switch will be left in the series connection when it is known that the load will be below the critical value for a long period of time, for example for several months in the summer or for a few years after the transformer is first put into service. If the load is expected to rise above the critical value the manual switch will be left in the parallel connection as might happen during the winter or when the load on the transformer has risen above the critical value since being put into service.

When the winding switching is carried out automatically, a current and voltage transducer such as that shown in Figure 3 may be used. A wire 24 carrying the whole or part of the winding current is threaded through a magnetic core 25 so as to cause the core to be magnetised but the core may be biassed by a voltage coil 26 associated with an impedance 27 acting so as to oppose the magnetisation of the core caused by the flow of current. The voltage coil 26 is optional but the wire 24 or an equivalent means of magnetisation by the current is essential. The resulting magnetisation of the core causes a signal to be picked up in the coil 28.This signal may be fed into a series of circuits of the type indicated in Figure 4 which rectify it, compare this with a reference voltage and accordingly may trigger the gates of thyristor pairs or other solid-state switches such as triacs, which can be 21 and 22 in Figure 2, for currents above the critical level, or solidstate switches 23 for currents below this level.

Alternatively a digital system may be used to control the winding switching automatically. In that case a current transducer 29 in Figure 5 supplies voltage signals to an A/D converter that in turn sends out a digital code that is compared with a reference voltage in digital form stored in memory 30. If the incoming voltage is less than the reference voltage in digital code an output is created to operate those switches that provide a form of connection suitable for no-load to medium load. If the incoming voltage is greater than the reference voltage switches operate to bring in the medium load to full load form of connection.

When the winding switching is carried out manually the arrangement shown in Figure 6(a) and (b) may be used. A similar switching arrangement is used on all windings that are to be switched and on all phases. All the switches used are ganged together mechanically if operated by hand. If operated by an electrical drive such as a solenoid all switches are jointly operated and interlocked. Figure 6(a) shows switching contact 32 joining terminals 34 and 35, to correspond to switch 21 in Figure 2 being closed, and contact 33 joining terminals 36 and 37 to correspond to switch 22 in Figure 2 being closed. Contacts 32 and 33 slide together on a mechanism and, as shown in Figure 6 (b) 32 then joins contacts 35 and 36 to correspond to switch 23 being closed in Figure 2.The use of 33 to join 37 and 38 in Figure 6 (b) may allow an indicator or interlock system to be activated when the switch is in this position.

Variations can be made on the particular winding arrangement described above and illustrated in Figure 2 while following the general arrangements shown in Figure 7. This shows one phase of one winding supplied through terminals T3 and T4. Sections 39 and 40 have equal numbers of turns but they may have a different number of turns from section 41 which may not contain any turns at all.

Sections 39 and 40 may be placed in parallel by the closing of switches 42 and 43 with 44 open or may be placed in series by closing switch 44 but leaving 42 and 43 open.

Electro-mechanical switches may be closed manually or by remote control using an electrical actuator. Semi-conductor switches such as thyristors or triacs may be used for automatic operation but mechanical bypass switches may also be placed in parallel with them to reduce power losses.

Claims (6)

1. A magnetically-cored transformer with at least one winding which has a plurality of sections said sections being provided with at least one switch operable to connect said sections in different shunt and/or series arrangements in such a manner that lower power losses are produced in the transformer over the normal range of electrical loadings through control of the magnetic flux density in the core and of the effective resistance of the windings according to loading.

2. A magnetically-cored transformer having a plurality of sections in each winding capable of being connected by switches into a combination of shunt and/or series arrangements as in Claim 1 means of sensing the electrical loading on the transformer and switches being controlled by said sensing means to enable the electrical power and energy losses in the core and windings taken together to be as small as the said winding sections and said switches will permit at any electrical loading on the transformer detected by the said sensing means.

3. A magnetically-cored transformer having a plurality of winding sections on each phase of each winding the sections being interconnected by switches as in Claim 1 whereby the switches can be operated manually including remote manual operation to cause the power and energy losses for the expected loading on the transformer to be as small as the said winding sections and said switches will permit under the loading expected on the transformer at the relevant time.

4. A magnetically-cored transformer having a plurality of winding sections on each phase of each winding said sections being interconnected by switches that are operated automatically at one or more current loadings as in Claim 2 said switches being semiconductor switches controlled in response to signals derived from a current transducer or from another indicator of the electrical loading on the transformer said signals being rectified and compared with reference levels in a comparator the output of said comparator being used to operate pulse circuits that trigger said switches.

5. A magnetically-cored transformer having a plurality of winding sections on each phase of each winding said sections being interconnected by switches that are operated automatically at one or more current loadings the switches being semiconductor switches controlled by signals derived from a current transducer or from any other indicator of the loading on the transformer as in Claim 4 said signals being rectified and fed to an A/D converter the digital output being compared with one or more reference levels stored in memory and appropriate switches triggered according to the relative values of said digital output and said reference level.

6. A magnetically-cored transformer substantially as herein described with reference to and as shown in the accompanying drawings.