reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

would increase the spacing between them and thus reduce the series capacitance C s . C g would be effectively unchanged, so that the ratio C g /C, would increase and the voltage gradient become greater still. The most effective method of controlling the increased stress at the line end is clearly to increase the series capacitance of the winding, since reducing the capacitance to earth is not very practicable*. Figure 3.22 shows several methods by which the series capacitance can be increased. The first (Fig 3.22 (a)) uses an electrostatic shield connected to the line end and inserted between the two HV discs nearest to the line end. The second (Fig 3.22 (b)) winds in a dummy strand connected to the line lead but terminating in the first disc. Both of these arrangements effectively bring more of the winding turns closer physically to the line end. Thirdly, a system of interleaving (Fig 3.22 (c)) can be used, whereby the winding turns can be taken from the line end further into the body of the winding rather than simply winding in series. This usually involves winding two or more strands in parallel and then reconnecting the ends after winding to give the interleaving arrangement required. It has the advantage over the first two methods that it does not waste any space, since every turn remains active. However, the cost of winding is greatly increased by the large number of joints. It is possible by adjustment of the degree of interleaving, to achieve a nearly linear distribution of impulse voltage through-

'Footnote: A short squat winding tends to have a lower capacitance to ground than a tall slim winding, so such an arrangement would have a better intrinsic impulse strength. There are, however, so many other constraints tending to dictate winding geometry that manufacturers are seldom able to use this as a practical means of obtaining the required impulse strength.

out the winding. In view of the high cost of interleaving, the designer aims to minimise this and, where possible, to restrict it to the end sections of the winding.

A further problem can occur at the neutral end of the winding, since it is possible for reflections of the impulse wave to be produced which can give rise to oscillatory conditions which, depending on their magnitude and phase relationships, can produce comparable stresses to those which occur at the line end. In addition, if some of the tapping winding is not in circuit, which happens whenever the transformer is on other than maximum tap, the tapping winding will then have an overhang which can experience a high voltage at its remote end. The magnitude of the impulse voltage appearing both across the neutral end sections and within the tapping winding overhang will be at a minimum when the initial distribution is linear, as can be seen from Fig 3.21 (c), and this is usually assisted by a further interleaved section at the neutral end. To some extent, the magnitude of the impulse voltage seen by the tapping winding due to overhang effects is dependent on the size of the tapping range, so this must be borne in mind when deciding the tapping range required.

1,4.11 Thermal considerations

When the resistive and other losses are generated in the transformer windings heat is produced. This heat must be transferred into, and taken away by, the transformer oil. The winding copper retains its mechanical strength up to several hundred degrees Celsius. Transformer oil does not degrade below about 140 ° C, but paper insulation deteriorates with greatly increasing

(a)

o,t

DISTANCE FROM LINE LEAD

(n)

OSCILLATION

FOLLOWING

INITIAL DISTRIBUTION

INITIAL

DISTRIBUTION

-4— OVERHANG

FIG. 3.21 Distribution of impulse voltage within winding

rapidity if its temperature rises above about 90 ° C. The cooling-oil flow, therefore, must ensure that insulation temperature is kept below this figure as far as possible.

The temperature at which no deterioration of paper insulation occurs is about 80 ° C. It is usually not economic or practical, however, to limit the insulation temperature to this level at all times. Insulation life would greatly exceed transformer design life and, since ambient temperatures and applied loads vary, a maximum temperature of 80 ° C would mean that on many occasions the insulation would be much cooler than this. Thus, the critical factor in determining the life of a transformer is the working temperature of the insulation or, more precisely, the temperature of the hottest part of the insulation or/tot spot. The problem

II

(a)Electrostatic shield

111111•1111111•1111LINE END

•••11•111••••

•11••••••••

(b) Dummy Strand E]

111•11•11• II • • • J11

M, 11 MI 11 MI

11 11 11 • IN

(b)Interleaving of 2 strands

over 4 discs

FIG. 3.22 Types of winding stress control

is to decide what temperature the hot spot should be allowed to reach. Various researchers have considered this problem and all of them tend to agree that the rate of deterioration or ageing of paper rapidly increases with increasing temperature. In 1930, Montsinger [4] suggested that the life of insulation would be halved for every 8 ° C increase between 90 and 110° C and this rate has been generally accepted, although some authorities now consider that a value of 6 ° C is more appropriate for present-day insulation materials.

It must be recognised that there is no generally accepted temperature at which insulation may be allowed to operate, nor is there agreement between transformer designers as to the precise hot spot temperature that should be accepted in normal operation. In fact, it is now recognised that there are other factors affecting insulation life, such as the moisture content, acidity and oxygen content of the oil, all of which tend to be dependent upon the system of breathing employed. Nevertheless BS171 and other international specifica-

dons set down limits for permissible temperature rise which aim at a life of about thirty years for the transformer. Such documents assume that a lifetime of this magnitude would be obtained with a hot spot

temperature of about 98 ° C.

It must also be recognised that the specified temperature rise can only be that value which can be measured, and that there will usually be, within the transformer. a hot spot which is hotter than the temperature tha can be measured and which will really determine the life of the transformer.

Study of Cie permitted temperature rises given in BS171 shows that a number of different values are permitted and that these are dependent on the method of oil circulation. The reason for this is that the likely difference between the value that can be measured and the hot spot, which cannot be measured, tends to vary according to the method of oil circulation. Those listed in BS171 are:

•Natural.

•Forced, but non-directed.

•Forced and directed.

Natural circulation utilises the thermal head produced by the heating of the oil which rises through the windings as it is heated and falls as it is cooled in passing

through the radiators.

With forced circulation, oil is pumped from the radiators and admitted to the bottom of the windings to pass through the vertical ducts formed by the strips laid 'above' and 'below' the conductors. In referring to axial ducts within windings, the expressions 'below' and 'above' mean 'nearer to the core' and 'further from the core', than the winding turns respectively. Radial ducts are those which connect these. In a non-directed design, flow through the horizontal ducts which connect the axial ducts above and below is dependent entirely on thermal and turbulence effects and the rate of flow through these is very much less than in the axial ducts (Fig 3.23 (a)).

With a forced and directed circulation,

washers are inserted into the windings which alternately close off the axial ducts above and below the conductors, so that the oil in passing through the winding must also weave its way through the horizontal ducts. This arrangement is illustrated in Fig 3.23 (b).

The average temperature rise of the winding is measured by its change in resistance compared with that at a known ambient temperature. Since some of the winding at the bottom of the leg is in cool oil and some at the top is in the hottest oil, there will be a difference from the average at either extreme by an amount equal to half the difference in temperature of the inlet and outlet oil. In addition all the conductors may not be equally exposed to the oil. In the diagram of Fig 3.24, which represents a group of conductors surrounded by vertical and horizontal cooling ducts, the four conductors at the corners are cooled directly

(a) Non-directed flow

41111111111111111111

(c) Directed flow

Flo. 3.23 Directed and non-directed oil flow

on two faces, whilst the remainder are cooled only on one. Further, unless the oil flow is forced and directed, not only will the heat transfer via the horizontal surfaces be poorer, due to the poor oil flow-rate, but this oil could well be hotter than the general mass of oil in the vertical ducts.

The temperature of the winding hot spot is thus the sum of the following:

•Ambient.

•Measured (specified) rise by resistance.

•Half difference between inlet and outlet oil.

•Difference between average and maximum winding/ oil temperature gradient.

VERTICAL COOLING

DUCTS

FIG. 3.24 Winding hot spots

Typical values are:

explained in BS Code of Practice CP 1010 [5], which deals with the subject of loading of power transformers. The system works well in practice since very few transformers are operated continuously at rated load. Even in power stations where loads tend to be more constant than for many other applications, loads vary as the unit load is varied or as the unit is started up and shut down, and ambient temperatures vary seasonally.

There is one exception to this loading pattern. This is the generator transformer of a large base-load unit. Ideally, this operates at near to its designed rating continuously, apart from its periodic maintenance shutdown. This will be discussed further in Section 2 of this chapter dealing with the subject of generator transformers.

1.4.12 Performance under short- circuit

The effects of short-circuit currents on transformers, as on most other items of electrical plant, fall into two categories:

It must be stressed that since items (c) and (d) cannot be covered by specification, they are typical values only and actual values will differ between manufacturers and so, therefore, will the value of hot spot temperature. It will be noted also, that the hot spot temperatures derived significantly exceed the figure of 98 ° C quoted above as being the temperature which corresponds to normal ageing. It will also be seen that the figure used for ambient temperature is not the maximum recognised in BS171, which permits an ambient temperature of 40 ° C, giving a hot spot temperature of 120°C. Such temperatures are permissible because the maximum ambient temperature occurs only occasionally and for a short time.

When a transformer is operated at a hot spot temperature above that which produces normal ageing due to increase either in ambient or loading, then insulation life is used up at an increased rate. This must then be offset by a period with a hot spot temperature at or below that for normal ageing, so that the total use of life over this period equates to the norm. This is best illustrated by an example: if two hours are spent at a temperature which produces twice the normal rate of ageing then four hours of life are used in this period. For the balance of those four hours (i.e., 4 — 2.2) the hot spot temperature must be such as to use up no life, i.e., below 80 ° C, so that in total four hours of life are used up. Jlhjs principle is fully

•Thermal.

•Mechanical forces.

It is a fairly simple matter to deal with the thermal effects of a short-circuit. This is deemed to persist for a known period of time, usually 3 or 5 s, allowing for clearance of the fault by back-up protection. During this brief time, it is safe to assume that all the heat generated remains in the copper. Therefore knowing the mass of the copper, its initial temperature, and the heat input, the temperature which it can reach can be easily calculated. It simply remains to ensure that this is below a permitted maximum which for oilimmersed windings is taken to be 250 ° C, in accordance with Table III of BS171: Part 5: 1978.

Mechanical short-circuit forces are more complex. First, there is a radial force which is a mutual repulsion between LV and HV windings. This tends to crush the LV winding inwards and burst the HV winding outwards. Resisting the crushing of the LV winding is relatively easy since the core lies immediately beneath and it is only necessary to ensure that there is ample support, in the form of the number and width of axial strips, to transmit the force to the core. The outwards bursting force on the HV winding is resisted by the tension in the copper, coupled with the friction force produced by the large number of HV turns which resists their slackening off. This is usually known as the 'capstan effect'. Since the tensile strength of the copper is quite adequate in these circumstances, the outward bursting force on the HV winding does not normally represent a problem either. An exception is any outer winding having a small number of turns, particularly if these are wound in a simple helix. This can be the case with an outer tapping winding or sometimes the HV winding of a unit transformer which,

FIG. 3.26 Forces within windings

to brace against the outward bursting forces. In addition, spreading the tapping turns throughout the full length of the layer would create problems in taking the HV line-lead away from the centre of the winding. Another factor which makes it difficult to obtain complete magnetic balance is the dimensional accuracy and stability of the materials used. Paper insulation and pressboard in a large winding shrink axially by several centimetres during dry-out and assembly of the windings. Although the manufacturer can assess the degree of shrinkage expected fairly accurately, and will attempt to ensure that it is evenly distributed, it is difficult to do this with sufficient precision to ensure complete balance.

Furthermore, shrinkage of insulation continues to occur in service and, although the design of the transformer should ensure that the windings remain in compression, it is even more difficult to ensure that such shrinkage will be uniform. With careful design the degree of unbalance will be small. Nevertheless it must be remembered that short-circuit forces are proportional to (current) 2 and that the current in question is the peak asymmetrical current and not the RMS value. Consequently, for a generator transformer, having an impedance of 16%, the magnitude of the force might be 116 times that occurring under normal full-load conditions, i.e., (6 x full load current) x asymmetry factor of 2.55. The effect of magnetic unbalance is to produce an additional component of radial leakage flux which acts in the same sense

throughout the winding. It thus produces a force which is upwards on one winding and downwards on the other (Fig 3.26 (b)).

Axial forces under short-circuit are resisted by transmitting them to the core. The top and bottom core frames incorporate pads which bear on the ends of the windings, these pads distributing the load by means of heavy-section pressboard or compressed laminatedwood platforms. The top and bottom core frames, in turn, are linked together by steel tie-bars which have the dual function of resisting axial short-circuit forces and ensuring that when the core and coils are lifted via the top core frames on assembly, the load is supported from the lower frames. These tie-bars can be seen in Fig 3.12 which shows a completed core before fitting of the windings. Since the precise magnitude of these forces depends much upon leakage flux, and the leakage flux pattern also determines the value of reactance, manufacturers nowadays have computer programmes for accurate determination of leakage flux which also, therefore, enable them to assess shortcircuit forces and accurately design for them.

1.5 Tappings and tapchangers

Almost all power station transformers incorporate some means of adjusting their voltage ratio by means of the addition or removal of tapping turns. This adjustment may be made on-load, as is usual in generator and station transformers, or by means of an off-circuit switch, or by the selection of bolted link positions with the transformer totally isolated. The degree of sophistication of the system of tap selection depends on the frequency with which it is required to change taps.

1.5.1 Uses of tapchangers

It is first necessary to examine the purposes of tapchangers and the way in which they are used. A more complete discussion of this subject will be found in Chapter 1 of this volume dealing with design and operation of the connections to the grid system and the auxiliary transformers forming the electrical auxiliary system, but the transformer engineer must know what is required of the plant and why.

Dealing first with the electrical auxilary system, the design of a suitable supply system for auxiliary plant must cater for all operating conditions, for example, unit start-up, full-load and emergency operation, and the outage of supply equipment. It must also optimise the various component impedances to achieve a suitably economical compromise between the conflicting requirements of fault-level limitation and acceptable voltage regulation.

This design task is greatly assisted by the use of off-circuit taps on the 11/3.3 kV and 3.3/0.415 kV auxiliary transformers, especially when precise information regarding loads and operating conditions cannot be established during the design period.

Supplies to the I-1V side of the station transformer at 132, 275 or 400 kV may be varied by ±10 07o to suit system loading conditions, under the direction of Grid Control and beyond the control of the power station operating staff. If the 11 kV station voltage is to be maintained sensibly constant, therefore, an onload tapchanger on the station transformer must be available to the station operators. It should be noted that if the stotion operator uses this tapchanger simply to maintain the station 11 kV system voltage constant as the system voltage varies, then the station transformer flux density remains constant. If, however, the operator also endeavours to compensate for regulation on the 11 kV station system, as he may need to do, then this will require an increase in the volts/turn and hence an increase in the flux density. The design flux density of the station transformer must take this operating condition into account.

The unit transformer 1-JV terminals are connected to those of the generator whose voltage is maintained within +5% of nominal by the action of the generator AVR. With such close control of its primary voltage, therefore, an on-load tapchanger is not necessary on the unit transformer. It is a wise precaution, however, to provide off-circuit taps so that some adjustment of the voltage ratio may be made should this be found to be necessary for the reasons outlined above with reference to auxiliary transformers.

The generator transformer is used to connect the generator whose voltage is maintained within ±5 0/o of nominal, to a 400 kV system which normally may vary by ±10%. This cannot be achieved without the ability to change taps on load. However, in addition to the requirement of the generator to produce megawatts, there may also be a requirement to generate or absorb VArs, according to the system conditions, which will vary due to several factors, for example, time of day, system conditions and required power transfer. Generation of VArs will be effected by tapping-up on the generator transformer, that is, increasing the number of [-IV turns for a given 400 kV system voltage. Absorption of VArs will occur if the transformer is tapped down. As with the station transformer, this mode of operation also leads to variation in flux density which must be taken into account when designing the transformer. The subject is more complex than for the station transformer however, and will be described in more detail in Section 2 of this chapter which deals specifically with the generator transformer.

1.5.2 Impedance variation

In discussion of the subject of leakage flux and shortcircuit forces, mention has already been made of the unbalance effect created by the provision of tappings. As tappings are added or removed from one of the windings without any compensating change on the other winding, there will be a change in the degree of 'out of balance', a change irt the leakage flux pattern

and a resulting change of impedance. The auxiliary system designer would, of course, prefer to be able to change the voltage ratio without affecting impedance but the best the transformer designer can do is to aim to minimise the variation or possibly achieve an impedance characteristic which is acceptable to the system designer rather than one which might aggravate his problems. Any special measures which the transformer designer is required to take is likely to increase cost and must therefore be totally justified by system needs.

Figure 3.27 represents a series of sections through the windings of a two-winding transformer having the tappings in the body of the HV winding. In all three cases the HV winding is slightly shorter than the LV winding in order to allow for the extra end insulation of the former. in Fig 3.27 (a) all tappings are incircuit, Fig 3.27 (b) shows the effective disposition of the windings on the principal tapping and Fig 3.27 (c)

DISPLACEMENT OF CENTRE

LINES OF HALF WINDINGS

(a) Maximum tap

(b) Pnncipal tap

x = DISPLACEMENT OF CENTRE

ci Minimum tap

FIG." 3.27 Effects of tappings within windings

when all tappings are out-of-circuit. It can be seen that, although all the arrangements are symmetrical about the winding centreline and therefore have overall axial balance, the top and bottom halves are only balanced in the condition represented by Fig 3.27 (b). This condition will therefore have the minimum leakage flux and hence the minimum impedance. Addition or removal of tappings increases the unbalance and thus increases the impedance. It can also be seen that the degree of unbalance is greatest in Fig 3.27 (c), so that this is the condition corresponding to maximum impedance. A plot of impedance against tap position would thus tend to be of the form shown in Fig 3.28 (a). It can be seen that the tap position for which the unbalance is minimum can be varied by the insertion of gaps in the untapped winding so that the plot is reversed (Fig 3.28 (b)) and, by careful manipulation of the gaps at the centre of the untapped winding and the ends of the tapped winding, a more or less symmetrical curve about the mean tap position can be

obtained. This is usually the curve which gives minimum overall variation.

From this, it will be apparent also that the variation will be reduced if the space which the taps occupy can be reduced to a minimum. While this can be achieved by increasing the current density in the tapping turns, the extent to which this can be done is limited by the need to ensure that the temperature rise in this section does not greatly exceed that of the body of the winding, since this would then create a hot spot. If it is necessary to insert extra radial cooling ducts in order to limit the temperature rise, then the space taken up by these offsets some of the space savings gained from the increased current density. The designer's control of temperature rise in the taps tends to be less than that which can be achieved in the body of the winding, where the designer can vary the number of sections by adjusting the number of turns per section, with a radial cooling duct every one or two sections. In the taps, the turns per section are dictated by the need to ensure that the tapping leads appear at the appropriate position on the outside of a section, hence one tap must span an even number of sections, with a minimum of two.

With the tappings contained in a separate layer, external to the HV winding, the degree of impedance variation throughout the tapping range tends to be less than for taps in the body of the HV winding. It can be seen from Fig 3.29 that the highest degree of bal-

ance will be achieved when there are no tapping turns in circuit, provided that the LV and I-1V winding are of the same length. But, as explained in Section 1.4.9 of this chapter, one of the occasions when it is necessary to provide a separate tapping layer is when

incling is star-connected and has graded insulation, an d with this arrangement both EIV and LV windings ■\ ill have similar applied voltage Lest levels so that it is logical tha they should have the same amount of end insulation and thus be of the same length. Furthermore, the physical size of the tapping winding is minimised and the minimum impedance will coincide with the mean tap position if it is arranged that the tapping winding is connected in a 'buck' and

ment. in this arrangement instead of all taps being additive to the minimum HV turns, at one extreme of the tapping range all taps are in circuit but of such a polarity as to subtract from the voltage induced in the main HV winding and at the other end of the range all are connected in the opposite sense, i.e., additive. This arrangement is frequently used on larger transformers, where the saving in space occupied by the tapping winding more than offsets the extra complexity of the on-load tapchanger required to provide buck/boost switching, and the reduced impedance variation is an added benefit.

1.5,3 Tapchanger mechanisms

The principal of on-load tapchanging was developed in the late 1920s and requires a mechanism which will meet the following two conditions:

•The load current must not be interrupted during a tapchange.

•No section of the transformer winding may be shortcircuited during a tapchange.

Early on-load tapchangers made use of reactors to achieve these ends but in modern on- load tapchangers these have been replaced by transition resistors which have many advantages. (In fact, the first resistortransition tapchanger made its appearance in 1929, but the system was not generally adopted in the UK until the 1950s. In the USA, the change to resistors is only now taking place in the 1980s.) The principle of all resistor-transition on-load tapchangers may be seen by reference to Fig 3.30. Alternate tapping connections are brought out from the tapping winding to two banks of selector switches SI and 52. The load current connection, which is usually the neutral in the case of neutral-end tapping windings of star-connected station or generator transformer HV windings, is taken from these selector switches via a diverter switch which Is arranged so that it connects to each bank of selector switches in turn, either solidly when the required tap has been selected, or via a transition resistor (or resistors) at the instant of changeover. This will be

LINE TERMINAL

S I

FIG. 3.30 Resistor transition on-load tapehanger

clarified by considering an actual tap change, say from tap 3 to tap 4 in the diagram. With tap 3 selected, the diverter switch is made to the right hand set of main and transition contacts MI and Ti. In order to change to tap 4, the selector switch of bank 52 must first move to contact 4. The diverter switch then starts to move to the left and, as it does so, contact M1 opens first, putting resistor RI in circuit. Further movement of the diverter switch bridges transition contacts Tl and 12, so that load current commences to flow from tap 4, and the section of the tapping winding between taps 3 and 4 will be circulating current through resistors RI and R2. As the diverter switch contact Ti separates, all the load current is transferred to tap 4, and flows via R2 until, finally, contact M2 is made, thus taking all current and shorting out R2. Should the tapchanger be required to return to tap 3, the sequence followed would be the reverse of the above. However, if the

next tapchange is to tap 5, the selector switch mechanism must be driven to the next contact 5 of bank Si before changing over the diverter switch. This is achieved by incorporating 'lost motion' into the drive train so that the initial output from the drive motor operates the selector switches only. For a fuller description of operating mechanisms the reader should consult a manufacturer's operation and maintenance manual.

The resistor-transition tapchangers described above brought great advantages to on-load tapchanging. By using a resistor to bridge the transformer taps, the currents to be switched are made to be in phase with the respective voltages across the diverter switch contacts, with the result that the arc extinguishes at current and voltage zero and the restrike voltage across the contacts does not build up to a maximum for a further quarter of a cycle. In addition, the diverter switch mechanism is made of the stored energy pattern, i.e., the drive motor winds up a spring which is then released when fully charged to change over the switch. The mechanism is thus highly reliable in that any drive failure before the spring is fully charged si mply results in the tapchange not taking place and, once released, the spring ensures that the diverter switch changes over and the transformer remains in a safe condition regardless of any subsequent failure of the drive train.

Current transfer in these modern tapchangers takes about 40-70 ms. This rapid operation, coupled with the resistive switching, ensures that contact erosion is minimised and reduces arc products which contaminate the oil, with the result that maintenance intervals can be increased and maintenance, when it is required, is si mplified.

1.5.4 Single compartment tapchangers

Although the quantity of arc products and extent of oil contamination produced by the diverter switches of a high speed resistor transition tapchanger are very much reduced as compared with reactor transition tapchangers, it is nevertheless practice to mount these in a separate compartment from the banks of selector contacts. This may be partly due to the fact that these tapchangers were developed from reactor gear in which the physical size of the changeover contacts and the extent of maintenance required necessitates a separate compartment, but it has the advantage that it reduces the quantity of oil which may have to be processed regularly and also makes access to diverter switches easier. On a large transformer having a large tapchanger, this arrangement adds little, if anything, to the overall cost. On small auxiliary transformers however, mounting diverter switches and their resistors separately not only adds greatly to costs but also to physical size, with consequent extra indirect costs.

The fact that the resistor transition diverter switch arrangement was inherently quicker and cleaner than

its predecessor, coupled with the objective of reducing the cost of the smaller units, led to the further development of the resistor transition principle to produce small single compartment tapchangers — both selector and diverter switch located in the same compartment — for smaller transformers.

The following description of the operation of a small single compartment tapchanger is based on that of the Ferranti DS Series shown diagrammatically in Fig 3.31 and pictorially in Fig 3.32. Speed of operation has been increased, compared with the separate compartment types, by the use of a single transition resistor and by the combination of selector and diverter switches into a single assembly which achieves the necessary changeover with the minimum of movement.

The stored energy device consists of a falling weight and tensioned spring wound up by a drive motor so that, when released, the weight falls very rapidly aided by the spring. The main current carrying contact (C) is shown (Fig 3.31 (a)) connecting tap contact Ti to the neutral point of the transformer winding, with the main auxiliary arcing contact a also in contact with T1 The switching sequence of changeover to tap contact 12 is as follows:

•When the stored energy mechanism operates and the moving contact assembly commences to travel, the main current carrying contact C opens and the circuit is maintained via the main auxiliary arcing contact a.

•Transitional contact t then makes contact with T2 and the bridging resistor R is then connected between tap contacts T1 and T2 (Fig 3.31 (b)).

•The main auxiliary contact a now breaks contact with Ti leaving resistor R momentarily in circuit to carry the load current.

•Contact a then makes contact with T2 whilst transitional contact t rolls round tap contact T2. (Fig 3.31 (c)). The bridging resistor R is thereby shorted out and the circuit is made via the main auxiliary arcing contact a.

• Transitional contact t breaks from T2.

•Finally, as the moving-contact assembly reaches the end of its travel, the main current carrying contact C closes on tap contact 12 (Fig 3.31 (d)).

The main current carrying contact C does not make or break current and should, therefore, last the lifeti me of the transformer.

1.5.5 1n- tank tapchangers

Both the separate compartment and single compartment tapchangers have been contained separately from the core and windings so that even the selector switches, which do not break current, are not operating in the