Industrial Power Transformers-- Transformer construction (part 5)

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Almost all 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 the case for many large transformers, 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 and the size and importance of the transformer.

At the start two definitions from the many which are set out in EN 60076-1: principal tapping is the tapping to which the rated quantities are related and, in particular, the rated voltage ratio. This used to be known as normal tapping and the term is still occasionally used. It should be avoided since it can easily lead to confusion. It should also be noted that in most transformers and throughout this book, except where expressly indicated otherwise, tappings are full-power tappings, that is, the power capability of the tapping is equal to rated power so that on plus tappings the rated current for the tapped winding must be reduced and on minus tappings the rated current for the winding is increased. This usually means that at minus tappings, because losses are proportional to current squared, losses are increased, although this need not always be the case.

Uses of tapchangers

Before considering the effects of tappings and tapchangers on transformer construction 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 a work dealing with the design and operation of electrical systems.

Aspects of tapchanger use relating to particular types of transformers will be discussed further in Section 7, but the basic principles apply to all transformer types and are described below.

Transformer users require tappings for a number of reasons:

• To compensate for changes in the applied voltage on bulk supply and other system transformers.

• To compensate for regulation within the transformer and maintain the out put voltage constant on the above types.

• On generator and interbus transformers to assist in the control of system VAr flows.

• To allow for compensation for factors not accurately known at the time of planning an electrical system.

• To allow for future changes in system conditions.

All the above represent sound reasons for the provision of tappings and, indeed, the use of tappings is so commonplace that most users are unlikely to consider whether or not they could dispense with tappings, or perhaps limit the extent of the tapping range specified. However, transformers without taps are simpler, cheaper and more reliable. The presence of tappings increases the cost and complexity of the transformer and also reduces the reliability. Whenever possible, therefore, the use of tappings should be avoided and, where this is not possible, the extent of the tapping range and the number of taps should be restricted to the minimum. The following represent some of the disadvantages of the use of tappings on transformers:

• Their use almost invariably leads to some variation of flux density in operation so that the design flux density must be lower than the optimum, to allow for the condition when it might be increased.

• The transformer impedance will vary with tap position so that system design must allow for this.

• Losses will vary with tap position, hence the cooler provided must be large enough to cater for maximum possible loss.

• There will inevitably be some conditions when parts of windings are in use, leading to less than ideal electromagnetic balance within the transformer which in turn results in increased unbalanced forces in the event of close up faults.

• The increased number of leads within the transformer increases complexity and possibility of internal faults.

• The tapchanger itself, particularly if of the on-load type, represents a significant source of unreliability.

One of the main requirements of any electrical system is that it should provide a voltage to the user which remains within closely defined limits regardless of the loading on the system. This despite the regulation occurring within the many supply transformers and cables, which will vary greatly from conditions of light load to full load. Although in many industrial systems, in particular, the supply voltage must be high enough to ensure satisfactory starting of large motor drives, it must not be so high when the system is unloaded as to give rise to damaging overvoltages on, for example, sensitive electronic equipment.

Some industrial processes will not operate correctly if the supply voltage is not high enough and some of these may even be protected by under-voltage relays which will shut down the process should the voltage become too low.

Most domestic consumers are equally desirous of receiving a supply voltage at all times of day and night which is high enough to ensure satisfactory operation of television sets, personal computers, washing machines and the like, but not so high as to shorten the life of filament lighting, which is often the first equipment to fail if the supply voltage is excessive.

In this situation, therefore, and despite the reservations concerning the use of tapchangers expressed above, many of the transformers within the public supply network must be provided with on-load tapchangers without which the economic design of the network would be near to impossible. In industry, transformers having on-load tapchangers are used in the provision of supplies to arc furnaces, electrolytic plants, chemical manufacturing processes and the like.

FIG. 40 shows, typically, the transformations which might appear on a section of public electricity supply network from the generating station to the user. The voltage levels and stages in the distribution are those used in the UK but, although voltage levels may differ to some degree, the arrangement is similar to that used in many countries throughout the world.

The generator transformer is used to connect the generator whose voltage is probably maintained within _5 percent of nominal, to a 400 kV system which normally may vary independently by _5 percent and up to _10 percent for up to 15 minutes. This cannot be achieved without the ability to change taps on load. However, in addition to the requirement of the generator to produce mega watts, 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 HV turns for a given 400 kV system voltage. Absorption of VArs will occur if the transformer is tapped down. This mode of operation leads to variation in flux density which must be taken into account when designing the transformer. The subject is fairly complex and will be described in more detail in Section 7.1 which deals specifically with generator transformers.

FIG. 40 Typical public electricity supply network

Interbus transformers interconnecting 400, 275 and 132 kV systems are most likely to be auto-connected. Variation of the ratio of transformation can not therefore be easily arranged since adding or removing tapping turns at the neutral end changes the number of turns in both windings. If, for example, in the case of a 400/132 kV autotransformer it were required to maintain volts per turn and consequently 132 kV output voltage constant for a 10 percent increase in 400 kV system voltage then the additional turns required to be added to the common winding would be 10 percent of the total. But this would be equivalent to 10 x 400/132 x 30.3 percent additional turns in the 132 kV winding which would increase its output from 132 kV to 172 kV. In fact, to maintain a constant 132 kV output from this winding would require the removal of about 17.2 percent of the total turns. Since 10 percent additional volts applied to 17.2 percent fewer turns would result in about 33 percent increase in flux density this would require a very low flux density at the normal condition to avoid approaching saturation under overvoltage conditions, which would result in a very uneconomical design. Tappings must therefore be provided either at the 400 kV line end or at the 132 kV common point as shown in FIG. 41. The former alternative requires the tapchanger to be insulated for 400 kV working but maintains flux density constant for 400 kV system voltage variation, the latter allows the tapchanger to operate at a more modest 132 kV, but still results in some flux density variation. Most practical schemes therefore utilize the latter arrangement. Alternatively these transformers may be used without tapchangers thereby avoiding the high cost of the tapchanger itself as well as all the other disadvantages associated with tapchangers identified above. The 'cost' of this simplification of the transformer is some slightly reduced flexibility in the operation of the 275 and 132 kV systems but this can be compensated for by the tappings on the 275/33 or 132/33 kV transformers, as explained below.

In the UK the 400 kV system is normally maintained within +-5 percent of its nominal value. If the transformers interconnecting with the 275 and 132 kV systems are not provided with taps then the variation of these systems will be greater than this because of the regulation within the interbus transformers.

The 275 and 132 kV systems are thus normally maintained to within +-10 percent of nominal. Hence 275/33 kV and the more usual 132/33 kV bulk supplies transformers must have tapchangers which allow for this condition. If, in addition, these transformers are required to boost the 33 kV system volts at times of heavy loading on the system as described in Section 2, that is when the 275 or 132 kV system voltage is less than nominal, it is necessary to provide a tap ping range extending to lower than +-10 percent, so it is common for these transformers to have tapping ranges of +-10 percent to +-20 percent. This runs counter to the aim of limiting the extent of the tapping range for high reliability in transformers, identified earlier, but represents another of the complexities resulting from the reduced system flexibility caused by omitting tap pings on the 400/132 kV transformers. Clearly tappings at the earthed neutral point of a star-connected 275 or 132 kV winding are likely to be more reliable and less costly than those operating at the 275 or 132 kV line end of a 400/275 or 132 kV interbus transformer.

FIG. 41 Alternative locations for tappings of 400/132 kV autotransformer

The greater degree of control which can be maintained over the 33 kV sys tem voltage compared with that for the 132 kV system means that 33/11 kV transformers normally need be provided with tapping ranges of only +-10 percent. As in the case of 132/33 kV transformers, however, the HV taps can still be used as a means of boosting the LV output voltage to compensate for system voltage regulation. In this case, this is usually achieved by the use of an open circuit voltage ratio of 33/11.5 kV, that is at no load and with nominal voltage applied to the HV the output voltage is higher than nominal LV system volts.

The final transformers in the network, providing the 11/0.433 kV trans formation, normally have a rating of 1600 kVA or less. These small low-cost units do not warrant the expense and complexity of on-load tapchangers and are thus normally provided with off-circuit taps, usually at +-2.5 percent and +-5 percent. This arrangement enables the voltage ratio to be adjusted to suit the local system conditions, usually when the transformer is initially placed into service, although the facility enables adjustments to be made at a later date should changes to the local system loading, for example, necessitate this.

FIG. 42 Typical variation of impedance with tap position for a two-winding transformer having taps in the body of one of the windings

Impedance variation

Variation of impedance with tap position is brought about by changes in flux linkages and leakage flux patterns as tapping turns are either added or removed from the tapped winding. Auxiliary system designers 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 minimize the variation or possibly achieve an impedance characteristic which is acceptable to the system designer rather than one which might aggravate his problems. It should be noted however that any special measures which the transformer designer is required to take is likely to increase first cost and must therefore be totally justified by system needs.

The magnitude and sense of the change depends on the winding configuration employed and the location of the taps. FIG. 42 shows typically the pattern of variation which may be obtained, although all of these options may not be available to the designer in every case. Figures 42(a) and (b) represent the type of variation to be expected when the taps are placed in the body of one of the windings.

FIG. 43 Effects of tappings within windings

FIG. 43 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. 43(a) all tap pings are in-circuit, FIG. 43(b) shows the effective disposition of the windings on the principal tapping and FIG. 43(c) when all the 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. 43(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. 43(c), so that this is the condition corresponding to maximum impedance. This enables an explanation to be given for the form of impedance variation shown in FIG. 42. FIG. 42(a) corresponds to the winding configuration of FIG. 43. 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 can be reversed ( FIG. 42(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 the degree of impedance variation throughout the tapping range tends to be less than for taps in the body of the HV winding but the slope of the characteristic can be reversed depending on where the taps are located. This is illustrated by reference to FIG. 44 which shows alternative arrangements having HV taps located either outside the main HV winding or inside the LV winding. Ampere-turn distributions for each extreme tap position is shown for both arrangements and also the resulting impedance variation characteristics. The arrangement having the taps located outside the HV winding is most commonly used and frequently the transformer will have a star-connected HV winding employing non-uniform insulation.

FIG. 44 Impedance variation with tap position with taps in a separate layer. In both cases HV winding is tapped winding

With this arrangement, described earlier in this section, the taps will probably have two sections in parallel and a centre gap to accommodate the HV line lead. The impedance characteristic shown in FIG. 44(b) will in this case be modified by the additional distortion of the leakage flux created by the center gap. This will probably result in an additional component of impedance and a resulting characteristic as shown in FIG. 45.

In the arrangements described above all the tappings are configured in a linear fashion, that is for each increasing tap position an equal number of tapping turns are added. However if these are contained in a separate layer, it is possible to configure these in a buck/boost arrangement as indicated in FIG. 46.

FIG. 45 Effect of gap in HV tapping winding on percentage impedance


FIG. 46 Connection of HV tapping winding in buck/boost arrangement


FIG. 47 Effect of type of tapping winding on impedance: Readers may wish to sketch the equivalent diagrams for the minimum tap condition. In this case the tapping winding makes no contribution to the total ampere-turns with the linear arrangement but adds negative ampere-turns with the buck/boost arrangement.


With this arrangement the taps are first inserted with a subtractive polarity, that is minimum tap position is achieved by inserting all taps in such a sense as to oppose the voltage developed in the main HV winding, these are removed progressively with increasing tap position until on mean tap all tapping turns are out and they are then added in the reverse sense until on maximum tap all are inserted. The advantage of this arrangement is that it reduces the physical size of the tapping winding and also the voltage across the tapping range. The reduction in size is beneficial whether this is placed inside the LV winding or outside the HV winding. In the former case a smaller tap winding enables the diameters of both LV and HV main windings to be reduced. In both cases it produces a small reduction in impedance, which is often useful in the case of large high-voltage transformers, as well as reducing the number of tapping leads. The reason for the impedance reduction will be apparent from a simple example: a transformer requires 1000 turns on principal tap with a tap ping range of _10 percent. With a linear arrangement this would have 900 turns in the body of the HV winding and 200 in the tapping winding. This is represented by FIG. 47(a). If a buck/boost arrangement were used the HV winding would have 1000 turns in the main body and 100 turns in the tap ping winding as shown in FIG. 47(b). Both arrangements utilize the same total number of turns but it is clear that the area of the ampere-turns diagram is less in the case of the buck/boost arrangement. The price to be paid for these benefits is a slightly more complicated and therefore more expensive tapchanger.

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:

(1) The load current must not be interrupted during a tapchange.

(2) No section of the transformer winding may be short-circuited 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 resistor-transition 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 only started to take place in the 1980s. Despite the fact that it was recognized that resistor transition had advantages of longer contact life, due to the relatively short arcing times associated with unity power factor switching, the centre tapped reactor type tapchanger was, in general, more popular because reactors could be designed to be continuously rated, whereas transition resistors had a finite time rating due to the high power dissipated when in circuit. This would have been of little consequence if positive mechanical tapchanger operations could have been assured but, although various attempts at achieving this were generally successful, there were risks of damage if a tapchanger failed to complete its cycle of operation.

With the earlier designs thermal protection arrangements were usually introduced, to initiate the tripping and isolation of the transformer. These early types of tapchangers operated at relatively low speeds and contact separation was slow enough for arcing to persist for several half-cycles. Arc extinction finally took place at a current zero when the contact gap was wide enough to prevent a re-strike. The arcing contacts were usually manufactured from plain copper.

The mechanical drive to these earlier tapchangers, both resistor or reactor types, was either direct drive or the stored energy type, the stored energy being contained in a flywheel or springs. But such drives were often associated with complicated gearing and shafting and the risk of failure had to be taken into account.

Most of these older designs have now been superseded by the introduction of the high-speed resistor-type tapchanger. Reliability of operation has been greatly improved, largely by the practice of building the stored energy drive into close association with the actual switching mechanism thus eliminating many of the weaknesses of earlier designs. The introduction of copper tungsten alloy arcing tips has brought about a substantial improvement in contact life and a complete change in switching philosophy. It is recognized that long contact life is associated with short arcing time, and breaking at the first cur rent zero is now the general rule.

The bridging resistors are short-time rated but with the improved mechanical methods of switch operation and the use of high performance resistance materials, such as nickel chrome alloy, there is only a negligible risk of resistor damage as the resistors are only in circuit for a few milliseconds. The switching time of a flag cycle, double resistor tapchanger (see below) is usually less than 75 ms.

A further advantage with high-speed resistor transition is that of greatly improved oil life. The oil surrounding the making and breaking contacts of the on-load tapchanger becomes contaminated with carbon formed in the immediate vicinity of the switching arc. This carbon formation bears a direct relationship to the load current and arcing time and whereas with earlier slow-speed designs the oil had to be treated or replaced after a few thousand operations a life of some 10 times this value is now obtainable.

The mid-point reactor type of tapchanger has some advantages over the high-speed resistor type, the main one being that since the reactor can be left in circuit between taps, twice as many active working positions can be obtained for a given number of transformer tappings, giving a considerable advantage where a large number of tapping positions are required and this arrangement is still used by North American utilities. A number of special switching arrangements including shunting resistors, and modification to the winding arrangement of the reactor to enable use of vacuum switches, have been introduced to improve contact life where reactors are employed, but there are definite limits to the safe working voltage when interrupting circulating currents.

Recommendations for on-load tapchanging have been formulated as EN 60214, Tap Changers - Part 1 Performance requirements and test methods was issued as a CENELEC document in 2003. The intention is that IEC 60542 Application Guide for on-load tapchangers will become Part 2 of EN 60214.

These documents are primarily written to set performance standards and offer guidance on requirements for high-speed resistor-type equipment.

In some of the earliest designs of tapchangers the transformer was equipped with two parallel tapping windings. Each tap winding was provided with a form of selector and an isolating switch. When a tapchange was required the isolating switch on one winding was opened, the load being transferred to the other tapping winding, the selector switch on the open circuit winding was then moved to its new position and the isolator reclosed. The second winding was treated in exactly the same manner and the operation was completed when both windings were finally connected in parallel on the new tapping position.

This scheme had the drawback that both halves of the windings were over loaded in turn, and the transformer had to be designed to restrict the circulating current which existed during the out of step mid-position. Any failure in the switching sequence or the switch mechanisms could be disastrous.

It is useful to explain the methods of tapchanging which have been used in the past and those which are in use today.

FIG. 48 On-load tapchanging by reactor transition

On-load tapchanging by reactor transition

The simplest form of reactor switching is that shown in FIG. 48. There is only a single winding on the transformer and a switch is connected to each tapping position. Alternate switches are connected together to form two separate groups connected to the outer terminals of a separate mid-point reactor, the windings of which are continuously rated. The sequence of changing taps is shown in the table on the diagram. In the first position, switch No. 1 is closed and the circuit is completed through half the reactor winding.

To change taps by one position, switch No. 2 is closed in addition to switch No. 1, the reactor then bridges a winding section between two taps giving a mid-voltage position. For the next tapchange switch No. 1 is opened and switch No. 2 is left closed so that the circuit then is via the second tap on the transformer winding. This particular type of tapchanger necessitates a relatively large number of current breaking switches which in turn produce a bulky unit and consequently a large oil volume is involved.

On-load reactor-type tapchanger using diverter switches

A modified type of reactor tapchanger is shown in FIG. 49. This arrangement uses two separate selectors and two diverter switches. The selectors and diverter switches are mechanically interlocked and the sequence of operation is as follows. A tapchange from position 1 to 2 is brought about by opening diverter switch No. 2, moving selector switch No. 2 from tap connection 11 to tapping connection 10 and then closing diverter switch No. 2.

A tapchange from position 2 to 3 initiates a similar sequence utilizing selector and diverter switches No. 3 in place of switch No. 2.

FIG. 49 On-load reactor-type tapchanger using diverter switches

FIG. 50 On-load reactor-type tapchanger with vacuum switch

On-load reactor-type tapchanger with vacuum switch

In some instances it is possible to utilize a vacuum interrupter in conjunction with a re-designed winding arrangement on the reactor-type tapchanger.

A typical schematic diagram for this type of unit is shown in FIG. 50.

The running position for tap 1 is shown in the diagram with all switches closed. A tapchange from tap position 1 to tap position 2 is as follows.

Diverter switch No. 2 opens without arcing and the load current flows via selector switch No. 2, vacuum switch No. 4, in parallel with the circuit via diverter switch No. 3, selector switch No. 3 through diverter switch No. 3. Vacuum switch No. 4 opens, selector switch No. 2 moves from tap connection 11 to tap connection 10, vacuum switch No. 4 closes, diverter switch No. 2 closes, completing the tapchange to tap position 2. A tap change from tap position 2 to tap position 3 utilizes selector No. 3, diverter switch No. 3 and vacuum switch No. 4 in a similar manner to that explained for the movement from tap position 1 to tap position 2.

Whenever vacuum switches are used, the problem of protection against loss of vacuum must be considered. In North America, two approaches to this problem have been considered. The first is the current balance method where a current transformer detects the current flowing through the vacuum switch. If this does not cease on opening the switch mechanically the tap changer locks out after one tapchange during which the selector contact is called upon to break load and circulating currents. The second method utilizes a transformer which applies a medium voltage across the vacuum gap between the closed contacts and a special metal contact sheath. If the gap breaks down, a relay ensures that the next tapchange does not take place. A series contact disconnects this voltage before each tapchange is initiated.

Diverter resistor tapchangers

The concept of enclosure of the arc is attractive in many ways since it prevents oil contamination and eliminates the need for a separate diverter switch compartment. Even though the contact life of a high-speed resistor tapchanger is longer than that of a reactor type, the question of using vacuum switching of resistor units has been seriously considered for many years. Several designs have been proposed utilizing the principle of removing the vacuum switches from the circuit and thereby from both current and voltage duties between tapchanges.

In the USA, on-load tapchangers are frequently fitted on the LV winding, and as stated in Clause 4.2 of ANSI C57.12.30-1977, 32 _ 5/8% steps are quite normal. To meet these conditions it is more economical to use a reactor for the transition impedance and to utilize the bridging position as a tapping. This reduces the number of tapping sections required on the transformer winding.

For this purpose, gapped iron cored reactors with a single centre tapped winding are employed. The voltage across the reactor is equal to that of two tap ping steps and the magnetizing current at that voltage is approximately 40-50 percent of the maximum load current. Figures 4.51 and 4.52 illustrate typical examples of North American practice employing reactor on-load tapchangers.

As previously mentioned, high-speed resistor-type tapchangers have now almost completely superseded the reactor type in many parts of the world since it is easier and more economical to use resistors mounted in the tap changer and the transformer tank need only be designed to accommodate the transformer core and windings.

FIG. 51 Three-phase reactor for a 200 MVA, 230/67 kV autotransformer with tapping at the LV line end (Federal Pacific Electric Co.)

In general high-speed diverter resistor tapchangers fall into two categories.

The first is referred to as the double compartment type, having one compartment containing the selectors which when operating do not make or break load or circulating currents and a second compartment containing the diverter switches and resistors. It is in this compartment that all the switching and associated arcing takes place and where oil contamination occurs.

FIG. 52 120 MVA, 230/13.8 kV, three-phase transformer with reactor pocket and the on-load tapchanger attached to the end of the tank (Federal Pacific Electric Co.)

It is usual therefore to ensure that the oil in this chamber is kept separated from that in the main transformer tank. Double compartment-type tapchangers can also be considered to be of two types:

(a) In-tank type.

(b) Externally mounted type.

In-tank-type tapchangers

In the UK for many years the practice has been to house even the selector switches, which do not make or break current, in a separate compartment from the main tank so that these are not operating in the same oil as that which is providing cooling and insulation for the transformer. The operating mechanism for the selector switch contacts and the contacts themselves suffer wear and require maintenance, contact pressures have to be periodically checked, minute metallic particles are produced and contaminate the oil. However, modern selector switch mechanisms have been developed since the early 1960s which need very little maintenance and cause very little oil contamination as a proportion of total quantity of oil in the main tank. These tapchangers have been designed for installation directly in the oil in the main tank, an arrangement which the manufacturers claim is cheaper, although the economic argument is a complex one.

They have the advantage that all tapping leads can be formed and connected to the appropriate selector switch contacts before the transformer is installed in the tank. With the separate compartment pattern, the usual practice is for selector switch contacts to be mounted on a base board of insulating material which is part of the main tank and forms the barrier between the oil in the main tank and that in the selector switch compartment. The tapping leads thus cannot be connected to the selector contacts until the core and windings have been installed in the tank. This is a difficult fitting task, requiring the tapping leads to be made up and run to a dummy selector switch base during erection of the transformer and then disconnected from this before tanking.

Once the windings are within the tank, access for connection of the tapping leads is restricted and it is also difficult to ensure that the necessary electrical clearances between leads are maintained. With in-tank tapchangers it is still necessary to keep the diverter switch oil separate from the main-tank oil.

This is usually achieved by housing the diverter switches within a cylinder of glass-reinforced resin mounted above the selector switch assembly. When the transformer is installed within the tank, removal of the inspection cover which forms the top plate of this cylinder provides access to the diverter switches. These are usually removable via the top of the cylinder for maintenance and contact inspection. Such an arrangement is employed by the German company Maschinenfabric Reinhausen (MR) for their OILTAP series described below.

Another claimed disadvantage of the in-tank tapchanger is that the selector switch contacts do, in fact, switch small capacitative currents thus generating gases which become dissolved in the oil. These dissolved gases can then cause confusion to any routine oil monitoring program which is based on dissolved gas analysis (see Section 6.7). In addition it is, of course, necessary to take a drive from the diverter switch compartment through to the selector switches and this usually requires a gland seal. There have been suggestions that this seal can allow contaminating gases to pass from the diverter switch compartment into the main tank thus distorting dissolved gas figures. This was such a serious concern of those traditionally preferring separate compartment tapchangers that before acceptance of IEC 214 as a CENELEC harmonization document an additional test was inserted into the Service duty test specification as a demonstration that hydrocarbon gases would not leak through the gland seal. This requires that the tapchanger undergoing service duty testing be placed in a chamber, not exceeding 10 times the volume of the diverter switch compartment, filled with clean new transformer oil. At the end of the test sequence a sample of oil from this chamber is required to be tested for dissolved hydrocarbon gases which shall not show a total increase greater than 10 ppm (EN 60214-1:2003 Clause

FIG. 53 Three-phase in-tank tapchanger. Exploded view showing interior of diverter switch compartment.

An example of an in-tank tapchanger is shown in FIG. 53. The unit illustrated is rated at 600 A, three-phase, it has a maximum volts per step of 3.3 kV and is suitable for transformers having a highest voltage for equipment from 72.5 to 300 kV. It can be fitted so as to provide a linear tapping range having 18 positions or, if used with a changeover selector, 35 positions maximum. In-tank tapchangers may also be utilized using three separate single-phase units; the advantage of this configuration lies in the fact that the phase to earth voltage only appears across the upper insulated housing which can be extended to pro vide appropriate insulation levels, whilst interphase clearances are determined by the design of the transformer. These clearances, together with an increase of the surrounding radial distance from the tank wall permit the working voltage to be extended to higher values more economically for certain applications than is the case with externally mounted tapchangers.

The diverter is designed as a three-pole segmental switch with the three sections spaced 120º apart. The sections of the diverter switches may be connected in parallel for currents up to 1500 A when the switch is used as a single-phase unit. When used on non-uniform insulation star-point applications the diverter becomes a complete three-phase switch for currents up to 600 A.

The whole diverter switch assembly may be lifted out of the upper housing for inspection or contact changing, and this housing is completely sealed from the oil in the main tank with the exception of the drive to the selector switches.

The selectors are built in a 'cage' whose vertical insulating bars retain the fixed contacts and the transformer tapping connections are bolted directly to these terminals, with the odd and even selectors concentrically driven by independent Geneva mechanisms. The cage design eliminates the need for a barrier board as on an externally mounted tapchanger, but access to the selectors necessitates removal of part or all of the transformer oil in the main tank.

If required, it is possible to install the equipment with separate tanks and barrier boards to improve selector accessibility but, of course, the main benefit of using an in-tank tapchanger is lost. FIG. 54 illustrates an in-tank-type tapchanger mounted from the tank cover and showing the leads from the HV winding.

FIG. 54 In-tank tapchanger showing connection of leads to diverter switches and arrangement for supporting tapchanger from top of tank (TCM Tamini)

In-tank tapchangers with vacuum diverter switches

As has been mentioned above reactor-type tapchangers which use vacuum switches have been used for a number of years. The use of these devices for switching transition resistors has until recently proved impracticable, particularly for in-tank tapchangers because their physical size does not permit their incorporation into a convenient package. There are many advantages in using vacuum switches, however, and some of these are set out below:

• The vacuum interrupter is hermetically sealed and so there is no interaction between the arc and the surrounding medium.

• The switching characteristics do not depend on the surrounding insulating medium.

• The arc voltage drop in a vacuum is considerably less than in oil. The switch therefore has lower energy consumption.

• Contact wear is reduced.

• Bi-products arising from switching in oil, particularly carbon, is eliminated, hence there is no need for periodic filtration of oil, no disposal problem for contaminated oil.

• Dielectric recovery time is rapid leading to very rapid switching.

FIG. 55 In-tank tapchanger with vacuum interrupters (MR)

FIG. 56 Switching sequence of resistor type OLTC with the same vacuum interrupters for the closing and opening side of the diverter switch - VACUTAP VV

It will be seen that the incentive to utilize vacuum switches is very great and as a result in 2000 MR introduced its VACUTAP® VV which achieves the necessary compactness by rethinking the operating logic to reduce the number of vacuum interrupters. The following description of the operation of the VACUTAP® VV is taken from an MR publication.

Generally, a conventional resistor-type tapchanger has different sets of switching contacts for the opening and the closing side of the diverter switch.

One way of reducing the number of vacuum interrupters needed is to use the same vacuum interrupters for the opening and the closing sides. This is the method that has been used in the VACUTAP® VV shown in FIG. 55.

The tapchanger incorporates two current paths. The main path comprises the main switching contacts (vacuum interrupter MSV) and the corresponding main tap-selector contacts MTS connected in series. The transition path comprises the transition contacts (vacuum interrupter TTV) with the corresponding transition tap-selector contacts TTS connected in series, and the transition resistor R.

The sequence of operation is shown in FIG. 56. In the initial position (step 1) at tap 1 both vacuum interrupters are closed. Consequently the interrupters are not exposed to a voltage stress. The tapchange operation starts with the opening of the transition tap-selector contacts TTS (step 2). The vacuum interrupter TTV in the transition path opens (step 3) before the transition tap-selector contacts TTS close on the adjacent tap eliminating the possibility of a pre-discharge arc.

Once the transition tap-selector contact TTS has reached the adjacent tap (step 4), the vacuum interrupter TTV closes (step 5) and a circulating current starts to flow.

The circulating current is driven by the voltage difference between the two adjacent taps and is limited by the transition resistor R. Subsequently, the vacuum interrupter MSV opens (step 6) transferring the current flow from the main tap-selector contacts MTS to the transition path. The load current now flows through tap 2. The main tap-selector contacts can now move load free to the adjacent tap (steps 7 and 8). The tapchange operation is finalized with the closing of the vacuum interrupter MSV, which shunts the transition path (step 9).

Tapchange operations in this direction (m to m x 1), here defined as 'raise', follow the described sequence of steps 1 through 9. Tapchange operations in the 'lower' direction follow the reverse order of events (step 9 to step 1).

There is no doubt that vacuum switches will become the norm in the very near future. MR are claiming 300,000 operations for their vacuum tapchangers before attention is required. For a network transformer this equates to a 40 year lifetime of service a duty. That number of operations is only likely to be exceeded in special applications such as arc furnesses and other such process transformers. Of course other tapchangers will continue to be used and encountered for many years and this is the reason for much of the description of oil-immersed resistor transition tapchangers which follows.

FIG. 57 Types of resistor transition tapchanging. (a) Pennant cycle; (b), (c) and (d) flag cycle

Oil-immersed resistor transition types

High-speed resistor tapchangers can be divided into two types, those which carry out selection and switching on the same contacts and generally use one resistor, and others which have selectors and separate diverter switches and which normally use two resistors. With a single resistor, load current and resistor circulating current have to be arranged to be subtractive, which dictates use with unidirectional power flow or reduced rating with reverse power flow.

When two resistors are employed the duty imposed on the diverter switch is unchanged by a change in the direction of power flow. Recently versions of the combined diverter/selector types have been developed having double resistors and thus overcoming the unidirectional power flow limitation.

The two types fall into two classes, single and double compartment tap changers. Most designs of the single compartment type employ a rotary form of selector switch and FIG. 57 shows diagrammatically the various switching arrangements for resistor-type changers. FIG. 57(a) illustrates the method employed for the single compartment tapchanger and is known as the pennant cycle, whilst Figs 4.57(b)-(d) show the connections when two resistors and separate diverter switches are employed and is known as the flag cycle. (The derivation of the terms 'flag cycle' and 'pennant cycle' and the precise definition of these terms are explained in EN 60214-1. They arise from the appearance of the phasor diagrams showing the change in output voltage of the transformer in moving from one tapping to the adjacent one. In the 'flag cycle' the change of voltage comprises four steps, while in the 'pennant cycle' only two steps occur.) Single compartment tapchangers were largely developed in order to provide an economical arrangement for medium sized local distribution transformers.

FIG. 58 Three-phase 400 A, 44 kV high-speed resistor-type double compartment tapchanger with the diverter tank lowered (Associated Tapchangers Ltd)

FIG. 59 Typical schematic and sequence diagram of a double resistor-type tapchanger.

On larger transformers, for example those used at bulk supply points, the on load tapchanging equipment is usually the double compartment type with separate tap selectors and diverter switches. The tap selectors are generally arranged in a circular form for a reversing or coarse/fine configuration, but are generally in line or in a crescent arrangement if a linear tapping range is required.

FIG. 58 illustrates a double resistor-type tapchanger and a typical schematic and sequence diagram arrangement is shown in FIG. 59. Switches S1 and S2 and the associated tapping winding connections are those associated with the selectors. These selectors are the contacts which do not make or break current and therefore can be contained in transformer oil fed from the main tank conservator. M1, M2, T1, T2, R1, R2 are the components of the diverter switch. Mounted on the diverter switch also are the main current carrying contacts which, like the selector switches, do not make or break current.

The schematic diagram indicates that the right-hand selector switch S1 is on tap position 1 and the left-hand selector switch S2 is on tap 2 whilst the diverter switch is in the position associated with tap 1. A tapchange from position 1 to 2 requires a movement of the diverter switch from the right-hand side to the left side whilst a further tapchange from tap position 2 to tap position 3 requires a movement of selector switch S1 from position 1 to position 3 before the diverter switch moves from the left-hand side to the right side. In order to produce this form of sequence the tapchanger utilizes a mechanism known as a lost motion device. The sequence diagram assumes the tapchanger to be fitted to the neutral end of the HV winding of a step-down transformer.

Load current flows from the main winding through S1 and M1 of the diverter switch to the neutral. Initiation of a tapchange causes the moving arcing con tact to move from the right-hand side to the left-hand side. At position (b) the moving contact has opened contact with the main fixed arcing contact 1; arcing will continue across the gap between these two contacts until the first current zero is reached. After this the current will flow through the diverter resistor R1. This current passing through R1 induces a recovery voltage between M1 and the moving arcing contact. The value of the recovery volt age is ILR1. Although initial examination at this point would suggest that the value of R1 be kept as low as possible in order to keep the recovery voltage down to a relatively low value, an examination at other positions produces a conflicting requirement to minimize the circulating current by maximizing the resistor value, and therefore the actual value of the diverter resistor is a compromise.

At position (c) the moving arcing contact is connected to both transition resistors R1 and R2. A circulating current now passes between tap position 2 and tap position 1 via R2-R1. The value of this circulating current is the step voltage between tap positions 2 and 1 divided by the value of R1 plus R2.

Hence there is a requirement to make R1 plus R2 as high as possible to limit the circulating current.

At position (d) the moving contact has now moved far enough to have bro ken contact with T1. Arcing will again have taken place between these two contacts until a current zero is reached. The recovery voltage across this gap will be the step voltage between the tap positions 2 and 1 minus the voltage drop across R2. It should be noted that when changing from tap 1 to tap 2 position (b) produces a similar condition to that which occurs at (d) but the recovery voltage between the transition contact of R2 and the moving contact is the step voltage plus the voltage drop across R1. At position (e) on the sequence diagram the tapchange has been completed and load current IL is now via S2-M2 to the neutral point of the winding.

If the sequence is continued through to the end of the tapping range it can be seen that the more onerous conditions of current switching and high recovery voltages occur on alternate sides. Should the power flow be reversed the same conditions will apply but occur on the other alternate positions of switching. The diagram shown for the movement between two tap positions is of the same configuration shown in EN 60214 for the flag cycle. For the single compartment tapchanger using only one diverter resistance there is considerable difference between that sequence and that of the double resistor unit.

FIG. 60 Typical schematic and sequence diagram of a single resistor-type tapchanger

Referring to FIG. 60 an explanation of the single resistor switching sequence is as follows. Assuming that the tapchanger is in the neutral end of the HV winding of a step-down transformer then position (a) is the normal operating position 1. Initiation of a tapchange movement causes the transitional arcing contact to make connection with the fixed arcing contact of tap position 2, the load cur rent still passing to the neutral via tap position 1 but a circulating current now flows from tap position 2 to tap position 1. FIG. 60(c) now shows the position when the main arcing contact has left tap position 1, and it should be noted that the current interrupted by the opening of these contacts is the difference between IL and IC the load and the circulating currents. The recovery voltage between the moving arc contact is the step voltage minus ILR, the voltage drop across the diverter resistance. The main arcing contact continues its movement until it too makes connection with the fixed arcing contact of tap position 2; when this is achieved the load current now flows to the neutral via tap position 2 and the transitional arcing contact moves to an open position.

There is a difference of function when moving from a higher-voltage tapping position to a lower position and this is explained as follows. FIG. 60(d) is the normal operating position for tap 2. When a tapchange is initiated the transitional arcing contact moves from its open position to tap position 2, the main arcing contact moves off towards tap position 1. When it leaves tap position 2 arcing takes place, the current interrupted is IL, and the recovery voltage between the main arcing moving contacts and the tap position 2 is ILR.

FIG. 60(f ) shows the condition when the main moving arcing contact has made connection at tap position 1, load current flow is via the main winding and tap position 1 to the neutral. Circulating current flows from tap position 2 to tap position 1; thus, when the transitional moving contact leaves tap position 2 the current interrupted is the circulating current and the recovery voltage is the step voltage.


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