Industrial Power Transformers-- Transformer construction (part 6)

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FIG. 61 shows the switching sequence for a single compartment tap changer which uses double resistor switching. Diagram (1) shows the condition with the transformer operating on tap position 1 with the load current carried by fixed and moving contacts. The first stage of the transition to tap position 2 is shown in diagram (2). Current has been transferred from the main contact to the left-hand transition resistor arcing contact and flows via resistor R1. The next stage is shown in diagram (3) in which the right-hand transition contact has made contact with the tap 2 position. Load current is now shared between resistors R1 and R2 which also carry the tap circulating current. In diagram (4) the left-hand arcing contact has moved away from tap 1 interrupting the circulating current and all load current is now carried through the transition resistor R2. The tapchange is completed by the step shown in diagram (5) in which main and transition contacts are all fully made on tap 2. A single compartment tapchanger utilizing this arrangement is shown in Figs 62(a) and 62(b).

As indicated above, when the tapping range is large or the system voltage very high, thus producing a considerable voltage between the extreme tappings, it is an advantage to halve the length of the tapping winding and to introduce a reversing or transfer switch. This not only halves the number of tappings to be brought out from the main winding of the transformer but also halves the volt age between the ends of tapping selector switch as shown in FIG. 63.

FIG. 61 Switching sequence for single compartment tapchanger (Associated Tapchangers)

FIG. 62(a) A small single compartment tapchanger suitable for 300 A, 44, 66 and 132 kV applications (Associated Tapchangers)

In diagram B the tapped portion of the winding is shown divided into nine sections and a further untapped portion has a length equal to ten sections. In the alternative diagrams C and D a section of the transformer winding itself is reversed. The choice of the tap changer employed will depend on the design of the transformer. In diagrams A, B and C the tappings are shown at the neutral end of the star-connected winding and in diagram D the tapchanger is shown connected to an autotransformer with reversing tappings at the line end of the winding.

In the three examples where a changeover selector is shown, the tapping selectors are turned through two revolutions, one revolution for each position of the changeover selectors, thus with the circuits shown 18 voltage steps would be provided.

As also mentioned previously variation of impedance over the tapping range can often be reduced by the use of reversing arrangements or the coarse/fine switching circuits described earlier.

Dielectric tests

The working levels of voltage and the insulation test levels to which the tap ping windings and thus the on-load tapchanger are to be subjected will have a great deal of bearing on the type of tapchanger selected by the transformer designer. It will be readily appreciated that a tapchanger for use at the line end of a transformer on a 132 kV system will be a very different type of equipment from an on-load tapchanger for use at the grounded neutral end of a star-connected 132 kV winding. The test levels to which both of these on-load tapchangers are likely to be subjected vary considerably. EN 60214-1 resolves this problem by identifying two classes for on-load tapchangers based on their duty. Class I covers those tapchangers that are designed for use at the neutral star point of windings: Class II tapchangers are those designed for use at a position other than the neutral point of windings.

FIG. 62(b) One phase of moving selector switch assembly for above tapchanger showing scissor contact mechanism and changeover selector (at top) for coarse/fine or reversing regulation (Associated Tapchangers)

There are then five dielectric withstand requirements which define the duty of a tapchanger. These are:

(1) to earth;

(2) between phases (where applicable);

(3) between the first and last contacts of the tap-selector switch and, where fitted, of the changeover selector;

(4) between any two adjacent contacts of the tap selector;

(5) between open diverter switch contacts.

Unlike the dielectric test voltages set out in EN 60076-3, which gives a range of rated lightning impulse withstand voltages and power frequency test volt ages for each value of highest voltage for equipment, EN 60214-1 gives only one. These are set out in Tables 1(a) and 4.1(b).

FIG. 63 On-load tapchanging circuits for resistor transition using diverter switches

Table 1(a) Rated withstand voltages - Series I based on European practice

Table 1(b) Rated withstand voltages - Series II based on North American practice

FIG. 64 Three-phase, HV earthed neutral end, 132 kV tapchanger (Ferranti Engineering Ltd)

FIG. 65(a) Three single-phase, fully insulated tapchangers fitted to the 132 kV tapping points of a 240 MVA, 400/132 kV autotransformer (Hawker Siddeley Power Transformers Ltd)

The effect of the differing requirements for Class I and Class II can be seen by reference to Figs 64 and 65(a) which indicate the basic difference due to the insulation requirements between an earthed neutral end tapchanger for a 132 kV system compared with a line end tapchanger for the same voltage.

In FIG. 64 the selectors are in the compartment which runs along the side of the transformer and the diverter switch compartment is mounted at the end of the selector compartment. Examination of FIG. 65(a) illustrates a 240 MVA, 400/132 kV three-phase autotransformer with three individual 132 kV line end on-load tapchangers. The selector bases are mounted on the transformer tank and the diverter switches are contained in the tanks which are mounted on the top of the 132 kV bushings. FIG. 65(b) is a cross-sectional view of the tapchanger illustrated in FIG. 65(a). The main tank housing the selector switches are arranged for bolting to the transformer tank together with the terminal barrier board. Mounted directly on this tank is the porcelain bushing which supports the high-speed diverter switch assembly.

FIG. 65(b) Cross-sectional drawing of the tapchanger illustrated in FIG. 65(a)

The main supporting insulation is a resin-bonded paper cylinder mounted at the base of the selector tank, and the mechanical drive is via a torsional porcelain insulator within this cylinder. Connections from the selector switches to the diverter switch are made by means of a double concentric condenser bushing and the mechanical drive shaft passes through the centre of this bushing.

On the UK Grid System, as in many other parts of the world, there are many 275/132 kV and 400/132 kV autotransformers installed where the on-load tapchangers are at the 132 kV point of the auto-winding. Earlier designs employed a reversing arrangement as shown in FIG. 66(a) utilizing a separate reversible regulating winding. More recently a linear arrangement has been used with the tapping sections of the winding forming part of the main winding as shown in FIG. 66(b).

FIG. 66 Diagrams of three-phase 400/132 kV and 275/132 kV autotransformers with 132 kV high-speed resistor-type tapchanger

In either case the tapping winding is usually a separate concentric winding.

As mentioned earlier in this section, because of the high cost, particularly of the porcelain insulators required for the line-end tapping arrangements, earthed neutral end tappings have also been used more particularly on the 400/132 kV autotransformers despite the fact that this introduces simultaneous changes in the effective number of turns in both primary and secondary and also results in a variation in the core flux density. The arrangement also introduces the complication of variable tertiary voltages. The latter can be corrected by introduction of a tertiary booster fed from the tapping windings.

On-load tapchangers have to be designed to meet the surge voltages arising under impulse conditions. In earlier HV tapchangers it was quite a common practice to fit non-linear resistors (surge diverters) across individual tappings or across a tapping range.

These non-linear resistors have an inherent characteristic whereby the resistance decreases rapidly as the surge voltage increases. In modern tapchangers this characteristic has, in general, been eliminated by improvement in design and positioning of contacts, such that appropriate clearances are provided where required. There is also now a much better understanding of basic transformer design and in particular the ways of improving surge voltage distribution to ensure that excessive values do not arise within tapping windings.

If a bank of three single-phase transformers is used to make up a three-phase unit, then each phase must have its own tapchanger. This is often the case for large generator transformers. These need to be coupled so as to ensure that all three remain in step and, whilst it is possible to make this coupling electrically, it is far preferable and more reliable to use a single drive mechanism with a mechanical shaft coupling between phases. Assuming that the units have tap pings at the neutral end of a star-connected HV winding, it is also necessary to make the HV neutral connection externally, usually by means of a copper busbar spanning the neutral bushings of each phase.

Another method of voltage regulation employed in transmission and distribution systems is one in which shunt regulating and series booster transformers are used. The former unit is connected between phases while the latter is connected in series with the line. Tappings on the secondary side of the shunt transformer are arranged to feed a variable voltage into the primary winding of the series transformer, these tappings being controlled by on-load tapchanging equipment.

The frame size or equivalent kVA of each transformer is equal to the throughput of the regulator multiplied by the required percentage buck or boost.

It should be noted that the voltage of the switching circuit of the regulator transformer to which the on-load tapchanger is connected can be an optimum value chosen only to suit the design and rating of the tapchanging equipment.

This arrangement of transformers is described as the series and shunt regulating transformer. It is normally arranged for 'in-phase' regulation but can also be employed for 'quadrature' regulation, or for both. FIG. 67 shows the connections for a typical 'in-phase' and 'quadrature' booster employing two tapchangers. Such a unit can be used for the interconnection of two systems for small variations of phase angle. Fuller descriptions of phase shifting transformers and quadrature boosters and their applications are given in Section 7.5.

FIG. 67 Diagram of connections for a three-phase 'in-phase' and 'quadrature' booster

FIG. 68 Arrangement of links under oil used to provide off-circuit taps on the HV winding of a 650 MVA, 20.9/345 kV, generator transformer supplied to the USA (Peebles Transformers)

Off-circuit tapchangers

As explained earlier in this section, the off-circuit tapping switch enables accurate electrical system voltage levels to be set when the transformer is put into operation. Once selected, the transformer may remain at that setting for the remainder of its operating life. The simplest arrangement is that in which the power transformer tappings are terminated just below oil level and there changed manually by means of bolted swinging links or plugs mounted on a suitable terminal board. The drawback to this arrangement is that it necessitates removing the transformer tank cover or handhole cover. It is, however, extremely simple, reliable and is the cheapest tapchanging device. It is important to design the tapchanging link device with captive parts as otherwise there is always the danger that loose nuts, washers, etc. may fall into the tank whilst the position of the taps is being altered. FIG. 68 shows one phase of this arrangement used to provide off-circuit taps on a 345 kV transformer. In this situation it is necessary to incorporate stress shielding into both the bridging link and the open ends of the unconnected tapping leads.

Most off-circuit tapping switches use an arrangement similar to the selector switch mechanism of the on-load tapchanger, employing similar components, but if these selector contacts are not operated occasionally contact problems can occur. This can be particularly problematical for higher current-rating devices.

An example is the case of power station unit transformers. On some large stations these can have ratings as high as 50 MVA at 23.5/11 kV. The 23.5 kV HV side is connected to the generator output terminals whose voltage is maintained within +-5 percent of nominal by the action of the generator automatic voltage regulator. The transformer is normally only in service when the unit is in operation and under these conditions its load tends to be fairly constant at near to rated load. An on-load tapchanger is therefore not essential and would reduce reliability but off-circuit taps are desirable to enable fine trimming of the power station electrical auxiliary system voltage to take place when the station is commissioned. For a transformer of this rating the HV current can be up to 1300 A which for trouble-free operation demands a very low contact resistance. If this is not the case heating will take place resulting in a build-up of pyrolytic carbon which increases contact resistance still further. This can lead to contact arcing and, in turn, produces more carbon. Ultimately a runaway situation is reached and the transformer will probably trip on Buchholz protection, shutting down the associated generator as well. To avoid the formation of pyrolytic carbon on high-current off-circuit tapchangers, it is vital that the switch has adequate con tact pressure and that it is operated, off-circuit, through its complete range during routine plant maintenance or preferably once per year to wipe the contact faces clean before returning it to the selected tapping. Because of these problems, the UK Central Electricity Generating Board in its latter years specified that ratio adjustments on unit transformers and other large power station auxiliary transformers, which would, hitherto, have had off-circuit tapping switches, should be carried out by means of links under oil within the transformer tank.

The links need to be located at the top of the tank so that access can be obtained with the minimum removal of oil, but provided this is specified, tapchanging is relatively simple and reliability is greatly improved. In fact, the greatest inconvenience from this arrangement occurs during works testing, when the manufacturer has to plan his test sequence carefully in order to minimize the number of occasions when it is necessary to change taps. More tapchanges will probably be made at this time than throughout the remainder of the transformer life time. This problem does not, of course, arise on the many small distribution and industrial transformers of 1 or 2 MVA or less, operating at 11/0.433 kV. These have an HV current of less than 100 A which does not place high demands on contact performance when operating under oil. Very conveniently, therefore, these can be provided with simple off-circuit switches enabling the optimum ratio to be very easily selected at the time of placing in service. It is nevertheless worthwhile operating the switches, where fitted, whenever a routine maintenance is carried out, particularly where the transformer is normally operating at or near full load when the oil temperature will consequently be high.


FIG. 69 (a) Schematic drawing of a Geneva mechanism and drive; (b) 180degree lost motion device and (c) schematic drawing of 180º lost motion device and Geneva drive

42. Diverter switch driving crank; 75. Insulated driving shafts;

45. Driving chain and sprocket; 76. Flexible couplings;

46. Selector switch drive shaft; 77. Geneva arm;

74. Selector switch Geneva mechanism; 78. 180° lost motion segment


Construction of tapchangers

It is a fundamental requirement of all tapchangers that the selector and diverter switches shall operate in the correct sequence. One of the methods used is based on the Geneva wheel. FIG. 69 shows the mechanism and its main component parts. The drive shaft 46 is driven from the motor drive or manual operating mechanism via a duplex chain and sprocket 45, and is coupled at one end to the diverter drive 42, and at the other end to the selectors via the lost motion device 78 and the Geneva arm 77. Referring to Figs 69(b) and (c) the lost motion device operates as follows; the drive shaft 46 has a quad rant driving segment in contact at its left-hand side with a quadrant segment on the Geneva arm 77. If the drive shaft rotates in a clockwise direction then the Geneva arm will be driven. However, if the drive shaft rotates in an anti clockwise direction then no movement of the Geneva arm takes place until the drive shaft has rotated through 180º. During this 180º rotation the diverter switch driven by 42 will have completed a full operation. Further drive shaft rotation will move the appropriate Geneva wheel for a particular selector.

Examination of the operation of the four-position Geneva mechanism in FIG. 69(c) shows the following:

• The Geneva drive does not engage until the Geneva drive arm itself has passed through approximately 45º.

• The driven period of the selector shaft occupies only 90º of the movement of the Geneva arm and the selector rotation rate is not constant. Entry of the Geneva arm into the slot produces an initial slow start increasing to maxi mum velocity after 45º of rotation when the drive wheel centers are in line and reducing to zero as the Geneva arm rotates through the second 45º.

• The Geneva arm travels a further 45º after disengaging from the Geneva drive wheel before the completion of a tap sequence.

The tapchanger design arranges for the diverter switch operation to occur after the moving selector has made contact with the fixed selector. In order to pro vide a definite switching action of the diverter switch it is usual to provide some form of positive stored energy device to operate the diverter switch of the single compartment unit.

Examples of stored energy devices are a spring charged across a toggle which is tripped mechanically at a predetermined time. Alternatively a falling weight is driven to a top dead centre position by a motor or by manual operation and once at that position provides sufficient energy to complete the tapchange.

Highly reliable operation has been achieved and long contact life can be guaranteed; diverter switch contacts will now last generally for the useful life of the transformer itself. One type of three-phase single compartment tap changer suitable for 44 kV, 600A 17 positions is illustrated in FIG. 70. It is fitted with a low oil level and surge protection device which is shown at the top of the tapchanger housing.

FIG. 71 illustrates three single-phase 1600 A linear-type tapchangers mechanically coupled together and is suitable for connection at the neutral end of a 400 kV graded winding.

FIG. 72 shows an example of a three-phase roller contact diverter switch which would be housed in the diverter compartment of the tapchanger shown in FIG. 64.

FIG. 73 illustrates a three-phase tapchanger which can be used as a coarse/fine or reversing regulator up to 33 positions, alternatively 17 positions as a linear switch. It is rated at 600 A with a power frequency insulation level of 70 kV, 200 kV impulse and is suitable for use at the neutral end of a 132 kV winding. On the right-hand side of the tapchanger is the separate compartment containing the driving mechanism and incorporated into this chamber is the Ferranti 'integral solid-state voltage and temperature control unit'. This feature dispenses with the necessity of a separate tapchanger and cooling circuit control cubicle.

FIG. 70 600 A, 44 kV three-phase 17 position single compartment tapchanger (Ferranti Engineering Ltd)

FIG. 71 Three single-phase 19 position linear-type, 1600 A tapchangers mechanically coupled as a three-phase unit (Associated Tapchangers Ltd)

FIG. 72 Three-phase roller contact diverter switch rated 650 A, normally housed in the diverter compartment of the tapchanger shown in FIG. 64 having 70 kV test level (Ferranti Engineering Ltd)

FIG. 73 Three-phase, 600 A, single compartment tapchanger fitted with an integrated solid-state voltage and temperature control unit (Ferranti Engineering Ltd)

FIG. 74 Single-phase diagram showing the principle of line drop compensation

Control of on-load tapchangers Many advances have been made in the design of control circuits associated with on-load tapchanging. Mention has already been made of driving mechanisms and the fundamental circuits associated with the starting of the motor for carrying out a tapchange. Whilst these vary from one maker to another they are comparatively simple. In general, the motor is run up in one direction for a 'raise' tapchange and in the reverse direction for a 'lower' tapchange. In some cases a brake is employed to bring the motor to rest whilst in others clutching and de-clutching is carried out electrically or mechanically. It is, however, the initiation of the tapchange and the control of transformers operating in parallel where the main interest lies and where operational problems can arise if the tapchangers are 'out-of-step.' Manual operation must always be available for emergency use and in some cases tapchangers are supplied for hand operation only.

Many installations are designed for simple pushbutton control but there has been a tendency towards unattended automatic voltage control at substations so that a predetermined constant or compensated busbar voltage can be maintained. In general, with these schemes a tapchanger is provided on a transformer for maintaining a predetermined outgoing voltage where the incoming voltage is subject to variations due to voltage drops and other system variations.

It is reasonable to expect that with the advent of digital control it will become possible to perform all the operations necessary for the control and operation of tapchangers and the monitoring of their performance by a single device using digital computer technology coupled to low burden output voltage and current transformers, thereby enabling very accurate control to be obtained with much simplified equipment. At the present time, however, the basic control devices remain within the class which is generally termed 'relays,' even though these may utilize solid-state technology, and tapchanger control continues to operate on principles which have developed since the early days of on-load tapchanging, with individual circuit elements performing discrete functions. The following descriptions therefore describe these traditional systems.

Voltage control of the main transformer requires a voltage transformer energized from the controlled voltage side of the main transformer. The voltage transformer output is used to energize a voltage relay with output signals which initiate a tapchange in the required direction as the voltage to be controlled varies outside predetermined limits. It is usual to introduce a time delay element either separately or within the voltage relay itself to prevent unnecessary operation or 'hunting' of the tapchanger during transient voltage changes.

The 'balance' voltage of the relay, namely the value at which it remains inoperative, can be pre-set using a variable resistor in the voltage sensing circuit of the relay so that any predetermined voltage within the available range can be maintained.

Often it is required to maintain remote busbars at a fixed voltage and to increase the transformer output voltage to compensate for the line drop which increases with load and this is achieved by means of a line drop compensator.

This comprises a combination of a variable resistor and a tapped reactor fed from the secondary of a current transformer whose primary carries the load cur rent. By suitable adjustment of the resistance and reactance components, which depend upon the line characteristics it is possible to obtain a constant voltage at some distant point on a system irrespective of the load or power factor.

FIG. 74 shows the principle of the compensator which for simplicity is shown as a single-phase circuit. The voltage transformer is connected between lines and the current transformer is connected as shown to the variable resistance and reactance components. These are so connected in the voltage relay circuit that the voltage developed across them is subtracted from the supply voltage, then as load current increases the voltage regulating relay becomes unbalanced and operates the main regulating device to raise the line voltage at the sending end by an amount equal to the line impedance drop and so restore the relay to balance. The reverse action takes place when the load cur rent decreases. The regulating relay and compensator are usually employed in three-phase circuits, but since the relay voltage coil is single phase, usually connected across two phases, the only difference between the arrangement used and that shown in FIG. 74 is that the arrangement of the voltage and current transformer primary connections must be such as to provide the proper phase relation between the voltage and the current.

The voltage transformer is connected across the A and C phases and the cur rent transformer in the A phase. Different phases may be used provided the phase relationship is maintained. The compensation afforded by this method is not strictly correct since there is a 30º phase displacement between the voltage and the load current at unity load power factor. Since line drop compensation is usually a compromise this method is acceptable in many cases.

In FIG. 74 a single current transformer is shown in the line connection for the current supply to the compensator. It is usual practice to have an interposing current transformer in order to obtain the correct full-load secondary cur rent but, at the same time, provide protection against damage due to overloads or fault currents in the line. The interposing current transformer is specifically designed to saturate under such conditions, thus avoiding the introduction of high overload currents to the compensator circuit. If greater accuracy is desired another method may be used with this scheme - the voltage transformer is connected across A and B phases with the main current transformer primary in C phase. Alternative phases may be used provided the phase relationship between voltage and the current is maintained. With this connection, since the current and voltage are in quadrature at unity load power factor, the resistor and reactor provide the reactance and resistance compensation, respectively. In all other respects this compensator is identical with that described for the first scheme but there is no phase angle error.

For many years the automatic voltage relay (AVR) used was the balanced plunger electro-mechanical type and many of these are still in service. Nowadays a solid-state voltage relay is used. For the former type a standard arrangement of line drop compensator has the external series resistor and mean setting adjustment rheostat for the regulating element of the voltage regulator mounted in the compensator, which has three adjustable components providing the following: variation of 90-110 percent of the nominal no-load voltage setting, continuously variable range of 0-15 percent compensation for resistance and 0-15% reactive compensation.

If compensation is required for line resistance only, a simple potentiometer resistor is used instead of the complete compensator and the external resistor and mean setting adjustment are supplied separately. When the compensator has been installed and all transformer polarities correctly checked, the regulating relay may be set to balance at the desired no-load voltage. The resistance and reactance voltage drops calculated from the line characteristics may then be set at the appropriate values.

FIG. 75 Control cubicle with voltage regulating relay (Bonar Long Ltd)

FIG. 76 Simplified arrangement of the regulating element of the AVE 5 relay (GEC measurements)

A voltage control cubicle with voltage regulating relay and line drop compensation is shown in FIG. 75. The voltage regulating relay is of the balanced plunger electro-mechanical type and a simplified arrangement of the relay is shown in FIG. 76. The design of the solenoid regulating element ensures that the magnetic circuit is open throughout the operating range. Therefore, the reluctance of the circuit is now appreciably affected by movement of the core and the unit operates with a very small change in ampere-turns.

Basically the element consists of a solenoid C with a floating iron core guided by two leaf springs LS which permit vertical but not lateral movement.

Control spring S, which has one end anchored to the relay frame and the other attached to the moving iron core 'a', is carefully adjusted to balance the weight of the core and the magnetic pull of the solenoid holding the disc 'd' at the mid position 'f' with contact A between contacts B with nominal voltage applied.

When the voltage increases or decreases the magnetic forces move the core up or down along the axis of the solenoid. The moving core carries a contact A which makes with the 'high volt' and 'low volt' fixed contacts B.

Positive action is ensured by the 'hold on' device. This consists of an iron disc 'd' attached to the core, which moves between the poles of a permanent magnet M. Pole pieces 'e', 'f' and 'g' concentrate the flux of the permanent magnet, and therefore its influence on the disc 'd', at positions corresponding to high, normal and low positions of the core and tend to restrain it at these positions. An eddy current damper consisting of fixed magnet 'm' and moving cop per vane 'v' minimizes oscillations set up by momentary voltage fluctuations.

To eliminate errors due to the variations of coil resistance with temperature, a comparatively high value of resistance having a negligible temperature coefficient is connected in series with the coil.

FIG. 77 Static voltage regulating relays. (a) Brush STAR relay (Brush Electrical Machines Ltd); (b) GEC VTJC relay (GEC measurements)

FIG. 78 Master/follower circuit diagrams

There is an advantage in providing means by which a sudden wide change in voltage can be more quickly corrected and solid-state voltage relays can provide this characteristic. These relays have a solid-state voltage sensing circuit and an inverse time characteristic so that the delay is inversely proportional to the voltage change. Two such relays are the VTJC and STAR, both of which are illustrated in FIG. 77. They can be used with a line drop compensator and a voltage reduction facility to give specified load shedding features.

Where two or more transformers with automatically controlled on-load tapchangers are operating in parallel, it is normally necessary to keep them either on the same tapping position or a maximum of one tap step apart. If transformers are operated in parallel on different tappings circulating currents will be set up and in general one step is the most that can be tolerated.

Many different schemes of parallel control have been devised, several of which are in regular use. If it is considered necessary that all transformers must operate on the same tapping this can be achieved by a master-follower system or by a simultaneous operation method.

Master-follower control

With this type of control system one of the units is selected as the master and the remaining units operate as followers. Built-in contacts in the on-load tapchanger mechanisms are connected so that once a tapchange has been completed on the master unit each follower is initiated in turn from the inter connected auxiliary contacts to carry out the tapchange in the same direction as that carried out by the master. A simplified schematic diagram of the master and follower circuit is shown in FIG. 78. The disadvantage of master/follower schemes is their complexity, so that nowadays they are very little used.

With simultaneous operation, all tapchangers of a group are arranged to start their operation at the same time. This is a simpler arrangement although it is still necessary to provide lock-out arrangements to take care of any individual failure.

Circulating current control

Where two or more transformers of similar impedance are operated in parallel they will each provide an equal share of the load current. In the event of one of these transformers changing to a higher tapping position, a circulating current will flow between this transformer and the remaining units. This circulating current will appear as a lagging current from the unit which has changed taps.

It will be equally divided between the other transformers which are in parallel and will appear to these transformers as a leading current.

It is possible by judicious connection of current transformers to separate this circulating current from the load current and introduce it into components in the automatic voltage regulating circuit. These are so connected into the AVR circuit such as to provide an additional voltage to the AVR which has tapped up and a subtractive voltage to the remaining AVR's controlling the parallel connected transformers. Using this method and carefully adjusted components, transformers can be kept within close tapping positions of each other.

There has been much development in the supervisory control of system volt ages, and on some systems centralized control has been achieved by the operations of tapchangers by remote supervisory methods. This is usually confined to supervisory remote pushbutton control, with an indication of the tapchanger position, but more complicated schemes have been installed and are being satisfactorily operated where tapchangers are controlled from automatic relays on their respective control panels, with supervisory adjustment of their pre-set voltage and selection of groups operating in parallel, and with all necessary indications reported back by supervisory means to the central control room.

Runaway prevention

The danger with any automatic voltage control scheme is that a fault in the control circuitry, either the voltage-sensing relay or, more probably, fuse failure of a voltage transformer, can cause a false signal to be given to the control equipment thus incorrectly driving it to one end of the range. Such a fault not only causes incorrect voltage to be applied to the system fed by the transformer but can also result in the transformer itself having an incorrect voltage applied to it. For example, the failure of a fuse of a voltage transformer monitoring the transformer LV system will send a signal to the control scheme to raise volts.

This will result in the transformer tapping down on the HV side and it will continue to do so until it reaches the minimum tap position. The applied voltage on the transformer HV side could, in fact, be at or near nominal, or even above nominal, so that this can result in the transformer being seriously overfluxed.

Various schemes can be devised to guard against this condition, the most reliable being possible when two or more transformers are controlled in parallel.

In this situation the AVC scheme outputs for each one can be compared. If they attempt to signal their respective transformer tapchangers to become more than two steps out of step then both schemes are locked out and an alarm given. All such schemes can only be as reliable as their input information and the principal requirement of any reliable scheme such as the one described must be that controls compared should operate from independent voltage transformer signals. Where the provision of an independent voltage transformer signal is difficult, as can be the case for a single transformer with on-load tapchanger supplying a tail-end feeder, it is possible to utilize a VT fuse monitoring relay.

This usually compares phase voltages of the VT output and alarms if any one of these does not match the other two.

FIG. 79 Principle of the moving coil type of voltage regulator

Moving coil regulator

The moving coil regulator does not suffer from the limitations of the on-load tapchangers finite voltage steps and has a wide range of application. It can be used in both low- and medium-voltage distribution systems, giving a smooth variable range of control. A shell-type core carries two coils connected in series opposition mounted vertically above the other. An outer third coil is short-circuited and mounted concentrically so that it can be moved vertically from a point completely covering the top coil to a lower position covering the bottom coil. This arrangement produces an output voltage proportional to the relative impedance between the fixed and moving coil which is smoothly variable over the range. FIG. 79 illustrates the principle of the moving coil regulator and the core and windings of two three-phase 50 Hz regulators are shown in FIG. 80. They are designed for a variable input of 11 kV _ 15 percent, an output of 11 kV _ 1 percent and a throughput of 5 MVA.

The Brentford linear regulating transformer The Brentford voltage regulating transformer (VRT) is an autotransformer having a single layer coil on which carbon rollers make electrical contact with each successive turn of the winding. It can be designed for single- or three phase operation and for either oil-immersed or dry-type construction. The winding is of the helical type which allows three-phase units to be built with a three-limb core as for a conventional transformer.

FIG. 80 Core and coil assemblies of a three-phase, 5 MVA throughput 50 Hz moving coil regulator (Allenwest Brentford Ltd)

The helical winding permits a wide range of copper conductor sizes, winding diameter and length. The turns are insulated with glass tape and after winding the coils are varnish impregnated and cured. A vertical track is then machined through the surface insulation to expose each turn of the winding.

The chain driven carbon roller contacts supported on carriers operate over the full length of the winding to provide continuously variable tapping points for the output voltage.

As the contacts move they short-circuit a turn and a great deal of research has been carried out to obtain the optimum current and heat transfer conditions at the coil surface. These conditions are related to the voltage between adjacent turns and the composition of the material of the carbon roller contacts.

The short-circuit current does not affect the life of the winding insulation or the winding conductor. The carbon rollers are carried in spring-loaded, self aligning carriers and rotate as they travel along the coil face. Wear is minimal and the rolling action is superior to the sliding action of brush contacts. In nor mal use the contact life exceeds 100 km of travel with negligible wear on the winding surface. FIG. 81 illustrates how the sensitivity of a regulator may be varied to suit a particular system application. If it is required to stabilize a 100 V supply which is varying by _10 percent a VRT would have say 100 turns so that by moving the roller contact from one turn to the next the out put would change by 1 V or 1 percent. However, if the VRT supplies a transformer which bucks or boosts 10 percent, the roller contact needs to move 10 turns to change the voltage by 1 percent, hence the sensitivity of the regulator is increased 10 times.

FIG. 81 Diagram illustrating variation of sensitivity of a voltage regulating transformer

FIG. 82 Three-phase, 100 A, 72 kVA, 415/0-415 V regulator (Allenwest Brentford Ltd)

FIG. 83 Schematic diagram of one phase of a three-phase Brentford interstep regulating unit employing an 8-position selector switch. Output voltage range is 0-100 percent. T - Main tapped transformer; R - Brentford stepless regulator; MPB1, MPB2 - double wound booster transformers connected in series; (a) to (h) - tappings on main transformer; S1 to S8 - 8-position selector switch

The contacts are easily removed for inspection by unscrewing the retaining plate and turning the contact assembly away from the coil face: contacts are then lifted vertically out of their carrier. Replacement is straightforward and with normal usage an operating life of 3-5 years can be expected.

Linear voltage regulators are available in ratings up to 1 MVA as a single frame and up to 15 MVA with multiple unit construction. Also on HV systems designs of regulators can be combined with on-load tapping selector switches connected to the transformer windings to provide power ratings in excess of 25 MVA.

Control of regulators over the operating range can be arranged for manual, pushbutton motor operation or fully automatic control regulating the output by means of a voltage-sensing relay.

FIG. 82 shows the core and windings of a 72 kVA three-phase regulator designed for an input of 415 V, 50 Hz and a stepless output of 0-415 V with a current over the range of 100 A. For the smaller low-voltage line-end boosters built into rural distribution systems, the regulator is often a single-sided equipment, and contact is made only to one side of the helical winding. For larger units, and those for networks up to 33 kV, the regulator is used in conjunction with series-booster and shunt-connected main transformers to give a wider range of power and voltage capabilities.

The schematic diagram, FIG. 83, shows the basic connection for an inter step regulating equipment designed to provide stepless control of its output voltage from 0-100 percent.

For the purposes of simplifying the explanation, the main transformer is auto-wound and provided with 8 tappings but depending upon the rating up to a maximum of 16 tappings can be used. Also for those applications where because of other considerations it is necessary to use a double wound transformer, it is often more economical for a restricted voltage range to utilize tappings on the primary winding, and not employ a separate tapped autotransformer.

Also provided in the equipment is a co-ordinating gearbox which mechanically synchronizes the operation of the switches and the regulator. The tap pings on the autotransformer are connected to the selector switches S1 to S8 and the regulator and booster transformers are arranged to act as a trimming device between any two adjacent tappings. For example, if switches S1 and S2 are closed and the regulator is in the position shown, then the secondary winding of booster transformer MPB1 is effectively short-circuited and the voltage at the output terminal is equal to tap position (a), that is zero potential.

To raise the output voltage, the contact of the regulator is moved progressively across the winding and this action changes the voltage sharing of the two booster transformers until MPB2 is short-circuited and the output voltage is equal to tap position (b). Under these conditions switch contact S1 can be opened, because effectively there is no current flowing through it and switch S3 can be closed.

To increase the output voltage further, the contact of the regulator is moved to the opposite end of the winding when S2 opens and S4 closes, and this procedure is repeated until the maximum voltage position is reached, which corresponds to switches S7 and S8 being closed with the secondary winding of MPB2 short-circuited. For this application the regulator is double sided, having both sides of the regulating winding in contact with the roller contacts and driven in opposite directions on either side of the coil to produce a 'buck' and 'boost' output from the regulator which is fed to the low-voltage side of the series transformer. The most common arrangement of this is shown diagrammatically in FIG. 84 but a number of alternative arrangements with patented 'double boost' connections are available.

FIG. 84 Diagram of connections of a regulator employed in conjunction with series and main transformers


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