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ON-LOAD TAP CHANGER TYPE DGeneral Manual

C O N T E N T S

1. Specification Details ..................................................................................... 2

2. Description of the On Load Tap Changer ..................................................... 4

3. Principal Parts of the On Load Tap Changer ................................................ 5

3.1 Diverter Switch Oil Tank .................................................................... 5

3.2 Diverter Switch .................................................................................. 6

3.3 Transition Resistors .......................................................................... 7

3.4 Tap Selector .......................................................................................7

3.5 Delta Tap Changer ............................................................................ 9

4. Electrical Characteristics ......... ................................................................... 9

4.1 Electrical Strength ............................................................................. 9

4.2 Short Circuit Withstand ..................................................................... 11

4.3 Tap Changing Cycle .......................................................................... 11

5 Motor Drive ................................................................................................... 14

6. Drying Out the On Load Tap Changer... ....................................................... 15

7. Inspection ..................................................................................................... 15

8. Protective Relay ........................................................................................... 16

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On Load Tap Changer Type D

1. Specification Details

Layout: Installed fully sunk in the transformer tank.(Can also be mounted in a separate tank).Diverter switch operates in its own oil space (diverter switch oil tank isolatedfrom the transformer oil.

Applications: On load tapchanger D III 200, D Ill 400, D Ill 500, as star point tapchanger forthree phase transformersand On load tapchanger D Ill 200, D Ill 400D Ill 500for application todelta connected transformersorOn load tapchanger D 1 400, D I 500, D I 600, D I 800 and D I 1200 assingle phase tap changer for single phase transformers or for three phasetransformers in any mode of connection. Star point tap changers are usedmainly for transmission system transformers and single phase tap changersfor auto transformers and industrial transformers supplying furnaces andelectrolysis plants.

Insulation: Insulation level voltage to earth 30, (45), 60, (80), 110, 150kV. Tap selectorSeries 60, 110,150.

A.C. test voltage: To earth: according to insulation level voltages to IS, BS, lEC, VDE, ASA, etc.(50Hz 1,2/50s) Internal selector insulation: withstand voltages for all insulation distance onapplication.

Current rating: 200, 400, 500 A when star or delta connected400, 500, 600, 800, 1200 A single phase in any connection.

Rated Frequency: 50........60 Hz

Short circuit withstand: Thermal test: 20 times the rated current (rms), duration 3 seconds. Dynamictest: 50 times the rated current (peak value).

Step voltage Particulars of permissible step voltages on application. The values depend ontap selector insulation and circular pitch.

Number of taps: Tap selector with preselector (coarse tap selector or reversing switch): 8,10,12,14,16 steps with 3 mid position (up to a maximum of 33 workingpositions differing in voltage).9, 11, 13, 15, 17 steps with one mid position (up to a maximum of 35 workingpositions differing in voltages).Tap selector without preselector:4 5 6, 7, 8 steps (up to a maximum of 17 working positions differing involtage).Driving Motor drive as standard equipment can be started and stopped bypush buttons from the drive cabinet and also from the control room.Manual operation by crank available if needed.

Remote position indication.Versions for parallel operation and for connection to voltage regulators.

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Fig. 13 shows an on-load tap changer Type D in the form of D III400 three phase unitwith an insulation level voltage of 110 kV to earth and a 110 series tap selector for + 13

steps conforming to the design specified in section 1. The principal parts are numbered

consecutively and detailed in thelist belonging to Fig.12. The same part numbers are given

in brackets throughout the text in the this publication . Models of the on load tap changer

type D designed for higher or lower insulation level voltages or tap selector series result in

correspondingly larger or smaller vertical dimensions of the diverter switch or tap selector.

As in the familar Jansen on load tap changers in the higher range of ratings, which have

become firmly established since 1926 the type D on load tap changer likewise retains thesame subdivision into a tap selector and a diverter switch giving high speed change over ,

both units however have been further developed to confer high switching capacity and

short circuit withstand and long span,as is described in further details below.The tap

selector and diverter switch are mounted as a unit in the transformer tank.

The diverter switch has its own oil tank, whilst the tap selector is immersed in the same oil

as the transformer. The tap selector and diverter switch are thus housed in seperate oil

chambers. The only fittting projecting above the transformer cover is the tap changer

head. This is no higher than the metal bases of the H.T. bushings and thus allows full use

to be made of the available space above the transformer head where they are readily ac-

cessible.

As shown in Fig.2. and 12, the on load tap changer designed on these principles is a unit

which hangs from the head (103) and can be bolted either to the transformer cover or toan annexe on the side of the transformer tank after welding on a mounting flange(101).

Fig. 1

2.Description of the On - Load Tap Changer

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When filling the transformer tank with oil ,the air which collects insidethe mounting flange (101) must be released through a small pipe

(102) to atmosphere (valve plug).

The on-load tapchanger head (103) contains the principal items ofthe driving mechanism and the position indicators for the diverterswitch and tap selector. The driving mechanism is only visible afterremoving the cover (106) on the on-load tap changer head whereasthe position indicators can be seen through a window (114) evenwhen the cover is in position. The parts mainly concerned are theprimary gear unit (121) together with the two Geneva drives (401)giving alternate operation of the two tap selector contact systems(408, 409) and the spring-operated energy accumulator (304)providing a high speed drive to the diverter switch. In addition, there

is an important electrical supervisory device, namely the protectiverelay RS 2001.

Directly suspended from the previously mentioned on-loadtapchanger head, which in turn bolts to the mounting flange on thetransformer cover, is the diverter switch oil tank (203). Screwed tothe base (207) of the latter is the tap selector (400) which at oneend carries the preselector (500) and which is driven via the tubulardriving shafts (402) from the mechanism (121, 401) in the on-loadtapchanger head.

3 Principal Parts of the On-Load Tap Changer

3.1 Diverter Switch Oil Tank

The diverter switch (300) is housed in its own oiltank (203). The oil in this tank is kept entirelyseparated from the transformer oil. The oil is, of course, of the same type as that used in thetransformer tank, since ordinary transformer oil is perfectly suitable for use as a switch oil.

Owing to the contamination brought about in service by arcing, however, the switch oil is kept

separate from the transformer oil.

It is impossible for the contaminated switch oil, to pass from the diverter switch compartmentinto the surrounding transformer oil (yellow).

The diverter switch oil tank consists of high-grade insulation which in the vertical directionwithstands the full working voltage to earth (under earth fault conditions also). Grading rings(208) fitted outside and inside the oil tank (bare up to 45 kV, insulated from 60 kV upwards)promote satisfactory field distribution with a low voltage gradient along the vertical insulationpath. Its connection to the conservator ensures that the diverter switch oil tank is kept completelyfull of oil at all times.

Fig . 2

Load Tap Changer D III 400, 60 kV,

Tap Selector Series 110

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3.2 Diverter Switch

The most important part of the on-load tapchanger is the diverterswitch. On the rear cover the diverter switch unit together withthe spring-operated energy accumulator is shown withdrawnfrom the oil tank. Details are shown in Fig. 3 and 4. The mainparts of the snap action diverter switch are the transition contacts(323), the transition resistors (315) and the short circuit-proofmain contacts (322). The diverter switch has been designed inparticular for maximum dependability and freedom from contactbounce. The kinematic principle of the contact system combinedwith the high speed operation achieved by the spring-operatedenergy accumulator gives short arcing times which minimizecontamination of the switch oil. To extend their working life thediverter switch contacts are faced with a sintered tungsten-copper

alloy, for 400 A and 500 A Units.

Along with the transition resistors associated with it, the diverterswitch is attached to the bottom end of the vertical diverter switchsuspension cylinder (307) which is made of fibre reinforcedplastic (FRP). At the top end of this cylinder is the diverter switchhead (301) with the driving mechanism consisting of the diverterswitch drive coupling (303), the spring-operated energyaccumulator (304) and the tubular driving shaft (308) (couplingthe energy accumulator to the contact system). This has enabledthe energy accumulator drive to be accommodated closealongside the main gear unit (121) in the on load tapchangerhead (103) where it is accessible at any time for inspection.

Fig. 3

Diverter Switch Unit, 21000

301

307

309

311

Three basic designs of Diverter switch are available.

Diverter Switch Type D 21000:

This diverter switch Fig .12 has a current carrying capacity of 200A

per phase . This is used in Tap Changers type D III 200, D I500 and D

I 600.

Diverter Switch Type D 04 66:

This diverter switch has a current carrying capacity of 400A per phase.

This is used in Tap Changers type DIII 400, D I800 and D I 1200.

A reinforced design of D 04 66 is used in tap Changers type D III 500

Fig . 4

Diverter Switch Type 21000

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Diverter Switch type D 0444:This is basically a single phase switch with a current carrying capacity of 400A and is used in

Delta tapchanger of 400 A capacity.As in the case of D 04 66, a reinforced version is available for current carrying capaci ty of 500A,and this is used in tapchangers type D III 500 Delta.A modified form of D 0444 is used in Delta Switch D III 200 Delta.

3.3 Transition ResistorsThese are wound with high-grade chrome-nickel alloy wire of circular section and consist ofpaper laminate panels, the long sides of which are provided with porcelain carriers. Each of thethree segments of the diverter switch has four transition resistors making twelve for the diverterswitch as a whole. These resistors are mounted as a singe group in an insulated drum and areconnected to the diverter switch contacts by screws which cannot work loose. The insulationdistances between the individual resistor panels make it necessary for the resistor drum to be

filled for the greater part with oil; only a small space is occupied by the active resistanceelements. This open type of construction allows very rapid cooling of the resistors which onlycarry the load during the transition time. The resistors are so designed that there is no need tolimit the number of tap changes which can be made in close succession.

3.4 Tap SelectorThe function of the tap selector is to prepare the tapchange by connecting the diverter switch tothe required tap on the transformer (Fig. 6). Duringthis stage the contact system of the diverter switchstays in its original position until the tap selector(Fig. 5) has arrived at the new tap. Thereupon thecontact system effects the change-over to bring the

new tap into operation.For this purpose the tap selector contacts (407)which connect to the transformer taps are arrangedone above the other on the vertical bars of theinsulator cage (404). Via these tap selectorcontacts and rotatable contact bridges (411 , 41 0)the necessary inter-connections to suit operatingconditions are made between the transformer tapsand central take-off rings (411). The latter areconnected by conductors (412) and sliding contacts(204) to the contacts (328, 329) of the diverterswitch. The current path between the diverterswitch and the starpoint output terminal (circularbus-bar 324) includes three further pairs of slidingcontacts.The alternate connecting of even-numbered andodd numbered tap selector contacts to the diverterswitch contacts takes place on the same switchingprinciples as employed in Jansen switches sinceas long ago as 1926. With this arrangement the tapselector contacts for each phase are divided intotwo groups odd-numbered contacts 1, 3, 5. andeven-numbered contacts 2, 4, 6... which aredisposed in two planes on separate cage bars. Twoshortci rcui t proof contact br idges (411, 410)

cooperate with these groups of contacts and aremoved alternately from one position to the next inunison with

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the diverter switch action by means of a Geneva mechanism (401). This mechanism is housed in the On

load tap changer head (103) where it is readily accessible for checking, and it drives the two

contact bridges of the tap selector via a pair of insulated shafts (402) and a reduction gear unit

(403)

The space-saving layout of the even-numbered and odd-numbered groups of tap selector contacts

and the central position of the tap selector conductors (412) from the take-off rings to the

conductors at the bottom of the tap selector are features which make for a compact arrangement.

The transverse forces brought about by the large number of leads connected to the tap selector

are resisted by the metal structure of the selector the central suspension tube of which (413)

joins together the upper and lower tap selector cage ring.The tap selector can be buil t for various

numbers of positions. Depending on the circuit arrangement used for connection the regulating

winding of the transformer to the tap selector there are types with 8,10,12,14 or16 tapping

positions, with three mid-positions, and others with 9,11,13,15 or 17 tapping positions and

only one mid-position. Fig. 6 shows the principal basic circuit arrangements for tap selectors of

any circular pitch. The numbers of tapping positions indicated apply to tap selectors with

preselector.

The on load tapchanger may be provided with a reversing switch or coarse tap selector (502,

504, 506) mounted on the side of the tap selector to keep the interconnecting leads short. The

reversing switch gives buck and boost connection of the regulating winding relative to the main

winding of the transformer (see Fig. 6 top circuit diagrams), whilst the coarse tap selector

connects the regulating winding, either direct or through a coarse tap winding, to the main winding

(see Fig. 6, basic circuit diagrams). While the tap selector contact K is in operation, the reversing

switch and coarse tap selector are actuated by a driving mechanism (501) running in unison

with the motion of the tap selector contact bridge (411) for the odd-numbered contacts. The

position of the reversing switch or coarse tap selector can be checked at any time from the

position indicator (119) visible through a window (116) in the on-load tapchanger head. Alongside

to right and left, the position of the tap selector bridges controlling the odd-numbered and even-

numbered contacts can be seen from the numbered dials on the two Geneva units (401).

The tap selector and reversing switch or coarse tap selector are immersed in the transformer

oil. Insulation distances in the vertical direction are protected against electrical over-stressing

by projecting electrodes. In the horizontal direction all insulation paths are through oil. These

arrangements give a high standard of protection against surge conditions.

3.5 Delta Tap Changers

In the case of tap changers intended for use in Delta connected transformers, it is necessary to

provide phase to phase insulation of appropriate value, in both the diverter switch and tap selector.

In these cases, three separate diverter switches are necessary, with insulated supports and

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drive shafts between them.

If the interphase insulation required is small, it is possible to house the three diverters, which

are now axially separated, as also the tap selector in one column, and such delta units look

about the same for external appearance as star or single phase units.

Where interphase clearance required is high, it may become necessary to mount the diverter

switch, and tap selectors in two different columns (See Fig.15)

4 Electrical Characteristics of On-Load Tap Changer

4.1 Electric Strength

The electric strength of the insulation to earth corresponds with the stipulated test voltages of

the national and international standards systems including IS, BS, VDE, ASA, IEC etc. The

withstand voltage to earth of the various insulation paths is higher than, or at least equal to the

test values referred to above.

The insulation of the on-load tapchanger to earth can be dimensioned independently of the internal

insulation of the on-load tapchanger and, in particular, independently of the insulation of the tap

selector. The latter is governed by the voltage stressing of the transformer and is graded accordingly

in the various sections. The internal electric strength of the tapchanger is determined primarily of

the impulse withstand and a.c. withstand voltage of the tap slector. The dimensioning of the tap

selector is based in the majority of cases, however on the impulse voltage stressing of the regulating

winding. The internal electric strength of the tap selector is provided by allowing appropriate

insulation distances in oil and along the selector bars. Through suitable grading of these distances

(tap selector contact circle diameter and tap selector length) three different tap selector series

have been developed (60, 110 and 150) so that the transformer manufacturer can match the tap

selector size to the voltage stressings of the regulating winding arising in service.

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The standard of insulation quality achieved is confirmed by the excellent results given during years of

service by many thousands of on-load tapchangers operating under the most diverse conditions in large

transformers upto 220 kV. The transformer manufacturer is responsible for ensuring correct coordinationbetween the electric strength of the on-load tapchanger and the voltage stressing arising in the transformer.

4.2 Short Circuit Withstand

The current paths and all contacts in the on-load tapchanger will withstand a thermal short circuit loading

at 20 times the rated current for 3 seconds without suffering any damage. They will also withstand dynamic

loading up to 50 times the rated current (peak value) without damage. The values specified in the IEC

recommendations are thus greatly surpassed.

4.3 Tap Changing Cycle

The diverter switch effects the no-break transfer of the current from the tap in service to the tap which hasbeen pre-selected. During the change-over pure ohmic resistance is cut in on the circuit. This gives

optimum arc extinction conditions at the critical points in the diverter switch, since the current through the

switch is always in phase with the recovery voltage regardless of the power factor (cos) of the transformer

load. The change-over action of the diverter switch D is illustrated in the diagram showing a tap change

sequence (Fig. 7) and in the oscillogram of a tapchange at the rated load (Fig. 8, 9). The following description

outlines the relationship between the various positions of the onload tapchanger and the characteristics of

the oscillograms. The diverter switch of the D Ill 200 on load tap changer (200 A rated current) does not

have the auxiliary contacts a2and b

2or the associated transition resistors R

2.

As will be seen from the above state-by-state description, a tap change involves three circuit-closing and

three circuit-opening actions. A tapchange can therefore be divided into three steps.The first step lasts from t

1to t

2(positions 2 to 4), the second from t

3to t

5(positions 4 to 6) and the third

from t5to t

7(positions 6 to 8).

Each such step must have a duration of not less than one half cycle of the system frequency. In the

example quoted this is the case.

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s

Fig.8 Oscillogram of a tap change (single phase)

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Top changer position Oscillogram

1 Main contact at working position A.

Transition from 1 to 2 Main contact A opens.

Current transfer from main contact A to transition contact a takes place

without arc formation, hence not recognizable in oscillogram (current IA).

2 Auxiliary contact a2closes.

Transition contact a opens.

Starting at time t1, current l

aforms arc, recognizable in oscillogram by occurrence

of arc voltage UA.

Transition from 2 to 3 current laflows as arc from t

1, to t

2, extinguished at current zero (t

2). UA occurs

as arc voltage from t1, to t

2.

3 UAoccurs as recovery voltage from t

2to t

3at transition contact voltage drop

across R1and R

2.

4 Auxiliary contact b2closes, recognizable in oscillogram by of commencement

of current Ib2

(t3).

Auxiliary contact a1opens, current l

a1occurs as arc starting at t

3.

Transition from 4 to 5 Current Ia1

occurs as arc at auxiliary contact a1 from t

3 to t

4, extinguished at

current zero (t4).


4to t

5at auxiliary contact a

1, brought about

by voltage drop across R2and R

2connected in parallel.

6 Auxiliary contact b1closes, recognizable in oscillogram by commencement of

current Ib1

(t5

).

Auxiliary contact a2opens, current, I a

2occurs as arc starting at t

5.

Transition from 6 to 7 Current Ia2

occurs as arc at auxiliary contact a, from t5to t

6, extinguished at

current zero (t6).


6to t

7at auxiliary contact a

2being equal to

the sum of the voltage drop across R2and R

1plus the tap voltage.

8 Transition contact b closes, recognizable in oscillogram by commencement of

current lb(t

7).

Auxiliary contacts b2and b

1shorted by the closing of transition contact b so that

no arc occurs on the opening of auxiliary contact b2.

9. Main contact B closes thereby taking up working position B and passing working

current; not recognizable in oscillogram.

Tap change completed.

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5. Motor Drive

The on load tapchanger type D is operated by a motor drive unit MA2 mounted outside the transformertank. The motor drive MA2, however, is limited to on load tap changers having not more than 35 tapping

positions. All the components which belong to the motor drive are contained in a sheet metal housing

designed for outdoor mounting and consisting of a case and a door.

The Drive shaft is the mechanical connection between motor drive and tap changer head. The bevel gear

serves for diverting motion from the vertical to the horizontal direction. The vertical drive shaft has to be

mounted be.tween motor drive and Bevel Gear and the horizontal drive shaft between bevel gear and tap

changer.

The motor drive unit is powered by a 415V, 50 Hz three phase motor (phase sequence RYB clockwise)

and with a stepping mechanism for direct and remote operation. Over travel in either direction is preventedby comprehensive mechanical and electrical safety features. The equipment also includes a six figure

counter showing the number of operations performed by the diverter switch, and a dial indicating the

present position of the on load tapchanger. Remote indication of on load tapchanger setting at any time

can be given by a digital display unit.

The built in electric heater prevents condensate forming in the drive housing.

For emergency operation and for setting up the motor drive unit when putting the on load tap changer into

service, a crank handle is provided. When pushed on to its shaft, this handle interrupts the motor circuit.

Re-engagement of the motor drive is brought about by pushing back the locking pin on the shaft after the

crank handle has been removed.

Fig. 10Motor Drive MA2

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6 Drying Out the On-Load Tap Changer

The undermentioned drying out times should be lengthened accordingly if a fairly long period of storage isunavoidable.

Prior to being put on load, the tapchanger must be dried out. The following are the minimum requirements

for this process :

6.1 Heat up in air from room temperature to a maximum temperature of 110oC. The heating must be

continued for at least 12 hours and a uniform rate of temperature rise must be ensured (< 10o/h).

6.2 Drying by air circulation at a maximum temperature of 110oC for not less than 20 hours.

6.3 Drying for at least 100 hours in vacuum at a residual pressure < 1 mm of mercury without oil,

at a maximum temperature of 110oC.

6.4 Impregnating with prepared oil.

The figures stated above represent the minimum drying out times which are necessary in order to maintain

or restore the electical properties of the fibre laminate to a satisfactory standard. Over rapid dehumidification

under conditions of high vacuum and high temperature is to be avoided, otherwise localized deterioration

in the bonding of the material may result. The figures given under 6.1 and 6.2 for the preliminary drying

stage should therefore be adhered to strictly.

If the on-load tapchanger and transformer have to be dried out simultaneoulsy and if the latter has to be

given an extra long period of drying, the final drying stage mentioned under 6.3 above may also be carried

out under conditions other than those specified in regard to pressure, temperature and duration, subject

to observance of the minimum requirements. Even considerably longer drying times need give rise to no

anxiety. On no account, however, should the maximum temperature of 110oC mentioned earlier be exceeded.

To prevent freeze drying the drying out temperature should not be below 60 oC.

7 Inspection

In the design and construction of the on load tapchanger special attention has been paid to ensuring that

all parts subject to wear in the tapchanger itself and in the motor drive unit, such as the gearing, energy

accumulator unit and diverter switch, are readily accessible for checking. as with all switchgear it is

advisable that on load tap changers also should be given regular checks at intervals depending on the

prevailing duty conditions (see appropriate inspection instructions). After the oil shut-off cock (123) has

been closed, the few litres of oil drained off via the oil suction plug (201) and the cover (106) removed, thediverter switch unit can if necessary be withdrawn from the oil tank (203) for checking. This is a simple

matter because the connections between the diverter switch and the oil tank are made by sliding contacts

(317).

After the diverter switch unit has been checked over and overhauled if necessary it goes back into the oil

tank again without any possibility of being wrongly positioned. The electrical connections are restored by

the sliding contacts already mentioned. At the same time the connection between the main gear unit (121)

and the driving crank of the energy accumulator unit (304) is automatically made at a readily visible point

in the on-load tapchanger head by means of the coupling (303) (Fig. 12). The tapchanger is now ready for

service again. All mechanical movements can be observed with the on-load tapchanger head (103) opened,

and the cover (106) need only be refitted after it has been verified that the mechanical functioning of the

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8 Protective Relay RS 2001

The protective device needed for disconnecting a regulating transformer in which a fault condition hasoccured includes an oil Surge Operated relay RS 2001 specially developed by us for use with on load tapchangers. The protective relay RS2001 (see Fig. 11) reacts to faults in the oil space of the diverter swtich.Under normal load and at the overload permitted for the unit the gases given off do not cause the protectiverelay to operate. In the event of unduly high pressure surges and in response to a specific rate of oil flowthe lever system linked to the flap is operated.

energy accumulator unit together with the latching devices and associated indicating gear is correct.

Even with the cover closed, the position indicator is still visible through the window (114). Experience

shows that under normal conditions skilled personnel should be able to carry out the necessary inspection

on a three-phase on load tapchanger and of the motor-drive operating it in about eight hours.

Fig. 11. Protective Relay RS 2001

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