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Wednesday, December 12, 2012

CHAPTER 1 POWER TRANSFORMERS

In onshore installations main electric power is taken in bulk from the National Grid through Area Boards at voltages up to

33kV. In offshore installations however main power must be generated locally, and this is usually done at 6.6kV.

Note that 6.6kV is a nominal rating in practice generation is more usually at 6.8kV to allow for voltage drop in the network.

There are instances of generation at other levels, but these are always high voltages. Some large loads a re fed directly from

the HV system,but for most purposes the supplies are needed at low voltage, typically 440/250V offshore and 415/240V

onshore. These are provided through 3-phasepower transformers.

FIGURE 1.1THREE PHASE OIL FILLED TRAN SFORMER

1.1 GENERAL

1.2 CONSTRUCTION

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1.2.1 General

Power transformers are always enclosed in a tank or si milar protection. They may be liquid cooled, air cooled or dry type

(encapsu lated or open). If liquid cooled, the coolant may be mineral (hydrocarbon) oil, silicone oil or some artificial liquid

such as Askarel.

The internal construction of all power transformers is si milar. The windings are stacked around a 3-limbed laminated iro

magnetic core, the low-voltage windings innermost and the high-voltage windings outside them - the best arrangement for

insulation. It can be seen in the cut-away portion of Figure 1.1. Ducts are arra nged through both windings on each limb to

assi st cooling. The terminations of the windings ar e brought out to cable boxes for external connections (s ee para. 1.8) or,

for large outdoor transformers, to terminal bushings.

1.2.2 Liquid (Oil) Filled Transformers

The largest bulk-power transformers are usually in a single tank, completely filled with oil and with a header tank called a

conservator on the roof. This mai ntains a sta tic head of pressure on the oil and also allows free expansi on and contractio

The transformer of Figure 1.1 is of this type.

In the pipe connecting the main tank to the conservator there is often inserted a device called a Buchholz Relay. It has two

elements: one traps and collects any small gas bubbles evolved at a winding due to the early stages of a possible breakdown o

insulation. If suffici ent gas has accu mulated, a float switch gives an alarm. The other element is a pi voted vane. If a major

fault occurs inside the tank, the displaced oil surges past the vane, causing it to swing, make a contact and trip the supply

breaker. The Buchholz relay is further described i n the manual Electrical Control Devices.

The oil coolant is heated by theI2Rlosses of the currents in all the windings and also by the iron losses. It circulates throug

a closed cooling system by thermosyphon action (i n a few cases by pumpi ng). Heat is extracted from the coolant through

radia ting tubes or fins either by natural convection, by forced cooling from fans or, more rarely, by water cooling. The

cooling of transformers is dealt with more fully in para. 1.6. The whole arrangement can be seen in Figure 1.1.

Faciliti es are provided for oil filling, draining and sa mpling for test. Oil samples are taken periodically for insulation testi

in the laboratory, where they are examined for deterioration or water pollution.

A tapp ing switch, normally for off-load use only, is usually fitted for changin g the transformer taps. The larger s ystem

network transformers may have on-load tap-changing gear - see para. 1.9 for details.

Smaller oil-filled transformers are usually sealed, with an air space above the oil instead of a conservator, to allow expansio

when hot. In ap pearance they would look like the Askarel-filled type shown in Figure 1.2.

Silicone oil is increasi ngly used instead of mineral oil because it is non-flammable. The construction is however similar.

1.2.3 Liquid (Askarel) Filled Transformers

In onshore installations where transformers can be installed at a distance from other plant, the fire risk is no greater than

normal, and mineral oil-filled transformers are generally used. Offshore however the fire risk is cru cial, and other designs

of transformer are necessary.

For this reason power transformers on offshore installations do not use mineral oil as the insulating and cooling medium.

Instead, a non-flammable silicone oil or a non-flammable liquid such as Askarel is used which is sometimes described by a

trade name s uch as Pyroclor or Pyralene.

The basic transformer design using silicone instead of mineral oil is no different from that described for oil-filled

transformers. But Askarel, apart from its fire-resistan t properties, has other characteristics , namely:

it is expensive,

it evaporates readily when in contact with air,

it is very penetrating,

it is heavy,

it is toxic and must not be allowed to come into contact with the skin or eyes,

it must on no account be discharged into the sea because it is non-biodegradable.

The external appearance of a typical Askarel-filled transformer is shown in Figure 1.2. Its internal construction is a sdescribed in p ara. 1.2.1, but, additionally, the transformer is hermetically sealed to prevent loss of liquid by evaporati on. In

some designs the main cover flange joint is welded up, as is also the filler plug after filling with liquid, and sometimes even

the drain plug. The expansion s pace above the liquid is fi lled with dry air or inert gas and ha s suffici ent volume to ensure

that the pressure insid e the tank is limited to a safe value even at maximum temperature and expans ion. A

pressur e/vacuum gauge is fitted. I t indicates zero at normal temperature, a positive pressure at high temperature and a

partia l vacuum at low temperature. This gauge is a constant monitor on the state of the sealing. A sight-glass also gives a

direct measure of the liquid level at all temperatures.

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3/11



TYPICAL 2 000kVA SEALED POWER TRANSFORMER (ASKAREL FILLED)

The off-load tappi ng switch spindle has five positi ons. Because Askarel is a penetrating liquid it is not easy to provide an

adequate seal where the operating shaft of the tapp ing switch comes through the tank-wall of the transformer. Some design

rely on a simple packing gland whereas others back this up by enclosing the whole tapping switch handle in an auxiliary bo

with a bolted cover. This box i s filled with As karel and pr ovides a s econd barrier to p revent liquid from leaking out of the

main tank. With the latter design it is necessa ry to drain and open up the auxi liary box to get at the tapp ing switch handle.

If a fault develops inside the transformer, the Askarel breaks down and forms a gas leading to a gradual or sudden rise in

pressur e, depending on the severity of the fault. To prevent the transformer tank from splitting, a s pring-loaded Qualitrol

pressure relief device on the top operates to release the excess pressure at the same time contacts within the device close to

trip the incoming sup ply and give an alarm. Pressures encountered in normal service are not high enough to operate the

device, which is ess entially an emergency relief valve. The Qualitrol device is fu rther described in the manual Electrical

Control Devices.

In case the tank should ever crack or split and so spill the Askarel, these transformers are always erected with a sill around

their mounting places. The sill is hi gh enough to contain the entire filling of its transformer and to prevent the toxic liquid

spreadi ng. After a spill the liquid must be collected and put into containers, along with mopping-up rags, and s ent ashore

for dispos al. ON NO ACCOUNT MAY THEY BE DROPPED INTO THE SEA. Personnel cleaning up must wear protective

clothing, gloves and goggles.

Though widely used offshore at first, Askarel-filled transformers are gradually being phased out in favour of silicone oil-

filled types because of the toxic nature of Askarel.

1.2.4 Dry Type Transformers

On some offshore installations transformers are used where the windings are encapsulated in epoxy resin and the whole

block is air cooled. To assi st the cooling, air d ucts are a rranged through the solid encap sulation.

Such transformers are often given a dual rating (e.g. 2 000/2 500k VA): the lower one where the cooling air circulates

naturally, and the higher one where it is as sisted by fans. It is arra nged that the fans start automatic ally when the loading o

the transformer exceeds the lower rati ng.

Dry-type power transformers can readily be built into their own LV distribution switchboards to form a single unit, thereb

saving LV cable boxes and cables and bringing the incoming feeder copperwork right up to the transformers LV terminals.

Small low-voltage transformers used as part of other equipment - for example, in battery chargers or inverters - are usually

open type, air cooled without any enclosure. They may be 3-phase or single-phase. Such open-type transformers are

protected by being housed within the parent equipments main enclosure.

The capacity of transformers is always given in kVA or MVA, because the heating depends only on the actual current and is

not affected by the power factor of that current.

The ratings of most offshore power transformers extend over a range of about 400kVA to

2 500kVA, depending on their duty. In onshore installations the ratings may go up to 30MVA or more National Grid sizes

may go up to 750MVA.

FIGURE 1.2

1.3 RATINGS

(15)

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A transformer i s desi gned to give a nominal secondar y voltage from a nomina l primary voltage - for example 11 000/415V or

6 600/440V. Due to voltage drop within the transformer its elf, the actual turns ra tio must be somewhat lower than this i f t

nominal secondary voltage is still to be achieved at full load. In the two examples cited above the turns ratio ( that is, the no-

load ratio) would need to be about 11 000/435V and 6 600/460V respectively.

Alone among electrical plant, trans formers are required by Bri tish Standar ds to have their no-load r ating dis played on thei

nameplates (generators and motors have their full-load ratings). The nameplate figure is therefore sometimes misleading i

that it su ggests a 435V or 460V system, whereas the nominal s ystem voltage is s till 415V or 440V. In some documentation

only nominal voltages are normally used, notwithstanding any transformer name-plate figures. Errors due to this

misunderstanding may often be found on other drawings.

In a manner similar to generators, transformers present impedance to the flow of through-currents. This imp edance is

measured by the percentage voltage applied at rated frequency to the primary winding necessary to circulate full rated

current in the secondary when short-circuited.

The effect of transformer impedance is to cause an internal voltage drop when load current is passed through the

transformer. Exactly as with a generator, the greatest drop is cau sed when a reactive current passes through the reactance

of the transformer. (The vectorial treatment of impedance loading on a generator is fully covered in the manual Electrical

Generation Equipment, Chapter 4.)

The internal drop due to load current cau ses a reduc tion of the secondary terminal voltage below its open-circu it level. This

reduction, usually expressed as a percentage of the no-load voltage, is termed the regulation of the transformer under the

stated load conditions . Since the impedance of a transformer i s almost wholly reactive, it follows that the greatest regulatio

occurs when a highly reactive load is applied.

IfEis the nominal rated line voltage applied to the primary winding, and if Escis the line voltage which, when applied to the

primary, will circulate full rated current in the short-circuited secondary, then

Z =

Esc

E

Escis called the impedance voltage of the transformer and is usually expressed as a percentage ofE. This same percentage

gives the value ofZ, which is the percentage impedance of the transformer.

This impedance is almost pure inductive reactance and ranges in value from about 5% to 10% for the sizes of transformers i

use. The measured percentage impedance is marked on each transformer nameplate and is us ed, together with other circu

impedances, to calculate the symmetrical short-cir cuit level on the low-voltage system. (See the manual Electrical

Protection.)

At the instant of s witching on a tra nsformer, while the core is unfluxed and therefore offers no reactance, a large inrush

current will flow which, although transient, may achieve a value of up to five times full-load primary current. This

disappears quickly after switch-on.

Once the core is magnetised, the impedance of a transformer to fault currents is constant this contrasts with a generator

whose reactance changes from su btransient through transi ent to synchronous as a fault pr ogresses.

1.5.1 Dry Type Transformers

The maximum temperature to which the windings of dry-type transformers may be allowed to rise depends on the type of

insulating material round the conductors. These transformers are classified according to the insulating material used, and

to each class is allotted a maximum ultimate temperature. The classification is as follows (according to BS 171 : 1970 and BS

2757 : 1956):

Clas s Typ ical Insula ti ng Materi al Ulti ma te Temp erature

A Impregnated cotton, silk, etc. paper

enamel105oC

E Pap er lami na tes ep oxi es 1200C

B Glass fibre, asbestos (unimpregnated)

mica130oC

F Glass fibre, asbestos, epoxy impregnated 155oC

H Glass fibre, asbestos, silicone impregnated 180oC

C Mica, c er amics, glass, with inor ganic

binders> 180oC

It should be noted that the classi fication letters do not follow an alphabetical sequence. This i s because there were originall

only three classes - A, B and C. Later intermediate classes were added, and it was decid ed not to disturb the original well

understood three.

Certain of the higher-temperature materials may be hygroscopic and therefore not always suitable in any particular

environment, particularly where dampness is severe.

It should be particularly noted that the classification depends on the ultimate temperature to which the insulating material

may be subjected, for it is this whi ch determines whether or not it will suffer damage when heated. It does not depend on

temperature rise alone. If, for instance, the ambient temperature is 40oC, a Class B material may be used if the designed

temperature rise will not exceed 90oC, so making the ultimate maximum temperature 130 oC. Designed temperature rises

must therefore take into account the greatest expected ambient temperature in which the transformer will operate.

1.4 IMPEDANCE VOLTAGE AND REGULATION

1.5 INSULATION

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5/11



1.5.2 liquid Filled Transformers

Liquid-fi lled transformers are not classi fied for insulation as are the dry type. There is an overall requirement that the

temperature rise of the windings shall not exceed 65 oC, and that the temperature rise at the top of the liquid shall not excee

60oC if the transformer is sealed or has a conservator.

The cooling system of a given transformer is identified by a 4-letter code, as follows:

1st and 3rd letter: kind of cooling medium

2nd and 4th letter: kind of circulation

The code symbols for the first and third letters are:

Mineral oil O

Synthetic insulatin g liquid L

Gas G

Water W

Air A

Solid insulant S

The code symbols for the second and fourth letters are:

Natural circulation N

Forced circulation F

Examples of the use of this code are:

Oil-filled, thermosyphon circulation, natural ventilation ONAN

Askarel-filled, thermosyphon circulation, natural ventilation LNAN

Dry-type encapsulated, fan cooled SNAF

Oil-filled, pumped circulation, water cooled by pump OFWF

A 3-phas e transformer has a 3-limb core. For transformers designed to BS 171 the terminals of the windi ngs mounted on

each limb are identified by a letter as shown in

Table 1 overleaf.

FIGURE 1.3

WINDING AN D TERMINAL MARKINGS

TABLE 1. D ESIGNATING LETTERS FOR 3-PHASE TRANSFORMERS

Designating Letter

1.6 COOLING

1.7 THREE PHASE TRANSFORMER CONNECTIONS

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6/11



WindingLimb 1 Limb 2 Limb 3

High Voltage A B C

Low Voltage a b c

Tertiary (if fitted) 3A 3B 3C

The external connections to the high- and low-voltage windings are brought out of the tank

through bushings. These terminals are labelled using letters approp riate to the winding concerned as s hown in Figure 1.3.

When viewed from a pos ition faci ng the high-voltage side of the transformer, the phase sequence is A-B-C from left to right.

The subscript numbers identify the winding terminations, including tappings, numbered in the direction of the applied or

induced voltage at a given instant.

Three-phase windings can be connected in delta, star or zig-zag (not very common) the star or zig-zag connection must be

chosen if a star-p oint is required to provide a neutral for a 4-wire system or for earthing. A common arrangement for 3-

phase power transformers in both onshore and offshore installations is for delta-connected high-voltage windings and star

connected low-voltage windings, with the star-point brought out to provide a neutral and earth for the low-voltage system.

A delta-connected winding is designated by the letter D, a star-connected windi ng by Y and a z ig-zag windi ng by Z. Capit

letters are us ed for the high-voltage windings and lowercase for the low-voltage. Thus Dy stands for d elta HV/star LV Yy f

star HV/star LV, and s o on. When the star-point of a star-connected winding is brought out it is designated YN for a high-

voltage or yn for a low-voltage winding.

The winding connections for a delta/star transformer having a delta-connected high-voltage winding are shown in Figure

1.4, which also shows the vector relationship between the voltage applied to each high-voltage winding and the induced

voltage in each correspondi ng low-voltage winding, the reversal between secondary and p rimary being ignored.

Taking the phase-to-neutral vector of A phase high-voltage as reference vector at

12 oclock, the corresponding a p hase low-voltage vector leads by 30oand i s therefore at 11 oclock. Thus the vector symbol i

this particular connection arrangement is Dy11, which describes the high- and low-voltage winding connections and theangular disp lacement between primary and s econdary voltages. Other winding arrangements are sometimes used, and for

full particulars of these, together with their vector symbols, reference should be made to BS 171 - Specification for Power

Transformers.

In the cas e shown above the vector symbol is sometimes wri tten Dyn11 to draw a ttention to the neutrals being brought out

on the secondary (low-voltage) side.

Transformers of different vector groups must not in general be paralleled. If all the prima ries are sup plied from a common

source, the secondaries of differing groups s uch as Dy11, Dy1, Yy0 will have different phase relationship s. For example,

there will be 60odifference between Dy1 and Dy11 (which leads on it), or 30odifference between Yy0 and Dy1 (which lags on

it). Such out-of-phase secondar ies must never be paralleled, even though their pri maries may be in pa rallel.

The exception is that groups with the same clock numbers, such as Dy11, Yd11, Yz11, may be paralleled, provided that there i

no other objection, since the secondaries are all in phase.

FIGURE 1.4

VOLTAGE VECTOR SYMBOLS

1.8 CABLE BOXES

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7/11



The terminals of large transformers which are connected to external lines are brought out through ceramic bushings in the

cover (shown in Figure 1.1). The terminals of other transformers have to be connected to cables. This ap plies parti cularly t

transformers on offshore installations and to most transformers in onshore oil installations.

The windings are connected to cables through cable boxes fixed to the transformer tank. If cables are used on both HV and

LV sides, the cable boxes would be on opposite sides of the tank, as seen in Figure 1.2.

On most transformers the current on the HV side is low enough to be carried by a single, 3-core cable which enters the HV

box through a sealing gland a nd divi des ins ide. Sometimes small current transformers are a lso fitted insi de the cable box.

On the LV side cu rrents are much heavier and often exceed 3 000A. The LV cable box is therefore much larger. In order to

carry such cur rents, three, or sometimes four, single-core cables are required for each phase. This results in the cable runsin the area of a trans former being very heavy and often difficult to accommodate. In addi tion there may be two similar-si ze

cables to carry the neutral, or 4th-wire, current.

On some installations the LV cable box is dispensed with and the windings are connected directly to the switchboard by

busbar-type copperwork in an enclosed duct which is brought right up to the si de of the transformer.

1.9.1 General

Tappings are usually provided to vary the transformers turns ratio by up to 5%. The correct tap is set when the installatio

is fi rst commissi oned and should not need to be changed for a considerable time. However, as the system load grows over

the years, the tapping may need to be changed to maintai n the secondary working voltage. This i s normally done on all

phases together by means of a switch on the transformer tank and must only be carried out off-load and isolated - that is,

with the transformer dead on both sides. Changes of tap s ettings may be carried out only by Authorised Persons, and then

only on the instructi ons of the Engineering Department. All tap changers on offshore and onsh ore oil installations are of th

off-load type.

In the larger shore networks on-load tap changers may be used to maintain system voltage they are usually remotely

controlled from a Control Centre and are desc ribed in pa ra. 1.9.3. On-load tap c hangers are not used on offshore or onshor

oil installations but may be employed on the networks supplying onshore plants.

1.9.2 Off Load Tap Changers

It is usual to provide four additional tappings with off-load tap changers, making a total of five, at 2% intervals, so that th

turns ratio vari es by 2% 5%. Tappi ngs are always placed on the high-voltage side this allows the lowest poss ible curre

rating for the tapping switch itself. Thus an 11 000/415V transformer with four such extra tapp ings would be shown on a

drawing as 11 000 2% 5%/415Vand would actually give 11 000/394, 405,415, 425 or 436V on load. In order to rais e the

secondary voltage it is necessary to go to a lower (i.e. negative) HV tap.

The tap-changer switch handle can be seen in Figure 1.2. I t must always be kept padlocked against unau thorised oraccidental operation.

1.9.3 On Load Tap Changers

Large network transformers which are provided with on-load tap changing normally have a much larger number of taps in

smaller steps. The princi ple used is make-before-break: this means that the new tap must be connected before the old tap i

broken, otherwise there would be a break in su pply and an interrup tion of full-load current by the tappi ng switch.

The difficulty with this simple idea is that, during the transition period while both taps are made, a small number of turns

the transformers HV winding are short-circuited by the two taps, and a heavy current will flow through them.

Arrangements are therefore made to insert resis tance temporarily into this short-circui ted loop to limit the current until t

tap change is complete and the short-circuit removed. Figure 1.5 shows in principle how this is done.

FIGURE 1.5

ON LOAD TAP CHANGING

A, B and C are adj acent taps on an HV winding. In (a) the tapping i s on A, a nd it i s desi red to move it, on load, to B.

The moving member consists of a main contact M and two transition contacts P and Q which are connected to M each

through a resistance. In positi on (a) M carries the full load, and P and Q are not in contact.

1.9 TAP CHANGING

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In the first part (b) of the transition the main contact M is still on tap A. Contact Q moves to B and contact P is still on A. Q

and M now short-circuit the HV turns between A and B, but the short-circuit current is limited by the lower half of the

resistance. Meanwhile M is still carrying the load current from tap A.

At the next stage (c) the moving member has travelled on, and the main contact M leaves tap A. P and Q now shar e the load

current which pa sses through both halves of the resistance. These two halves also limit the current in the shorted turns

between A and B.

At the next stage (d) the mai n contact M has moved to tap B, so that it is once again ca rrying the load current, but now from

the new tap. P however is s till on tap A, so that the current from the shorted turns is limi ted by the upper half of the

resistance.

Finally the moving member is at position (e), where the main contact M is on B and carrying the load, while P and Q are out

of contact, as they were in position (a), but now on the new tap.

During these transition stages the load current has never been interrupted, nor has the main contact ever been called upon

to break any large current. Moreover the current in the short-circui ted turns is always limited by one or both halves of the

resistance.

In some designs of tap changer the transition resis tors are replaced by reactors. These have a similar limiting effect but ar

not a source of heat. They also cancel each other out magnetically in stage (c) when both are sharing the load.

During stage (c) the full-load current pas ses momentarily through both halves of the resistance. To keep them to a

reasonable size, they must be short-rated. This p oses the problem that, if the driving motor power should be lost at the

moment the mechanism reached stage (c), it would stick there and a rapid burnout of the resistors would follow, with

inevitable damage to the short-circui ted turns. Steps must therefore be taken to prevent this hap pening.

The philosophy is that the power to operate the tap-changer mechanism must never do so directly but should be used only tstore energy. When a tap cha nge is called for, that energy is released and is su fficient to complete the change on its own, eve

if the external power supply fails.

The stored-energy tap-changer mechanis m is us ually of one of two types - spring-opera ted or flywheel-operated. In the

former a motor winds and charges a spri ng. A tap change cannot begin until the spri ng is fully charged, and, once released,

it completes the change on its own.

In the flywheel type a motor runs u p a flywheel on receipt of a tap -change signal. When the wheel is up to full speed the mot

is d isconnected and a clutch engages. The kinetic energy of the flywheel completes the change on its own.

On-load tap changers and their operating mechanisms are usually separate assemblies bolted to the transformer tank,

through which the tappings from all three phases are brought out into the changer compartment. This too is usually oil fille

but separate from the mai n tank, so that the tap changer can be dra ined for mai ntenance without having to drai n the main

tank.

Provision is mad e for manual operation, if that should be necessary, by inserting an operating handle. The speed of the tap

change remains the same as with power operation, since the same stored energy is released.

FIGURE 1.6

AUTO TRANSFO RMER CONNECTION

Where a transformer ra tio is fairly close - for examp le 3:1 or less - there is mu ch advantage in both cost and weight in

combining the primary and secondary windi ngs, as in Figure 1 6(a). Such an arrangement is called an auto-transformer.

In Figure 1.6(a) the secondary winding is combined with the primary, one terminal of each being common the other

secondary terminal is effectively a tap on the primary wind ing. This ar rangement gives a step-down effect, like a

1.10 AUTO TRANSFORMERS
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potentiometer, depending on the primary/ secondary turns rati o. Since the primary and secondary currents are in

opposi tion, the net current in the common part is less than the secondary current alone. F or example, if the primary curre

were 100A and the ratio 3:1, the secondary cu rrent would be 300A, and the net current in the common pa rt would then be t

difference, namely 200A. This p art of the winding could therefore be of lighter construction than would be needed if the

transformer had been of the normal double-wound type. Also, because of the closer linkage between the primary a nd

secondary windings, there is less leakage reactance, and the reactance of an auto-transformer is in general less than that of

its double-wound counterpart.

Although Figure 1.6(a) shows a voltage step-down arra ngement, an auto-trans former can equally be used for steppi ng up

(unlike a potentiometer), as in Fi gure 1.6(b). This is possi ble because the primary flux still links the whole of the secondary

winding, so d eveloping in it the full emf determined by the secondary turns. Use of an auto-transformer is a very economic

way of converting, for example, control supplies from 110V to 220V or vice versa.

Because a double-wound transformer provides complete electrical isolation between the two sides, an earth fault on one sidis not carri ed over into the other. This is not the case however with an auto-transformer. Both sides are electrically

connected through the common terminal and the tap.

It is important for reasons of safety that, if one line is earthed on one side, that earth should be applied to the common

terminal so that it is also app lied to both sides, as shown in F igure 1.6(c). In that case, if the primary voltage were 220V and

the secondary 110V, the common earth would ensu re that the live secondary terminal would never be more than 110V to

earth.

A safety haza rd would exis t if an auto-transformer were wrongly connected, as s hown in Fi gure 1.6(d). Here the earthed li

is not the common one, with the result that there is now no direct earth on the 110V system, one line being at 110V and the

other at 220V to earth - a possibly dangerous situation when the secondary circuit is switched in one pole only.

This error can easily arise when domestic equipment which has been designed for the USA 110V system is adapted to opera

from the UK 240V supp ly. Any such adap tations should always be carefully checked for polarity.

1.11.1 Manufacturers Tests

All power transformers are s ubjected to extensive tests by the manufacturers before delivery to the customer. While

operators and maintenance staff are not responsible for carrying out these tests, it is obviously an advantage to have some

knowledge of them. They are summari sed overleaf. If more details are required, reference should be made to BS 171:1970 -

Specification for Power Transformers.

Tests by the manufacturer are of three kinds:

Routine A test to which each individ ual transformer is subjected.

Type A test made on a transformer which is representative of other transformers, to demonstrate that they comply

with speci fied requirements not covered by routine tests.

Special A test other than a type test or a routine test, agreed by the manufacturer and the purcha ser, and applic able on

to one or more transformers of a particular contract.

Routine tests comprise:

(a) Measurement of winding resista nce, using a d.c. source and taking account of temperature.

(b) Ratio, polarity and phase relationships, in which the voltage ratio is measured on each tappi ng. The polarity of

single-phase transformers and the vector group symbol of 3-phase transformers are checked.

(c) Impedance voltage, using an a.c. source at the rated frequency, and carried out between pai rs of windings. Impedanc

voltage is defined in para. 1.4.

(d) Load losses, at rated frequency and carried out between pair s of windings.

(e) No-load losses and no-load current, measured at rated voltage and frequency.

(f) Induced overvoltage withstand: a test of dielectric insulation using a source of higher-than-rated frequency to avoid

excessive excitation current.

(g) Separate source voltage withstand: similar to (f) but using a source not less than 80% of the rated frequency.

(h) The insulation resistance of each winding in turn to all other windings, core and frame or tank, all connected togethe

and to earth. (Note: Where windings are star-connected or delta-connected insid e the transformer, phase-to-phas e

insulation tests cannot be carried out.)

Type tests and special tests are made only if specified by the purchaser. They include:

(j) Temperature rise test.

(k) Impulse-voltage withstand tests (with and without chopped waves).

(I) Measurement of zero-phase sequence impedance.

1.11.2 Users Tests

Operators and maintenance staff, while not responsible for the manufacturers tests referred to above, are required to appl

certain routine checks and tests to power transformers at the intervals laid down in the appropriate maintenance schedule

1.11 TRANSFORMER TESTING
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Posted by Emad Sedky at 8:27 PM

Labels: Electrical Distribution Equipments Part B

These routine tests include:

(a) Visua l examination of the transformer and its earthing resistor (if any), cable connections and earthing

arrangements for tightness, mechanical damage, corrosion and signs of overheating.

(b) Checking the oil or Askarel levels and insp ecting for leaks and clear drains.

(c) High-voltage insulation resistanc e test on the HV windings.

(d) Low-voltage insulation resista nce test on the LV windings.

(e) Simulation of overtemperature and overpressure by manual operation of the protection devices, and checking that t

alarm indications appear and the circuit-breaker trips.

The protection of electrical installations, including transformers, against damage caused by overload or fault conditions is

described in the manual Electrical Protection. To summarise, the protection provided for transformers may consis t of on

or more of the following:

HV Side Overcurrent

Earth fault

LV Side Restricted earth fault

General Overpressure (Qualitrol)

Overtemperature Buchholz (oil-filled only)

Differential

There is a range of small transformers, other than power transformers, which are used to operate measuring

instruments, meters and protective relays. They comprise voltage transformers (VT) and current transformers (CT)

and are covered in Chapter 4.

1.12 TRANSFORMER PROTECTION

1.13 INSTRUMENT TRANSFORMERS

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