Power Transformers

1.1 1.1 Definition:Definition:Transformers :transfer electrical energy at system voltage to electrical Transformers :transfer electrical energy at system voltage to electrical energy at the required voltage or higher voltage.energy at the required voltage or higher voltage.

- Current transformers Current transformers - Voltage transformersVoltage transformers

- Oil – immersed transformers Oil – immersed transformers

- Askarel immersed transformers Askarel immersed transformers - Dry type transformersDry type transformers

Instrument transformer Instrument transformer Distribution transformers:-Distribution transformers:-

1.2 Types of transformers1.2 Types of transformers

1.2.1 1.2.1 Oil immersed transformers :Oil immersed transformers :Oil immersed transformers have their cores & windings Oil immersed transformers have their cores & windings

immersed in mineral oil.immersed in mineral oil.

1.2.21.2.2 Askarel immersed transformers Askarel immersed transformersAskarel Oil immersed transformers have their cores and winding Askarel Oil immersed transformers have their cores and winding immersed in the synthetic cooling & insulating fluid Askeral immersed in the synthetic cooling & insulating fluid Askeral normally .normally .These transformers refferred to under the trade name of the fluid , These transformers refferred to under the trade name of the fluid , e.g “ colophon – immersed transformers “ or “ Pyroclor/Aroclor e.g “ colophon – immersed transformers “ or “ Pyroclor/Aroclor immersed transformers”immersed transformers”Askeral is a colourless , flame – resistant & explsion proof fluid , it Askeral is a colourless , flame – resistant & explsion proof fluid , it is made of chlorinated aromatic hydrocarbons . this density is is made of chlorinated aromatic hydrocarbons . this density is approx. 1.569 cm at 15.5 C° ambient temperature .approx. 1.569 cm at 15.5 C° ambient temperature .The electrical properties are a good as these of mineral oil The electrical properties are a good as these of mineral oil (transformer oil ) but Askarel – immersed transformers can be (transformer oil ) but Askarel – immersed transformers can be installed without special measures of fire protection .installed without special measures of fire protection .For chemichal reasons , mineral transformer oil & Askaral must For chemichal reasons , mineral transformer oil & Askaral must not be mixed .not be mixed .An oil immersed transformer cannot therefore be refilled with An oil immersed transformer cannot therefore be refilled with Askaral or an Askaral immersed transformer be refilled with oil .Askaral or an Askaral immersed transformer be refilled with oil .

1.2.31.2.3 Dry type transformers: Dry type transformers:Dry type transformers have no insulating & cooling fluid they are designed Dry type transformers have no insulating & cooling fluid they are designed according to the type of insulation of the windings into varnish – insulated & according to the type of insulation of the windings into varnish – insulated & silicon insulated dry type transformers.silicon insulated dry type transformers.

Self Cooling :Self Cooling :Distribution transformers are mainly manufactured with self cooling , with Distribution transformers are mainly manufactured with self cooling , with this kind of cooling the heat generated is dissipated by the natural air flow this kind of cooling the heat generated is dissipated by the natural air flow and by radiation .and by radiation .With forced air cooling the cooling air is circulated by fans . subsequent With forced air cooling the cooling air is circulated by fans . subsequent addition of fans to the transformer is only possible after consulting the addition of fans to the transformer is only possible after consulting the manufacturer . manufacturer . The limits of temperature rise permitted for various insulating materials are The limits of temperature rise permitted for various insulating materials are specified in ( VDE-0532 ) and these are based on the following limits of specified in ( VDE-0532 ) and these are based on the following limits of cooling air temperature : -cooling air temperature : - Maximum temperature of air 40h C .Maximum temperature of air 40h C . Daily mean temperature of the air 30h C .Daily mean temperature of the air 30h C . Yearly mean temperature of the air 20h CYearly mean temperature of the air 20h C . .

The temperature of the ambient air is measured at a distance of 1-2 m The temperature of the ambient air is measured at a distance of 1-2 m from the transformer .from the transformer .

1.2.41.2.4 Instrument transformers : Instrument transformers :- Current transformers- Current transformers- Voltage transformers- Voltage transformers

•Current transformers: Current transformers: Current transformer is transformer with small rate power (burden) , Current transformer is transformer with small rate power (burden) , whose primary windings are in series with the line circuit , and whose primary windings are in series with the line circuit , and secondary windings are connected to measuring instruments , secondary windings are connected to measuring instruments , electricity meters relays or control devices , current transformers electricity meters relays or control devices , current transformers isolate the measuring of protection circuits from the primary voltage isolate the measuring of protection circuits from the primary voltage and also protect the apparatus corresponding to the over current and also protect the apparatus corresponding to the over current response of the current .response of the current .

1.2.51.2.5 Voltage transformers : Voltage transformers : Voltage transformers are also of small power rate and operate at Voltage transformers are also of small power rate and operate at almost no – load . they isolate the primary high voltage from the almost no – load . they isolate the primary high voltage from the connected measuring or protected circuits.connected measuring or protected circuits.

TRANSFORMERSTRANSFORMERSDefinition :Definition : Transformers can be defined as a static electric machine which converts Transformers can be defined as a static electric machine which converts electric energy from one potential to another at the same frequency .electric energy from one potential to another at the same frequency .

It can also be defined as consists of two electric circuits linked by a It can also be defined as consists of two electric circuits linked by a common variable flux.common variable flux.

Theory of operation :Theory of operation :

The primary coil of the transformer is connected to a supply of sine The primary coil of the transformer is connected to a supply of sine wave voltage . an alternating sine wave current will flow in the wave voltage . an alternating sine wave current will flow in the primary . thus the primary m.m.f ( N.I ) will produce a common primary . thus the primary m.m.f ( N.I ) will produce a common

flux ( g ) which is also alternating and in phase with the current flux ( g ) which is also alternating and in phase with the current according to Faradayaccording to Faradays law the common flux interesting two coils s law the common flux interesting two coils will induce in them an alternating e.m.f ( e1 , e2 ) .will induce in them an alternating e.m.f ( e1 , e2 ) .

e1e1 is an e.m.f of self induction is an e.m.f of self induction e2e2 is an e.m.f of Mutual inductionis an e.m.f of Mutual induction

from 1,2 from 1,2 the transformation ratio the transformation ratio

1 dt

dN - e 1

1

2 dt

d N- e 21

2

1

2

1

N

N

e

e K

Applying Kirchoffs law on the primary circuit.V = - e1 V+ e1 = 0

From the secondary circuit e2 = v2

Equivalent circuit :

V.D e.m.f

2

1

2

1

2

1

V

V

N

N

e

e K

Io = 10 : 15 % of rated current .

111111 1 XIJ rI V E

22222 2 XIJ rI V E

Tansformer testing :Tansformer testing :

Determination of parameters :Determination of parameters :

Connect the primary to a source of alternating current at nomial voltage the Connect the primary to a source of alternating current at nomial voltage the secondary is open circuit – read the magnitude of ( Io , V1 , Po ) at no load .secondary is open circuit – read the magnitude of ( Io , V1 , Po ) at no load .

The impeadence of the circuit at no load .The impeadence of the circuit at no load .

01

o

1 Z Z I

V

I

V Z

o

10

Z Z o1

Z Z o1

Z1 can be neglected Z1 can be neglected

I

p r 2

o

oo

2

oo2

o Xr Z

2

o

2

oo r Z X

-Another method:Another method:

Io1

o V

Po cos

Po = V1 Io is Po = V1 Io is ggo o

0

1o I

V Z

Io1

1-

o V

Po cos

oo os Z Ro c

ooo sin Z x

For parallel circuit rm & JXm : Neglect Zm relation to Z1

ooo cos I I a

ooo sin I rI

V

Pcos

1

o1-

o

ma Io

V1

2r Ior

V X 1

m

-Short circuit test:Short circuit test:

Connect the primary to a reduced voltage ( from 15 – 20 % of V1 ) until the Connect the primary to a reduced voltage ( from 15 – 20 % of V1 ) until the primary current becomes near to the value of the full load current of the primary current becomes near to the value of the full load current of the primary .primary .

Short circuit the secondary winding .Short circuit the secondary winding .

Measure ; ( V1 ) sh.c ( I1 ) sh.c ( P1 ) sh.c Measure ; ( V1 ) sh.c ( I1 ) sh.c ( P1 ) sh.c

In the circuit ZIn the circuit Z2 2 , Z, Zo are connected in parallel o are connected in parallel

Zo is of the order of ( 10 )Zo is of the order of ( 10 )-2-2 relative to Zrelative to Z22

eqZ sh.cI

sh.cV

1

1

So the effect of Zo can be simplified to the show So the effect of Zo can be simplified to the show figure ( c ) . figure ( c ) .

Where ; Where ; Zeq = Req + Jxeq Zeq = Req + Jxeq

Req = r1 + r2Req = r1 + r2 xxeqeq = x = x

11 + x + x22

1 sh.c ) cos ( )sh.c I ( )sh.c V1 ( sh.c P1

V

P

P 1-cos sh.c

.I11

1

1

csh

sin eqZ eqx

cos Z R eqeq

2

22 K

rr

2

X X X

eq

21

21 r r

2

22 k

x x

D.C Test :D.C Test :

Connect the primary coil with a direct current supply . measure Connect the primary coil with a direct current supply . measure the applied voltage and the current . the applied voltage and the current .

ohmsr 1I

E

The effect of X1 , XThe effect of X1 , X00 will not appear when using direct current will not appear when using direct current

dt

Ldi X

I is the const. relative to timeI is the const. relative to time[ [ also the effect of ro will not appear also the effect of ro will not appear because it represent the eddy and hysteresis losses which are not existing because it represent the eddy and hysteresis losses which are not existing in the case of direct current they appear only when there is varialable flux in the case of direct current they appear only when there is varialable flux in the core . in the core . Similarly we can determine the resistance of the secondary ( r2 ) by Similarly we can determine the resistance of the secondary ( r2 ) by connecting the battery to the terminals of the secondary coil . connecting the battery to the terminals of the secondary coil .

Voltage regulation : Voltage regulation :

The voltage regulation is defined as the change in the secondary The voltage regulation is defined as the change in the secondary voltage of a loaded transformer when the load is removed . while the voltage of a loaded transformer when the load is removed . while the primary voltage is constant at itprimary voltage is constant at its nominal value .s nominal value .E = ( VE = ( V

22 ) ) n.ln.l – V – V

2 2 load load

In order to enable the comparison between transformers of different In order to enable the comparison between transformers of different working voltages , the voltage regulation in usually expected as percent working voltages , the voltage regulation in usually expected as percent or a per unit value related to the secondary voltage at load . or a per unit value related to the secondary voltage at load . usually the voltage regulations is determined for full load conditions . so usually the voltage regulations is determined for full load conditions . so to simplify equivalent circuit to simplify equivalent circuit

% 100

2V

2V - .2V V.Rε

L

LLn

unit per

2V

2V - .2V V.Rε

L

LLn

calculation the effect of I0 is neglected and we get the following simplified calculation the effect of I0 is neglected and we get the following simplified equivalent circuit and the corresponding to it vector diagram ( Kapp vector equivalent circuit and the corresponding to it vector diagram ( Kapp vector diagram ) diagram )

Io neglected :

eq1 Req21 X IJ I1 V V

eq1 eq112 X IJ RI V V

II1= 1= II 22

ZZq q = Req + JX= Req + JXeqeq = ( r = ( r

1 + 1 + rr2 2 ) + J( X) + J( X11 + X + X22 ) )

To calculate the voltage regulation the following value must be To calculate the voltage regulation the following value must be determined .determined .VV11,I,I11 cos cos gg11 and Z and Z

eqeq . .

ReqI cosV V Xeq I1112

1112 - sin V

l

lln

)(V

)(V .)V (

2

22

Note: Note: ( V( V22 ) )n.ln.l = V = V

11

l

lln

)(KV

)(KV .)V(K

2

22

2

21

2

22

V

VV

)V(

)V( .)V (

l

lln

where Vwhere V2 is calculated by eq ( 1 ) 2 is calculated by eq ( 1 )

some times the quantities known are V2 , I2 Cos some times the quantities known are V2 , I2 Cos gg2 , Zeq 2 , Zeq in this case To find the voltage regulation V1 we can calculated from the in this case To find the voltage regulation V1 we can calculated from the geometry as geometry as Note :Note :Parameters rParameters r2 , X2 , X2 , I2 , I2 , E2 , E2 as follows : 2 as follows :

I cosV V eq XI sin VR 2222

eq2221 -

K

I I 2

2 Where ;Where ;

r2 = k2 r2 , x2 = k2 x2 E2 = kE2

1

2

2

1

1

1

2

1

I

I

V

V

E

E

N

N k

in

in

input

output

PP

P

Pη losses -

req2

2 req 12 1212cu I I r r I IP cup

b

a

bcu

acu

PP

2

2

I

I

Transformer efficiency ( ) :

This means that if the cu losses are known at a certain load ( current ) , This means that if the cu losses are known at a certain load ( current ) , then the copper losses can be determined at another load .then the copper losses can be determined at another load .

I1 : nominal value ( full load value ) usually the copper losses are I1 : nominal value ( full load value ) usually the copper losses are determined from a short circuit test at a current equal to the full load or determined from a short circuit test at a current equal to the full load or nominal value , accordingly the equation can be written as :nominal value , accordingly the equation can be written as :

lflfcu

cu

PP

.I

I

. 2

2

2

II

.

f.l

cucu lfrequired pp

X I

I

f.l

let

f.lcucu PP ) ( X required 2

loss loss const.

cuout

out

PPη

cuP Pout

out

cos K.V.A

cos K.V.A

o

cuP Pout

out

o cos K.V.A

cos K.V.A

::تحسب كاألتى تحسب كاألتى ( ( وعموما الكفاءة عند أى حمل ) وعموما الكفاءة عند أى حمل )

in

losses -in p

pη

f.lo cu )(

ppη

f.l

f.l

X cos )K.V.A ( X

cos )K.V.A ( X

2

cos )K.V.A ( X

X cos )K.V.A ( X

2

f.l

f.l cu )( f.lo ppη

average efficiency for the transformer during 1 day ;average efficiency for the transformer during 1 day ;

hours 24ough energy thrinput Total

hours 24ough energy throutput Total

the input energy of the transformer through the day is equal to the Total the input energy of the transformer through the day is equal to the Total out put + Total losses per/day . out put + Total losses per/day . losses are const. or magnetic ( Plosses are const. or magnetic ( P

oo ) and are constant through the day . ) and are constant through the day .

the ( electrical orthe ( electrical or cu ) losses are variable according to the load ( cu ) losses are variable according to the load ( QQII22 ) . ) .

E.X : E.X : 100 K.V.A lighting transformer has a full load loss of 3 K.V.A , the 100 K.V.A lighting transformer has a full load loss of 3 K.V.A , the losses being equally devided between iron and copper . During a day the losses being equally devided between iron and copper . During a day the transformer operates , on full load for 3 hours , one half for 4 hours , the transformer operates , on full load for 3 hours , one half for 4 hours , the output being negligible for the reminder of the day calculate the all day output being negligible for the reminder of the day calculate the all day efficiency . efficiency .

SolutionSolution : : It should be noted that lighting transformers are taken to have a load p.f It should be noted that lighting transformers are taken to have a load p.f of unity iron losses for 24 hours = 1.5 x 24 = 36 K.W.h ( const. losses ) of unity iron losses for 24 hours = 1.5 x 24 = 36 K.W.h ( const. losses ) FL.cu losses = 1.5 K.W FL.cu losses = 1.5 K.W Cu loss for 3 hours on F.L = 1.5 x 3 = 4.5 k.w.hCu loss for 3 hours on F.L = 1.5 x 3 = 4.5 k.w.hCu loss for half F.L = 1.5 /4 k.w.h Cu loss for half F.L = 1.5 /4 k.w.h Cu loss for 4 hours at half the load = ( 1.5 / 4 ) x 4 = 1.5 k.w.h Cu loss for 4 hours at half the load = ( 1.5 / 4 ) x 4 = 1.5 k.w.h Total losses = 36 + 4.5 + 1.5 = 42 k.w.h Total losses = 36 + 4.5 + 1.5 = 42 k.w.h Total output = ( 100 x3 ) + ( 50 x 4 ) = 500 k.w.h Total output = ( 100 x3 ) + ( 50 x 4 ) = 500 k.w.h all day = 500 x 100 / 542 = 92.26 %all day = 500 x 100 / 542 = 92.26 %

Group numbers : Group numbers : The group number indicates the phase differience between primary and The group number indicates the phase differience between primary and secondary ( H.T and L.T ) line voltages in electrical degrees . It is secondary ( H.T and L.T ) line voltages in electrical degrees . It is sometimes determined as a clock reading each hour is equivalent to sometimes determined as a clock reading each hour is equivalent to 30° phase difference . 30° phase difference .

Y-Y connectios : Y-Y connectios :

( ( Y ) Y )

Parallel operation :Parallel operation :

1.1.the transformations ratios must be the the transformations ratios must be the same same

2.2.the group number must be the same the group number must be the same 3.3.the phase connection must be in same the phase connection must be in same

sequence sequence 4.4.short circuit impendence ( Zeq ) must be short circuit impendence ( Zeq ) must be

the same the same

In power station transformers are usually In power station transformers are usually working in parallel in order to enable the working in parallel in order to enable the connections or disconnection of any number connections or disconnection of any number of them according to their required load :-of them according to their required load :-The following conditions must be fulfilled The following conditions must be fulfilled for correct parallel operation : for correct parallel operation :

Transformers are produced locally as , three phase immersed in oil , Transformers are produced locally as , three phase immersed in oil , natural cooling , in door & outdoor ,with power rate from 50 up to natural cooling , in door & outdoor ,with power rate from 50 up to 10.000 K.V.A & voltages up to 22 K.V.A under licence of Siemens 10.000 K.V.A & voltages up to 22 K.V.A under licence of Siemens A.B.B & France TransfoA.B.B & France Transfo2.12.1 General characteristics : General characteristics :

the transformers described have the following general characteristics :the transformers described have the following general characteristics :Three – phase .Three – phase .Connection D/Y11Connection D/Y11 Frequency 50 Hz .Frequency 50 Hz .Natural cooling in oil or silicon .Natural cooling in oil or silicon .Continuous service .Continuous service .Indoor or out door installation .Indoor or out door installation .

Rated power :Rated power :

Normal rated power in KVA as follows . ( 25- 50- 100 – 160 – 250 – 400 – Normal rated power in KVA as follows . ( 25- 50- 100 – 160 – 250 – 400 – 500- 630 – 800 – 1000 – 1250 – 1600 – 2000 – 2500 – K.V.A -………. ) 500- 630 – 800 – 1000 – 1250 – 1600 – 2000 – 2500 – K.V.A -………. )

2.2.2 Formation of distribution 2 Formation of distribution transformers :transformers :

التوزيع محوالت التوزيع مكونات محوالت مكونات

•2.2.1 Iron core:2.2.1 Iron core: الحديدى :* * الحديدى :القلب القلبMade of cold rolled silicon steel sheets Made of cold rolled silicon steel sheets 0.3mm to minimize losses 0.3mm to minimize losses

الصلب رقائق من الحديدى القلب الصلب يصنع رقائق من الحديدى القلب يصنعبسمك البارد على المسحوب بسمك السيليكونى البارد على المسحوب السيليكونى

المغناطيسى 0.30.3 الفيض فى الفقد لتقليل المغناطيسى مم الفيض فى الفقد لتقليل مم

2.2.2 –Windings:2.2.2 –Windings:

High tension turns are made of High tension turns are made of copper wires of either circular cross copper wires of either circular cross sections varnish isolated or rectangular sections varnish isolated or rectangular cross sections isolated by sililose paper. cross sections isolated by sililose paper. Low tension turns are made of either Low tension turns are made of either noninsulated copper foils with insulating noninsulated copper foils with insulating paper in between or of rectangular wires paper in between or of rectangular wires

insulated by cylindrical paper sheetsinsulated by cylindrical paper sheets

الملفات :الملفات :

أسالك من العالى الجهد ملفات أسالك تصنع من العالى الجهد ملفات تصنع

بالورنيش معزول مستدير مقطع ذات بالورنيش نحاسية معزول مستدير مقطع ذات نحاسية

السيليوزى بالورق معزول مستطيل مقطع السيليوزى أو بالورق معزول مستطيل مقطع أو

شرائح من المنخفض الضغط ملفات شرائح وتصنع من المنخفض الضغط ملفات وتصنع

نحاس أسالك أو بالورق المعزول نحاس النحاس أسالك أو بالورق المعزول النحاس

األسطوانية . الورق بشرائح األسطوانية .معزولة الورق بشرائح معزولة

2.2.3-Tank :2.2.3-Tank :The transformer tank is made of The transformer tank is made of

corrugated steel .corrugated steel .

The corrugated tank surface is itself the The corrugated tank surface is itself the cooling surface. The tank is provided with cooling surface. The tank is provided with an additional steel reservoir for oil an additional steel reservoir for oil expansion, on which a piping device is expansion, on which a piping device is installed to transmit oil cock ,a hole for installed to transmit oil cock ,a hole for silicagel apparatus,and an oil level silicagel apparatus,and an oil level indicatorindicator..

الخارجى : الخارجى :التنك التنك

ليكون المعرج الصاج ألواح من التنك ليكون يصنع المعرج الصاج ألواح من التنك يصنع

للمحول المبرد المسطح هو التنك للمحول جسم المبرد المسطح هو التنك جسم

مصنع الزيت لتمدد بخزان التنك مصنع ويزود الزيت لتمدد بخزان التنك ويزود

توصيل مواسير عليه ويركب الصاج توصيل من مواسير عليه ويركب الصاج من

نقص لتعويض فتحة وبه للتنك نقص الزيت لتعويض فتحة وبه للتنك الزيت

السيلكاجل جهاز لتركيب وفتحة السيلكاجل الزيت جهاز لتركيب وفتحة الزيت

الزيت مبين الزيت وكذلك مبين وكذلك

2.2.4- Terminals :2.2.4- Terminals :

H.V.and L.V. terminals are brought out H.V.and L.V. terminals are brought out through porcelain bushings according to the through porcelain bushings according to the rated voltage. The insulators are fixed to the rated voltage. The insulators are fixed to the tank cover in such a way to ensure tank cover in such a way to ensure replacement without dismantling the replacement without dismantling the transformer cover. Cable end boxes on either transformer cover. Cable end boxes on either H.T. or L.T. side or both can be made if H.T. or L.T. side or both can be made if

requiredrequired..

التوصيل : التوصيل :أطراف أطرافالعالى الضغط أطراف العالى توصل الضغط أطراف توصل

الصينى من عوزال الى الصينى والمنخفض من عوزال الى والمنخفضهذه وتثبت التشغيل لجهد هذه مناسبة وتثبت التشغيل لجهد مناسبة

تسمح بطريقة التنك غطاء فى تسمح العوازل بطريقة التنك غطاء فى العوازلالطلب وعند الغطاء فتح بدون الطلب بتغييرها وعند الغطاء فتح بدون بتغييرهاللكابالت نهاية بصناديق تزويدها للكابالت يمكن نهاية بصناديق تزويدها يمكنالضغط جهتى من الطلب حالة الضغط فى جهتى من الطلب حالة فى

. فقط احداهما أو والمنخفض .العالى فقط احداهما أو والمنخفض العالى

2.2.5 –Tapchangers :2.2.5 –Tapchangers :

or distribution transformers , tap or distribution transformers , tap changers are externally for allowing changers are externally for allowing voltage regulation with ±5 % of the voltage regulation with ±5 % of the rated value in 5 equal steps of ± 2.5 % rated value in 5 equal steps of ± 2.5 % each , the tap changer is manually each , the tap changer is manually operated while current is offoperated while current is off

الجهد : الجهد :منظمات منظمات

منظمات التوزيع محوالت على منظمات يركب التوزيع محوالت على يركببتغيير تسمح مراحل خمس ذات بتغيير للجهد تسمح مراحل خمس ذات للجهد

حدود فى حدود الجهد فى منها% 55 ++الجهد منها% كل % % 2.52.5 ++كلفصل بعد المحول خارج من يدويا فصل ويعمل بعد المحول خارج من يدويا ويعملأن أى تماما الكهرباء عن أن المحول أى تماما الكهرباء عن المحول

الالحمل . على يعمل الالحمل .المنظم على يعمل المنظم

2.2.6- Oil :2.2.6- Oil :

transformers are filled with special oil transformers are filled with special oil ( Diala ( 5 ) or equal ) of high ( Diala ( 5 ) or equal ) of high insulating grade according to IEC insulating grade according to IEC specificationsspecifications

التبريد : .. التبريد :زيت زيت

درجة ذو محوالت بزيت المحوالت درجة تمأل ذو محوالت بزيت المحوالت تمألطبقا ( ) يماثلها ما أو ب ديالة عالية طبقا ( ) عزل يماثلها ما أو ب ديالة عالية عزل

IECIECلمواصفات لمواصفات

2.2.7 – Main accessories of distribution 2.2.7 – Main accessories of distribution transformers : transformers :

Oil expansion reservoir . Oil expansion reservoir . Thermometer pocket Thermometer pocket Oil drain cock Oil drain cock Oil level indicator Oil level indicator Lifting chackles Lifting chackles Four two – directional adjustable wheels Four two – directional adjustable wheels Earth screw Earth screw Name plateName plate

لمحوالت األساسية لمحوالت الملحقات األساسية الملحقاتالتوزيع :التوزيع :

الزيت تمدد الزيت خزان تمدد خزان الترمومتر الترمومتر جراب جراب أخذ أو الزيت تصريف أخذ صمام أو الزيت تصريف صمام

منه منه عينات عينات الزيت مستوى الزيت مبين مستوى مبين المحول لرفع المحول حلقات لرفع حلقات عجالت عجالت أربع أربع األرضى توصيل األرضى مسمار توصيل مسمار البيان البيان لوحة لوحة

2.2.8 – Additional accessories :2.2.8 – Additional accessories : Dehydrating breather Dehydrating breather Buchholz relay Buchholz relay Ordinary mercury thermometerOrdinary mercury thermometer

اإلضافية اإلضافية الملحقات الملحقات المتصاص السيلكاجل المتصاص جهاز السيلكاجل جهاز

الرطوبةالرطوبة المحول لحماية البوخهلز المحول جهاز لحماية البوخهلز جهاز عادى عادى ترمومتر ترمومتر

رقم شكل يلى رقم وفيما شكل يلى وفيما ) )11 ( نموذج ) يوضح نموذج يوضح

المحوالت المحوالت لمكونات لمكونات E محليا E المصنعة محليا المصنعة

3.1 The general data of transformers are: 3.1 The general data of transformers are: المواصفات العامة المواصفات العامةللمحوالتللمحوالت

المحوالت كانت إذا ما تحديد المحوالت يجب كانت إذا ما تحديد يجبالتوازى أوعلى منفرده تعمل التوازى المطلوبه أوعلى منفرده تعمل المطلوبهموجودة أخرى محوالت مع أو بعضها موجودة مع أخرى محوالت مع أو بعضها معالمعاوقه بيان يجب الحاله هذه المعاوقه وفى بيان يجب الحاله هذه وفى

الموجودة . للمحوالت الموجودة .الكليه للمحوالت الكليه

Determine wheather the ransformer will Determine wheather the ransformer will work in parallel or individual In case of work in parallel or individual In case of parallel work with old transformers, the parallel work with old transformers, the voltages and impeadance of them must be voltages and impeadance of them must be mentioned . mentioned .

. الجهد منظم التغيير الجهد .نسبة منظم التغيير نسبة Percentage regulation of tap changer .Percentage regulation of tap changer .

داخل يركب وهل المحول تركيب داخل مكان يركب وهل المحول تركيب مكانخارجة . أم خارجة .المبنى أم المبنى

The place in which the transformer will be The place in which the transformer will be installed and wheather indoor or outdoor . installed and wheather indoor or outdoor .

. التوصيل مجموعة التوصيل .رقم مجموعة رقم Connection group ( e.g DY II ) Connection group ( e.g DY II )

, , والذبذبة الخروج وجهد الدخول والذبذبة , , جهد الخروج وجهد الدخول جهدبالهرتز .بالهرتز .

Input voltage , output voltage in volts and Input voltage , output voltage in volts and cycle in Hertz . cycle in Hertz .

المحول المحول قدرة . . . قدرة أ. ف .ك أ. ف ك Power of transformer in K.V.A Power of transformer in K.V.A

3.2 Selection3.2 Selection: : المحوالت مواصفات تحديد المحوالت كيفية مواصفات تحديد كيفية3.2.1 Rated values :3.2.1 Rated values :

The rated values of the transformer such as power , voltage , The rated values of the transformer such as power , voltage , transformation ratio and impendence ratio are selected according to transformation ratio and impendence ratio are selected according to the requirements of the system .the requirements of the system .

i طبقا الكلية والمقاومة التحويل ونسبة والجهد كالقدرة األسمية القيم i تتحدد طبقا الكلية والمقاومة التحويل ونسبة والجهد كالقدرة األسمية القيم تتحددالكهربائية التغذية نظام الكهربائية لمتطلبات التغذية نظام لمتطلبات

3.2.2 Rated power :3.2.2 Rated power :

The rated power is found first by determining the peak effective power The rated power is found first by determining the peak effective power demand designed or measured , usually a margin is added to provide demand designed or measured , usually a margin is added to provide for the regular increase in power demand .for the regular increase in power demand .مطلوبة فعاله قدرة ألقصى القياس أو التصميم طريق عن اإلسمية القدرة مطلوبة تتحد فعاله قدرة ألقصى القياس أو التصميم طريق عن اإلسمية القدرة تتحد

نسبة إضافة نسبة مع إضافة فى ( ) ( ) مع العادية المستقبلية الزيادة الحتماالت القدرة فى من العادية المستقبلية الزيادة الحتماالت القدرة منالمطلوبة الكهربائية المطلوبة الطاقة الكهربائية ..الطاقة

When calculate the rated power (PN) we must take in consideration the When calculate the rated power (PN) we must take in consideration the anticipated power factor (COS anticipated power factor (COS ) )

المتوقع القدرة معامل قيمة مراعاة اإلسمية القدرة هذة حساب عند المتوقع ويجب القدرة معامل قيمة مراعاة اإلسمية القدرة هذة حساب عند . . ويجب

3.2.3 The impendence voltage(UK) :3.2.3 The impendence voltage(UK) :

the impeadence voltage the impeadence voltage UKUK is the voltage necessary at the input terminals at is the voltage necessary at the input terminals at rated frequency to cause the rated current to flow in the primary when the rated frequency to cause the rated current to flow in the primary when the terminals on the secondary are short circuited terminals on the secondary are short circuited

3.2.4 The rated impeadence voltage(UkN) :3.2.4 The rated impeadence voltage(UkN) :

the rated impeadance voltage (UKN) is the value of the impedance voltage the rated impeadance voltage (UKN) is the value of the impedance voltage on the principal tap when related to the rated voltage (UN) it is called UKN on the principal tap when related to the rated voltage (UN) it is called UKN and given in percentand given in percent

UkNUkN UNUNWhere ;Where ;

UKN = rated impeadence voltage % .UKN = rated impeadence voltage % .UKN = rated impeadence voltage in V.UKN = rated impeadence voltage in V. U.N = reated voltage in V.U.N = reated voltage in V.

X 100%X 100%UkN =UkN =

3.2.5 Selection of rated impeadence voltage :3.2.5 Selection of rated impeadence voltage :

In distribution system a rated impeadence voltage UKN= 4 % is preferred in order to In distribution system a rated impeadence voltage UKN= 4 % is preferred in order to keep the voltage drop small .keep the voltage drop small .

For larger industrial systems with greater power demand transformers with a rated For larger industrial systems with greater power demand transformers with a rated impeadence voltage of 6 % are used in order to limit the short – circuit stresses on the impeadence voltage of 6 % are used in order to limit the short – circuit stresses on the switch gear of the plant . switch gear of the plant .

3.2.6 Transformers loss PK:3.2.6 Transformers loss PK:

For transformers losses include no-load losses Po and load losses ( Pk ) the no load For transformers losses include no-load losses Po and load losses ( Pk ) the no load losses resulting from the continious magnetic flux reversal in the iron are practically losses resulting from the continious magnetic flux reversal in the iron are practically constant voltage impeadence of load .constant voltage impeadence of load .

The load losses ( copper losses ) constant of resistive losses in the windings and losses The load losses ( copper losses ) constant of resistive losses in the windings and losses due to stray fields , the load losses vary with the square of the load . the total losses of a due to stray fields , the load losses vary with the square of the load . the total losses of a transformer are :-transformer are :-

Pk a P P 2

OV

)K.V.A in Ppower rated

K.V.A in pa loadpart (factor load a

Pv = total losses in watt .Pv = total losses in watt ...Po = no-load in watt.Po = no-load in watt.

PK = load losses in W PK = load losses in W

3.2.7 3.2.7 - Efficiency : - Efficiency : the efficiency the efficiency of a distribution transformer can be calculated of a distribution transformer can be calculated

with reasonable accuracy from the following ; with reasonable accuracy from the following ; NN

where ; where ; = efficiency in % .= efficiency in % .PN PN = rated power in K.V.A .= rated power in K.V.A .Po Po = no-load losses in K.W ,= no-load losses in K.W ,Pk Pk = load losses in K.W .= load losses in K.W . = power factor . = power factor . a a = load factor . = load factor .

% 100 Po cos Watt P a

Pa Po - % 100

N

k2

Cos

* Example :-* Example :-

calculate efficiency (calculate efficiency () of a transformer at full load given the ) of a transformer at full load given the following information :following information :

PN = 500 K.V.A., Po = 1.1 K.W , Pk = 5.5 K.W , PN = 500 K.V.A., Po = 1.1 K.W , Pk = 5.5 K.W , Cos Cos = 0.8 , a = 1.0 = 0.8 , a = 1.0

= 98.36 %= 98.36 %

% 100 Po cos P a

P a Po - % 100

N

K2

% 100 K.W 1.1 8.0K.V 500 1

K.w 5.5 (1) K.W 1.1 - % 100

2

Pk

Po a

447.05.5(k.w)

(k.w) 1.1 a

% 100

K.W 1.1 8.0KVA 500 447.0

K.w 5.5 0.447 K.W 1.1 - % 100

2

3.2.8 Maximum efficiency: 3.2.8 Maximum efficiency:

the load factor ( a ) for the maximum efficiency of a transformer is defined as the load factor ( a ) for the maximum efficiency of a transformer is defined as follows ;follows ;

For the transformer in the above example this becomes ;For the transformer in the above example this becomes ;

Therefore maximum efficiency occurs with a load of ;Therefore maximum efficiency occurs with a load of ;Pa = PN.a Pa = PN.a Pa = 500 K.V.A Pa = 500 K.V.A 0.447 = 224 K.V.A 0.447 = 224 K.V.A And for this load the efficiency is ;And for this load the efficiency is ;

= 98.36 %= 98.36 % = 98.78 % .= 98.78 % .

3.2.9 Insulation class :3.2.9 Insulation class :

the insulation rating of a transformer is designed by numerals indicating the the insulation rating of a transformer is designed by numerals indicating the insulation voltage class in K.V & a letter insulation voltage class in K.V & a letter NN indicating that the transformer is indicating that the transformer is designed for use no a system where a neutral is not solidly earthed . designed for use no a system where a neutral is not solidly earthed . the insulation class corresponds to the values given in table no (1)* the insulation class corresponds to the values given in table no (1)*

Table no (1)* Table no (1)*

3.2.10 Maximum continuous operating voltage ( U.b) :3.2.10 Maximum continuous operating voltage ( U.b) :

transformers must be selected with an insulation class such that the voltage on which transformers must be selected with an insulation class such that the voltage on which

they are continuously used does not exceed the allocated operating voltage ( Ub ) .they are continuously used does not exceed the allocated operating voltage ( Ub ) .

Rated voltages (UN): Rated voltages (UN):

The rated voltages ( UN ) is the voltage present , at rated load , The rated voltages ( UN ) is the voltage present , at rated load , at the input windings and for which the transformer is at the input windings and for which the transformer is designed .designed .

The rated voltage ( UN ) on the output side is the voltage which The rated voltage ( UN ) on the output side is the voltage which appears at no load ( no load voltage Uo ) with rated voltage appears at no load ( no load voltage Uo ) with rated voltage and rated frequency on the input side .and rated frequency on the input side .

3.2.11 Adjustment of transformer ratio steps :3.2.11 Adjustment of transformer ratio steps :

to compensate for voltage fluctuations due to load variations in systems , the high to compensate for voltage fluctuations due to load variations in systems , the high voltage windings of distribution transformers have tapping brought out to terminals voltage windings of distribution transformers have tapping brought out to terminals ( as normally used on dry-type transformers ) or to a tap changer ( as normally used on ( as normally used on dry-type transformers ) or to a tap changer ( as normally used on oil – or Askeral – immersed transformers ) the tapping corresponding to the nominal oil – or Askeral – immersed transformers ) the tapping corresponding to the nominal voltage is normally situated in the middle of the tapping range and is referred to as the voltage is normally situated in the middle of the tapping range and is referred to as the principal tap .principal tap .

Rated tapping range :Rated tapping range :

the rated tap voltage range of a winding is the range between the highest tap voltage the rated tap voltage range of a winding is the range between the highest tap voltage and nominal or similary the lowest tap voltage and nominal under no load condition and nominal or similary the lowest tap voltage and nominal under no load condition with an excitation corresponding to the rated voltage on the principal tapwith an excitation corresponding to the rated voltage on the principal tap

Tap voltage range :Tap voltage range :

tap voltage ranges are laid down in DIN standard and expressed as a percentage rated tap voltage ranges are laid down in DIN standard and expressed as a percentage rated voltage ( e.g ± 4% ) V.D.E 0532 specifies that rated voltages and the adjustable voltage ( e.g ± 4% ) V.D.E 0532 specifies that rated voltages and the adjustable voltages are shown in Volts in the rating plate . for a transformer with rated voltage on voltages are shown in Volts in the rating plate . for a transformer with rated voltage on the input side of 20.000 volt and a rated tap voltage range of ± 5% , the rating plate the input side of 20.000 volt and a rated tap voltage range of ± 5% , the rating plate would show the values 21.000 V, 20.000 V and 19.000 Voltwould show the values 21.000 V, 20.000 V and 19.000 Volt . .

3.2.12 vector group number :3.2.12 vector group number :

The vector group symbol indicates the respective connections of the high voltage and The vector group symbol indicates the respective connections of the high voltage and low voltage transformer windings their relative phase displacement expressed as a low voltage transformer windings their relative phase displacement expressed as a clock – hour figure. clock – hour figure. the identification letters of the method of connection are given for the high voltage the identification letters of the method of connection are given for the high voltage winding ( OS ) in capital letters and for low voltage side ( us ) in small letters see table winding ( OS ) in capital letters and for low voltage side ( us ) in small letters see table no (2)* no (2)*

Table no (2)Clock – hour number :Clock – hour number :

the clock hour number indicate the multiple of 30° with which the vector of the low the clock hour number indicate the multiple of 30° with which the vector of the low voltage side is lagging when moving anti-clock wise in comparison to the high voltage side is lagging when moving anti-clock wise in comparison to the high voltage side of the corresponding terminal this angle between the voltage vectors can voltage side of the corresponding terminal this angle between the voltage vectors can have values between 0° & 360° .have values between 0° & 360° .

the terminals 2U,2V& 2W on the low voltage side are related to the terminals the terminals 2U,2V& 2W on the low voltage side are related to the terminals 1U,1V,1W on the high voltage side , the mark up U , V, W corresponding to DIN 1U,1V,1W on the high voltage side , the mark up U , V, W corresponding to DIN standard .standard .

Obtaining the clock hour number:Obtaining the clock hour number:

the clock hour number can be obtained by first drawing the vector diagrams of the the clock hour number can be obtained by first drawing the vector diagrams of the connections of the windings up one on top of the other and both on top of a clock face connections of the windings up one on top of the other and both on top of a clock face in such a way that the marking 1V of the high voltage side coincides with number 12 in such a way that the marking 1V of the high voltage side coincides with number 12 corresponding to 0 .corresponding to 0 .

the position 2V of the connection diagram of the low voltage side on the clock face the position 2V of the connection diagram of the low voltage side on the clock face gives the clock hour number of the vector group .gives the clock hour number of the vector group .Example: (fig 2).Example: (fig 2).

Fig no (2)Fig no (2)Vector group (DY5)Vector group (DY5)

* C – Y/S: * C – Y/S: High voltage side : C connection .High voltage side : C connection .Low voltage side : y connection Low voltage side : y connection

Clock – hour number 5 multiplied by 30° gives 150° Clock – hour number 5 multiplied by 30° gives 150° phase displacement between vector 1V of the high voltage phase displacement between vector 1V of the high voltage winding & the vector 2V of the low voltage winding .winding & the vector 2V of the low voltage winding .

preferred vector group :preferred vector group :

for distribution transformers , star or delta connections for for distribution transformers , star or delta connections for the high voltage windings and star or zizag connections the high voltage windings and star or zizag connections for the low voltage winding are preferred table (3)* shows for the low voltage winding are preferred table (3)* shows some group numbers some group numbers

Table no (3)Table no (3)

4.1 Indoor installation :4.1 Indoor installation :Indoor pattern fluid cooled transformers must be installed in covered Indoor pattern fluid cooled transformers must be installed in covered rooms which provide protection against rain , snow , dust & sand etc…. , rooms which provide protection against rain , snow , dust & sand etc…. , and good ventilation .and good ventilation .Dry type transformers : must be installed in closed rooms which dry & Dry type transformers : must be installed in closed rooms which dry & practically dust free the rooms should be easily accessible , to allow for practically dust free the rooms should be easily accessible , to allow for transport , operation maintenance & fire fighting .transport , operation maintenance & fire fighting .

4.2 Outdoor installation : 4.2 Outdoor installation : Fluid cooled transformers are suitable for outdoor installation when Fluid cooled transformers are suitable for outdoor installation when provided with suitable bushings & paint finish suitable for outdoor provided with suitable bushings & paint finish suitable for outdoor conditions.conditions.Measuring the temperature of insulating and cooling fluid the temperature Measuring the temperature of insulating and cooling fluid the temperature of the cooling and insulating fluid are measure near the top for this of the cooling and insulating fluid are measure near the top for this purpose, the caver of the transformer contains for thermometer pockets purpose, the caver of the transformer contains for thermometer pockets into which thermometers can be inserted these pockets are filled with into which thermometers can be inserted these pockets are filled with same type of cooling and insulating fluids used within the transformer same type of cooling and insulating fluids used within the transformer

4.3 Building dimensions of transformer station :4.3 Building dimensions of transformer station :The dimensions of the transformers are an important factor to house the The dimensions of the transformers are an important factor to house the transformer allowance should always be made for increase the power transformer allowance should always be made for increase the power demand in the future .demand in the future .The following table give an example of dimensions of transformers for a The following table give an example of dimensions of transformers for a given power [table (4) & fig no (3)*] .given power [table (4) & fig no (3)*] .

Fig no (3)Fig no (3)

Table no (4)Table no (4)Shows some ratings of Transformers And their dimensions and weightsShows some ratings of Transformers And their dimensions and weights

The height of the building to house a transformer & also provide the The height of the building to house a transformer & also provide the required access is dependant upon the height of the transformer , the type of required access is dependant upon the height of the transformer , the type of ventilation , the location cables & connections & the clearance necessary ventilation , the location cables & connections & the clearance necessary between live parts & earthed metal . for this type of transformer station the between live parts & earthed metal . for this type of transformer station the minimum height transformer should be the over all height of the minimum height transformer should be the over all height of the transformer plus 500mm ..transformer plus 500mm ..

4.4 Width of inspection passage way:4.4 Width of inspection passage way:

the length & width of a transformer station with service access should be the length & width of a transformer station with service access should be dimensioned such that for transformers with rated power of up to 630 KVA dimensioned such that for transformers with rated power of up to 630 KVA the inspection passage wais are at least 70 cm wide , for transformers of the inspection passage wais are at least 70 cm wide , for transformers of 800-1600K.V.A. the passage- ways should be at least 75 cm wide 800-1600K.V.A. the passage- ways should be at least 75 cm wide

4.5 Floor of transformer station for fluid cooled transformers :4.5 Floor of transformer station for fluid cooled transformers :

the floor of the transformer station can be made from either a reinforced the floor of the transformer station can be made from either a reinforced concrete slab with an opening in the centre or from girders of reinforced concrete slab with an opening in the centre or from girders of reinforced concrete slab construction is used . the cement grouting should have an concrete slab construction is used . the cement grouting should have an inclination of 1-2° in the direction of the collecting pit as shown in fig.(4) *inclination of 1-2° in the direction of the collecting pit as shown in fig.(4) *

fig no ( 4 )fig no ( 4 )example for the indoor example for the indoor

installation of a transformerinstallation of a transformer

4.6 Rails for transportation rollers :4.6 Rails for transportation rollers :

the international specifications states the recommendations for steel the international specifications states the recommendations for steel I section girders for supporting transformers the transportation I section girders for supporting transformers the transportation rollers rest on these girders which also incorporate a guide strip 2 rollers rest on these girders which also incorporate a guide strip 2 cm high fig. (5) * cm high fig. (5) *

Fig no (5)Fig no (5)Typical arrangement of girderTypical arrangement of girder

And guide strip for And guide strip for transportation rollerstransportation rollers

4.7 For oil immersed transformers:4.7 For oil immersed transformers:

Collecting tank and pits cooling and insulating fluid : Collecting tank and pits cooling and insulating fluid : For transformers with a rated power of 630 – K.V.A a collecting tank below the For transformers with a rated power of 630 – K.V.A a collecting tank below the transformer can be used providing it has capacity sufficient to hold the total fluid of transformer can be used providing it has capacity sufficient to hold the total fluid of the transformer floor can also be used as collecting tank when the door step & the transformer floor can also be used as collecting tank when the door step & ventilation openings are correspondingly high ventilation openings are correspondingly high

Collecting pit :Collecting pit :For transformers with rated power of 800 – 1600 K.VA, the collecting pit provided For transformers with rated power of 800 – 1600 K.VA, the collecting pit provided must have a capacity of approxi 2 m³ ( oil content of transformers ) .must have a capacity of approxi 2 m³ ( oil content of transformers ) .With a number of transformers each with rated capacity of between 800 – 1600 With a number of transformers each with rated capacity of between 800 – 1600 K.VA. a common collecting pit can be used providing this has a capacity at least 2 K.VA. a common collecting pit can be used providing this has a capacity at least 2 m³ . this pit can be situated out side the transformers , it is permitted to construct a m³ . this pit can be situated out side the transformers , it is permitted to construct a number of small inter connected pits providing the total capacity of these pits exceed number of small inter connected pits providing the total capacity of these pits exceed 2 m³ . a sum pit should be provided in the bottom of each pit to facilitate the pumping 2 m³ . a sum pit should be provided in the bottom of each pit to facilitate the pumping out of small amounts of water or oil .out of small amounts of water or oil .

Gravel, granite chipping layer :Gravel, granite chipping layer :Collecting pits & the oil carrying ducts for a common collecting pit must be screened Collecting pits & the oil carrying ducts for a common collecting pit must be screened above by a layer of gravel or granite chipping at least 20 cm , thick laid on a above by a layer of gravel or granite chipping at least 20 cm , thick laid on a galvanized iron grating to minimize the spread of fire fig no (5) *galvanized iron grating to minimize the spread of fire fig no (5) *

Outdoor collecting pit :Outdoor collecting pit :

A collecting pit must also be provided if transformers are installed A collecting pit must also be provided if transformers are installed outdoor in order to prevent seepage of cooling &insulating fluid into outdoor in order to prevent seepage of cooling &insulating fluid into the ground . the out door collecting pit must have a capacity of at the ground . the out door collecting pit must have a capacity of at least 1.2 times the fluid content of the transformer to allow for part least 1.2 times the fluid content of the transformer to allow for part filling of rain water or melted or melted snow , the pit must be filling of rain water or melted or melted snow , the pit must be pumped out regularly , other wise it would became filled with pumped out regularly , other wise it would became filled with rainwater .rainwater .

4.8 Ventilation of transformer rooms :4.8 Ventilation of transformer rooms :

When designing the room to house a self – cooled transformer it When designing the room to house a self – cooled transformer it must be borne in mind that the transformer heat losses must be must be borne in mind that the transformer heat losses must be dissipated inlet- & outlet air openings have to provided the air inlet dissipated inlet- & outlet air openings have to provided the air inlet should be either from underneath the transformer or as close as should be either from underneath the transformer or as close as possible to the floor level but never higher than the midpoint of the possible to the floor level but never higher than the midpoint of the transformer . the outlet opening should be as high as passible.inlet transformer . the outlet opening should be as high as passible.inlet and outlet openings should be arranged on opposite walls . The and outlet openings should be arranged on opposite walls . The efficiency of the ventilation increases with the difference in height efficiency of the ventilation increases with the difference in height between the middle of the transformer tank & the outlet openings, between the middle of the transformer tank & the outlet openings, fig (6)*.fig (6)*.

Fig no (6)Fig no (6)Arrangement of inlet Arrangement of inlet

and outlet air openingsand outlet air openings

4.9 Dimensioning of outlet air opening :4.9 Dimensioning of outlet air opening :

from curves given fig.(10) the dimensions of the required out let air from curves given fig.(10) the dimensions of the required out let air opening can be approximated the value obtained is for a free air opening can be approximated the value obtained is for a free air opening without screen & also allowing for an air temperature rise of opening without screen & also allowing for an air temperature rise of 15 °C within the room the values have been increased by for simple 15 °C within the room the values have been increased by for simple screens approx. 10% , for screens & shutters approx. 50 % .screens approx. 10% , for screens & shutters approx. 50 % .

Dimensions of inlet- air opening :Dimensions of inlet- air opening : The dimensions of the inlet – air opening can be 10 % smaller than The dimensions of the inlet – air opening can be 10 % smaller than

the dimensions of the outlet- air opening , ( plus the increase the dimensions of the outlet- air opening , ( plus the increase required for screen & shutters ) . to find the dimensions of inlet & required for screen & shutters ) . to find the dimensions of inlet & outlet air openings . outlet air openings .

Given :Given :

Height from station floor to center of outlet air opening 3100 mm , Height from station floor to center of outlet air opening 3100 mm , transformer rated power 400 K.V.A height from station floor to mid transformer rated power 400 K.V.A height from station floor to mid point of transformer tank 600 mm resulting difference in height h = point of transformer tank 600 mm resulting difference in height h = 3100 – 600 = 2500 mm .3100 – 600 = 2500 mm .Outlet air opening obtained from curves 0.9 m² Outlet air opening obtained from curves 0.9 m² Increase for simple screen + 10 % + 0.09 m² Increase for simple screen + 10 % + 0.09 m² Dimension of outlet air opening 0.99 m²Dimension of outlet air opening 0.99 m²Dimension of inlet air opening (10 %) 0.9 m²Dimension of inlet air opening (10 %) 0.9 m²

5.1 5.1 Important instructions for Important instructions for commissioning :commissioning :

مراعاتها يجب التى مراعاتها الخطوات يجب التى الخطواتاالختبارات بدء االختبارات قبل بدء قبل

والتشغيل :والتشغيل :1.1. cleaning the porcelain bushings – bus bars forcleaning the porcelain bushings – bus bars for

both H.T & L.T sides & tank surfaceboth H.T & L.T sides & tank surface التوصيل - - 11 وبارات الصينى العوازل التوصيل تنظيف وبارات الصينى العوازل تنظيف

والضغط العالى الضغط والضغط جهتى العالى الضغط جهتىومواسير التنك سطح وكذلك ومواسير المنخفض التنك سطح وكذلك المنخفض

التبريد.التبريد.

2.2. well fastening of porcelain bushing to well fastening of porcelain bushing to transformer cover and to be sure that transformer cover and to be sure that oil seals are in good conditionoil seals are in good condition

على - - 22 الصينى العوازل ربط من على التأكد الصينى العوازل ربط من التأكدالمرنة الجوانات وسالمة التنك المرنة جسم الجوانات وسالمة التنك جسم

المحوالت من الزيت تسرب تمنع المحوالت التى من الزيت تسرب تمنع التى

3.3. oil level for safe operationoil level for safe operation المحول ––33 فى الزيت مستوى أن من المحول التأكد فى الزيت مستوى أن من التأكدعن يقل ال بحيث للتشغيل الكافى عن بالقدر يقل ال بحيث للتشغيل الكافى بالقدرالتمدد خزان على مبين مستوى التمدد أدنى خزان على مبين مستوى أدنىعن الزيت تزويد فيمكن األمر احتاج عن واذا الزيت تزويد فيمكن األمر احتاج واذاالتمدد بخزان العليا الفتحة التمدد طريق بخزان العليا الفتحة طريق

المعادل أو األصلى الزيت نوع المعادل وبنفس أو األصلى الزيت نوع وبنفس لهله

4.4. in case that the transformer is not in case that the transformer is not used for long time the dielectric used for long time the dielectric strength of oil is to be checked & strength of oil is to be checked & after a year of operation it must be after a year of operation it must be not less than 20 K.V/2.5 mmnot less than 20 K.V/2.5 mm

44 - - لمدة المحول استعمال عدم حالة لمدة فى المحول استعمال عدم حالة فىعلى عام مرور بعد وكذلك على طويلة عام مرور بعد وكذلك طويلةعزل قوة اختبار اعادة يجب عزل التشغيل قوة اختبار اعادة يجب التشغيل

عن تقل أال ويجب عن الزيت تقل أال ويجب 2020الزيت/ ف. /ك ف. مم. مم. 2.52.5ك

5.5. to ensure safety of operation apparatus to ensure safety of operation apparatus like bucholz relay , thermometer , silica like bucholz relay , thermometer , silica gel and earthing screwgel and earthing screw

55 - - مثل الوقاية أجهزة سالمة من مثل التأكد الوقاية أجهزة سالمة من التأكدوالسيلكاجل الترمومتر ، والسيلكاجل البوخهلز الترمومتر ، البوخهلز

األرضى األرضى ومسمار ومسمار

6.6. to be sure that the silica – gel is blue to be sure that the silica – gel is blue ( i,e) able to remove humidity , if it ( i,e) able to remove humidity , if it became red it must be dried 140° till it is became red it must be dried 140° till it is blue again or must be changed .blue again or must be changed .

األزرق - - 66 السيلكاجل الملح لون من األزرق التأكد السيلكاجل الملح لون من التأكدتحول فإذا الرطوبة امتصاص يمكنه تحول حتى فإذا الرطوبة امتصاص يمكنه حتىيجب فإنه الوردى األحمر اللون يجب الى فإنه الوردى األحمر اللون الىلدرجة بتعريضه الملح تجفيف لدرجة إعادة بتعريضه الملح تجفيف إعادة

عن تزيد ال عن حرارة تزيد ال يستعيد ººمم140140حرارة يستعيد حتى حتىالملح بتغيير أو األزرق الملح لونه بتغيير أو األزرق لونه

7.7. terminals of warning and switching off in terminals of warning and switching off in the Buchholz relay must be connected to the Buchholz relay must be connected to the warning circuit and to the protection the warning circuit and to the protection circuits of the customer Here is a guide circuits of the customer Here is a guide connection diagram (fig.No.13) In case of connection diagram (fig.No.13) In case of small faults the gasses gathered around small faults the gasses gathered around the upper fault which closed , the warning the upper fault which closed , the warning circuit and the warning voice is heared , in circuit and the warning voice is heared , in case of big faults a big amount of gasses case of big faults a big amount of gasses move the lower float which shuts the move the lower float which shuts the tripping circuit and the current is switched tripping circuit and the current is switched off the trafo – warning and switching off the trafo – warning and switching circuits work on the customer protection circuits work on the customer protection current on voltages between 24 & 220 current on voltages between 24 & 220 Volts A.C or D.C . Volts A.C or D.C .

ونقطتى - 7 االنذار نقطتى توصيل يجببأجهزة البوخهلز جهاز روزتة فى الفصل

تعمل حيث العميل بمفاتيح والفصل االنذارحول الغازات تراكمت اذا االنذار صفارة

يفصل وحيث البوخهلز بجهاز العليا العوامةبالكهرباء المحول تغذية مفتاح الوقاية جهاز

واندفاع الدائرة فى قصر حدوث حالة فىالسفلية العوامة الى المحول من الغازات

البوخهلز . جهاز فىتيار على والفصل االنذار دائرتا وتعمل

جهد على العميل عند الوقاية أجهزة تشغيلأو 220الى 24من مستمر تيار فولت

متردد.رقم ( جهاز) 13شكل توصيل يوضح

والفصل اإلنذار بأجهزة البوخهلز

8.8. To be sure that the tape changer is in the To be sure that the tape changer is in the required position .required position .

الوضع - - 88 فى الجهد منظم تثبيت من الوضع التأكد فى الجهد منظم تثبيت من التأكدعليه . المحول تشغيل عليه .المراد المحول تشغيل المراد

9.9. In case of indoor installation of Trafo, the In case of indoor installation of Trafo, the dimensions of the Trafo room must be dimensions of the Trafo room must be suitable to the Trafo , size and aeration , suitable to the Trafo , size and aeration , air ducts must be made in opposite air ducts must be made in opposite directions .Covering the air ducts with a directions .Covering the air ducts with a metallic net is recommended metallic net is recommended

المبانى - - 99 داخل المحول تركيب حالة المبانى فى داخل المحول تركيب حالة فىمناسبة الغرفة أبعاد تكون أن مناسبة فيراعى الغرفة أبعاد تكون أن فيراعىفتحات عمل يحسن كما المحول فتحات لحجم عمل يحسن كما المحول لحجمأن ويحسن متضادين اتجاهين فى أن تهوية ويحسن متضادين اتجاهين فى تهوية

معدنية بشبكة التهوية فتحات معدنية تغطى بشبكة التهوية فتحات تغطى

رقم ( رقم (شكل والفصل) 77شكل االنذار بأجهزة البوخهلز جهاز توصيل والفصل) يوضح االنذار بأجهزة البوخهلز جهاز توصيل يوضح

5.2Transformer tests:5.2Transformer tests: االختباراتاالختبارات routine tests are carried out routine tests are carried out according to IEC standards :according to IEC standards :

االختبارات المحوالت على االختبارات تجرى المحوالت على تجرىلمواصفات طبقا لمواصفات الروتينية طبقا وتشمل :وتشمل :IECIECالروتينية

turns – ratio test turns – ratio test Insulation test between windings and Insulation test between windings and between winding and earth.between winding and earth.Induced high voltage test. Induced high voltage test. no – load losses test no – load losses test short circuit test short circuit test windings resistance measurementswindings resistance measurements

التحويل نسبة التحويل اختبار نسبة اختبار وبينها وبعضها الملفات بين العزل وبينها اختبار وبعضها الملفات بين العزل اختبار

األرض .األرض . وبين وبين من حلقة لكل التأثيرى بالجهد العزل من اختبار حلقة لكل التأثيرى بالجهد العزل اختبار

الملفات.الملفات. حالة فى الحديدى فىالقلب الفقد قياس حالة اختبار فى الحديدى فىالقلب الفقد قياس اختبار

حمل وجود حمل عدم وجود عدم الدائرة قصر الدائرة اختبار قصر اختبار الملفات مقاومة الملفات قياس مقاومة قياس

5.2.1 upon request following 5.2.1 upon request following tests can be made :tests can be made :

إجراء يمكن العميل طلب إجراء حسب يمكن العميل طلب حسبالتالية التالية اإلختبارات اإلختبارات

loading and heat test loading and heat test resistance of insulation to impulse resistance of insulation to impulse

test test noise lvelnoise lvel

الحرارة ودرجة التحميل الحرارة اختبار ودرجة التحميل اختبار الكهربائية للصدمات العزل تحمل الكهربائية اختبار للصدمات العزل تحمل اختبار الضوضاء مستوى الضوضاء قياس مستوى قياس

6.16.1 Arrangement of distribution transformers Arrangement of distribution transformers The distribution transformers can be arranged centrally in one station or The distribution transformers can be arranged centrally in one station or in a number of sub – stations distributed over the whole load area .in a number of sub – stations distributed over the whole load area .As a rule they should be placed at the centers of load thus the length of As a rule they should be placed at the centers of load thus the length of cables cross section of conductors and losses are kept within economic cables cross section of conductors and losses are kept within economic limits .limits .The centrally arranged system used for distribution systems where only The centrally arranged system used for distribution systems where only small extension of the area is likely and load intensity is high .small extension of the area is likely and load intensity is high .the load equalization occurs mainly on the low tension side on the bus the load equalization occurs mainly on the low tension side on the bus bars see fig (8)* in calculating the rated power of distribution bars see fig (8)* in calculating the rated power of distribution transformer .transformer .

Fig no (8)Fig no (8)Centrally Arranged distributionCentrally Arranged distribution Transformers in a sub-stationTransformers in a sub-station

6.2 6.2 Some requirements for operation :- Some requirements for operation :-

The peak load of the whole distribution system taking into account a The peak load of the whole distribution system taking into account a reserve in the event of one transformer being out of order the reserve in the event of one transformer being out of order the transformers should if possible be equal in size to facilitate operation transformers should if possible be equal in size to facilitate operation in parallel and interchangeability.in parallel and interchangeability.The decent tralized arrangement of distribution transformers is of ten The decent tralized arrangement of distribution transformers is of ten favored for wide supply area and for widely distributed centers of favored for wide supply area and for widely distributed centers of load neighborly substation can then be connected by ri ng main on load neighborly substation can then be connected by ri ng main on the low voltage side in this way load equalization between the sub the low voltage side in this way load equalization between the sub station Can be achieved and the supply can be maintained when station Can be achieved and the supply can be maintained when one of the transformers on the high voltage side of substation is out one of the transformers on the high voltage side of substation is out of order.of order.if necessary several transformers may be required as reserve , as if necessary several transformers may be required as reserve , as shown in fig ( 9 ) and fig ( 10 ) and fig (11 ) .shown in fig ( 9 ) and fig ( 10 ) and fig (11 ) .

Fig no (9)Fig no (9)Decentralized arrangement of distributionDecentralized arrangement of distribution

Transformers in several sub-stationsTransformers in several sub-stations

Fig no (10)Fig no (10)Ring-main system with ring Ring-main system with ring Circuit looped between sub-Circuit looped between sub-

stationsstations

Fig no (11)Fig no (11)diagram of ring-main diagram of ring-main system.system.a) Ring –main system a) Ring –main system with two ring circuitswith two ring circuitsb) Ring-main system with b) Ring-main system with terminal stationterminal station

6.3 6.3 Peculiarities of high voltage ring system:Peculiarities of high voltage ring system: this simple way to loop ring circuits into substations is via load break isolators this simple way to loop ring circuits into substations is via load break isolators the ring circuit can be opened at convenient points to confine a fault to a part the ring circuit can be opened at convenient points to confine a fault to a part of the circuit only ( e.g in substation b3 ) fig (12)*of the circuit only ( e.g in substation b3 ) fig (12)*By means of short circuit indicators in each sub-station affected can be By means of short circuit indicators in each sub-station affected can be determined thus the faulty section of the circuit can be readily found and determined thus the faulty section of the circuit can be readily found and isolated.isolated.

After isolating the faulty cable run at (d) ( worst fault condition ) & closing the After isolating the faulty cable run at (d) ( worst fault condition ) & closing the isolating point in substation b3 , normal service can be resumed under the isolating point in substation b3 , normal service can be resumed under the worst fault condition , with the fault in cable run ( d ) between the ring supply worst fault condition , with the fault in cable run ( d ) between the ring supply and the first substation the remaining cable has to supply current to the whole and the first substation the remaining cable has to supply current to the whole ring circuit and must therefore be rated for single supply if there is a load ring circuit and must therefore be rated for single supply if there is a load center with high load demand opposite the transfer station ( e.g a university , center with high load demand opposite the transfer station ( e.g a university , hospital in a rural area ) , it will found advantageous to provides a ring main hospital in a rural area ) , it will found advantageous to provides a ring main system with a terminal substation at the load center . this would ensure good system with a terminal substation at the load center . this would ensure good load equalization and high security of supply to, all sub station yet the number load equalization and high security of supply to, all sub station yet the number of ring circuits will be less fig. (11) of ring circuits will be less fig. (11)

6.46.4 Parallel operation: Parallel operation:

transformers will operate in parallel when they are connected to the same network transformers will operate in parallel when they are connected to the same network system, both on the high voltage as well as on the low voltage side .system, both on the high voltage as well as on the low voltage side .slight different conditions apply between operation in parallel bus bars and operation slight different conditions apply between operation in parallel bus bars and operation in parallel on systems networks .in parallel on systems networks .

6.5 6.5 general requirements for parallel operation :general requirements for parallel operation :

for satisfactory operation in parallel , especially on bus bars the following general for satisfactory operation in parallel , especially on bus bars the following general requirements apply ;requirements apply ; For transformers having the same vector group and clock hour number, terminals For transformers having the same vector group and clock hour number, terminals

having identical designations are paralleled . having identical designations are paralleled . The ratio of transformation must be identical .The ratio of transformation must be identical . All taping must have identical values on each transformer .All taping must have identical values on each transformer . The rated impeadence voltage must be practically the same within 10 % , the The rated impeadence voltage must be practically the same within 10 % , the

transformer with the smaller rated power should have the higher rated transformer with the smaller rated power should have the higher rated impeadence voltage , if possible the ratio of the rated power of transformers impeadence voltage , if possible the ratio of the rated power of transformers working in parallel should not be more than3:1 .working in parallel should not be more than3:1 .

for checking the phase relationship the 2N-terminal of the transformer to be for checking the phase relationship the 2N-terminal of the transformer to be connected to the transformer to be connected is connected to the 1N bus bar of the connected to the transformer to be connected is connected to the 1N bus bar of the system the phase relation is then check by using a voltmeter if the connection is system the phase relation is then check by using a voltmeter if the connection is correct the voltmeter will indicate zero , if the connection is correct the voltmeter correct the voltmeter will indicate zero , if the connection is correct the voltmeter will indicate values values of up to twice the phase voltage fig (13) * .will indicate values values of up to twice the phase voltage fig (13) * .

Fig no ( 13 )Fig no ( 13 )

6.6 6.6 Protection by fuses ( short circuit protection ) :Protection by fuses ( short circuit protection ) :

Distribution transformers are normally protected against short circuits by high Distribution transformers are normally protected against short circuits by high voltage H.R.C- fuses ( table No 5 ) can be used for the selection of fuses for voltage H.R.C- fuses ( table No 5 ) can be used for the selection of fuses for distribution transformers ; the maximum current inrush when switching on the distribution transformers ; the maximum current inrush when switching on the transformer has been taken into consideration .transformer has been taken into consideration .

Table no (5)Table no (5)Guiding values for the ratingGuiding values for the rating Of high-voltageOf high-voltage HRC-fusesHRC-fuses

6.7 Over load protection:6.7 Over load protection:

the over load protection of transformers is provided on the low voltage side the over load protection of transformers is provided on the low voltage side . the low voltage H.R.C- fuses or the thermally delayed over current trips . the low voltage H.R.C- fuses or the thermally delayed over current trips of the circuit breakers are selected according to the rated current IN of the of the circuit breakers are selected according to the rated current IN of the transformer . this selection normally provides adequate selectivity between transformer . this selection normally provides adequate selectivity between high – voltage and low voltage side . high – voltage and low voltage side .

6.8 Short circuit strength:6.8 Short circuit strength:

Distribution transformers need to be designed to with stand the effects of Distribution transformers need to be designed to with stand the effects of external short circuits with out damage .external short circuits with out damage .

Steady state R.M.S short circuit currentSteady state R.M.S short circuit current II..KK : :

The short circuit current Ik of a transformer is the steady state R.M.S. The short circuit current Ik of a transformer is the steady state R.M.S. value of current at the terminals following on all phases short circuit on the value of current at the terminals following on all phases short circuit on the terminals of the outgoing side and when the D.C component has decayed terminals of the outgoing side and when the D.C component has decayed with constant rated voltage applied to the input terminals . with constant rated voltage applied to the input terminals .

Calculation of steady state R.M.S short circuit current :Calculation of steady state R.M.S short circuit current :

For transformer operating at rated voltage & rated frequency and tappings For transformer operating at rated voltage & rated frequency and tappings adjusted to the principal tap , the continuous short circuit current Ik can be adjusted to the principal tap , the continuous short circuit current Ik can be calculated from the rated current IN & the rated impeadence voltage UKN . calculated from the rated current IN & the rated impeadence voltage UKN .

IN = rated current in A .IN = rated current in A .

UKN = rated independence voltage in % .UKN = rated independence voltage in % .For the calculation of short circuit currents of installation systems , the For the calculation of short circuit currents of installation systems , the

resistance of the system must be taken into consideration . for calculation of resistance of the system must be taken into consideration . for calculation of short – circuit current in three phase systems .short – circuit current in three phase systems .

6.11 Maximum permissible duration of short circuit :6.11 Maximum permissible duration of short circuit :

For values of Ik and time duration see table ( 6 ) the peak short circuit For values of Ik and time duration see table ( 6 ) the peak short circuit current is important in considering the dynamic stresses in the transformer & current is important in considering the dynamic stresses in the transformer & other operating equipment affected by the fault .other operating equipment affected by the fault .The peak short circuit current “ IS “ of a transformer is the first peak The peak short circuit current “ IS “ of a transformer is the first peak maximum transient value of current at the terminals of the outgoing side . maximum transient value of current at the terminals of the outgoing side . The first peak value of current Ik is the The first peak value of current Ik is the multiple of the R.M.S short multiple of the R.M.S short circuit current IKcircuit current IK

A.in current circuit short R.M.S k I

2 X

table no (6) table no (6) Values of Ik and time of durationValues of Ik and time of duration

2 X

2 I I KS

. ) peak value (A in current circuit short peak I S .A in current circuit short R.M.S epermissibl maximum IK

.factor impulse 2 X

6.12 Calculation of the peak short circuit current Is :6.12 Calculation of the peak short circuit current Is :

The permissible peak short circuit current Is of a transformer is :The permissible peak short circuit current Is of a transformer is :

Where : Where :

Impulse factor : the impulse factor of the reactance X to the Ohmeic resistance R , ( Impulse factor : the impulse factor of the reactance X to the Ohmeic resistance R , ( i.e ) dependent on the ratio of the reactance voltage Ui.e ) dependent on the ratio of the reactance voltage Uxx to the resistive voltage drop UR to the resistive voltage drop UR of the transformer . of the transformer .

Maintenance work is permitted only when the transformer is switched off and the Maintenance work is permitted only when the transformer is switched off and the terminals earthed .terminals earthed .

7.17.1 Dry type transformers: Dry type transformers:

Dry type transformers must be kept dust free & protected against pollution . at Dry type transformers must be kept dust free & protected against pollution . at regular intervals these should be cleaned with bellows or a vacuum cleaner . regular intervals these should be cleaned with bellows or a vacuum cleaner . terminals and bolted joints should also be checked regularly .terminals and bolted joints should also be checked regularly .

Drying out :Drying out :

The insulation resistance between windings & also windings to earth provides good The insulation resistance between windings & also windings to earth provides good indication of the condition of the insulation especially when a transformer has been indication of the condition of the insulation especially when a transformer has been out of service over a long period and many have absorbed moisture . subsequent out of service over a long period and many have absorbed moisture . subsequent drying out may therefore be necessary .drying out may therefore be necessary .

The insulation resistance at room temperature should not less than The insulation resistance at room temperature should not less than

For operating voltage: For operating voltage: Up to 1000 v 15 m. ohm.Up to 1000 v 15 m. ohm. Above 1000 v 25 m. ohmAbove 1000 v 25 m. ohm Above 5000 v 40 m. ohm .Above 5000 v 40 m. ohm .

The insulation resistance is normally measure with 1000 V or 2000 V instrumentThe insulation resistance is normally measure with 1000 V or 2000 V instrument

If the insulation resistance is below the above recommended values it can be assumed If the insulation resistance is below the above recommended values it can be assumed that moisture has been observed by the insulation .that moisture has been observed by the insulation .

7.3 The winding can be dried out as follows :7.3 The winding can be dried out as follows :

By heating in a drying oven at a temperature of approx. 80°C or with heaters By heating in a drying oven at a temperature of approx. 80°C or with heaters such as incandescent lamps , resister elements or radiators . the surface of such as incandescent lamps , resister elements or radiators . the surface of the windings must not exceed 100°C when using direct radiation .the windings must not exceed 100°C when using direct radiation .heating under short – circuit conditions with a maximum rated current the heating under short – circuit conditions with a maximum rated current the output side of the transformer is short – circuited and the input side connected output side of the transformer is short – circuited and the input side connected to a voltage approximately equal to the impedance voltage .to a voltage approximately equal to the impedance voltage .this voltage is adjusted such that the current flowing does not exceed the this voltage is adjusted such that the current flowing does not exceed the rated current stated on the nameplate is exceeded during drying the room rated current stated on the nameplate is exceeded during drying the room must be well ventilated during dying process . the value of insulation must be well ventilated during dying process . the value of insulation resistance should be measured repeatedly .resistance should be measured repeatedly .

7.27.2 Renovation of the operating room : Renovation of the operating room :During renovation work in the transformer room the transformer should be During renovation work in the transformer room the transformer should be switched off and earthed . switched off and earthed . It is important , particularly with dry type transformer to cover carefully to It is important , particularly with dry type transformer to cover carefully to protect from dust , paint , humidity and other pollution , when building and protect from dust , paint , humidity and other pollution , when building and painting work is in process , the room must be well ventilated and if necessary painting work is in process , the room must be well ventilated and if necessary heated .heated .Cast resin transformers are largely maintenance free and do not require Cast resin transformers are largely maintenance free and do not require drying .drying .

7.37.3 Oil – immersed transformers : Oil – immersed transformers :

the type of fluid used is stated on the rating plate .the maintenance of this the type of fluid used is stated on the rating plate .the maintenance of this type of transformer includes :type of transformer includes : Checking the level of fluid at the sight glass .Checking the level of fluid at the sight glass . Checking effectiveness of gaskets and quality of paint finish .Checking effectiveness of gaskets and quality of paint finish . Testing the insulating fluid of moisture content and checking or Testing the insulating fluid of moisture content and checking or

reactivating the silica – gel in the breather .reactivating the silica – gel in the breather .To check the moisture content of the insulating fluid a sample must be taken To check the moisture content of the insulating fluid a sample must be taken

to ensure that the measurement corresponds to the actual conditions of the to ensure that the measurement corresponds to the actual conditions of the fluid within the transformer , special care the highest degree of clean lines fluid within the transformer , special care the highest degree of clean lines is required when taking samples .is required when taking samples .1.1. A specimen bottle with large opening and glass stopper ( not cork ) is A specimen bottle with large opening and glass stopper ( not cork ) is

required required 2.2. The bottle must be cleaned with clean alcohol and well dried .The bottle must be cleaned with clean alcohol and well dried .3.3. The drain cook must be carefully cleaned and dried before taking a test The drain cook must be carefully cleaned and dried before taking a test

sample . sample . 4.4. The specimen bottle should be approximately half filled with the fluid The specimen bottle should be approximately half filled with the fluid

and rinsed with this fluid before a test sample is taken .and rinsed with this fluid before a test sample is taken ...

7.47.4 break down voltage : break down voltage : Eleven samples should be taken and a break down voltage test made with Eleven samples should be taken and a break down voltage test made with electrodes having a 2 – 5 mm gap for each sample the break down electrodes having a 2 – 5 mm gap for each sample the break down voltage test is repeated six times with a 2 minute interval between each . voltage test is repeated six times with a 2 minute interval between each . the break down voltage is the mean value of the results of tests 2 to 6 . the break down voltage is the mean value of the results of tests 2 to 6 . the transformers of insulation rating up to 30 k.v. the mean value the transformers of insulation rating up to 30 k.v. the mean value acceptable is for new oil or a skarel 60 k.v. minimum and for used oil 30 acceptable is for new oil or a skarel 60 k.v. minimum and for used oil 30 k. v. minimum .k. v. minimum .if the measured values fall below the above mentioned minimum values if the measured values fall below the above mentioned minimum values the insulating fluid must be changed or re conditional by use of special the insulating fluid must be changed or re conditional by use of special filter presses or for small trans formers the complete transformer can be filter presses or for small trans formers the complete transformer can be dried under vacuum the manufacturers of transformers and the electricity dried under vacuum the manufacturers of transformers and the electricity boards have the necessary plant and devices for the conditioning and boards have the necessary plant and devices for the conditioning and drying of the insulating fluid .drying of the insulating fluid .for refilling is necessary the fluid used must be identical to the original it for refilling is necessary the fluid used must be identical to the original it is recommended to test the new fluid for break down voltage before is recommended to test the new fluid for break down voltage before refilling refilling