Power Transformers

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

1.2 Types of transformers •Distribution transformers:- Oil – immersed transformers - Askarel immersed transformers - Dry type transformers

•Instrument transformer - Current transformers - Voltage transformers

1.2.1 Oil immersed transformers : Oil immersed transformers have their cores & windings immersed in mineral oil.

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

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

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

 Maximum temperature of air 40h C .  Daily mean temperature of the air 30h C .  Yearly mean temperature of the air 20h C . The temperature of the ambient air is measured at a distance of 1-2 m from the transformer .

1.2.4 Instrument transformers : - Current transformers - Voltage transformers •Current transformers: Current transformer is transformer with small rate power (burden) , whose primary windings are in series with the line circuit , and secondary windings are connected to measuring instruments , electricity meters relays or control devices , current transformers isolate the measuring of protection circuits from the primary voltage and also protect the apparatus corresponding to the over current response of the current . 1.2.5 Voltage transformers : Voltage transformers are also of small power rate and operate at almost no – load . they isolate the primary high voltage from the connected measuring or protected circuits.

TRANSFORMERS Definition : Transformers can be defined as a static electric machine which converts 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 common variable flux.

Theory of operation : The primary coil of the transformer is connected to a supply of sine wave voltage . an alternating sine wave current will flow in the 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 according to Faraday′ s law the common flux interesting two coils will induce in them an alternating e.m.f ( e1 , e2 ) .

N1d φ e1 = dt

e1 e2

(1)

is an e.m.f of self induction is an e.m.f of Mutual induction

dφ e1 = - N 2 dt

( 2)

from 1,2 ∴ the transformation ratio

e1 N1 K= = e2 N 2

Applying Kirchoff′ s law on the primary circuit. V = - e1 V+ e1 = 0

∑e.m.f

= ∑V.D

From the secondary circuit e2 = v2



e1 N1 V1 K = = = e 2 circuitN: 2 V2 Equivalent

Io = 10 : 15 % of rated current .

 1 = − E 1 + I1r1 + JI1X1 V 1

V2′ = − E 2′ + I′2 r2′ + JI′2 X′2

Tansformer testing : Determination of parameters :

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 . The impeadence of the circuit at no load .

V1 = Z 1 +Z 0 Io

 Z1 < < Zo Z0 ≅

V1 Io

 Z1 < < Zo

Z1 can be neglected

po ro = ( Io ) 2

Zo = r 2 o + X o Xo

2

= Zo − ro

2

2

-Another method: Po = V1 Io is go

V1 Zo ≅ I0 cos φo ≅

Po V1 Io

Ro = Zo cos φo

Po φo = cos V1 Io -1

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

x o = Zo sin φo

I o a = I o cos φo Io r = Io sin φo φo =cos V

rm 2 =

1

Ioa

-1

 Po  V  1

   

V1 Xm = Ior

-Short circuit test:

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 . Short circuit the secondary winding . Measure ; ( V1 ) sh.c ( I1 ) sh.c ( P1 ) sh.c In the circuit Z′ 2 , Z′ o are connected in parallel Zo is of the order of ( 10 )-2 relative to Z′ 2 So the effect of Zo can be simplified to the show figure ( c ) . V1sh.c = Z eq I1sh.c Where ; Zeq = Req + Jxeq

Req= =x r1 x + x+′ 2r2′ eq 1

P1 (sh.c ) = ( V1 )sh.c ( I1 )sh.c ( cos φ ) sh.c φsh.c

=cos

P1

-1

xeq = Zeq sin φ

 P1   sh .c V1I1 

Req = Zeq cos φ

r1 ≅ r′2

Xeq X1 ≅ X′2 2 r2 =

r2′

K

2

x′2 x2 = 2 k

•D.C Test :

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

E = r1 ohms I

The effect of X1 , X0 will not appear when using direct current

  X →

Ldi  dt



I is the const. relative to time[ also the effect of ro will not appear 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 core . Similarly we can determine the resistance of the secondary ( r2 ) by connecting the battery to the terminals of the secondary coil .

Voltage regulation :

The voltage regulation is defined as the change in the voltage of a loaded transformer when the load is removed primary voltage is constant at it′ s nominal value . E = ( V2 ) n.l – V2 load

secondary . while the

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

(V2 )n.L - V2 L ε V.R = × 100 = (V2 )L    

   

(V2 )n.L - V2 L ε = V.R (V2 )L    

   

%

= per unit

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

•Io neglected : I1= I ′ 2 Zq = Req + JXeq = ( r1+ r′ 2 ) + J( X1 + X′ 2 ) To calculate the voltage regulation the following value must be determined . V1,I1 cos g1 and Zeq .

 1=V  ′2 + I1Req + JI1 Xeq V

 ′2 = V  1 + I1Req + JI1 Xeq V 2 V2 = V1cosφ 1 − I1Req + V1 sinφ 1 - I1 Xeq

(

) (

Note: ( V′ 2 )n.l = V1

ε = ( V )n.l − (V )l 2

2

(V2 )l

ε = (K V )n.l − (KV )l 2

2

(KV2 )l

′

′

′

ε = ( V )n.l − (V )l = V − V 2

(V2′ )l

2

1

V2′

2

)

where V′ 2 is calculated by eq ( 1 ) some times the quantities known are V2 , I2 Cos g2 , Zeq in this case To find the voltage regulation V1 we can calculated from the geometry as Note : Parameters r′ 2 , X′ 2 , I′ 2 , E′ 2 as follows :

V 1 =

( V′ cosφ

Where ;

2

I′2 =

) (

− I′2Req + V′2 sinφ 2 - I′2 Xeq 2

2

I2 K

r′ 2 = k2 r2 , x′ 2 = k2 x2 E′ 2 = kE2

N1 E1 V1 I 2 k= = ≅ ≅ N 2 E1 V2 I1

)

Transformer efficiency (η ) :

Poutput Pin - losses η= = Pin Pinput ∴ Pcu ∝ I2

2 ′ ′ pcu = I ( r1 + r2 ) = I1 req = I 2 req 2 1

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

( Pcu )a = ( I )a ( Pcu )b ( I )b 2

2

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 nominal value , accordingly the equation can be written as :

Pcu = ( I ) ( Pcu) f .l ( I) f .l 2

2

(

pcu ) required = ( pcu ) f .l  I   If.l 

2

I =X If.l Pcu required = X 2 ( Pcu ) f.l let

η=

Pout Pout + const.loss + cu loss

=

( K.V.A) out cosφ ( K.V.A) out cosφ + Po + P

cu

: ‫ ( ىىىى ىىىىىى‬η ) ‫وعموما الكفاءة عند أى حمل‬

: ‫ىىىى ىىىىىى‬ ( K.V.A ) out cos φ = ( K.V.A ) out cos φ + Po + Pcu

X ( K.V.A ) f.l cosφ η= X ( K.V.A ) f.l cosφ + po + X 2 ( pcu )f.l

pin - losses η= pin

X ( K.V.A ) f.l cosφ − po − X 2 ( pcu )f.l ηΧ = X ( K.V.A ) f.l cosφ

average efficiency for the transformer during 1 day ;

Total output energy through 24 hours η= Total input energy through 24 hours the input energy of the transformer through the day is equal to the Total out put + Total losses per/day . losses are const. or magnetic ( Po ) and are constant through the day . the ( electrical or cu ) losses are variable according to the load ( QI2 ) . E.X : 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 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 efficiency .

Solution :

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 ) FL.cu losses = 1.5 K.W Cu loss for 3 hours on F.L = 1.5 x 3 = 4.5 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 Total losses = 36 + 4.5 + 1.5 = 42 k.w.h Total output = ( 100 x3 ) + ( 50 x 4 ) = 500 k.w.h η all day = 500 x 100 / 542 = 92.26 % Group numbers : The group number indicates the phase differience between primary and secondary ( H.T and L.T ) line voltages in electrical degrees . It is sometimes determined as a clock reading each hour is equivalent to 30° phase difference .

Y-Y connectios :

(∆ Y)

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

1.the transformations ratios must be the same 2.the group number must be the same 3.the phase connection must be in same sequence 4.short circuit impendence ( Zeq ) must be the same

Transformers are produced locally as , three phase immersed in oil , 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 A.B.B & France Transfo 2.1 General characteristics : the transformers described have the following general characteristics : Three – phase . Connection D/Y11 Frequency 50 Hz . Natural cooling in oil or silicon . Continuous service . Indoor or out door installation .

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

2.2 Formation of distribution transformers :

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

2.2.2 –Windings: High tension turns are made of copper wires of either circular cross sections varnish isolated or rectangular cross sections isolated by sililose paper. Low tension turns are made of either noninsulated copper foils with insulating paper in between or of rectangular wires insulated by cylindrical paper sheets

‫مكونات محولت التوزيع‬

: ‫* القلب الحديدى‬ ‫يصنع القلب الحديدى من رقائق الصلب السيليكونى المسحوب‬ ‫مم لتقليل الفقد فى الفيض المغناطيسى‬0.3 ‫على البارد بسمك‬

: ‫الملفات‬ ‫تصنع ملفات الجهد العالى من أسلك نحاسية ذات مقطع‬ ‫مستدير معزول بالورنيش أو مقطع مستطيل معزول بالورق‬ ‫السميليوزى وتصمنع ملفات الضغمط المنخفمض ممن شرائح‬ ‫النحاس المعزول بالورق أمو أسملك نحاس معزولمة بشرائح‬ . ‫الورق السطوانية‬

: ‫التنك الخارجى‬

2.2.3-Tank : The transformer tank is made of corrugated steel . The corrugated tank surface is itself the cooling surface. The tank is provided with an additional steel reservoir for oil expansion, on which a piping device is installed to transmit oil cock ,a hole for silicagel apparatus,and an oil level indicator.

2.2.4- Terminals :

‫يصنع التنك من ألواح الصاج المعرج ليكون جسم التنك‬ ‫هو المسسطح المبرد للمحول ويزود التنك بخزان لتمدد‬ ‫الزيت مصنع من الصاج ويركب عليه مواسير توصيل‬ ‫الزيست للتنسك وبسه فتحسة لتعويسض نقسص الزيست وفتحة‬ ‫لتركيب جهاز السيلكاجل وكذلك مبين الزيت‬

: ‫أطراف التوصيل‬

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

.

both can be made if required

‫توصل أطراف الضغط العالى والمنخفض الى عوزال‬ ‫من الصينى مناسبة لجهد التشغيل وتثبت هذه العوازل‬ ‫فى غطاء التنسك بطريقسة تسسمح بتغييرهسا بدون فتح‬ ‫الغطاء وعنسد الطلسب يمكسن تزويدهسا بصسناديق نهاية‬ ‫للكابلت فسى حالسة الطلسب مسن جهتسى الضغسط العالى‬ .‫والمنخفض أو احداهما فقط‬

2.2.5 –Tapchangers : or distribution transformers , tap changers are externally for allowing voltage regulation with ±5 % of the rated value in 5 equal steps of ± 2.5 % each , the tap changer is manually operated while current is off 2.2.6- Oil : transformers are filled with special oil ( Diala ( 5 ) or equal ) of high insulating grade according to IEC specifications

: ‫منظمات الجهد‬ ‫يركب على محولت التوزيع منظمات للجهد ذات خمس‬ ‫ كل منها‬%5 + ‫مراحل تسمح بتغيير الجهد فى حدود‬ ‫ ويعمل يدويا من خارج المحول بعد فصل‬% 2.5 + ‫المحول عن الكهرباء تماما أى أن المنظم يعمل على‬ . ‫اللحمل‬ : ‫ زيت التبريد‬. ‫تملم المحولت بزيمت محولت ذو درجة عزل عالية‬ IEC ‫)ديالة ب أو ما يماثلها ( طبقا لمواصفات‬

2.2.7 – Main accessories of distribution transformers : Oil expansion reservoir . Thermometer pocket Oil drain cock Oil level indicator Lifting chackles Four two – directional adjustable wheels Earth screw Name plate

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

: ‫الملحقات الساسية لمحولت التوزيع‬ ‫خزان تمدد الزيت‬ ‫جراب الترمومتر‬ ‫صمام تصريف الزيت أو أخذ عينات منه‬ ‫مبين مستوى الزيت‬ ‫حلقات لرفع المحول‬ ‫أربع عجلت‬ ‫مسمار توصيل الرضى‬ ‫لوحة البيان‬

       

‫الملحقات الضافية‬ ‫ جهاز السيلكاجل لمتصاص الرطوبة‬ ‫ جهاز البوخهلز لحماية المحول‬ ‫ ترمومتر عادى‬

‫وفيمككا يلككى شكككل رقم ) ‪( 1‬‬ ‫يوضح نموذج لمكونات‬ ‫المحولت المصنعة محليًا‬

3.1 The general data of transformers are: ‫للمحولت‬  Power of transformer in K.V.A  Input voltage , output voltage in volts and cycle in Hertz .  Connection group ( e.g DY II )  The place in which the transformer will be installed and wheather indoor or outdoor .  Percentage regulation of tap changer .  Determine wheather the ransformer will work in parallel or individual In case of parallel work with old transformers, the voltages and impeadance of them must be mentioned .

‫المواصفات العامة‬

‫أ‬.‫ف‬.‫ ك‬. ‫ قدرة المحول‬ . ‫ والذبذبة بالهرتز‬, ‫ وجهد الخروج‬, ‫ جهد الدخول‬ . ‫ رقم مجموعة التوصيل‬ ‫مكان تركيب المحول وهل يركب داخل المبنى أم‬ . ‫خارجة‬



. ‫ نسبة التغيير منظم الجهد‬ ‫يجب تحديد ما إذا كانت المحولت المطلوبه تعمل‬ ‫منفرده أوعلسى التوازى مع بعضها أو مع محولت‬ ‫أخرى موجودة وفى هذه الحاله يجب بيان المعاوقه‬ . ‫الكليه للمحولت الموجودة‬



3.2 Selection: ‫كيفية تحديد مواصفات المحولت‬ 3.2.1 Rated values : The rated values of the transformer such as power , voltage , transformation ratio and impendence ratio are selected according to the requirements of the system . ‫تتحدد القيم السمية كالقدرة والجهد ونسبة التحويل والمقاومة الكلية طبقًا لمتطلبات نظام التغذية‬ ‫الكهربائية‬ 3.2.2 Rated power : The rated power is found first by determining the peak effective power demand designed or measured , usually a margin is added to provide for the regular increase in power demand . ( ) ‫تتحد القدرة السمية عن طريق التصميم أو القياس لقصى قدرة فعاله مطلوبة مع إضافة نسبة‬ . ‫من القدرة لحتمالت الزيادة المستقبلية العادية فى الطاقة الكهربائية المطلوبة‬ When calculate the rated power (PN) we must take in consideration the anticipated power factor (COS ) . ‫ويجب عند حساب هذة القدرة السمية مراعاة قيمة معامل القدرة المتوقع‬

3.2.3 The impendence voltage(UK) : the impeadence voltage UK is the voltage necessary at the input terminals at rated frequency to cause the rated current to flow in the primary when the terminals on the secondary are short circuited 3.2.4 The rated impeadence voltage(UkN) : 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 and given in percent UkN =

UkN X 100% UN

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

3.2.5 Selection of rated impeadence voltage : In distribution system a rated impeadence voltage UKN= 4 % is preferred in order to keep the voltage drop small . 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 switch gear of the plant . 3.2.6 Transformers loss PK: 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 constant voltage impeadence of load . 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 transformer are :-

PV = PO + a 2 Pk Pv = total losses in watt .. Po = no-load in watt.

a = load factor ( PK = load losses in W

part load pa in K.V.A ) rated power P in K.V.A

3.2.7 η - Efficiency : the efficiency η of a distribution transformer can be calculated with reasonable accuracy from the following ; N where ; η PN Po Pk Cos φ a

= efficiency in % . = rated power in K.V.A . = no-load losses in K.W , = load losses in K.W . = power factor . = load factor .

Po + a 2 Pk η = 100 % × 100 % a PN Watt cos φ + Po

* Example :calculate efficiency (η ) of a transformer at full load given the following information : PN = 500 K.V.A., Po = 1.1 K.W , Pk = 5.5 K.W , Cos  = 0.8 , a = 1.0 Po + a 2 PK η = 100 % × 100 % a PN cos φ + Po 1.1 K.W + (1)2 × 5.5 K.w η = 100 % × 100 % 1 × 500 K.V × 0.8 + 1.1 K.W η = 98.36 %

3.2.8 Maximum efficiency: the load factor ( a ) for the maximum efficiency of a transformer is defined as follows ;  Po  a=   Pk   For the transformer in the above example this becomes ;

1.1 (k.w) = 0.447 5.5(k.w) Therefore maximum efficiency occurs with a load of ; Pa = PN.a Pa = 500 K.V.A × 0.447 = 224 K.V.A And for this load the efficiency is ; a=

1.1 K.W + ( 0.447 ) × 5.5 K.w η = 100 % × 100 % 0.447 × 500 KVA × 0.8 + 1.1 K.W 2

η = 98.36 % η = 98.78 % .

3.2.9 Insulation class : the insulation rating of a transformer is designed by numerals indicating the insulation voltage class in K.V & a letter N indicating that the transformer is 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)*

Table no (1)*

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

Rated voltages (UN): The rated voltages ( UN ) is the voltage present , at rated load , at the input windings and for which the transformer is designed . The rated voltage ( UN ) on the output side is the voltage which appears at no load ( no load voltage Uo ) with rated voltage and rated frequency on the input side .

3.2.11 Adjustment of transformer ratio steps : to compensate for voltage fluctuations due to load variations in systems , the high 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 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 principal tap .

Rated tapping range : 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 with an excitation corresponding to the rated voltage on the principal tap

Tap voltage range : 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 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 would show the values 21.000 V, 20.000 V and 19.000 Volt .

3.2.12 vector group number : The vector group symbol indicates the respective connections of the high voltage and low voltage transformer windings their relative phase displacement expressed as a clock – hour figure. 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 no (2)*

Clock – hour number :

Table no (2)

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 of the corresponding terminal this angle between the voltage vectors can have values between 0° & 360° . 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 standard .

Obtaining the clock hour number: 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 in such a way that the marking 1V of the high voltage side coincides with number 12 corresponding to 0 . 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 . Example: (fig 2). * C – Y/S: High voltage side : C connection . Low voltage side : y connection Clock – hour number 5 multiplied by 30° gives 150° phase displacement between vector 1V of the high voltage winding & the vector 2V of the low voltage winding . preferred vector group : for distribution transformers , star or delta connections for the high voltage windings and star or zizag connections for the low voltage winding are preferred table (3)* shows some group numbers

Fig no (2) Vector group (DY5)

Table no (3)

4.1 Indoor installation : Indoor pattern fluid cooled transformers must be installed in covered rooms which provide protection against rain , snow , dust & sand etc…. , and good ventilation . Dry type transformers : must be installed in closed rooms which dry & practically dust free the rooms should be easily accessible , to allow for transport , operation maintenance & fire fighting . 4.2 Outdoor installation : Fluid cooled transformers are suitable for outdoor installation when provided with suitable bushings & paint finish suitable for outdoor conditions. Measuring the temperature of insulating and cooling fluid the temperature of the cooling and insulating fluid are measure near the top for this purpose, the caver of the transformer contains for thermometer pockets into which thermometers can be inserted these pockets are filled with same type of cooling and insulating fluids used within the transformer 4.3 Building dimensions of transformer station : The dimensions of the transformers are an important factor to house the transformer allowance should always be made for increase the power demand in the future . The following table give an example of dimensions of transformers for a given power [table (4) & fig no (3)*] .

Fig no (3)

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

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 ventilation , the location cables & connections & the clearance necessary between live parts & earthed metal . for this type of transformer station the minimum height transformer should be the over all height of the transformer plus 500mm .. 4.4 Width of inspection passage way: 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 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 4.5 Floor of transformer station for fluid cooled transformers : 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 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) *

fig no ( 4 ) example for the indoor installation of a transformer

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

Fig no (5) Typical arrangement of girder And guide strip for transportation rollers

4.7 For oil immersed transformers: Collecting tank and pits cooling and insulating fluid : 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 the transformer floor can also be used as collecting tank when the door step & ventilation openings are correspondingly high

Collecting pit : 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 ) . 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 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 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 .

Gravel, granite chipping layer : 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 galvanized iron grating to minimize the spread of fire fig no (5) *

Outdoor collecting pit : A collecting pit must also be provided if transformers are installed outdoor in order to prevent seepage of cooling &insulating fluid into 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 filling of rain water or melted or melted snow , the pit must be pumped out regularly , other wise it would became filled with rainwater . 4.8 Ventilation of transformer rooms : When designing the room to house a self – cooled transformer it must be borne in mind that the transformer heat losses must be dissipated inlet- & outlet air openings have to provided the air inlet should be either from underneath the transformer or as close as possible to the floor level but never higher than the midpoint of the transformer . the outlet opening should be as high as passible.inlet and outlet openings should be arranged on opposite walls . The efficiency of the ventilation increases with the difference in height between the middle of the transformer tank & the outlet openings, fig (6)*.

Fig no (6) Arrangement of inlet and outlet air openings

4.9 Dimensioning of outlet air opening : 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 without screen & also allowing for an air temperature rise of 15 °C within the room the values have been increased by for simple screens approx. 10% , for screens & shutters approx. 50 % .

Dimensions of inlet- air opening : The dimensions of the inlet – air opening can be 10 % smaller than the dimensions of the outlet- air opening , ( plus the increase required for screen & shutters ) . to find the dimensions of inlet & outlet air openings .

Given : 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 point of transformer tank 600 mm resulting difference in height h = 3100 – 600 = 2500 mm . Outlet air opening obtained from curves Increase for simple screen + 10 % Dimension of outlet air opening Dimension of inlet air opening (10 %)

0.9 m² + 0.09 m² 0.99 m² 0.9 m²

‫الخطوات التى يجب مراعاتها قبل بدء‬ ‫الختبارات والتشغيل ‪:‬‬ ‫‪ -1‬تنظيمف العوازل الصمينى وبارات التوصميل جهتى‬ ‫الضغط العالى والضغط المنخفض وكذلك سطح التنك‬ ‫ومواسير التبريد‪.‬‬

‫‪5.1 Important instructions for‬‬ ‫‪commissioning :‬‬ ‫‪1. cleaning the porcelain bushings – bus bars for‬‬ ‫‪both H.T & L.T sides & tank surface‬‬

‫‪ -2‬التأكمد ممن ربمط العوازل الصمينى علمى جسمم التنك ‪2. well fastening of porcelain bushing to‬‬ ‫وسلمة الجوانات المرنة التى تمنع تسرب الزيت من ‪transformer cover and to be sure that‬‬ ‫المحولت‬ ‫‪oil seals are in good condition‬‬ ‫‪–3‬التأكد من أن مستوى الزيت فى المحول بالقدر الكافى‬ ‫للتشغيمل بحيمث ل يقمل عمن أدنمى مسمتوى ممبين على‬ ‫خزان التمدد واذا احتاج المر فيمكن تزويد الزيت عن‬ ‫طريمق الفتحة العليما بخزان التمدد وبنفمس نوع الزيت‬ ‫الصلى أو المعادل له‬

‫‪3. oil level for safe operation‬‬

‫‪ - 4‬فى حالمة عدم اسمتعمال المحول لمدة طويلمة وكذلك ‪4. in case that the transformer is not used‬‬ ‫بعد مرور عام على التشغيل يجب اعادة اختبار قوة ‪for long time the dielectric strength of‬‬ ‫عزل الزيت ويجب أل تقل عن ‪ 20‬ك‪.‬ف‪2.5/‬مم‪.‬‬ ‫‪oil is to be checked & after a year of‬‬ ‫‪operation it must be not less than 20‬‬ ‫‪K.V/2.5 mm‬‬

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

، ‫ التأكمد ممن سملمة أجهزة الوقايمة مثمل البوخهلز‬- 5 ‫الترمومتر والسيلكاجل ومسمار الرضى‬

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

‫ التأكمد ممن لون الملمح السميلكاجل الزرق حتمى يمكنه‬-6 ‫امتصمماص الرطوبمة فإذا تحول الممى اللون الحمر‬ ‫الوردى فإنمه يجمب إعادة تجفيمف الملمح بتعريضه‬ ‫ حتى يستعيد لونه‬º‫م‬140 ‫لدرجة حرارة ل تزيد عن‬ ‫الزرق أو بتغيير الملح‬

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

‫ يجب توصيل نقطتى النذار ونقطتى الفصل فى‬- 7 ‫روزتة جهاز البوخهلز بأجهزة النذار والفصل بمفاتيح‬ ‫العميل حيث تعمل صفارة النذار اذا تراكمت الغازات‬ ‫حول العوامة العليا بجهاز البوخهلز وحيث يفصل جهاز‬ ‫الوقاية مفتاح تغذية المحول بالكهرباء فى حالة حدوث‬ ‫قصر فى الدائرة واندفاع الغازات من المحول الى العوامة‬ . ‫السفلية فى جهاز البوخهلز‬ ‫وتعمل دائرتا النذار والفصل على تيار تشغيل‬ 220 ‫ الى‬24 ‫أجهزة الوقاية عند العميل على جهد من‬ .‫فولت تيار مستمر أو متردد‬ ‫( يوضح توصيل جهاز البوخهلز‬13) ‫شكل رقم‬ ‫بأجهزة النذار والفصل‬

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

‫ التأكد من تثبيت منظم الجهد فى الوضع المراد تشغيل‬- 8 . ‫المحول عليه‬

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

‫ فى حالمة تركيمب المحول داخمل المبانمى فيراعمى أن‬- 9 ‫تكون أبعاد الغرفمة مناسمبة لحجمم المحول كمما يحسن‬ ‫عمل فتحات تهوية فى اتجاهين متضادين ويحسن أن‬ ‫تغطى فتحات التهوية بشبكة معدنية‬

‫شكل رقم )‪ (7‬يوضح توصيل جهاز البوخهلز بأجهزة النذار والفصل‬

‫الختبارات‬ ‫تجرى علقى المحولت الختبارات الروتينيقة طبققا لمواصفات‬ ‫‪ IEC‬وتشمل ‪:‬‬ ‫‪‬‬

‫اختبار نسبة التحويل‬

‫‪‬‬

‫اختبار العزل بين الملفات وبعضها وبينها وبين‬ ‫الرض ‪.‬‬

‫‪‬‬

‫اختبار العزل بالجهد التأثيرى لكل حلقة من الملفات‪.‬‬

‫‪‬‬

‫اختبار قياس الفقد فىالقلب الحديدى فى حالة عدم وجود حمل‬

‫‪‬‬ ‫‪‬‬

‫اختبار قصر الدائرة‬ ‫قياس مقاومة الملفات‬

‫حسب طلب العميل يمكن إجراء الختبارات التالية‬ ‫‪‬‬ ‫‪‬‬ ‫‪‬‬

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

‫‪5.2Transformer tests:‬‬ ‫‪routine tests are carried out according‬‬ ‫‪to IEC standards :‬‬ ‫‪turns – ratio test‬‬ ‫‪Insulation test between windings and‬‬ ‫‪between winding and earth.‬‬ ‫‪Induced high voltage test.‬‬ ‫‪no – load losses test‬‬ ‫‪short circuit test‬‬ ‫‪windings resistance measurements‬‬

‫‪5.2.1 upon request following‬‬ ‫‪tests can be made :‬‬ ‫‪loading and heat test‬‬ ‫‪resistance of insulation to impulse‬‬ ‫‪test‬‬ ‫‪noise lvel‬‬

‫‪‬‬ ‫‪‬‬ ‫‪‬‬

6.1 Arrangement of distribution transformers The distribution transformers can be arranged centrally in one station or 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 cables cross section of conductors and losses are kept within economic limits . The centrally arranged system used for distribution systems where only 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 bars see fig (8)* in calculating the rated power of distribution transformer .

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

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

Fig no (9) Decentralized arrangement of distribution Transformers in several sub-stations

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

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

6.3 Peculiarities of high voltage ring system: 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 of the circuit only ( e.g in substation b3 ) fig (12)* 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 isolated. 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 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 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 , 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 load equalization and high security of supply to, all sub station yet the number of ring circuits will be less fig. (11)

6.4 Parallel operation: 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 . slight different conditions apply between operation in parallel bus bars and operation in parallel on systems networks . 6.5 general requirements for parallel operation : for satisfactory operation in parallel , especially on bus bars the following general requirements apply ;  For transformers having the same vector group and clock hour number, terminals having identical designations are paralleled .  The ratio of transformation must be identical .  All taping must have identical values on each transformer .  The rated impeadence voltage must be practically the same within 10 % , the transformer with the smaller rated power should have the higher rated impeadence voltage , if possible the ratio of the rated power of transformers working in parallel should not be more than3:1 . 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 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 will indicate values values of up to twice the phase voltage fig (13) * .

Fig no ( 13 )

6.6 Protection by fuses ( short circuit protection ) : 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 distribution transformers ; the maximum current inrush when switching on the transformer has been taken into consideration .

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

6.7 Over load protection: 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 of the circuit breakers are selected according to the rated current IN of the transformer . this selection normally provides adequate selectivity between high – voltage and low voltage side .

6.8 Short circuit strength: Distribution transformers need to be designed to with stand the effects of external short circuits with out damage .

Steady state R.M.S short circuit current I.K : 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 terminals of the outgoing side and when the D.C component has decayed with constant rated voltage applied to the input terminals .

Calculation of steady state R.M.S short circuit current : For transformer operating at rated voltage & rated frequency and tappings adjusted to the principal tap , the continuous short circuit current Ik can be calculated from the rated current IN & the rated impeadence voltage UKN .

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

= rated current in A .

UKN = rated independence voltage in % . For the calculation of short circuit currents of installation systems , the resistance of the system must be taken into consideration . for calculation of short – circuit current in three phase systems .

6.11 Maximum permissible duration of 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 & other operating equipment affected by the fault . 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 . The first peak value of current Ik is the X 2 multiple of the R.M.S short circuit current IK

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

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

IS = I K × 2

Where :

IS = peak short circuit current in A ( peak value ) .

I K = maximum permissible R.M.S short circuit current in A .

X 2 = impulse factor . Impulse factor : the impulse factor X 2 of the reactance X to the Ohmeic resistance R , ( i.e ) dependent on the ratio of the reactance voltage Ux to the resistive voltage drop UR of the transformer .

Maintenance work is permitted only when the transformer is switched off and the terminals earthed . 7.1 Dry type transformers: Dry type transformers must be kept dust free & protected against pollution . at regular intervals these should be cleaned with bellows or a vacuum cleaner . terminals and bolted joints should also be checked regularly . Drying out : The insulation resistance between windings & also windings to earth provides good 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 drying out may therefore be necessary . The insulation resistance at room temperature should not less than For operating voltage:  Up to 1000 v 15 m. ohm.  Above 1000 v 25 m. ohm  Above 5000 v 40 m. ohm . The 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 that moisture has been observed by the insulation .

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 such as incandescent lamps , resister elements or radiators . the surface of the windings must not exceed 100°C when using direct radiation . heating under short – circuit conditions with a maximum rated current the output side of the transformer is short – circuited and the input side connected to a voltage approximately equal to the impedance voltage . 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 must be well ventilated during dying process . the value of insulation resistance should be measured repeatedly .

7.2 Renovation of the operating room : During renovation work in the transformer room the transformer should be switched off and earthed . It is important , particularly with dry type transformer to cover carefully to 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 heated . Cast resin transformers are largely maintenance free and do not require drying .

7.3 Oil – immersed transformers : the type of fluid used is stated on the rating plate .the maintenance of this type of transformer includes :  Checking the level of fluid at the sight glass .  Checking effectiveness of gaskets and quality of paint finish .  Testing the insulating fluid of moisture content and checking or reactivating the silica – gel in the breather . 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 fluid within the transformer , special care the highest degree of clean lines is required when taking samples . 1. A specimen bottle with large opening and glass stopper ( not cork ) is required 2. The bottle must be cleaned with clean alcohol and well dried . 3. The drain cook must be carefully cleaned and dried before taking a test sample . 4. The specimen bottle should be approximately half filled with the fluid and rinsed with this fluid before a test sample is taken ..

7.4 break down voltage : 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 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 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 k. v. minimum . if the measured values fall below the above mentioned minimum values 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 dried under vacuum the manufacturers of transformers and the electricity boards have the necessary plant and devices for the conditioning and drying of the insulating fluid . 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 refilling