TRIBHUWAN UNIVERSITY
INSTITUTE OF ENGINEERING
PULCHOWK CAMPUS
A
Detailed Design of
Single Phase Core Type Distribution Transformer
Submitted to:
Er . Purushottam khadka
Department of Electrical Engineering
Submitted by:
Sobit Thapa
072 BEL 338
Date of Submission: 2074/11/21
Question:
Design a 100 KVA, 50 Hz, 3300/400 V, single phase core type oil
immersed, natural cooled distribution transformer. The mean
temperature rise of oil not to exceed 35ºC.
DESIGN OF TRANSFORMER
Design of core:
We know Voltage per turn is,
Constant for volt per turn (K)=0.75 (from standard data table for single
phase core type distribution transformer)
E t=0. 75√ 100
=7.5 volt/turn
= 0.03377Wb Using
the cold roll si steel
Area of iron, Ai= Φm/Bm =0.02518 m2 =25.18*103 mm2
Using three stepped core:
[Assuming ks=0.9] Ai=
0.6d2 so
d=204.857 mm
Dimensions of core section, Largest
stamping
a=0.9d=184.371 mm
Middle stamping
b=0.7d=143.4 mm
Smallest stamping
c=0.42d=86.039 mm
Design of Window:
For 100 KVA,3.3kv
Window space factor
Taking value of current density =2.5 A/mm2
We have,
Q=2.22*f*Φ m*Aw*kw*δ*10
-3
So,
Aw=0.0356m2=35.608*103 mm2 Taking the
ratio Hw/Ww =2.5
2.5*Ww2 = 35.608*103 mm2
.. . Ww = 119.345 mm Hw = 298.363 mm
Distance between adjacent limbs:
D=Ww+d
D=119.345+204.857 =324.202 mm
Design of Yoke:
Taking area of yoke: Ay=Ai= 25.18*103 mm2
Gross area of yoke: Agy = 25.18*103 /0.9 = 27.977103 mm2
Taking rectangular section of yoke,
Dy=a =184.371 mm
Hy=Agy / Dy =151.74 mm
Overall core dimension
Overall height of frame (H):
H=Hw + 2*Hy
H=601.843 mm
Width of frame (W):
W=D+a
W=508.573 mm
Design of LV winding (Secondary winding):
Secondary voltage Vs = 400 V
No. of turns =T2= Vs /Et = 400/= 53..33≈53 turns Current on
secondary Is= 125/0.4 = 250
Current density δ =2.5 A/mm2
Area of a secondary conductor =a2 = 250/2.5 = 100 mm2
Taking four strips of rectangular conductors having thickness = 3.5mm
and width = 10 mm
now, new area of conductor = 4*3.5*10 = 140mm2 new current
density = 250/140 = 1.78 A/mm2
considering increase in thickness and width due to paper insulation, new
thickness = 4 mm and new width = 10.5 mm
Taking two layers of secondary conductor of 24 winding each
dimension of single insulated conductor = 4*10.5 = 42mm2 Axial depth
of L.V winding = 24*10.5 =252mm
Clearance on each side of yoke = mm
Radial depth of L.V winding = 2*4*3.5 = 28mm
Inside diameter of L.V winding = 20.23 + 2*0.536 = 21.016cm
Outside diameter of L.V winding = 21.016+2*0.35*4 = 23.816 cm
mean diameter of L.V winding = =22.416cm mean length of L.V
winding = П* 22.416 = 70.421 cm
Design of High voltage Windings
Taking consideration of fact that the transformer to be designed is self
cooled and Distribution transformer selecting current density δ = 2.5
A/mm2
now number of turns to be made in primary winding is
Tp
= 440 turns
again considering 5.5% tapping , total no of turns = 1.055*440
=464turns
Current flowing in Primary winding is
Ip
= 30.30 A
Area of primary conductor is
ap
= 12.12 mm2
Diameter of primary conductor = 3.92 mm
Considering fine insulated covering having new d=4 mm
Now, new area of conductor=
New current density = 1 A/mm2
Considering 4layers of primary conductor having 69 turns each,
Axial depth of H.V winding = 69*4 = 276 mm
Clearance on each side of yoke = mm
Radial depth of H.V winding = 4*2.41 = 9.64mm
Inside diameter of H.V winding = 204.857 + 2*1.49 = 207.837mm
Outside diameter of H.V winding = 207.837+2*3.37 = 214.577mm
mean diameter of H.V winding = = 211.207mm
mean length of H.V winding = П* 211.207=6.635 cm
Calculation of Operating Characteristic
Calculation of Resistance
Resistivity of copper at 75°𝐶𝐶 (𝜌𝜌) = 0.021𝛺𝛺 mm2⁄𝛷𝛷
resistance of primary winding is
rp
resistance of secondary winding is
rs
Now,
Total resistance referred to primary side
Rp
Per unit voltage drop in the resistance of transformer
Er pu
Leakage reactance
Height of the winding
= 264 mm
P.U. leakage reactance
AT= Ampere turn of either winding = 30.30* 440=13332 a
= 10 mm b1= 28 mm b2= 29.975 mm
= .0.0000458p.u.
P.U. impedance =√0.00004582 + 0.00532 = 0.53 p.u.
Regulation ϵ
ΕrCosφ+ ΕxSin φ
Per unit regulation at unity pf :0.0053∗1=0.0053 pu
Per unit regulation at 0.8 pf 0.0053∗0.8+0.0000485∗0.6=0.00422pu
= 0.42%
Calculation of Losses:
Copper loss
I2R loss = I12 * R1
= (30.30)2 *0.58
= 519.237 W
Taking 7.5% as stray loss
Total copper loss =Pc = 1.075*519.237 = 558.1801W
Core loss
Taking density of laminations for CROS = 5*103 kg/m3
Weight of two limbs
= 2* height of window* iron area*density
= 2*0.298* 0.02518*5*103 kg
= 87.8782kg
Weight of two yokes
= 2*iron area of yoke (Ayi) * width of frame (W) * density
= 2*0.02518*0.508*5*1000
=127.9144kg
As CROS is used so taking area of yoke to be equal to area of limb
Bm for yoke =Bm for limb= 1.5 Wb/m2
From the graph specific core loss = 1.01 W/Kg
Total core loss =Pi= 1.01*(87.8782 +127.98)
= 218.06 W
Total Loss at full load =iron loss + copper loss =558.1+ 218.1
=776.2W
Calculation of Efficiency:
At unity pf = %
At 0.8 pf = = 99.03%
Calculation of No load current
Total magnetizing mmf = 2* at* (Lc + Ly)
From graph the values of magnetizing mmf
𝑎t= 𝑎tc = 𝑎ty = 160 𝐴⁄m and
Length of core Lc = Hw = 0.256𝛷
Length of yoke = Ly = 𝑊 = 0.512𝛷
Total magnetizing mmf
= 2*160*(0.256+0.512)
= 245.76 AT
Magnetizing current
Loss component of current
Total no load current
No-load current as % of full load current is
Design of cooling system
Tank Design:
Overall frame height = 601mm
Allowing 50 mm at the base and about 170 mm oil level above the core.
Thus total = 732 mm up to oil level. Allowing another 500 mm for lead,
etc.
Height of the tank= 1321mm
Width of the = Outside dia. of H.V. coil+ end clearances to
ank tank accommodate lead, etc. =214.57 + 2* 45
Length of the tank =D+ outside dia. of H.V. coil + tank
end clearances =324.202+477.59+2*40
=881.792
Temperature Rise
For dissipation of heat only lateral surfaces of the tank are considered.
Total loss dissipating surface of the tank, St=2* 1.155 *1.5 + 2 *0.57*1.5
= 5.175m2
Full load loss to be dissipated = 776.2 W
Total specific loss dissipation due to convection and radiation is 12.5
W/m2/˚C temperature rise.
Temperature rise in ˚C= ˚C
Since the temperature rise is within limit of 35 ˚C so the provision of oil
tubes is not needed.
Weight of L.V. winding
Number of turns= 16
Length of mean turn= 1232.73 mm a2=1500mm2
Using density of copper = 8.89* 103 kg/m3
Weight of L.V. winding = 16 * 1.232*1500*10-6*8.89* 103=262.8kg
Weight of H.V. winding
Number of turns= 264
Length of mean turn= 1413.28 mm a1=90.9 mm2
Weight of L.V. winding = 264 * 1.413*90.9*10-6*8.89* 103=301.44 kg
Total weight of copper in transformer =564.3 kg
