The document is a lab report on the design of a 1000 KVA, 11/66 kv, 50 Hz, three-phase, core type distribution transformer. It provides details on the core design, window design, winding designs for the high voltage and low voltage coils, resistance and reactance calculations, efficiency calculations, regulation calculations, loss calculations, and tank design including the number of cooling tubes required. The transformer is designed to have a maximum temperature rise of 40°C and tappings of ±2.5% and ±5% on the high voltage winding.

1. INSTITUTE OF ENGINEERING
PURWANCHAL CAMPUS
DHARAN-8, SUNSARI
NEPAL
A
LAB REPORT
ON
TRANSFORMER DESIGN
Submitted by: Submitted to: Gyan Kafle
Bikash Gyawali (072/BEL/05) Asst. Lecturer
Chet Raj Kandel (072/BEL/09) Department of
Bishop Poudel (072/BEL/07) Electrical Engineering
Akash Pandey (072/BEL/02) Purwanchal Campus Dharan

2. Designa 1000 KVA, 11/66 kv,50 HZ, 3-phase , Δ/Y , core type, oil immersed natural
cooleddistribution transformer .The transformer is provided with tappings ±𝟐 𝟐
𝟏
% ,
±𝟓 𝟐
𝟏
% on the HV winding. Maximum temperature rise not to exceed40℃.
CORE DESIGN:
For 3-𝜙 core type distribution transformer
We use K=0.45
Voltage per turn Et =k√ 𝑄 =0.45√400 = 9V
∴ Flux in the core 𝜙 𝑚 =
Et
4.44×𝑓
=
9
4.44×50
=0.04054054Wb
Hot rolled silicon steel grade 92 is used
so, the Bm is assumed to be 1.3Wb/m2
Net iron area Ai =
0.04054054
1.3
= 30.1185 × 103mm2
Using cruciform core, Ai = 0.56d2
Diameter of circumscribing circle d = √
30.1185 ×10 3
0.56
= 235mm
a=0.85d = 0.85×235=200mm
b= 0.53d = 0.53 × 235 =125mm
Window Design:
The window space factor for 400 KVA transformer is given by formula
kw =
12
30 +𝐾𝑉
=
12
30+66
= 0.125
Current density (𝛿 ) =1.5 Wb/mm2 (assumed)
Q= 3.33fBmkw 𝛿AwAi× 10 -3
Aw =
Q
3.33f 𝐵 𝑚 𝑘 𝑤 𝛿𝐴𝑖×10 −3 =
Q
3.33 ×50×1×0.125×1.5 ×10 6× 0.04054054 ×10 −3 = 0.20611 m2
Aw =206. 11 × 103 mm2
Hw×Ww = 206. 11 × 103
2.5 W 𝑤
2
= 206. 11 × 103
Ww = 290mm
Hw = 720 mm
Aw = Hw×Ww = 290× 720 = 208×103 mm3
D= Ww+d
= 290+235
= 525 mm
Yoke Design
The area of yoke is taken as 1.2 times that of limb
Flux density in yoke = 1.3/1.2= 1.08 Wb/m2
Net area of yoke = 1.2×31.1185 ×103 mm2 = 37.422×103 mm2
Gross area of the yoke =
37.422×103
0.9
= 41.58 ×103 mm2

3. Taking the yoke as Dy = a= 200mm
Hy =
41.58×103
𝑎
= 200mm
Overall Dimensions of frame
Height of frame (H) = Hw + 2Hy = 720 + 2×200 = 1120mm
Width of frame W = 2D +a =2× 525 + 200 = 1250mm
Depth of frame = a= 200mm
LV Winding
Star connection
Secondary line voltage = 11kv
Secondary phase voltage Vph =
11 ×1000
√3
= 6350V
Number of turns per phase =Vph/Et =6350/9 = 705
Secondary phase current =
400 ×1000
3×6350
= 21 A
A current density= 2.3 A/mm2
Area of secondary conductor, as =
21
2.3
= 9.13mm2
From the table (IS 1897-1962) using a bare conductor = 6.0 × 1.9 mm2
Area of bare conductor as = 9.29mm2
𝛿s = 21/9.29 = 2.26 A/mm2
The dimensions of insulated conductor = 6.5 × 2.4 mm2
Using 7 layers of helical winding.
The no of turns along axial depth =100+1=101
Axial depth of L.V = 101× 6.5= 656.5mm
Height of window= 720mm
Clearance = (720-656.5)/2 = 31.75mm
Radial depth
bs= no of layers × radial depth of conductor + insulation between layers
= 7× 2.4 +6×0.5 = 19.8 mm
Inside diameter of L.V winding= d+2× insulation = 235+ 2× 1.5 = 238mm
Outside diameter of L.V winding = 238+ 2× 19.8 = 277.6 mm
H.V Winding:
Primary voltage (line) = phase voltage = 66000kV (in delta)
No of turns Tp = 66000/9 = 7334
For 5.5% tapping
Tp max= 1.055 × 7334 = 7737
Using 20 coils,
Turns per coil = 7737/20 = 387
Using 19 normal coils of 392 turns and one reinforced coil of 289 turns

4. Tp=19× 392+ 289= 7737
Taking 24 layers per coil, we have
Turns per layer = 16
H.V winding phase current Ip =
400×1000
3×6600
= 2.02 A
It is below 21 A so cross over coils are used for H.V winding
Taking current density of 2.4 A/mm2
ap = 2.02/2.4 = 0.841 mm2
Diameter = (
4
𝜋
× 0.841)1/2
Diameter of bare conductor =1.034 mm
Using paper covered conductor of (IS: 1897 -1962)
Bare diameter = 1.06mm and insulated diameter =1.26mm with fine covering
Modified area of conductor=
𝜋
4
× (1.26)2
= 1.25mm2
Actual current density is = 2.02/1.25= 1.616 A/mm2
Axial depth of one coil = 16 × 1.26 = 20.16mm
The spacers used between adjacent coils are 5mm in height.
Axial length of H.V winding = no of coils × axial depth of each coil + depth of spacers
=20× 20.16 + 20× 5 = 503.2 mm
The height of window = 720 mm
The clearance = (720- 503.2)/2 = 108.4 mm
Radial depth of H.V coil bp = 24 × 1.26 + 23 × 0.5 = 41.74mm
The thickness of insulation between H.V and L.V is given by = 5+ 0.9KV = 5+0.9× 66 = 64.4 mm
Inside diameter of H.V winding = outer diameter of L.V + 2× thickness of insulation between L.V and H.V
= 277 + 2× 64.4 = 406 mm
Outside diameter of H.V winding = 406 + 2× 41.74 = 490mm
Clearance between windings of two adjacent limbs = 525mm- 490mm=35mm
Resistance
Mean diameter of primary winding =
406 +490
2
= 448mm
Lmtp = 𝜋 × 448 × 10-3 m = 1.41m
Resistance of primary winding at 75℃ rp =
𝑇𝑝 𝜌×𝐿 𝑚𝑡𝑝
𝑎 𝑝
=
7334×0.021×1.41
1.246
= 174.29 Ω
Mean diameter of secondary winding =
238 +277 .6
2
= 257.8mm
Lmts = 𝜋 × 257.8 × 10-3 m = 0.81m
Resistance of secondary winding at 75℃ rs =
705×0.021 ×0.81
9.29
= 1.29 Ω
Total resistance referred to primary side Rp = 174.29 + (
7334
705
)2 × 1.29 = 313.9Ω
P.U resistance of transformer 𝜀r =
𝐼 𝑝 𝑅 𝑝
𝑉𝑝
=
1.29×313 .9
66000
= 0.00613 Ω

5. Leakage Reactance
Mean diameter of windings =
238+490
2
= 364mm
Length of mean turn Lmt = 𝜋 × 364 × 10-3 m = 1.143 m
Height of winding Lc =
𝐿 𝑐𝑝+ 𝐿 𝐶𝑠
2
=
503 .2+656.5
2
= 579.85mm
Width of duct, a = (364- 277.6)/2 = 43.2 mm
Leakage reactance of transformer referred to primary side
Xp = 2𝜋f µ0 Tp
2 𝐿 𝑚𝑡
𝐿 𝑐
(a+
𝑏 𝑝+ 𝑏 𝑠
3
)
= 2𝜋 ×50×4 𝜋 × 10−7
× (7334)2 ×
1.206
0.579
(a +
𝑏 𝑝+ 𝑏 𝑠
3
)
= 2𝜋 ×50×4 𝜋 × 10−7
× (7334)2 ×
1.206
0.579
(43 +
41.74+ 19.8
3
) × 10-3
= 2808 Ω
P.U leakage reactance 𝜀x = (2.02 × 2808)/66000 = 0.086
P.U impedance 𝜀s = √0.006132 + 0.0862 = 0.08621
Regulation
P.U regulation 𝜀 = 𝜀r cos ∅ + 𝜀x sin ∅
Per unit regulation at unity power factor 𝜀 = 𝜀r = 0.00613
At zero p.f lagging 𝜀 = 𝜀x = 0.086
At 0.8 pf lagging 𝜀 = 𝜀r cos ∅ + 𝜀x sin ∅ = 0.00613 × 0.8 + 0.086× 0.6 = 0.056504
Losses
Copper loss:
At 75℃ = 3I2
Rp = 3× 2.02 2
× 313.9 = 3842 watt
Taking 15% stray loss = 1.15 × 3842 = 4420 watt
Core loss
Taking the density of laminations = 7.6 × 103
kg/m3
Weight of 3 limbs = 3× Hw × Ai × 7.6 × 103
= 3× 0.72 × 0.0301185 ×7.6 × 103
= 494.42kg
The flux in the limbs is 1.3 Wb/mm2
From the specific core loss is 2.03 W/kg
Therefore core loss in limbs = 494.42 × 2.03 = 1003.67 Watt
Weight of two yoke = 2 × length of yoke × area of the cross section × density of laminations
= 2 × 1.250 × 0.037422× 7.6 × 103
= 711 kg
Corresponding to 1.08 Wb/m2
≈ 1 .0 Wb/ m2
, specific core loss =1.2 W/kg
Therefore core loss in yoke = 711 × 1.2 = 853.2 Watt
Total core loss Pi
=
1003.7+ 853.3 =1856.9 ≈ 1857 watt
Efficiency:
Total losses at full load = Pc + Pi = 4420 + 1857 = 6277 watt
Efficiency at full load and unity power factor =
400×1000
400000+6277
=98.45%
For maximum efficiency, x2
PC = Pi

6. ∴ x =√
1857
4420
= 64.81%
Tank Design
Height of frame H =1120 mm
Allowing for 50 mm for the base and 200 mm for oil
Oil level = 1120 + 50+ 200 = 1370 mm
Allowing another 250 mm for leads etc
Height of tank Ht = 1370 + 250 = 1620 mm
The height of tank is taken as 1.65m
Allowing a clearance of 75 mm along the length
Length of the tank Lt = 2D + De + 2l= 2×525 +490 + 2×75 =1690 mm
The length of tank is taken as 1.7m
The width of the tank Wt = De + 2× clearance = 490 + 2× 90 = 670mm ≈ 0.68m
Loss dissipating surface of the tank St = 2(length + width) × height = 2(1.7 + 0.68) × 1.65=7.854m2
Temperature rise =
total loss
12.5 ×𝑆𝑢𝑟𝑓𝑎𝑐𝑒
=
6277
12.5 ×7.854
= 64℃
So we need to design tank with tubes
TANK WITH TUBES
We have
St be the dissipating surface of the tank
The total heat dissipating by tank walls only = 12.5St W/℃
Let the area of tubes = xSt
Total area of tank walls and tubes = St + xSt = St(1+x)
Loss dissipated =
12.5+8.8x
𝑥+1
W/m2 -℃
Now x=
1
8.8
(
𝑃𝑖 +𝑃𝑐
𝑆𝑡 𝜃
− 12.5) =
1
8.8
(
6277
7.854× 35
− 12.5) = 1.175
Area of tubes = xSt = 1.175 × 7.854 = 9.23 m2
Using 50mm diameter tubes spaced 70 mm apart. The average length of tubes is assumed as
1.4m
Dissipating area of each tubes =𝜋 ×0.05 × 1.4 = 0.22m2
Number of tubes to be provided = 9.23/0.22 = 42
Arrangement of tubes:
Along length – 2 rows 11 and 10 tubes
Design Sheet
KVA rating = 400KVA, Phase = 3-phase, Frequency = 50HZ, Delta/star

7. line voltage [
𝐻. 𝑉 660000𝑉
𝐿. 𝑉 11000𝑉
] phase voltage[
𝐻. 𝑉 660000𝑉
𝐿. 𝑉 6350𝑉
]
line current [
𝐻. 𝑉 3.5𝐴
𝐿. 𝑉 21𝐴
] phase current [
𝐻. 𝑉 2.02𝐴
𝐿. 𝑉 21𝐴
]
Type – Core Type of cooling: Oil Natural
Core Symbols Quantity
1 Material 0.35 mm thick 92 Grade
2 Output constant k 0.25
3 Voltage constant Et 9V
4 Circumscribing circle diameter d 235mm
5 Number of steps 2
6 Dimensions a= 200 mm, b= 125mm
7 Net iron area Ai 30.1185 × 10 3mm2
8 flux density Bm 1.3Wb/m2
9 flux ɸm 0.04054054Wb
10 Weight 494.42Kg
11 Specific iron loss 2.03 W/kg
12 Iron loss 1003.37W
Yoke
1 Depth of yoke Dy 200mm
2 Height of yoke Hy 200mm
3 Net yoke area 37.422×103 mm2
4 Flux density 1.08 Wb/m2
5 Flux 0.04054054Wb
6 Weight 711 kg
7 Specific iron area 1.2W/kg
8 Iron loss 853.2 W
Windows
1 Number 2
2 Window space factor Kw 1.25
3 Height of window Hw 720mm
4 Width of window Ww 290mm
5 Window area Aw 206. 11 × 103 mm2
Frame
1 Distance between adjacent limbs D 525 mm
2 Height of frame H 1120mm
3 Width of frame W 1250mm

8. 4 Depth of frame Dy 200mm
Figure: Overall dimensions of 400KVA, 66/11KV 3 phase, 50Hz, coretype, distribution
transformer

9. Windings L.V H.V
1 Type of winding Helical Cross over
2 Connection Star Delta
3 Conductor
Dimensions-bare 6.0 × 1.9 mm2
1.06mm
insulated 6.5 × 2.4 mm2 1.26mm
Area 9.29mm2 1.25mm2
Number in parallel
4 Current density 2.26 A/mm2 1.616 A/mm2
5 Turns per phase 705 7334(7735)at 5.5% tap
6 Coils total number 3 3× 20
per core leg 1 20
7 Turns per coil 705 19 of 392 turns, 1 0f 289
per layer 101 16
8 Number of layers 7 24
9 Height of winding 656.8mm 503.2
10 Width of winding 19.8mm 41.74mm
11 Insulation between layers 0.5mm press board 0.5mm paper
between coils 5.0 spacer
12 Coil diameter inside 238mm 406mm
Outside 277.6mm 490mm
13 Length of mean turn 0.81m 1.41m
14 Resistance at 75°C 1.29 Ω 174.29Ω
Insulation
1 Between L.V winding and core
2 Between L.V winding and H.V winding
3 Width of duct between L.V and H.V
Tank
1 Dimensions- Height Ht 1.65m
Length Lt 1.7m
Width Wt 0.68m

10. 2 Oil level 1.37m
3 Tubes
42 tubes
(11 and 10 in parallel)
4 Temperature 64℃
Impedance
1 P.U Resistance 0.00613
2 P.U Reactance 0.086
3 P.U impedance 0.08621
Losses
1 Total core loss 1857W
2 Total copper loss 4420W
3 Total losses at full load 6277W
4 Efficiency at full load and unity p.f 98.45%