Cooling of Transformer
Unit 3
Electrical Machine Design
Cooling of transformers
 Losses in transformer-Converted in heat energy.
 Heat developed is transmitted by,
 Conduction
 Convection
 Radiation
The paths of heat flow are,
From internal hot spot to the outer surface(in contact with oil)
From outer surface to the oil
From the oil to the tank
From tank to the cooling medium-Air or water.
Cooling of transformers
Methods of cooling:
1.
2.
3.
4.
5.
6.
7.
8.
Air Natural (AN)-upto 1.5MVA
Air Blast (AB)
Oil natural (ON) – Upto 10 MVA
Oil Natural – Air Forced (ONAF)
Oil Forced– Air Natural (OFAN) – 30 MVA
Oil Forced– Air Forced (OFAF)
Oil Natural – Water Forced (ONWF) – Power plants
Oil Forced - Water Forced (OFWF) – Power plants
Cooling of transformers
Transformer Oil as Cooling Medium
 Specific heat dissipation due to convection is,
 conv
 
 40 . 3 

H 
1
4
2 0
W /m . C
where,  - Temperatur
H  Height
e difference
of dissipatin
of the surface relative
0
to oil, C
g surface, m
 The average working temperature of oil is 50-600C.
 For   20 0 C & H  0 .5 to 1m ,
 conv  80 to 100 W/m . C .
2 0
 The value of the dissipation in air is 8 W/m2.0C. i.e, 10 times less
than oil.
Cooling of transformers
Temperature rise in plain walled tanks
 Transformer wall dissipates heat in radiation & convection.
 For a temperature rise of 400C above the ambient temperature of 200C, the
heat dissipations are as follows:
 Specific heat dissipation by radiation,rad=6 W/m2.0C
 Specific heat dissipation by convection, conv=6.5 W/m2.0C
 Total heat dissipation in plain wall 12.5 W/m2.0C
 The temperature rise,
 
Total losses
 Specific heat   Heat dissipatin g 

 

Dissipatio
n
surface
of
tank

 


Pi  Pc
12 . 5 S t
St – Heat dissipating surface
 Heat dissipating surface of tank : Total area of vertical sides+ One half area of
top cover(Air cooled) (Full area of top cover for oil cooled)
Design of tanks with cooling tubes
 Cooling tubes increases the heat dissipation
 Cooling tubes mounted on vertical sides of the transformer would
not proportional to increase in area. Because, the tubes prevents
the radiation from the tank in screened surfaces.
 But the cooling tubes increase circulation of oil and hence improve
the convection
 Circulation is due to effective pressure heads
 Dissipation by convection is equal to that of 35% of tube surface
area. i.e., 35% tube area is added to actual tube area.
Design of tanks with cooling tubes
Let, Dissipating surface of tank – St
Dissipating surface of tubes – XSt
Loss dissipated by surface of the tank by radiation and convection
= 6  6 .5 S  12 .5 S
t
t
 Loss dissipated by 
135
 XS t  8 . 8 XS

  6 .5 
100
 tubes by convection 
 Total loss dissipated

 by walls and tubes
t

  12 . 5 S t  8 . 8 XS t  12 . 5  8 . 8 X S t  (1)

Actual total area of tank wall s and tubes  S t  X S t  S t (1  X )
Design of tanks with cooling tubes
2
Loss dissipated
per m of dissipatin
g surface

Total losses dissipated
Total area
2
Loss dissipated
per m of dissipatin
Temperatur
e rise in
Transforme
r with cooling
Total losses, Ploss  Pi  Pc
g surface
8 .8 X 
Pi  P c
 St
S t (12 . 5  8 . 8 X )
S t (1  X )


Total loss
 
tubes 
Loss Dissipated
 (3)
From (1) and (3), we have,  
(12 . 5  8 . 8 X ) 

Pi  P c
S t (12 . 5  8 . 8 X )
Pi  P c
 St
 12 . 5


1  Pi  P c

X 
 12 . 5 

8 .8   S t

(12 . 5  8 . 8 X )
(1  X )
 (2)
Design of tanks with cooling tubes
Total area of cooling
Let ,

1  Pi  P c
1  Pi  P c


St 
tubes 

12
.
5

12
.
5
S
 (5 )

t 


8 .8   S t
8 .8  


l t  Length of tubes
d t  Diameter
of tubes
 Surface area of tubes   d t l t
Total number
of tubes, n t 
nt 
Total area of tubes
Area of each tube
 Pi  P c

 12 . 5 S t   ( 6 )

8 . 8 d t l t  

1
 Standard diameter of cooling tube is 50mm & length depends on the
height of the tank.
 Centre to centre spacing is 75mm.
C4
HT
D
D
C3
WT
LT
C2
Doc
C1
Design of tanks with cooling tubes
 Dimensions of the tank:
Let, C1 – Clearance b/w winding and tank along width
C2 - Clearance b/w winding and tank along length
C3 – Clearance b/w the transformer frame and tank at the
bottom
C4 - Clearance b/w the transformer frame and tank at the top
Doc – Outer diameter of the coil.
Width of the tank, WT=2D+ Doc +2 C1 (For 3 Transformer)
= D+ Doc +2 C1 (For 1 Transformer)
Length of the tank, LT= Doc +2 C2
Height of the tank, HT=H+C3+ C4
Design of tanks with cooling tubes
 Clearance on the sides depends on the voltage & power
ratings.
 Clearance at the top depends on the oil height above the
assembled transformer & space for mounting the
terminals and tap changer.
 Clearance at the bottom depends on the space required
for mounting the frame.
Design of tanks with cooling tubes
Voltage
kVA Rating
Up to 11kV
Clearance in mm
C1
C2
C3
C4
<1000kVA
40
50
75
375
Upto 11 kV
1000-5000kVA
70
90
100
400
11kV – 33kV
<1000kVA
75
100
75
450
85
125
100
475
11kV – 33kV 1000-5000kVA