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BUSHINGS
Introduction
The bushings are insulators which are used to take high voltage conductors through earthed
barriers such as walls, floors, metal tanks etc
...
Electrical insulation for the rated voltage and for service overvoltages
...
Mechanical support for the conductor and external connections
...
Stress control and avoid damages
...

Difference between bushings and insulators: The insulators are used to insulate two different
voltage points whereas bushings are used to conduct one side voltage to the other side through
zero potential body
...

I
...
Based on Insulation Material:
a)
b)
c)
d)
e)
f)
g)

Synthetic resin-bonded paper (s
...
b
...
Based on Applications:
a)
b)
c)
d)
e)

Transformer bushing
Alternator bushing
Switchgear bushing
Wall and roof bushing
Cable terminations and Joint box bushings

I
...


Fig-1: Simple bushing
a - radial clearance
b - axial clearance
Fig-2: Voltage distribution in simple bushing
Clearances a and b are selected to suit the electric strengths of the insulating material and
surrounding media
...
The voltage distribution is
non-uniform in radial and axial direction, and is shown in figure 2
...

Condenser bushing or Capacitance bushing:
The bushing in which cylindrical conducting layers are arranged coaxially within the
insulating material
...
Condenser bushings
may be one of several types:
i
...
r
...
p) insulation

ii
...
i
...


Epoxy resin impregnated paper

In synthetic resin-bonded paper bushings, the paper is first coated with synthetic resin then
wound into a cylindrical form under heat and pressure and inserting conducting layers at
appropriate intervals
...

Oil-impregnated bushings are also made in the similarly winding raw paper, inserting
equipotential layers in the appropriate positions, after vacuum drying, the winding is
impregnated with oil
...
i
...
r
...
p may be enclosed in porcelain and glass for outdoor applications
...


Composite bushing:
The bushing which has two or
more coaxial layers of different
insulating materials
...

Composite bushings have been
designed for application on oil filled
transformers and reactors
...
The
condenser consists of an electrical grade paper wound over a central winding tube
...
The condenser core is then heat and vacuum dried and subsequently
impregnated with an electrical grade epoxy
...

Compound-Filled bushing:
A bushing in which the space between the major insulation (or conductor where the major
insulation is used) and the inside surface of the protective weather casing (usually porcelain) is
filled with a compound having better insulating properties
...
CLASSIFICATION BASED ON MATERIALS
Synthetic resin-bonded paper (s
...
b
...
So a laminated construction is obviously very suited to it
...
In s
...
b
...

In practice, the s
...
b
...
Thus
s
...
b
...


Oil impregnated paper bushing:
These are made by the similarly winding raw paper, inserting equipotential layers in
appropriate position
...
For bushings
the winding may be spirally wound with paper tape thus allowing longer bushing to be made
than with s
...
b
...

Dielectric losses are less than s
...
b
...
i
...

Oil-Filled Bushing:
A bushing in which the space between the major insulation (or the conductor where no
major insulation is used) and the inside surface of a protective weather casing (usually porcelain)
is filled with insulating oil
...
In the manufacturing process, creped paper tape or sheet is wound onto a conductor
...
The raw paper insulator is dried in an autoclave under a strictly controlled
heat and vacuum process
...
As a 525 kV bushing
may have a core greater than 6 m in length, it is important that the resin has low viscosity and
long pot life to ensure total impregnation
...
The resulting insulation is dry, gas tight and void
free, giving a bushing with low dielectric losses and good partial discharge performance
...
The spacing between
individual layers varies between the peaks and troughs of the creping
...
RIP bushings are designed to operate with a radial
stress of about 36 kV/cm
...


Solid bushing:
A bushing in which the major insulation is provided by ceramic or analogous material
...

Alternator requires bushing upto 33KV
...
r
...
p Duresca and Copar finds the recent application
for hydrogen cooled alternators
...

Depends upon the current rating different types of bushings are available, namely
1
...
Gas cooling upto 31KA
3
...

Transformer bushings:
Bushings are required on the primary and secondary side of the
transformer one side contact with air and the other side with oil
...

Transformer requires terminal bushing consists one porcelain
shell with or without oil filling are used for voltages up to 66kv
...

A bushing called as re-entrant bushing, which by its design,
requires less clearance to the turret wall, then the conventional bushings
...
It is used with synthetic resin
bound paper of voltage rating, conventional bushings are preferred
...

For high current application at HAEFELY: 36KV, 25KA
...
At higher voltages synthetic resin bound paper bushing with
porcelain protection at the outer end and sometimes at the lower end for protection against
carbon contamination of the oil, oil impregnated paper bushing are used
...


In recent years switchgear design have been developed which has cast-resin imulation
instead of porcelain or s
...
b
...
this material allows more complicated shapes of bushings, which
can be used with advantage to simplify the construction of switchgear
...
Duresca is a material made from crape paper
tape impregnated with epoxy resin and incorporating metallic grading layers
...

Copar is bostically a mineral loaded cast epoxy resin with its condenser grading built uip
on a corrugated paper skeleton
...

Wall and Roof bushings:
Pollution is the major problem when working at outdoors one
effective way of overcoming this problem is greasing the
porcelain insulators but this is very costly business
...

building such as this need wall bushings which at 132 KV
invariably have an s
...
b
...
Wall bushings are usually
filled with current transformers so that there is no need for
separate post type current transformer
...

Cable end and Joint bushing:
A variety of bushings are used in cable end boxes
from 3
...

s
...
b
...


DESIGN OF BUSHING:
A – Air end clearance
B – Oil end clearance
Let, Vr – Rated voltage (rms)
Vp
...
f
1
...
f = 2
...
0 kV/cm (rms)
VLI = 4
...
5 kV/cm (pk)
2
...
f = 8 - 10 kV/cm (rms)
VLI = 15 - 20 kV/cm (pk)
Size of oil end decreases 3 times as that of air end
...
Creepage distance:
(16 – 32) mm/kV of Vr
For clean surface = 16 mm/kV (rms)
For polluted surface = 32 mm/kV (rms)
4
...
5 kV/mm at Vr in air
2
...
5 – 5
...
r
...
p
E = 35 kV/cm (pk) ------ for o
...
p
5
...
,
a
...

c
...


Current rating
Mechanical strength required
Thermal effect
Voltage (sometimes)

6
...
r
...
p
...
Number of foils or floating electrode:
It depends on one minute power frequency voltage and kV rating of the bushing
...
25 – 1
...

For capacitance bushing, n = (Vp
...
Length of the earth layer:
First foil length (l1) = 80% of air end clearance

 = l1/ln
 = 3
...
29

for minimum external diameter

 = 4
...
44

for minimum total volume

3
...


Let ‘Ue’ be the AC inception of partial discharge at the edges of the conduction layer or
floating electrode
...
,

Ue = K (S/r)0
...

Partial discharge test, Pollution test etc
...


ANSOFT RESULTS:
1
...
With floating electrode:

Equipotential lines

E (V/m)

E-Plot

Distance (mm)

DESIGN OF BUSHINGS – Non-Condensor and Condensor type :
Design a bushing for 72 kV, 800 A and p
...

I
...
Air end clearance A :
Vp
...
2 – 3
...
2 – 140/3)
= (63
...
67) cm
VLI = 4
...
5 kV/cm (pk)
= (325/4
...
5)
= (67
...
09) cm
A = Max [63
...
67, 67
...
09] = 67
...
Oil end clearance B :
Vp
...
5 – 14) cm
VLI = 15 - 20 kV/cm (pk)
= (325/15 – 325/20)
= (21
...
25) cm
B = Max [17
...
67, 16
...
67 cm
3
...
567 mm
r0 = 11
...
Creepage distance :
(16 – 32) mm/kV of Vr
Creepage distance for clean surface = 16 * 72 = 1152 mm
Creepage distance for polluted surface = 32 * 72 = 2304 mm
Given that
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5
...
r
...
p, E = 20 kV/cm (pk)
For o
...
p, E = 35 kV/cm (pk)
In this case, the maximum value has been taken, i
...
E = 35 kV/cm
6
...
128 * ln (rn/1
...
87 cm
i
...

rn = 148
...
Length of the earth layer :
For minimum insulation thickness,  = l1/ln = 3
...
8/ln = 3
...
8/3
...
7*0
...
15
ln = 17
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Condensor Bushing:
1
...
f *
n = (140 *

) / (12 kV)
) / 12

= 16
...

Thus, 17 intermediate floating electrodes are required
...
Length of the earth layer or first layer :
First foil length (l1) = 80% of air end clearance
= 0
...
8*67
...
16 cm
3
...
44
4
...
44*1
...
1 mm
5
...
1-11
...
28 mm
δ = 0
...
128 + 0
...
356 cm
l1 = 54
...
356 + 0
...
584 cm
For radial grading,

l1r1 = l2r2 = l3r3 = ……
...
16*1
...
584
l2 = 46
...


r0 = 1
...
370 cm

r1 = 1
...
160 cm

r2 = 1
...
364 cm

r3 = 1
...
530 cm

r4 = 2
...
000 cm

r5 = 2
...
381 cm

r6 = 2
...
423 cm

r7 = 2
...
960 cm

r8 = 2
...
878 cm

r9 = 3
...
094 cm

r10 = 3
...
549 cm

r11 = 3
...
198 cm

r12 = 3
...
006 cm

r13 = 4
...
947 cm

r14 = 4
...
000 cm

r15 = 4
...
148 cm

r16 = 4
...
377 cm

r17 = 5
...
676 cm

Comparison of results:
Parameters

Non-Condensor bushing

Condensor bushing

Air end clearance ‘A’

677 mm

677 mm

Oil end clearance ‘B’

216
...
7 mm

Radius of the conductor

11
...
28 mm

Outer radius of bushing

148
...
1 mm

Inception voltage Ue :

Ue = K (S/r)0
...
1 for mineral oil
S = 0
...
5
Inception voltage Ue = 20*(0
...
1)
= 6
...

Length of the conductor l0 = (A+B) = 893
...
6 mm
Mean length (l01) = (l0 + l1)/2 = (893
...
6)/2 = 717
...
854*10-12)*2
...
718] / ln (1
...
128)
C0 = 0
...
6 + 463
...
62 mm
C1 = 2π0rl / ln (r2/r1)

= [2π*(8
...
1*0
...
584/1
...
378 nF
(l23) = (l2 + l3)/2 = (463
...
3)/2 = 434
...
854*10-12)*2
...
4345] / ln (1
...
584)
C2 = 0
...
77 + 146
...
265 mm
C17 = 2π0rl / ln (r16/r17)
= [2π*(8
...
1*0
...
004/4
...
377 nF
Hence, we prove that the capacitance between the foils is almost equal and thus the
voltage distribution will be uniform
...

Between l1 and l2: S = (l1 – l2)/2
S = (54
...
364)/2
S = 3
...
5
Inception voltage, Ue = 20 (3
...
1) = 27
...
103 kV
V2 =

=
= 55
...
103 - 55
...
513 kV
ii
...
377 – 14
...
3505 cm
0
...
3505/2
...
171 kV
Voltage drop across foils l16 and l17,
V16 =

=
= 2
...
254 - 0
= 2
...
Hence the design value is acceptable

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