POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
Chapter four
Over head insulator
The overhead line conductors should be supported on the poles or towers
in such a way that currents from conductors do not flow to earth through
supports i.e., line conductors must be properly insulated from supports.
This is achieved by securing line conductors to supports with the help of
insulators.
The insulators provide necessary insulation between line conductors and
supports and thus prevent any leakage current from conductors to earth.
*The function of the insulators are :.
1- Insulate the conductors from each other and from the towers under
highest voltage and under bad air estimate circumstance
2-Carry the conductors under the bad estimate mechanical stresses
*The materials that used for made the insulators of overhead line are
1- porcelain
2-Tougheneed glass.
Although the two above materials are brittle and inelastic but it are the
best materials that used to insulate the conductors.
Types of insulator:
1-Pin type insulators
Pin type insulators are used for transmission and distribution of electric
power at voltages upto 33 kV. Beyond operating voltage of 33 kV, the
pin type insulators become too bulky and hence uneconomical.
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
2-Suspension- type insulator.
The cost of pin type insulator increases rapidly as the working voltage is
increased. Therefore, this type of insulator is not economical beyond 33
kV. For high voltages (>33 kV), it is a usual practice to use suspension
type insulators consist of a number of porcelain discs connected in series
by metal links in the form of a string. The conductor is suspended at the
bottom end of this string while the other end of the string is secured to the
cross-arm of the tower. Each unit or disc is designed for low voltage, say
11 kV. The number of discs in series would obviously depend upon the
working voltage. For instance, if the working volt-age is 66 kV, then six
discs in series will be provided on the string.
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
3. Strain insulators.
When there is a dead end of the line or there is corner or sharp curve, the
line is subjected to greater tension. In order to relieve the line of
excessive tension, strain insulators are used. For low voltage lines (< 11
kV), shackle insulators are used as strain insulators. However, for high
voltage transmission lines, strain insulator consists of an assembly of
suspension insulators as shown in Fig. 8.8. The discs of strain insulators
are used in the vertical plane. When the tension in lines is exceedingly
high, as at long river spans, two or more strings are used in parallel.
4. Shackle insulators.
In early days, the shackle insulators were used as strain insulators. But
now a days, they are frequently used for low voltage distribution lines.
Such insulators can be used either in a horizontal position or in a vertical
position. They can be directly fixed to the pole with a bolt or to the cross
arm. The conductor in the groove is fixed with a soft binding wire.
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
*The eclectic behavior of each Insulator is as like a capacitance
Appeared in the string
1- C ( self capacitance )
Between the cap of insulator and the pin in the bottom of insulator and its
value of about 30 pf
2- Ce ( air capacitance )
Where Ce is another capacitance between the tower and the connection
point between the pin of each insulator and the cap of the next insulator
and it is have a value of about ( K where k (0.1
The insulator make the distribution of voltage through the insulator of the
string is different (where the insulator that nearest to the conductor has a
higher voltage then the next and up)
*There is another capacitance ( air capacitance ) between the conductor
and the connection point between the pin of each insulator and the cap of
the next insulator and is very little and it can be neglected.
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(1)
At point B
(2)
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
At point C
(3)
At point D
(4)
If we have n insulator
(5)
(6)
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
Where V is the voltage of line (string voltage ƪ is the efficiency of the
string.
Also there is another method (general method ) to solve this problem,
where
(7)
(8)
Where X is the number of insulator that you need to get the voltage across
it.
*Methods of increasing the string efficiency
The string efficiency increase when there is a uniformly distribution
voltage across the insulator, that means when if V (phase votage
of the conductor)
Is equal to n ( n : number of insulator and is the voltage of the last
insulator )
We can get ƪ = 100 % ( maximum efficiency )
The voltage across the capacitors ( insulator) are equally ( uniformly). But
this case is a theoretical case because of the air capacitance (ce) that make
the voltage across the insulator are not uniformly. Also the string
efficiency decrease if the number of insulator (n) increase because of the
not equally .
Distribution voltage across the insulators. Many method are used to
increase the string efficiency such as :.
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
1-Reducing the value of k :.
When
We can reduce the value of k by reducing the value of by increasing
the length of cross- arm for the tower but this method leads to reduce
rigidity for the cross-arm and increase the cost
2-Grading of units :.
This method depend on the change
Of the self capacitance where
The capacitance that near to
The conductor is greater than
The others due to the location
Of insulator in the string and
The capacitance (self capacitance)
That near to the cross-arm is
Less than the others
If we have four insulator (n =4) and the self capacitance that nearest to
the cross- arm is C then we make a grading for self capacitance in order to
get an equal voltage across the insulators (maximum efficiency)
(
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
At point A
(9)
At point B
(10)
At point C
Ƶ= y+3kc
Ƶ= c(1+3k)+3kc
(11)
And so on
This method is difficult in practical application because of different value
of the capacitance .
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
3) Guard Ring :.
It is possible to make the voltage distribution across the units of string of
insulator uniformly by a guard ring which is a ring of conductor put or
placed across or around the nearest insulator to the conductor .
The main reason of different the Voltage across the insulator of The
string is the air capacitance (Ce) And so that the guarding reduce Ce by
adding a number of Air capacitance between the Conductor or guard ring
and Metal point of insulator and these air capacitances are in opposite by
the effect with Ce and make the voltage across the insulators more
uniformly .
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
Ex1) Over string of five insulator and the ratio of Ce/C=0.1. If the string
is connected to line of voltage 33kv.
1- What is the voltage distribution over the unit of string and what
will be the string efficiency.
2- When the string is supplied by a guard Ring and this lead to add a
two air capacitance of value 0.2c,0.1c respectively to the nearest to
the conductor. Find the new voltage distribution and the new
efficiency.
So1)
0.1c
A
0.1c cc
B
(1) 0.1c
C
c
0.1c
D
c
(2)
Conductor
(3)
(4)
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
*Also we can solve it by
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
2) After the guard Ring
c
0.1c
A
At point C
c
0.1c
B
0.1c c
C 0.1c
c
0.1c
D 0.2 c
c
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
At point D
1.299
Now
After guard Ring Before guard Ring
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Ex2) Each of the 4- insulator have after guard ring
C
0.6C
A cf
3C=5Ce
Calculator cd, cf, cg in 0.6C C cg
B
0.6C C cd
C
Conductor
So1) at point A
At point B
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
At point C
Ex3) over string of 3 insulators and the ratio of if the string is
connected to line voltage of 33kv. Also if the break down voltage
of each insulator is 7kv.
1- Find the voltage distribution over the unit of the string and what is
the string efficiency.
2- When the string is supplied by a guard ring and this load to add at air
capacitance of 0.15 c,0.15c respectively to the nearest to the
conductor . find the new voltage distribution and the new efficiency
So1)
= 19050 voltage 0.15c C
0.15c C V
*1
C
*2
conductor
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
We see that the break down voltage is 7000 volt
Then the case will be C
0.15C
V
C
conductor
(Break down voltage)
Then
The string failed to insulator the conductor from the tower
2) at point A
0.15C C 0.15C
C 0.15C
B
0.15C
C
At point B
conductor
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
This string successes in insulation the conductor
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
Ex4) Over string of 3- Transmission line has four insulator and
Ce=0.12C. if the voltage between the metol cap of the last insulator and
the earth is 33kv.
1- Find the voltage distribution across the insulators and what is the
string efficiency
2- If the string is supplied by a guard ring and this lead to add a two
Air capacitance of o.2C, 0.1 C respectively to the nearest to the
conductor. Find the new voltage distribution and the new string
efficiency
So1)
C
0.12c
C
0.12c
V
0.12c C
K= 0.12
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
KV
At point B
(1)
POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.
C
0.12C
A
0.12C C
B 0. 1C
0.12C C
0. 2C
C
At point C
Also as F before
