A Suspension Insulator is
basically a string of number of porcelain disc connected in series though
metallic link. It shall be noted that the number of discs in Suspension Insulator
can be increased or decreased by adding extra discs or removing a disc. Figure-a
below shows a typical Suspension Insulator.

As discussed in above paragraph,
porcelain disc remain in between the metallic links, therefore each disc acts
as a Capacitor. Therefore, if we draw the equivalent circuit for a suspension
insulator, it will be as shown in figure-b.

Thus when such type of
insulator is connected to hold a power conductor carrying electrical power at
high voltage, a charging current will flow though the series connected
Capacitors. As the charging current through each of the Capacitor (Porcelain
disc) is same, therefore the potential distribution across each Porcelain disc
will be same i.e. each Porcelain disc will have equal voltage stress. Let us
assume that the voltage at which power conductor carrying electrical power be
V, then for suspension insulator, potential across each Capacitor i.e.
Porcelain disc will be V/3.

But actually things do not
work as we think normally. Here also in case of Suspension Insulator, the
potential gradient across each disc is not same rather the disc nearer to the
power conductor will face highest voltage stress while the disc nearer to the tower
body will face the lowest voltage stress.

**Why?**

Actually, in addition to
self capacitance of porcelain disc there also exists Capacitance in between the
metallic link of suspension insulator and grounded tower body. This capacitance
is known as Shunt Capacitance. Now due to this Shunt Capacitance, the charging
current through each porcelain disc will no longer be same rather it will
decrease as we move from the disc nearer to the power conductor to the disc farthest
from the power conductor as shown in figure below.

As can be seen from the
figure above,

**I**

_{3 }> I_{2}> I_{1}

_{}
Therefore,

**V**_{3}> V_{2}> V_{1}_{}

_{}
Though each disc is designed
to withstand same voltage stress, say 11 kV, but the disc nearer to the power
conductor is much stressed, say 16 kV, while the disc farther from the
conductor is less stressed. Thus proper utilization of disc is not achieved due
to shunt capacitance. Some discs are underutilized while disc nearer to
conductor is over utilized which may lead to damage of the disc. Thus a term
called String Efficiency originates from this philosophy.

**String Efficiency of Suspension Insulator String:**

String Efficiency shows the
utilization of suspension insulator. More the utilization of discs of insulator
more will be the string efficiency.

String Efficiency is defined
as the ration of conductor voltage and the voltage across the disc nearest to
conductor multiplied by number of discs.

Thus,

**String Efficiency = Conductor Voltage / (nxvoltage across the disc nearest to conductor)**

n = Number of discs in
suspension insulator.

Here comes the Grading Ring
or Guard Ring. Grading Ring or guard Ring equalizes the potential distribution
across each disc in Suspension Insulator. Thus if the voltage across each disc
becomes equal then String Efficiency will be 100%.

Shall we calculate String
Efficiency and voltage distribution in string insulator? No….But still I will
show you the calculation, please give me your extra 10 minutes.

**Calculation of Voltage Distribution:**

C

_{1}= Shunt Capacitance
C = Self Capacitance

Let us assume that C

_{1}= KC, where K is some constant.
As obvious from the figure
above,

I

_{2 }= I_{1}+i_{1}_{}

⇒V

_{2}wC = V_{1}wC + V_{1}wC_{1}_{}

⇒
V

_{2}wC = V_{1}wC + V_{1}wKC
⇒

**V**_{2}= V_{1}(1+K)**………………….(1)**

Similarly,

I

_{3}= I_{2}+ i_{2}_{}

⇒
V

_{3}wC = V_{2}wC + (V_{1}+ V_{2})wC_{1}_{}

⇒
V

_{3}wC = V_{2}wC + (V_{1}+ V_{2})wKC
⇒
V

_{3}= V_{2}+ (V_{1}+ V_{2}) K
Putting the value of V

_{2}from equation (1),
= V

_{2}+ (V_{1}+ V_{2})K
= KV

_{1 }+ V_{2}(1 + K)
= KV

_{1}+ V_{1}(1+K)^{2}………….[from equation (1)]
= V

_{1}[1+3K + K^{2}]
Thus,

**V**

_{3}= V_{1}[1+3K + K^{2}] ……………..(2)

Now,

Voltage between conductor
and earth V,

= V

_{1}+ V_{2}+ V_{3}_{}

Putting the value of V2 and
V3 from (1) and (2),

= V

_{1}[3 + 4K + K^{2}]
= V

_{1}(1+K)(1+3K)
So,

**V = V**

_{1}(1+K)(1+3K) ………………….(3)

Therefore,

V

_{1}= V / (1+K)(1+3K)
V

_{2}= V_{1}(1+K)
V

_{3}= V_{1}(1+3K+K^{2})
Also,

**String Efficiency = (Vx100) / (3xV3) %**

From the above expression of
V1, V2 and V3, we observe that more the value of K, more non uniform is the
potential distribution across the discs and hence less is the efficiency of
string.

Also, as the number of discs
increases in the suspension insulator, the inequality in voltage distribution
increases. Thus shorter string has more efficiency than larger string.

## 5 comments:

What is w in VwC

w is omega i.e. angular frequency.

How to find out the number of insulator disc in suspension type string ????

Mitesh Kavar, generally one disc can withstand a voltage stress of 11 kV. So the number of disc can be roughly calculated by dividing the phase voltage level by 11.

I have seen 11 or 10 discs on 132 kv side. What is the reason?

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