1. Reducing ratio of shunt capacitance to self capacitance
2. by grading the insulators
3. use of guard ring to provide static shielding.
1. Reducing Ratio of Shunt Capacitance to Self Capacitance
The voltage across the
line unit depends on the value of k which is the ratio of shunt capacitance to
self capacitance. The string efficiency us dependent on the voltage across the
line unit. Lesser the value of k, higher is the string efficiency and more
uniform is the potential distribution. The voltages across the various units of
string are almost equal for very low values of k.
Now k = C1 /C i.e. the
ratio of shunt capacitance to self capacitance. So to reduce the value of k, C1 must be reduced. This is possible by increasing the distance between the
insulator and the earth i.e. tower. This can be achieved by increasing the
length of cross-arms as shown in the Fig.1. More the length d of the cross-arm,
less is the value of C1 and less is the value of k.
But this method has
practical limitations such as :
1. Use of long cross-arm
increase the cost.
2. Due to long cross-arm
overall strength of the tower reduces.
Hence the practice, the
minimum value k which can be achieved by this method is 0.1.
And due to this
limitations, this method is rarely used in practice.
2. Grading the
insulators
By correct grading of
the insulators, more uniform voltage distribution across the string can be
achieved. In the method of grading, the insulators are so selected that the
self capacitances i.e. mutual capacitance of the various units are different and
the values of mutual capacitances decrease from the line unit. So top unit has
minimum mutual capacitance while the line unit has maximum mutual capacitance.
The voltage for the given current across the capacitance is inversely
proportional to the capacitance. So more the capacitance, lesser is the voltage
across the capacitance.
Thus keeping line unit
capacitance to be maximum, current through it is minimum. This reduces the
voltage across the line unit.
So by probably grading
the insulators i.e. by using different sized insulators in a string, uniform
voltage distribution can be achieved and string efficiency can be improved. The
grading is shown in the Fig. 2.
The design of such
string using different sized insulators is practically complicated and
inconvenient. Hence the method is used only for very high voltage system such
as 200 Kv and above. Though the design of such string is inconvenient, using
standard insulators for most of the units and larger units adjacent to the line
conductor, better results can be obtained.
3. Use of Guard Ring
In this method a large
metal ring surrounding the line unit and connected to the metal part of the
bottom of the line unit is used. Such a ring called ''guard ring''. The guard
ring is shown in the Fig.3. This is also called static shielding of the string.
Earlier it has been
mentioned that capacitance between insulator and the line is neglected. But use
of guard ring increases the capacitance between the metal part of the insulator
and the line.
These capacitance are
shown as C'2,C'3 in the Fig.3. These capacitances are greater for the lower units.
Due to this, voltage across these units is reduced. An equal distribution of
voltage is not possible by this method.
But the guard ring used
can be designed in such a way that shunt capacitance current I1, I2, I3 etc. are equal
to the currents through newly introduced capacitors i.e. i1, i2, i3 etc. as shown
in the Fig.3. Due to this, charging current through the mutual capacitors
remains same, giving uniform voltage distribution. But such a design is
practically difficult.
Thus the primary aim of
the guard ring is to reduce the electrical stress on the lower units.
