A string insulator contains multiple insulator discs (usually porcelain discs) connected in series through metallic links. Thus, each insulator disc lies between two metal links, forming a capacitor. Capacitance provided by these discs is called as mutual capacitance. If mutual capacitances due to these discs were acting alone, then charging current would have been same through all the discs and consequently voltage across each disc would have been the same. However, capacitance also exists between each metallic link and the support tower also (air dielectric between metal links and metal tower). This capacitance is called as shunt capacitance. As these shunt capacitances also take charging current, it will get divided at each link and will not be the same through all the discs. Therefore, voltage across each disc will be different. Voltage across the disc nearest to the conductor will be maximum, while that across the farthest disc will be minimum.
Thus, voltage distribution across the discs of a string insulator is not uniform. Voltage across the disc nearest to conductor will be much higher than all the other discs in the same string. This unequal potential distribution is undesirable and usually expressed in terms of string efficiency.
String efficiency = (voltage across the string) / (n * voltage across the disc nearest to the conductor)
……. n = number of discs in the string insulator.
If the string efficiency is lower, the voltage distribution across each disc will be much unequal. In that case, the disc nearest to the conductor will be highest stressed and may breakdown or cause a flash-over and the other discs will also breakdown in succession.
Why string efficiency for a DC system is 100%?
As we know, capacitive reactance Xc = 1 / (2*pi*f*C)
Frequency of DC current is zero, i.e. f=0
Hence, capacitive reactance for DC is infinite.
So, for a DC system, once the mutual and shunt capacitances of insulator strings are charged, it won’t take charging current at all anymore. Hence, voltage will be uniformly distributed across each disc. That is why string efficiency for a DC system is 100%.