Life of Capacitors

 

The life of capacitor is affected by

• Temperature
• Voltage
• Current

Temperature

The temperature depends upon following parameters

( 1 ) Ambient temperature at which capacitor is operated

( 2 ) Over current flows through capacitor

( 3 ) Dielectric and resistive power loss in the capacitor

• As the temperature increases, there is fast degradation of the dielectric. The life of capacitor is reduced to half for every 10 degree increases in the temperature. 
• The life of capacitor reduces for faster degradation of the dielectric. There is expansion of impregnation and dielectric material if the temperature increases beyond its limit. 
• The capacitor may be burnt in the worst condition. The classification of capacitor is done according to ambient maximum temperature operating condition.

Symbol	Maximum ambient temperature
A	40
B	45
C	50
D	55

 Voltage

• If the voltage increases beyond its specific limit, the dielectric of capacitor may breakdown. 
• This will result in capacitor may short circuit permanently.

Current flow through capacitor

• If the voltage increases beyond its specific limit, the capacitor current increases because the X_C of the capacitor remains same. 
• This will reduce the life of capacitor.
• Voltage limit for capacitor

+ 10%	8 hours in 24 hours
+15%	30 min in 24 hours
+20%	5 min in 24 hours
+30%	1 min in 24 hours

 Current

( 1 ) Over load current

• The over load current of capacitor is due to over voltage and harmonics. This may lead to local hot spot and internal short circuit of the capacitor.

( 2 ) Inrush current

• When capacitor is switched on, a very high inrush current flows which may be 100 times rated current of capacitor. 
• If the inrush current is not limited during frequent switching operation, it may leads to life of capacitor.