Surge Protection Capacitors
Surges and Protection against surges:
Electrical networks experience surges wherein a voltage or a current rises rapidly to unsafe values and destroys the dielectric insulation. These, along with partial discharges which these start, are blamed for the major portion of failures of electrical equipment of all types.
As per modern thinking, most of the surges are current – sourced as against the normal voltage sourced electric power supply. An amount of let off energy, determines this current which flows to ground – irrespective of the circuit resistance.
If a contact of a lighting conductor stripe is bad, it creates dangerous voltages – rather than reducing the current. This rapid rate of rise of current is responded by a magnetic circuit (of all types of transformers) with an equally rapidly rising flux, a back EMF and a very high induced voltage.
This voltage causes breakdowns, flash overs, partial discharges and so on. This surge has two or three parameters which lead to electrical break – down:
Rate of rise of current or voltage.
Energy contained within a surge which dictates.
The current flowing in a surge.
Any capacitor can not be charged to a full surge voltage instantly. It will take our indefinite amount of current to do so.
Thus, it takes time to get charged. This time slopes down the almost vertically advancing surge were – though not substantially.
Even a small reduction in di/dt reduces the magnetically induced voltages from an infinite value to a finite value and this is how surge capacitors help.
Surge capacitors by themselves are protective on small voltage spikes – with limited involved energies. They have to be supplemented with lighting arrestors which can ground large amounts of surge energies.
The surge capacitors are normally, single terminal, body grounded type. If these get connected across a system with ungrounded neutral, there is a possibility of the line terminal getting full line voltage – instead of a phase voltage, should one of the phases get shorted (under a surge).
Besides they are subjected to high rate of charging when they cater to surges. As such they are rated at the line voltage or slightly higher – even though normally they will operate at phase voltage.