PART IV - Power Capacitors and Reactive Power Controls
Chapter 25. Making capacitor units and ratings of switching devices
• Making a capacitor element
• Special purpose capacitors
• A critical review of internally protected capacitor units
• Self-healing capacitors
• Making a capacitor unit from elements
• Making capacitor banks from capacitor units
• Rating and selection of components for capacitor duty
• Fast discharge devices
Making a capacitor element
A capacitor unit is made up of a number of capacitor elements stacked together in combination of series and parallel configurations to add up to the required voltage rating and VAr size. A capacitor element can be formed with any of the available dielectric materials, as noted below, depending upon the voltage rating of the capacitor unit, the likely voltage variations, harmonic contents in the supply system, ambient temperature and switching operations.
Self-healing (SH) feature
An electric breakdown is possible as a result of thermal or electrical over-load or at the end of useful life of capacitor unit. This results in a small arc that causes the evaporation of the metallized layer in the region of breakdown within few microseconds. The gas pressure generated at the spot associated with high temperature blows the vapours out of the breakdown region creating a non-conducting isolated region that is highly resistive. During and after the break down capacitor is fully functional. Reduction in capacitance due to self-healing is less than 100 pF (p = 10–12).