A typical industrial converter basically consists of an AC/DC section (to convert the AC voltage of the grid into a DC voltage) and a DC/AC section either at variable frequency (motor drive) or fixed frequency (generators or UPS).
These two parts are connected through a DC bus (link circuit) where capacitors are required in order to filter the high frequency components (DC Link Capacitors).
Most important requirements for these capacitors are:
- capability to withstand high currents at frequencies above 1000 Hz;
- high energy density (Joule/dm3).
Electrolytic Capacitors banks are used up to a voltage of 2000V, but their limits are:
- maximum working voltage across each capacitor limited to about 450÷500V;
- maximum current, especially at high frequency, limited by the high ESR (Equivalent Series Resistance) typical of this technology.
For these reasons, in general, Electrolytic Capacitors have to be connected in series/parallel to form banks able to withstand the voltages and the currents required by the application.
Polypropylene film capacitors are able to overcome these limits and in most cases they are able to replace favourably electrolytic capacitors in applications where the voltage is above 500Vdc.
Main advantages of Metallized Film Capacitor are:
- high current per capacitance (A/μF);
- high voltage per element;
- high capability to withstand over voltages up to 2 times the rated voltage;
- more than 10 years estimated lifetime in the temperature range -25/+70 °C;
- easy connections and low equivalent inductance;
- non polar dielectric;
- no leakage of dangerous or poisonous electrolytes.