Capacitor Peak Voltage
The voltage that appears on a capacitor when used in a harmonic filter is non-sinusoidal. Due to significant harmonic voltage drops across the series connected filter reactor, significant harmonic voltage may appear at the capacitor. In some cases, the true rms voltage may not seem much different than the bus voltage plus the boosting effect of the reactor, but the crest factor of the voltage can be significantly higher than the normal 1.414. In the worst cases, harmonic voltage at various harmonic frequencies can be approximated as the arithmetic sum of all peak voltages at each harmonic frequency. This cannot be revealed if one considers only the rms voltage. Capacitors must be capable of handling the peak voltage as well as the rms voltage.
Capacitor High Altitude Capability
Although altitude (elevation above sea level) may only be a consideration in a small percentage of capacitor and harmonic filter applications, it is an indication of the internal relative temperature rise of a capacitor. Most capacitors are suitable for use at elevations up to 1000 meters. Above this, one would typically derate by using higher voltage rated capacitors so the capacitor will operate with reduced internal temperature rise. Capacitors suitable for use at higher elevations will generally be those with lower internal temperature rise under normal operating conditions.
FRAKO Type DD60 capacitors are designed for use at up to 4000 meters (13,124 feet) above sea level.
Capacitor Ambient Air Temperature
People normally think of ambient temperature as room air temperature in the area where electrical equipment will be installed. When it comes to capacitors however, it is the temperature of the air surrounding the capacitor itself, inside the enclosure, that really matters. The internal temperature of an enclosure can be 10°C to 20°C hotter than the room air temperature. Heat is the destroyer of electrical and electronic equipment. Capacitor systems often involve reactors, contactors, control transformers, fuses plus power and control circuit wiring. Each of these contributes to the internal temperature rise of the enclosure and thus to temperature of the air surrounding the capacitors. The location of the capacitors also has a lot to do with the temperature of the surrounding air. It makes a difference whether capacitor are installed above or below major heat producers (ie: reactors), or where they are located in the stream of air flow.
To complicate matters, capacitor temperature ratings can be misleading. For example, some capacitors are rated – 40/D, 55°C max. What does this mean? If you think this capacitor is rated for use at 55C you would be wrong. The highest mean over any 24 hour period should not exceed 45C, and the highest mean over the course of one year shall not exceed 35°C. For a short term the temperature may reach 55°C, but the requirement is only one time for up to 48 hours with sinusoidal voltage applied. It is much better to use a capacitor with a continuous ambient temperature rating that is higher than the maximum ambient temperature in the enclosure. If a capacitor is operated in an ambient temperature that is 10°C above its rating, its life will be reduced by at least 50%.
FRAKO Type DD60 capacitors handle 60°C ambient continuously.