Practical capacitors are available commercially in many different forms. The type of internal dielectric, the structure of the plates and the device packaging all strongly affect the characteristics of the capacitor, and its applications. Values available range from very low (picofarad range; while arbitrarily low values are in principle possible, stray (parasitic) capacitance in any circuit is th. Fuses are used on power systems up to 115,000 volts AC. High-voltage fuses are used to protect instrument used for electricity metering, or for small power where the expense of a is not warranted. A circuit breaker at 115 kV may cost up to five times as much as a set of power fuses, so the resulting saving can be tens of thousands of dollars.
The fuse, by its design, avoids absorbing all of the available energy on the series group. This fuse is used for capacitor banks with a large number of parallel capacitors. It can be used on applications with essentially infinite parallel stored energy, as long as sufficient back voltage can be developed to force the current to extinguish.
Most capacitor fuses have a maximum power frequency fault current that they can interrupt. These currents may be different for inductive and capacitively limited faults. For ungrounded or multi-series group banks, the faults are capacitive limited.
The capacitor must be able to absorb this energy with a low probability of case rupture. Fuses are usually applied with some continuous current margin. The margin is typically in the range of 1.3 to 1.65 per unit. This margin is called the fusing factor.
For high voltage capacitor fuses, this is generally defined as 8.3, 15.5 or 23 kV, the distribution system maximum voltages. Other voltage ratings may be available for special applications. When a capacitor fails, the energy stored in its series group of capacitors is available to dump into the combination of the failed capacitor and fuse.
Over the years, a set of terms has been developed to apply capacitor fuses. The concept of applying fuses should be a simple engineering task; however, fuse operation is a non-linear function. The resistance of fuse elements changes non-linearly as they melt and clear.
What are the different types of power capacitor units?
There are two primary classifications of power capacitor units: Internally fused units consist of elements that are each protected by a series connected fuse inside the capacitor enclosure. As an element fails, the internal fuse protecting that element clears.