The fuse protecting the capacitor is chosen such that its continuous current capability is equal to or greater than 135% of rated capacitor current for grounded-wye connected racks, and 125% for ungrounded-wye racks. This overrating includes the effects of overvoltage, capacitor tolerance, and harmonics.
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.
Since internal fuses are hidden from view and most units contain at least 20 but can have as many as 100 elements, detecting one or two failed elements in a large internally fused capacitor bank requires very sensitive unbalance relaying equipment.
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.
Either of these two effects can impede the proper operation of the fuse. In the event of a capacitor fault, excess current will flow through the fuse of the faulted unit. This current causes the fuse element to melt and vaporize. An arc will form across the vaporized section within the fuse tube.
The available energy is calculated by assuming that the parallel capacitance is charged to 1.1 times the crest of the AC rated voltage (kV AC) (j = 1⁄2 CV2). For shunt capacitor applications, the energy is equal to 3.19 joules per kVar. The available energy is then compared to the rating of the fuse and capacitor unit.
Fuse rating for Capacitor Circuits. For the capacitor circuit, the fuse rating is equal to 1.5 times of the full load reactive current by considering staring inrush current, circuit harmonics and capacitor tolerances. The formula will be. Capacitor …