Internally protected capacitor units

This is applicable to both LT and HT capacitors. But it is more important in HT banks, which are relatively much larger and are built of a number of single units eonnected in series-parallel. These may encounter much higher fault currents in the event of a severe internal fault, even in one unit and are thus rendered more vulnerable to such ruptures. This phenomenon is more applicable to units that are externally protected w here (he intensity of fault may be more severe, than internally protected units. 

Protection with internal fuses is easier, as fuses are provided for each element which can contain the severity of the fault well within the safe zone in all probability. Some users even recommend capacitor units 250/300 kVAr and above with internal fuses only. Figure 26.1 shows a typical operating band of (he internal fuses for an internally protected unit. It demonstrates a sufficient margin between the operation of (he fuses and (he shell s safe zone. The fuse characteristics are almost the same for all manufacturers. 

A critical review of internally protected capacitor units 25/812... 

For loads, such as on motors where the capacitor unit is being switched with the machine, or where close monitoring of the capacitor health is a prerequisite to avoid an eventual outage of the capacitor unit by gradual depletion of its capacitance, internally protected capacitor units may be preferred. 

Mtiking a ctipacitor element A critictil review of internally protected capacitor units Self-healing capacitors Making a capacitor unit from elements Making capacitoi banks from eapticitor units Rating... 

Shell rupture protection is a vital consideration in externally protected capacitor units. Since there is no control over small internal faults until they become major fault, protection can be provided only for the whole unit and the entire unit has to be dismantled after such a fault. In fact, the capacitor bank may have lo be shut down completely to replace the lost unit with a new one lo avoid an imbalance, besides making up for the lost capacitance. 

Generally, the fault must be cleared well within Zone I and for which the protective scheme must be chosen. As discussed in Section 25.4.2, protection of capacitor units with external fuses is not easy. It is not practical to contain a mild internal fault as isolation of the units is not possible on mild internal faults until the fault current rises to the level of the fuse s operating range (Figure 26.2 illustrates this). By then enough time will have elapsed to cause severe damage to the unit. 

---