Capacitors self-healing type

Shunt power capacitors of the self healing type for a.c. systems having rated voltage up to and including 1000 V 13340/1993 BS EN 608.31 -1/1998... 

Shunt power capacitors of non self-healing types up to and including 1000 V General performance, testing and rating. Safety requirements. Guide for installation and operation 13585-1/1994 BS EN 60931-1/1996... 

The metallized film capacitors have the characteristic of self-healing. On a small dielectric failure the capacitor element is not rendered completely unserviceable. After clearing the fault, the affected capacitor element returns to the circuit and the capacitor unit functions normally. Only the punctured area is eliminated from the element and causes a negligibly small reduction in its capacitance value. Such a characteristic is termed self-healing and such capacitors are known as the self-healing type. 

For LT capacitor units up to 1000 V For both self-healing and non-self-healing types ... 

Shunt capacitors for a.c. power system rated voltage above 1000 V (non-self-healing type) Endurance testing 13925-2/1994 BS 7264-2/1990... 

Traditionally, off-line X-caps were of special metallized film + paper construction whereas Y-caps were a specially constructed disc ceramic type. However we can also find X-caps that are ceramic, as we can find Y-caps that are film type. It s a choice dictated by cost, performance, and stability concerns. Film capacitors are known to always provide much better stability over temperature, voltage, time, and so on — than most ceramics. In addition, if they are of metallized construction, they also possess self-healing properties. Note that ceramic capacitors do not have any inherent self-healing property. However, ceramic Y-caps are specifically designed never to fail shorted under any condition, as this would pose a serious safety hazard. 

Second type of self-healing process came about in tantalum capacitors with polypyrrole cathode (Harada 1997). This material thermally decomposes at about 300°C. When increased current is flowing through the defect region, heat is generated causing the decomposition of polypyrrole, which becomes less-conductive and finally to be insulating. 

Panasonic offers both polyester and metallised polyester film capacitors. The non-inductive, high stability, epoxy coated polyester types are also claimed to offer high volumetric efficiency and low losses. Additionally the metallised polyester film types are claimed to be self-healing. Their structure is composed of a number of discrete units to confine and isolate any dielectric breakdown. Panasonic also supplies miniaturised versions whose height and length dimensions are less than 8 mm with a corresponding thickness less than 5 mm. 

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