Vasai East Vasai Thane 401208
Mon-Fri 08:00 AM - 05:00 PM
Vasai East Vasai Thane 401208
Mon-Fri 08:00 AM - 05:00 PM

Internal Construction of HT Capacitor Unit


In an external fuse design, one entire capacitor unit gets disconnected on failure of even one internal element. This prevents case rapture that can be caused due to carbonization and formation of gases that may occur due to failure of element inside the capacitor unit. This allows the rest of the capacitor bank to remain in operation though with a reduced capacitor rating.

External fuse unit Externally fused capacitor bank
Externally-Fused Capacitor Design

This design as shown in figure 3(f)), places many elements in series within the capacitor unit and each series group consists of a few elements in parallel. Hence one element failure leads to a cascading failure of elements leading to shorting of the entire unit. Externally fused capacitor banks have a perceived visual advantage since the failed unit is identified by the blown fuse. When a fuse operates in a capacitor bank, the bank may have to be taken out-of-service to replace the fuse otherwise other healthy capacitor units will experience overvoltage and overstressing.


Internal fuses shown in figure 3(g) are not current-limiting fuses but intended to isolate failed capacitor element in a capacitor unit and allow normal operation of the remaining healthy elements within the unit. An element failure and subsequent isolation removes only a small part of the capacitor unit and allows the capacitor unit and bank to remain in service. The design philosophy of internally-fused capacitor units places a large number of elements in parallel in each series group within a capacitor can. A failed element will trigger momentary discharge of healthy elements connected parallel thorough the failed element.. This causes a slight increase in the voltage across the parallel elements. But the capacitor element is designed to withstand this voltage increase and the unit can be left in service indefinitely. Instantaneous disconnection of a failed element prevents the unit from being exposed to sustained arcing, minimizing the risk of capacitor-can rupture.

Internally-fused type capacitor unit Internally fused type capacitor bank
Internally- Fused HT capacitor

Some of the advantages of internally-fused capacitors are:
 There is no need for fuses, fuse rail assemblies, or insulators and connection becomes easier.
 The output of the capacitor unit is not limited which results in a substantial reduction of the total number of capacitor units in a bank design.
 The bank design is very compact and contains fewer live parts, making it very easy to cover and insulate the connections (if required). This design reduces the exposure to faults from animals.
 The design philosophy of internally-fused capacitor banks enables many units in series within the bank, as shown in Figure 3(g). This design is ideal choice for harmonic filter applications where bank capacitance fluctuations must be minimized.


Fuse-less capacitor units eliminate capacitor fusing. The role played by fuses in previous generations of capacitor designs has become secondary because of the high quality insulating materials used in present day capacitors. The design philosophy of fuse-less capacitor units, shown in Figure 3 (h), consists of a few elements in parallel and many in series. This design is similar to the one used for externally-fused capacitors. Figure 3 (h) shows the design philosophy of a fuse-less capacitor bank where the capacitor units are connected in series. Failure of an individual capacitor element, a rare occurrence, leads to a very small voltage increase on the remaining series elements in that string. This is because the small voltage increase is shared by all the series elements in the string.

Fuse-less capacitor unit Fuse-less capacitor bank
Fuse-less design HT capacitor

Thus, fuse-less bank design has the same advantages as the internally-fused capacitor bank design. In addition, the fuse-less design produces lower losses than the fused design since there are no losses associated with capacitor fuses. However as the bank configuration requires many series groups to achieve the advantages of this design. Hence this designs only suitable for voltages of 66 KV and above.



Having seen the different design it seems the internal element fuses are the most favoured design. But many times there is always a debate on the choice between external fuse design and internal fuse design. The comparison between these two designs is given below :

Parameter Of Comparison External Fusing Internal Fusing
Fuse isolates only the failed / faulty element No Yes
Capacitor unit is isolated on fuse operation Yes No
Reduction in Bank output after fuse operation Considerable Negligible
Reduction in unit life after fuse operation Unpredictable Marginal
Bank with cells of higher kVAr Out–put No Yes
Reliability offuseco–ordination/unbalance protection Unpredictable Very reliable
Case rupture protection Yes No
Economy in cost of installation Yes No
Space requirement for installation More Less
Visual indication of fuse operation Yes No
Terminal to case insulation protection Yes No
Consistency in fuse operation / protection No Yes
Fuse is subject to routine electrical testing along with the unit Yes No
Fuse is protected from environmental hazards No Yes
Cost of structural steel & weight Higher Lower
Convenience of assembly and connections Cumbersome Convenient
External electrical clearances Are Less Are More
Locating failed unit in event of failure of unit(s) Easy Cumbersome