EXTERNALLY-FUSED HT CAPACITORS :
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.
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.
INTERNALLY-FUSED HT CAPACITORS :
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.
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 HT CAPACITORS :
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.
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.
COMPARISON BETWEEN INTERNAL FUSE AND EXTERNAL FUSE DESIGN HT CAPACITORS :
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|