Types of Capacitor Bank
Types of Capacitor Bank Definition
Capacitor banks are defined as groups of capacitors connected together to improve the power factor in electrical systems.
Externally fused capacitor bank.
Internally fused capacitor bank.
Fuse less capacitor bank.
Externally Fused Capacitor Bank
In this type of capacitor bank, each capacitor unit has an external fuse. If a unit develops a fault, its external fuse will blow. This disconnection allows the bank to continue operating without interruption. These capacitor units are connected in parallel.
With several capacitor units connected in parallel per phase, the failure of one unit doesn’t greatly affect the bank’s performance. The phase with the missing unit will have lower capacitance, causing higher voltage in the other two phases. If each unit has a low enough capacity, the voltage imbalance will be minimal. This is why the VAR rating per capacitor unit in a bank is limited to a specified amount.
In the externally fused capacitor bank, the faulty unit can easily be identified by identifying blown out fuse unit through visual inspection.The rating of capacitor unit is typically from 50 KVAR to 40 KVAR.The main drawback of this type of capacitor bank is that, on failure of any fuse unit, there will be unbalance sensed, even all capacitor units of the bank are healthy.
Internally Fused Capacitor Bank
The entire capacitor bank is built as a single arrangement, with multiple capacitor elements connected in parallel and series according to the bank’s rating. Each element is individually protected with a fuse, all housed within the same casing, making it an internally fused capacitor bank. Each element has a very small rating, so if one fails, it doesn’t significantly affect the bank’s performance. These banks can operate satisfactorily even if more than one element is out of service.
The main drawback of this bank is that, on failure of many number of capacitor elements, the entire bank is to be replaced. There is no scope of single unit replacement.The main advantages is that, it is quite easy to install and also easy to maintain.
Fuse-less Capacitor Bank
In this type of capacitor bank, the required number of fuse units is connected in series to form a capacitor string. The necessary number of these strings is then connected in parallel to form a capacitor bank per phase. Three similar per-phase banks are connected in star or delta to create a complete three-phase capacitor bank.
The units in these strings are not protected by any internal or external fuses. If one unit in a string fails due to a short circuit, the current through the string doesn’t change much because many other capacitors are connected in series. The bank can continue running for a long time before the faulty unit needs replacement, which is why fuses aren’t needed to isolate faulty units immediately.
Advantages of Fuse Less Capacitor Bank
The main advantages of fuse less capacitor bank are,
They are less expensive then fused capacitor banks.
They require less space compared to fused capacitor bank.
Less chance of bird fault, snake fault or rat fault as the inter connecting wire can be insulated properly in fuse less capacitor bank.
Disadvantages of Fuse Less Capacitor Bank
There are also some disadvantages of fuse less capacitor bank.
Any earth fault in the bank, unit, such as bushing fault, insulation failure between tank and live part of the capacitor, should be cleared immediately by tripping of circuit brake associated with this bank as there is no provision of any fuse.
For replacement of any capacitor unit, only identical spare is required. It cannot be managed by available standard capacitor unit. So, there must be sufficient stock of identical capacitor units available at site which is an extra investment.
Some time it becomes difficult to locate actual faulty unit of the bank only by visual inspections. Then the time required to replace actual faulty unit will be higher.
Sophisticated relay and control system are essential for fuse less capacitor bank. The relay system of the bank should also be capable of tripping the circuit breaks associated with it in the event of input power failure to the relay.
External reactor is required to limit transient current in the capacitor.
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