Unit of a capacitor bank is normally called capacitor unit. The capacitor units are manufactured as single phase unit. These single phase units are connected in star or delta to farm a complete 3 phase capacitor bank. Although some rare manufacturers 3 phase capacitor unit but normally available capacitor units are single phase type. The
- Externally fused capacitor bank.
- Internally fused capacitor bank.
- Fuse less capacitor bank.
Let us discuss these types of capacitor bank one by one.
Externally Fused Capacitor Bank
In this type of capacitor bank, the fuse unit is provided to each capacitor unit externally. On fault in any unit the externally provided fuse of that unit will be blown out. As the fusing system disconnects the faulty capacitor unit, the bank will continue it’s service without any interruption. In this type of capacitor units are connected in parallel.
As there are several capacitor units connected parallelly per phase of the capacitor bank, on failure of one unit, there will be not much affect on performance of entire bank. As one capacitor unit is missing in one phase the capacitance of that phase will be lower than that of other two phases. This will result higher voltage in other two phases of the bank. If the capacity of one capacitor unit in a bank is low enough then in absence of any unit in the bank there will not be much voltage unbalance. This is why the VAR rating per capacitor unit in a bank is restricted up to a specified limit.
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.
Interally Fused Capacitor Bank
The entire capacitor bank is constructed in a single arrangement. As per rating of entire bank, several capacitor elements are connected parallel and series. Each of the capacitor elements is individually protected with fuse unit. As the fuses and capacitor elements are placed inside same casing, the bank is called internally fused capacitor bank. In this type of capacitor bank, each capacitor element is very small in ratings, so that if any of the elements are out of service, there will be no remarkable affect in the performance of the bank. Internally fused capacitor bank can run satisfactorily even more than one capacitor elements are 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 required number of fuse units is connected in series to farm a capacitor string. Then required number of required number of these strings is connected in parallel to farm capacitor bank per phase. Then three similar per phase bank are connected in star or delta to farm entire 3 phase capacitor bank. The units of the capacitor strings are not at all protected by any internal or external fusing arrangement. In this system, if one of the unit of a string fails due to short circuit, there will be no remarkable change in current through this string as there are many other capacitor connected in series along this path. As the effect of short circuited unit in the string is small enough, the bank can be run up to prolong time before replacement of faulty unit. This is the reason, why fuse is not required to isolate the faulty unit from the system in this type of capacitor bank immediately after unit becomes faulty.
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.