Protection of Capacitor Bank

Like other electrical equipments, shunt capacitor may also be subjected to internal and external electrical faults. Hence this equipment is also to be protected from internal and external faults. There are numbers of schemes available for protection of capacitor bank, but during applying any of the schemes, we should remember the initial investment on that capacitor for economical point of view. We should compare the initial investment and the cost of the protection applying on it. There are mainly 3 types of protection arrangement that are applied to a capacitor bank.

• Element Fuse.

• Unit. Fuse.

• Bank Protection.

Element Fuses

Manufacturers of capacitor unit commonly provide inbuilt fuse in each element of the unit. In this case, if any fault occurs in any element itself, it is automatically disconnected from rest of the unit. In this case, the unit still serves its purpose, but with smaller output. In smaller rated capacitor bank only these inbuilt protection scheme is applied to avoid the expenditure of other special protective equipments.

Unit Fuse

The unit fuse protection is generally provided to limit the duration of arc inside a faulty capacitor unit. As the arc duration is limited, there is less chance of major mechanical deformation and huge production of gas in the faulty unit, and hence the neighborhood units of the bank are saved. If each unit of a capacitor bank is individually protected against fuse, then in case of failure of one unit, the capacitor bank can still be running without interruption before removing and replacing the faulty unit.

Another major advantage of providing fuse protection to each unit of the bank is that, it indicates the exact location of the faulty unit. But during choosing the size of the fuse for this purpose, it should be taken into consideration that the fuse element must withstand the excessive loading due to harmonics in the system. In the view of that the current rating of the fuse element for this purpose is taken as 65% above the full load current. Whenever the individual unit of capacitor bank is protected by fuse, it is necessary to provide discharge resistance in each of the units.

Bank Protection

Although in general fuse protection is provided with each of the capacitor units, but when a capacitor unit is under fault and the associated fuse element is blown out, the voltage stress increases to the other capacitor units connected in series in same row. Generally, each capacitor unit is designed for withstanding 110% of its normal rated voltage. If any other capacitor unit further becomes out of service, in the same row where previously one unit is damaged, the voltage stress upon other healthy units of that row will increase further and easily crosses the limit of maximum allowable, voltage of these units.

Hence it is always desirable to replace damaged capacitor unit from the bank as soon as possible to avoid excess voltage stress on the other healthy units. Hence, there must be some indicating arrangement to identify the exact faulty unit. As soon as the faulty unit is identified in a bank, the bank should be removed from the service for replacing the faulty unit. There are several methods of sensing unbalance voltage caused by failure of capacitor unit.
The figure below is showing the most common arrangement of capacitor bank protection. Here, the capacitor bank is connected in star formation. Primary of a potential transformer is connected across each phase. The secondary of all three potential transformers are connected in series to form an open delta and a voltage sensitive relay is connected across this open delta. In exact balanced condition there must not be any voltage appears across the voltage sensitive relay because summation of balanced 3 phase voltages is zero. But when there would be any voltage unbalancing due to failure of capacitor unit, the resultant voltage will appear across the relay and the relay will be actuated for providing an alarm and trip signals.

The voltage sensitive relay can be so adjusted that up to a certain voltage unbalancing only alarm contacts would be closed and for certain higher voltage level the trip contacts along with alarm contacts would be closed. The potential transformer connected across the capacitors of each phase also serves for discharging of the bank after being switched off.
In another scheme, the capacitors in each phase are divided into two equal parts connected in series. Discharge coil is connected across each of the parts as shown in the figure. In between the secondary of discharge coil and the sensitive voltage that unbalances the relay an auxiliary transformer is connected which serves to regulate the voltage difference between secondary voltages of discharge coil under normal conditions.
Here the capacitor bank is connected in star and the neutral point is connected to the ground through a potential transformer. A voltage sensitive relay is connected across the secondary of the potential transformer. As soon as there is any unbalance between the phases, the resultant voltage will appear across the potential transformer and hence the voltage sensitive relay will be actuated beyond a preset value.

Here, the capacitor bank of each phase is divided into two equal parts connected in parallel and the star points of both parts are interconnected through a current transformer. The secondary of the current transformer are connected across a current sensitive relay. In case any misbalancing occurs between the two parts of the bank, there would be a unbalance current flowing through the current transformer and hence the current sensitive relay will actuate. In this scheme for discharging the bank after switching off, discharge coil may be connected across the capacitors in each phase.
In another scheme of protection of capacitor bank, the star point of a three phase capacitor bank is connected to the ground through a current transformer and a current sensitive relay is connected across the secondary of the current transformer. As soon as there is any unbalancing between the phases of capacitor bank, there must be a current flowing to the ground through the current transformer and hence the current sensitive relay will be actuated to trip the circuit breaker associated with the capacitor bank.

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