What is a Capacitor Bank and why is it used?

In electrical substations, an interconnected system of multiple capacitors is used for improving the power factor of the system, this interconnected system of capacitors is referred to as a capacitor bank. In short, a capacitor bank is device which consists of multiple capacitors connected in parallel or series and provide reactive power for improving the power factor of the electrical system.

Capacitor banks are important components in utility and industrial substations as they are useful in improving power factor and efficiency of the power supply system.

Read this article to learn the basic concepts related to a capacitor bank, its construction, working, types, advantages, disadvantages, and applications.

What is a Capacitor Bank?

A capacitor bank is nothing but a combination of multiple capacitors connected in series or parallel to obtain a desired value of capacitance for improving the power factor of an electrical power supply system. Therefore, the primary function of a capacitor bank is to improve the power factor of the system and minimize the energy losses.

Capacitor banks are important components in substations because they play a crucial role in improving the overall efficiency of an electrical substation.

How Does a Capacitor Bank Work?

An electrical capacitor is the core component of a capacitor bank. Thus, the working principle of a capacitor bank is based on the working of a capacitor. From the basics, we know that a capacitor consists of metallic plates separated by a dielectric material and stores electrical energy in the form of electrostatic field.

When a capacitor is connected to an alternating current supply, it charges and discharges in each cycle of AC. During first half cycle, the capacitor charges and during another half cycle, it discharges.

When a capacitor charges, it takes a leading current from the power supply and hence takes leading reactive power from the power supply. During this charging period, it gives out a lagging reactive power. This lagging reactive power is supplied to the electrical load whose power factor is to be improved.

Therefore, a capacitor bank if connected in an electrical system, it compensates the requirement of lagging reactive power and hence improves the power factor of the system.

Types of Capacitor Banks

The following three types of capacitor banks are commonly available for power factor correction −

• Internally Fused Capacitor Banks

• Externally Fused Capacitor Banks

• Fuseless Capacitor Banks

Let’s discuss the features and characteristics of each type of capacitor banks in detail.

Internally Fused Capacitor Banks

In internally fused capacitor banks, the capacitors are combined in series and parallel combinations, these combinations are done on the basis of rating of the capacitor bank. Each capacitor is protected by its own internal fuse. For this reason, it is termed as internally fused capacitor bank.

The capacitors and fuse units of the internally fused capacitor banks are housed within a same enclosure. Hence, these capacitor banks are fairly small in size, however it also depends on the rating of the bank.

If one or more capacitors fail, the operation of the capacitor bank is not affected. The internally fused capacitor banks are known for their easy installation and less maintenance requirement.

Externally Fused Capacitor Banks

In the case of externally fused capacitor banks, the fuse units for protecting each capacitor are provided in a separate enclosure which is located outside of the capacitor bank’s enclosure.

When any of the capacitor element fails in the capacitor bank, its external fuse blows out and isolates the defective capacitor from the bank. This mechanism allows the capacitor bank to keep operating without any interruption even when a specific unit of capacitors is damaged.

In externally fused capacitor banks, the capacitors are generally connected in parallel combination inside each stage of the capacitor bank. Therefore, the failure of any one capacitordoes not affect the operation of rest of the system. However, it can affect the capacitance balance in a three-phase system.

Fuseless Capacitor Banks

Fuseless capacitor banks are designed by connecting multiple capacitors in series and then multiple series strings of capacitors are connected in parallel to design the capacitor bank.

These are called fuseless capacitor banks because there is no internal or external fuse unit is provided for protecting the capacitor units. In these capacitor banks, the protection against fault is provided by the sequence arrangement of capacitor itself. Therefore, any of the capacitor strings fails due to a fault, the flow of current through the string remains almost unaffected.

How to Calculate the Size of a Capacitor Bank?

The procedure for calculating the size of capacitor bank for an electrical system is explained below.

Step (1) − First of all determine the power factor angle of the circuit for the given power factor.

$$\mathrm{PF_1\:=\:cos\:\phi_1}$$

$$\mathrm{\phi_1\:=\:cos^{-1}\:(PF_1)}$$

Step (2) − Calculate the angle of the new power factor required.

$$\mathrm{\phi_2\:=\:cos^{-1}\:(PF_2)}$$

Step (3) − Calculate the reactive power required for power factor improvement.

$$\mathrm{Q_C\:=\:P\times (tan\phi_1\:-\:tan\phi_2)}$$

Step (4) − Finally, calculate the size of the capacitor bank i.e., capacitance.

$$\mathrm{C\:=\:\frac{Q_C}{V^2\times 2\pi f}}$$

Advantages of Capacitor Bank

Capacitor banks improve the power factor of the system which helps in minimizing the penalties of utility companies. They improve the voltage regulation of the system and thus help in stabilizing the voltage within specified limits. Capacitor banks reduce the currents in the circuit and thereby reduce the energy losses in the system.

Capacitor banks enhance the load carrying capacity of the power distribution network. By reducing the energy losses and penalties, capacitor banks also reduce the operation of the system.

Disadvantages of Capacitor Banks

The upfront cost of capacitor banks is high. The installation of capacitor banks adds extra cost in the overall cost of the system.

For reliable operation, capacitor banks require regular maintenance which again increases the cost of electrical system. If the capacitor banks are not properly sized according to the system, then it may cause resonance and problem of overvoltage and faults.

Capacitor banks can have charge stored in it even when they are disconnected from the system. This poses a risk of electric shock if not properly managed.

Applications of Capacitor Banks

Capacitor banks are primarily used in substations for power factor improvement. The following are some common facilities where capacitor banks are used for improving power factor include − Industrial substations in factories and plants, Transmission and distribution substations in power companies, Large commercial building to minimize energy cost, HVAC (Heating, Ventilation, and Air Conditioning) systems to reduce energy consumption, Renewable energy power plants for power factor and power quality improvement, etc.

Conclusion

From the above discussion, we can conclude that capacitor bank is an important component in an electrical power supply system, as it improves the power factor and reduces the energy losses. Consequently, capacitor banks help in reducing the energy cost and improving the power quality.