What is a Capacitor Bank: How it Works, Circuit, and Applications

What is a Capacitor?

Before discussing a capacitor bank, it is imperative to know about the capacitor, which is the basic building block of a capacitor bank.

A capacitor can be referred to as a small storage tank for electric charges. It is constructed from two metal plates separated by a non-conductive substance, typically ceramic or plastic. One plate of the capacitor is deposited with positive charges, and the other plate is deposited with negative charges when these plates are connected to a power source. Once charged, these plates start acting like two terminals of a battery. The capacitor has the ability to hold charges until these are required by some attached load posing low potential or some pathway for current flow is established. Upon releasing its stored energy to a connected load or circuit, the capacitor helps to smooth out voltage spikes and provides a sudden burst of force when needed.

Figure 1: Types and Structure of Capacitor

What is a Capacitor Bank?

An arrangement of capacitors used to store electrical energy in the form of static charges is called a capacitor bank. In this arrangement, capacitors are connected in series and parallel. A capacitor bank will begin the cycle of charging and discharging as soon as it is connected to the electrical system, maintaining voltage levels of the system and thus stabilizing it. They provide the sudden voltage required for the startup of some machinery or to compensate for voltage dips upon disturbance/fault at a generation plant. This way, it allows the system to function uninterruptedly, even with variations in supplied power. Capacitor banks are frequently used in power plants, substations, industries, and certain residential areas to increase the dependability and effectiveness of electrical systems.

Figure 2: A Capacitor Bank

Components of Capacitor Bank

To understand the workings of a capacitor bank, it is essential to know about its construction and various components. Here, typical components of a capacitor bank are discussed briefly.

Capacitors

Capacitors are the most important part of capacitor banks, as their name implies. When needed, these capacitors release the electrical energy they have stored. These capacitors are connected in series and/or parallel to increase the total capacitance and energy-storing capacity.

Resistor

Resistors are among the most crucial components in a capacitor bank. When a capacitor is charged, the inrush current could be very high, could cause damage to the insulation of wires, or affect semiconductor devices negatively. In fact, semiconductor devices have small sizes, and they can’t dissipate heat quickly, compromising the reliability of such devices. So, in capacitor-based circuits, a resistor is usually installed to avoid problems caused by inrush current. As the overall resistance of the system increases, the current will decrease (I=V/R). Similarly, the same could happen in capacitor bank-based systems, which have much higher capacitance. So, a resistor is also used in series with a capacitor bank to control the flow of inrush current.

Switching Devices

These banks need some switching devices, such as transistors or relays, to regulate their connectivity with the electrical systems. Switching devices in a capacitor bank would help prevent leakages, improve efficiency, and enhance the safety of the whole system.

Protective Equipment

Safety should always come first while handling electrical energy because voltage surges or dips could be dangerous and cause accidents. In order to prevent any short circuits or voltage spikes, several safety and protective components are also installed.

Control, Monitoring, and Communication Systems

To enhance accuracy and precision, almost all modern electrical systems employ feedback-based controllers (or multiple controllers) and a rigorous monitoring system. For this reason, a lot of capacitor banks also contain integrated microcontrollers and monitoring devices. Communication devices complying with specific communication protocols are also used for various purposes, e.g., communication of monitored data to the control room of the plant.

Internal Circuit of Capacitor Bank

Here, the basic circuit representation of a capacitor bank is shown where capacitors are connected in series and parallel. As the number of capacitors is increased in parallel, capacitance also increases. Then, sets of parallel capacitors are connected in series. If each set of parallel capacitors is considered a battery, a series connection causes an overall voltage rise.

Figure 3: Internal Circuit of a Capacitor Bank

Working Mechanism

As discussed earlier, capacitor banks are made by connecting numerous capacitors in series and parallel to create a storage device with a large capacity. This bank begins to charge as soon as it is attached to a power source, but because of the feedback-controlled mechanism, it will never overcharge because the controller will cut off the bank’s supply by activating the switching devices. Later on, the capacitor bank’s controller is notified when the main electrical system has a power outage or voltage dip. As a result, the link with the main electrical system is reestablished, and the capacitor bank starts to be discharged. This loop will keep on till the system keeps functioning. The controller also keeps getting measurements from the monitoring system to keep the system healthy. These measurements might include inrush current, the temperature of switching devices, the voltage across capacitors, etc.

Figure 4: Flow Chart to Represent Working of Capacitor Bank-Based System

What is Power Factor and Power Factor Correction by Capacitor Banks?

The actual power utilized or consumed by the system (measured in kW) divided by the total power provided to the system (measured in kVA) is known as the power factor.

Power Factor=(power consumed by the system/power supplied to the system)

The difference between supplied and consumed power is power loss, so it can be said that the power factor establishes the system’s power efficiency. A lower power factor means that more power was supplied to the system, but consumption was lower, which corresponds to lower efficiency. The power factor has a range of 0 to 1.

In a pure inductive circuit, current lags voltage by 90 degrees. Contrarily, current leads voltage by 90 degrees in a pure capacitive circuit. The more lagging the current is, the lower the power factor and the higher the losses.

From the above discussion, it can be inferred that due to inductive behavior of most of the loads, electrical systems typically have low power factors and high losses. Therefore, to improve system efficiency and power factor, capacitor banks are used, which lessen the system’s inductive effect by reducing lag in current. This, ultimately, raises the power factor. So, we can say that capacitor banks reduce power losses by improving or correcting the power factor.

Need of Capacitor Bank in Substation

They are commonly used for these three reasons:

• Power Factor Correction: Substations are home to large inductive loads such as transformers and motors. Industrial and domestic loads, powered through substations, also have inductive loads majorly. Such loads pull down the power factor as explained above, decrease efficiency, and cause power loss. A sizable capacitor bank is added to the system to counteract this effect. While the inductor causes power lag, the capacitor causes power lead. Thus, the capacitor bank eliminates the inductive behavior and increases system efficiency.
• Voltage Regulation: It is imperative that substations supply steady and consistent voltages, but noise and electrical fluctuations at the input and output side make it challenging to maintain voltage at steady levels. By absorbing voltage spikes from the input side (and even regenerating energy in special cases) or delivering the instant high voltage requirements to output, the capacitor bank helps regulate voltage and stabilize the electrical system.
• Transient Response: The abrupt shift in electrical power brought on by a change in voltage or current is known as a transient response. Because the capacitor banks provide reactive power, the system’s total transient response may be improved.

Figure 5: Capacitor Banks at a Substation

Other Applications

However, the employment of capacitor banks at substations stands as their most important application. They are also used in various other applications, though their size might change according to the size of the system.

• Cars: These banks are used in electric or hybrid vehicles to store electricity. These are also used to capture regenerative braking energy as voltage spikes are instant. The purpose of these banks is to charge and absorb voltage spikes so that the vehicle can store more energy and improve its efficiency.
• Industrial Machinery: As was already mentioned, all inductive loads cause power to dissipate, which increases heat production and power losses. So, industries are required to install their own capacitor banks to decrease power loss, minimize heat losses, and protect devices.
• Commercial Buildings: Commercial buildings have a lot of inductive loads, like fans, pumps, blinds, etc., much like an industrial setting. These structures also have capacitor banks to lower the building’s power loss and raise its overall power factor.

Conclusion

In conclusion, capacitor banks play a pivotal role in enhancing the efficiency and stability of electrical systems across various applications. By mitigating power losses through power factor correction, regulating voltage in substations, and improving transient responses, capacitor banks contribute significantly to the reliability of power supply. These devices exemplify their versatility, whether utilized in industrial machinery, electric vehicles, or commercial buildings. As technology continues to advance, they remain integral in supporting the seamless operation of diverse electrical systems, underscoring their importance in the evolving landscape of power management.

Fatima Razzaq

Fatima Razzaq is a freelance technical writer who served as an electrical engineering lecturer at Air University—a federally chartered public sector research university in Pakistan. Razzaq holds a Bachelor’s degree with distinction in electronic engineering from Ghulam Ishaq Khan Institute of Engineering Sciences and Technology (GIKI) and a Master’s degree in Sustainable Transportation and Electrical Power Systems from the University of Nottingham, Universidad de Oviedo, and La Sapienza University of Rome. Razzaq’s diverse work experiences in academia and industry continue to inform her prolific technical writing journey in the areas of electrical engineering, storage mechanisms, power electronics, electric vehicles, energy, and related topics.

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