Capacitor Bank 101: Everything You Need to Know

Capacitor banks have been used since the development of electricity applications among the general public and have proven to be beneficial in home appliances, commercial machines, and industrial equipment.

A capacitor bank is an electrical device used in power systems to store and release electrical energy. By creating an electric field between two conductive plates, a capacitor bank can store energy there. Capacitor banks are often used to improve the power factor of a system, reduce reactive power usage, and control voltage levels, controlled by a power factor correction controller to provide appropriate results. A capacitor bank connects multiple capacitors of the same capacitance together in series or parallel. While a single capacitor acts to store electrical energy, a capacitor bank acts to store a larger volume of electrical energy. By increasing the number of capacitors in a capacitor bank, the capacity of the capacitor bank to store electrical energy can be increased.

Today, capacitors are used in a wide variety of technical applications, from small MEMS devices to large wind farms. Capacitor banks can be used for both AC and DC power. In AC power applications, capacitor banks are used to correct power lag factors or offset phase shifts. On the other hand, in DC power applications, capacitor banks are used to increase the total stored energy or boost ripple current capacity.

Wherever capacitor banks are used, they have a simple, fundamental role: to store electrical energy and regulate energy flow while supporting power factor correction, harmonic filtering, and transient suppression.

This article will first introduce the basic knowledge necessary to understand capacitor banks, and then understand their types and uses. At the end of the article, I also give some precautions when choosing and installing them.

1. Theoretical knowledge of capacitor banks

To understand the importance of capacitor banks, we must first understand the electrical system. In all electrical energy distribution systems, electrical loads can be divided into three categories, namely resistive (such as incandescent lamps, and heaters), inductive (such as motors, air conditioners, refrigerators), and capacitive (such as capacitors).

Of the three types of loads mentioned above, the most common load in modern systems is the inductive load. Common loads include general lighting, transformers, AC induction motors, and furnaces, which not only draw real power from the power supply but also reactive power (KVAR). The common characteristic of these inductive loads is that they use windings to generate an electromagnetic field. The inductive field enables the motor to operate, and a certain amount of electrical energy is required to maintain the magnetic field.

Meanwhile, there are three types of power sources in the power distribution system, namely active power, reactive power and apparent power.

• Active power is work done and is measured in watts.
• Reactive power generates magnetic fields, which are necessary for the proper functioning of equipment. Reactive power is calculated as VAR.
• Active power and reactive power together make up apparent power. Its unit is a volt-ampere.

Real power is divided by apparent power to determine the power factor. The power factor is a measure of how efficiently an AC power system uses the power it supplies. The power factor of a system can range from 0 to 1, depending on the type and number of loads connected to it. A low power factor indicates a high reactive power demand and low power utilization. A high power factor indicates a low reactive power demand and high power utilization.

Power factor correction can reduce line losses and improve system efficiency. By improving the power factor, current can be reduced and losses can be minimized, thereby increasing voltage levels and improving system performance.

Correction can also improve system capacity and reliability, improve power factor, reduce apparent power demand, and provide more active power to the load.

It is also often overlooked that many utility companies charge additional fees or impose penalties on consumers with low power factor because this places more strain on the transmission and distribution network and increases its operating costs. By improving the power factor, these fees or penalties can be avoided or reduced, thereby reducing the electricity bills for consumers.

2. Purpose of capacitor banks

While power factor correction, mentioned earlier, is sufficient to justify the use and efficiency benefits of capacitor banks, here are some other common uses for capacitor banks.

• Used for shunts in power systems. Shunting means that current flows through the path of least resistance to protect the power system. In other words, shunt capacitor banks can improve the efficiency of power distribution systems.
• Used to store electrical energy. This application is very useful for equipment that requires on-demand reactive power to keep the machine running.
• Helps solve the system power lag problem at a lower cost by changing the power grid. Helps reduce the phase difference between voltage and current.
• Capacitor banks are also used in micro-electromechanical systems (MEMS), which use radio frequency and wireless space to bring convenience to humans. Smartphones are the best example.

In addition to common uses in industrial manufacturing, capacitor banks are also active in scientific research and weapons manufacturing. Scientists are studying the use of capacitor banks to make electromagnetic armor and weapons and also to generate high-intensity electromagnetic fields required for scientific research.

3. Types of capacitor banks

There are many types of capacitor banks, but we can roughly divide them into the following three types based on their physical structure and working principle.

1). Internal fuse capacitor bank

Internally fused capacitor banks are set up with capacitor elements combined in parallel and series depending on the rating. Each capacitor element is individually protected by its own fuse unit. As the name suggests, both the capacitor element and the fuse unit are housed in the same housing. The unique capacitor elements in these capacitor banks are quite small, depending on their rating.

If a capacitor element fails, the functionality of the capacitor bank is not affected. This redundant design allows the capacitor bank to continue to operate even if one or more capacitor elements fail. Capacitor banks with internal fused diodes are easy to install and require little maintenance.

2). External fuse capacitor bank

In an externally fused capacitor bank, each capacitor unit is protected by a fuse located outside the capacitor housing. If any capacitor unit fails, the external fuse will blow, isolating the faulty capacitor from the system. This special design allows the capacitor bank to continue to operate uninterrupted even if some units fail.

In this configuration, capacitor banks are typically connected in parallel in each phase of the bank. If one capacitor bank fails, the overall function of the bank is only minimally affected. However, the loss of capacitor product in a single phase will cause a slight drop in the capacitance of that phase, potentially affecting the balance between the three phases.

3). Capacitor bank without fuse

In a fuseless capacitor bank, capacitor strings are made up of multiple capacitor units connected in series. These strings are now connected in parallel to form a capacitor bank for each phase. To form the entire three-phase capacitor bank, three such capacitor banks need to be connected in a star or delta configuration.

In this setup, there are no external or internal fuses to protect the capacitor strings. Therefore, if a string fails due to a short circuit or other fault, the current flowing through that string is barely affected. This is because the sequential arrangement of the capacitors ensures that the overall path remains intact regardless of the failure of a specific unit.

4. Size and design of capacitor banks

The performance and efficiency of the system depend largely on the design and size of the capacitor bank. Several important factors are listed below:

• Rated voltage: When designing a capacitor bank, the system voltage range it supports must be considered.
• Rated Current: The maximum current that a capacitor bank can handle without performance degradation or damage determines its rated current.
• Capacitance: The energy that a capacitor bank can store determines its capacitance, usually expressed in farads (F).
•  Power Factor Correction: The value of capacitance required is determined by the level of power factor correction, which is the purpose of building the capacitor bank.
• Harmonic filtering: By eliminating harmonics in the system, capacitor banks can help reduce power losses and improve power quality.
• Switching arrangements: Different switching arrangements can be used when designing capacitor banks to provide different levels of control and protection.
• Physical Size and Arrangement: The physical size and configuration of the capacitor bank will be determined by the specific needs of the system, such as the amount of available space and cooling requirements.

In general, a thorough examination of the system’s electrical requirements and characteristics should serve as the basis for capacitor bank design and sizing.

5. Circuit safety and precautions

Circuit protection and safety are key considerations when integrating capacitor banks into power systems. The following important measures contribute to the safe operation of capacitor banks:

• Overcurrent protection: If an overcurrent condition occurs, the capacitor bank and the rest of the system should be protected by an overcurrent protection device such as a fuse or circuit breaker.
• Short Circuit Protection: To protect the capacitor bank and the rest of the system in the event of a short circuit, short circuit protection devices should be added. These can be fuses or circuit breakers.
• Discharge resistor: After the capacitor bank is offline, a discharge resistor should be added to safely release the energy in it.
• Ground fault protection: In order to protect the system in case of a ground fault, a ground fault protection device, namely a residual current device (RCD), should be installed.
• Warning signs: To warn against illegal entry and to warn of the presence of high voltage, warning signs should be placed near the capacitor bank.
• Ventilation: Adequate ventilation must be provided to ensure effective cooling of the capacitor bank and reduce the risk of fire or explosion.
• Safety Protocols: Comprehensive safety protocols should be developed and followed when installing and maintaining capacitor banks, and appropriate protective equipment and clothing should be used.

6. Conclusion

Capacitor banks are useful devices that store electrical energy and regulate the flow of electrical energy in power systems. They can provide or absorb reactive power as needed, thereby improving the system’s power factor, voltage regulation, system efficiency, capacity, reliability, and stability. Capacitor banks can be connected in series or parallel with the load or to specific points in the system, depending on their purpose and design. Capacitor banks require proper sizing, installation, protection, and maintenance to ensure their optimal performance and safety.