In the past, capacitor banks were relegated to isolated, low-tech, high-fenced public power stations. Today, capacitor bank applications have scaled down to nano-sized MEMS devices and outward to ocean-based wind-farm substations. Regardless of their usage, capacitor banks perform the same functions of storing and smoothing out electrical energy. This article will examine the basics of capacitor banks and their usage in a wide range of modern applications.
A Definition
As the name implies, a capacitor bank is merely a grouping of several capacitors of the same rating. Capacitor banks may be connected in series or parallel, depending upon the desired rating. As with an individual capacitor, banks of capacitors are used to store electrical energy and condition the flow of that energy. Increasing the number of capacitors in a bank will increase the capacity of energy that can be stored on a single device.
Typical Applications
Our modern world of electronics requires a lot of energy. To meet this demand, energy must be stored electrically for easy access. Capacitors are ideal for storing large electrical energy charges as well as conditioning the flow of energy as needed.
Here are some of the typical uses for capacitor banks:
• Shunt Capacitor: A shunt is a mechanism that allows electric current to pass around another point in the circuit by creating a low-resistance path. In electrical noise bypass applications, capacitors are used to redirect high-frequency noise to ground before it can propagate throughout the system, but especially to the load. Shunt capacitor banks are used to improve the quality of the electrical supply and thus improve the efficiency of the power systems (Fig. 1).
Figure 1: Here’s a capacitor bank, specifically a shunt capacitor bank. (Source: Vishay Intertechnology)
• Power-Factor Correction: In transformers and electric motors, capacitor banks are used to correct power-factor lag or phase shift in alternating-current (AC) power supplies. The power factor of an AC power system is a comparison of the power used by the load, called the “real power,” divided by the power supplied to the load, known as “apparent power.” In other words, the power factor is the ratio of the useful work performed by a circuit compared to the maximum useful work that could have been performed at the supplied voltage and amperage.
In electric power distribution, capacitor banks are used for power-factor correction. These banks are needed to counteract inductive loading from devices like electric motors and transmission lines, thus making the load appear to be mostly resistive. In essence, power-factor correction capacitors increase the current-carrying capacity of the system. By adding capacitive banks, you can add additional load to a system without altering the apparent power. Banks can also be used in a direct-current (DC) power supply to increase the ripple-current capacity of the power supply or to increase the overall amount of stored energy.
• Store Energy: Like individual capacitors, capacitive banks store electric energy when it is connected to a charging circuit and release that energy when discharged. Capacitors are commonly used in electronic devices to maintain power supply while batteries are being changed. For modern consumer devices like mobile phones, high-storage capacity is needed in a very small volume due to limited space. This poses a challenge since increased capacitance typically means an increase the area of the plates, represented as “A” in Fig. 2.
Figure 2: The miniaturization of capacitive banks is due to the introduction of new materials between the plates of the capacitor that increase the permittivity “k” of the dielectric material. (Source: Article Author)
As the equations reveals, another way to increase the capacitance is to increase the dielectric strength. The “k” element is the relative permittivity of the dielectric material between the plates. For free space, “k” equals unity or one. For all other media, “k” is greater than one. Film and electrolyte capacitors are typical examples of devices suited to these applications.
Large to Small to Exotic
Capacitor bank applications run the gamut from the very large to the very small. One of the more unusual large applications is a wind-farm substation application. The Lincs Wind Farm is a 270 MW offshore wind farm 8 km (5.0 mi) off Skegness on the east coast of England (Fig. 3). The energy generated offshore is transferred to the grid via the Walpole onshore substation located in Norfolk County. The Siemens High-Voltage Capacitor Plant has provided a total of six single-phase fuseless capacitor banks as well as six single-phase banks with internally fused capacitors.
Figure 3: Lincs Offshore Wind Farm. (Source: Mat Fascione via Geograph)
In practice, power-capacitor bank installations can be grouped into one of three areas: internally fused, externally fused or fuseless. For internally fused capacitors, the individual can containing the bank is constructed from series groups of parallel capacitor elements, each element individually fused within the can. Conversely, externally fused capacitor banks consist of groups of parallel capacitors that are designed to be operated with a common external fuse.
This external fusing can potentially cause problems if a failure occurs in one of the winding elements, in which case the entire unit must be disposed. According to Brad Henderson, Regional Sales and Marketing for Vishay’s ESTA Power Capacitor Division, the use of internally fused capacitors is one of the latest trends in modern capacitor-bank technology. “One of the biggest design challenges is to change the mindset of the end customers accustomed to using the older style externally fused capacitor banks historically used here in the Americas,” explains Henderson.
Finally, fuseless capacitor banks also use an external fuse. However, they usually contain more elements than a typical fused capacitor. Thus, a shorting in one element doesn’t cause cascading failures throughout the can.
At the opposite end of the scale are small applications such as for smartphones and storage devices. Small-power capacitor banks are used in conjunction with large-capacitance super-capacitors to reduce the charging time of a mobile phone. A super-capacitor is capable of holding hundreds of times more electrical charge than a standard capacitor and is sometimes used as low-voltage rechargeable battery.
In the radio-frequency (RF) and wireless spaces, tiny micro-electro-mechanical systems, or MEMS, tunable capacitor banks are used to augment or replace full-size electromechanical tunable capacitors. Hundreds of tiny MEMS capacitors of various values are controlled and tuned digitally via a serial peripheral interface (SPI). These switched-capacitor banks can be combined into one package, thus increasing the tunable range of the overall system.
One of the more exotic uses of capacitor banks is in pulsed power and weapons systems. Research has been conducted on low-inductance high-voltage capacitor banks that can supply huge pulses of current for many pulsed-power applications. Capacitor banks with a high energy density (more than 1 J/cm3) and modern semiconductor switches can be used to create compact energy amounting to several hundreds of kilo-Joules (kJ) and generating high-amplitude, pulse currents.
Applications for these high-density capacitor banks includes electromagnetic forming, Marx generators, pulsed laser, pulse-forming networks, radar, fusion research, and particle accelerators. Experimental work continues using banks of capacitors as power sources for electromagnetic armor and electromagnetic railguns and coil guns.