Power capacitors: fundamentals of power capacitors

A capacitor is a device that stores energy within an electric field. This is achieved by having two oppositely charged electrical conductors separated by dielectric materials.

Power capacitors are constructed of several smaller capacitors commonly referred to as “elements,” “windings” or “packs.” These elements are formed from multiple layers of aluminum foil (conductors) and polypropylene film (dielectric) wound together. When interconnected, multiple elements combine to function as a single capacitor unit. Elements are connected in series based on rated voltage, and in parallel based on required kvar. The completed module is enclosed in a hermetically sealed tank, and any air from the unit is removed and replaced with a dielectric fluid. Units include bushings with terminal caps, that are used as connection points and to maintain electrical creepage and clearance requirements.

In distribution systems, these capacitors provide reactive power to offset inductive loading from devices like motors, arc furnaces and lighting loads. The incorporation of capacitors into a power distribution system offers economical and operational benefits including increasing system load capacity, reducing losses and improving power factor. 

There are two primary classifications of power capacitor units:

• Internally fused units consist of elements that are each protected by a series connected fuse inside the capacitor enclosure. As an element fails, the internal fuse protecting that element clears. After the fuse clears, the voltage on elements in parallel with the failed element rises and the voltage on the capacitor unit rises.
  
• Unfused units do not include any fuse protection within the unit. They are commonly utilized in fuseless or externally fused bank systems. Unfused units are available in three different product duties, the primary performance differences are summarized here.

A capacitor switch is used to energize and deenergize capacitor units and banks. 

Switches could simply be used for localized maintenance purposes to take equipment offline for service: this may be achieved utilizing hot sticks, control toggles or electronic interfaces. Switches are often utilized as a safety feature to remove a bank from service when operation is beyond intended design points. Switches can also be combined with relays, sensors and control packages to provide a significantly more sophisticated offering that will engage and disengage a bank under specific application conditions (temperature, time of day, system loading, etc.)

The two primary insulation methods for capacitor switches are oil and vacuum designs. Users will typically differentiate between the two based on their specific application needs: expected number of operations, ambient temperature, speed and rating requirements.

A capacitor bank is an array of multiple capacitor units combined in series and parallel connections to meet overall system needs. These units are commonly housed in a metallic frame, where each level is referred to as a block. Typically, units are connected in series to meet the maximum operating voltage, and in parallel to achieve the necessary kvar requirements.

Banks are available in a variety of orientations to meet user footprint and application needs. 

The units may be oriented vertically, horizontally or flatwise. 

Banks may be constructed:

• where one block (frame) houses multiple phases
• where each phase is isolated within its own block(s) and arranged next to one another
• where each phase is isolated within its own block(s) and then stacked on top of each other

A pole-mounted capacitor bank consists of a frame that is mounted directly to a utility pole. These banks most commonly will consist of a 3-phase design where each phase has 1 to 3 units (3 to 9 units per bank). They are easy to install, maintain and locate for optimal performance. They can house a full range of accessories and control options and are available in fixed or switched offerings.     

Metal-enclosed capacitor banks feature a metallic surround housing that serves as a protection barrier. This enclosure protects the public and personnel against inadvertent contact to energized equipment. In addition, the system is less susceptible to animal and pollution-related outages. These banks are suitable for indoor, outdoor and underground installations, and are available in multiple step configurations allowing for scalability to meet system needs. Users benefit from decreased installation labor as this package is pre-assembled and shipped ready to be installed.

Mobile capacitor banks are designed to be transportable to provide rapid deployment  and flexibility to address  user needs without having to install permanent equipment. 

Mobile banks are available with a variety of trailer options: flatbeds, single-drop, double-drop. They are scalable from single-trailer to multiple-trailer arrangements depending on application needs. Most commonly, these type of banks are utilized for emergency outage support, maintenance applications, temporary construction power or to provide loading assistance while delaying larger capital investments.

Open air capacitor banks are the most common and historically proven design. These banks are typically mounted to a rigid pad and may consist of an elevating structure to provide clearances to operators and adjacent equipment. Open air capacitor banks utilize a range of frame structures and configurations that can be scaled and configured to meet application needs. They are available in externally fused, fuseless and internally fused configurations. These banks provide economical and reliable methods for reducing losses and improving power quality. 

There are additional specialty application banks to meet more custom requirements:

• Filtering products that counteract the negative effects of harmonics and provide a more stable system
• High-Voltage Direct Current (HVDC) transmission banks
• Flexible Alternating Current Transmission Systems (FACTS)
  • Series compensation: Static Synchronous Series Compensator (SSSC), Thyristor Controlled and Switched Series Capacitor (TCSC and TSSC)
  • Shunt compensation: Static Synchronous Compensator (STATCOM) and Static VAR Compensator (SVC)