Pros & Cons of Different Types of Capacitor Banks

Capacitor Banks and Harmonic Filter Banks can be purchased and specified in four forms, as follows:

1.      Metal-Enclosed: Banks of this type are totally enclosed in a steel enclosure and are usually located within a fenced-in substation or switchgear room.

2.      Pad-Mounted: Banks of this type are totally enclosed in a steel enclosure with the tamperproof integrity requirements set forth in ANSI C37.12.28. These banks are usually “dead-front” and are located in areas that are accessible to the general public. All of the features that are available in a Metal-Enclosed bank are available in a Pad-Mounted bank.

3.      Stack-Rack: Banks of this type have exposed live parts and are supported on a steel structure. These banks are usually located in fenced-in substations.

4.      Pole-Top: Banks of this type have exposed live parts and are typically supported on a wood power pole.

These four types of banks provide the same function in regards to power factor correction, harmonic filtering, and voltage control. Their physical design, installation, appearance, and operational features however, can be dramatically different. This document provides information concerning the pros and cons of each of these types of Capacitor Banks and Harmonic Filter Banks. It is important that specifying engineers and buyers understand the key difference between the four banks so that an appropriate type of bank will be purchased.

Metal-Enclosed Banks & Pad-Mounted Banks

Figure one shows a typical Metal-Enclosed bank. Metal-Enclosed banks offer the same features as pad-mounted banks and come fully assembled, tested, and ready for interconnection. Just about any conceivable control or equipment feature is available in this style of a bank. Key features that are typically included with a Metal-Enclosed Capacitor Bank and Harmonic Filter Bank are as follows:

Main-Incoming Air Disconnect Switch: This switch provides safe access into the bank and the visible disconnecting means required by the National Electric Code (NEC – Article 460-24(b)(2)). All non-utility power consumers are required to follow this requirement in the NEC. 

Ground Switch: This switch grounds the load side terminals of the Main-Incoming Air Disconnect Switch during maintenance. It also provides a means of discharging all trapped charge on the capacitors.  For industrial applications, the ground switch is required when the disconnecting switch of the capacitor bank forms one of the six service disconnects (Article 230-204d of the NEC).

Main-Incoming Current-Limiting Fuses: These fuses provide main bus protection for the entire capacitor bank and harmonic filter bank.  The fuses can be safely changed while the Main-Incoming Air Disconnect Switch is open.

Vacuum Switches & Contactors: Vacuum Switches and Contactors allow the bank to be switched on and off in increments.

Transient Inrush & Harmonic Filter Reactors: Transient Inrush reactors limit the magnitude and frequency of transients associated with capacitor bank switching.  Harmonic Filter Reactors tune capacitor banks to specific frequencies to lower harmonic distortion and improve power quality. The filter reactors will typically be of the iron-core type, while the transient inrush reactors will be of the air-core type.

Individually Fused Capacitors: Metal-Enclosed Capacitor Banks are typically provided with double bushing capacitors that are protected from case rupture by current limiting fuses.

Blown Fuse Detection: Blown fuse detection systems provide an external indication (typically by a strobe light or an externally located enunciator panel) of a blown fuse condition.

Automatic Controls: Metal-Enclosed banks are typically provided with automatic controls that are mounted in a control compartment as shown in Figure 1. These controls turn the banks on and off based upon power factor or other signals (i.e. voltage, temperature, current, KW or kvar pulse) in accordance with system requirements.

Key Interlock System: A key interlock system is typically provided to control ingress, egress, and proper operation of the bank.

Figure 1 - 10 MVAR 34.5kV Metal-Enclosed Harmonic Filter

Bank Mounted on Concrete Pad

Pros –

Metal-Enclosed Banks/Pad-Mounted Banks

• The Main-Incoming Switch, Ground Switch, and Main Incoming Fuses meet the NEC requirements for “Service Entrance Gear”.  An interposing or upstream breaker or switch is not required to connect this type of bank to an electric utility service point.

• A Metal-Enclosed bank comes fully assembled, tested, and ready for interconnection. No field assembly is required. Therefore installation costs and problems associated with mistakes during installation are very low.

• Maintenance costs for Metal-Enclosed banks are low.  Special equipment such as bucket trucks and hot-line tools, as well as the trained personnel that are necessary to utilize them, are not required due to the construction and controls features that are provided with Metal-Enclosed banks.

• A Metal-Enclosed bank significantly reduces the risks and the associated liability of trespassing or tampering public and un-trained employees becoming exposed to electrical hazards. With Metal Enclosed banks, all live parts are contained in a grounded, key-interlocked enclosure and no external hardware is accessible. 

• Pad Mounted banks do not need to be located within a fenced-in substation.

• Metal-Enclosed Banks and Pad-Mounted Banks have a small footprint.

• Wildlife and air-born settlements that can cause faults and insulator tracking do not readily affect Metal-Enclosed Banks.

• Metal Enclosed Banks provide good corrosion resistance to all electrical components as they are protected by an 11-guage galvaneal enclosure that is painted with a Marine Epoxy-Based Paint. This paint provides excellent chemical resistance to splash, spillage, fumes, and weather for acidic, alkaline, salt solutions (acid, neutral, and alkaline salt solutions), fresh water, solvents, and petroleum product environments.

• Metal-Enclosed Banks utilize current limiting fuses that dramatically reduce damage associated with faults.  Current limiting fuses also provide for the best coordination with upstream protective devices and therefore reduce the liability associated with a system-wide power outage.

• Metal-Enclosed Banks are esthetically pleasing due to their low profile and small footprint. In addition, they can be painted to match the surrounding architecture.

• Components within Metal-Enclosed Banks are shaded from direct sunlight and Ultra-Violet rays. 

• For filter banks, the magnetic field associated with the filter reactors will be confined to the iron-core. 

Cons –

• Initial equipment cost may be higher and therefore justification that includes future liability, installation cost, maintenance costs, and operating cost may be required.

• In dusty environments intake filters may require routine cleaning.

• Metal-Enclosed Banks are not available in voltages higher than 34.5kV.

• Standard features of Metal-Enclosed banks vary between manufacturers. An informed purchase may require a higher level of research.

• Components are only partially visible though enclosure windows when banks are energized.

• Metal-Enclosed Banks require more expensive current limiting type of fuses.

• The Metal-Enclosed Bank can be difficult to expand.  Expansion capability should be designed into the bank during the specification stage of the bank. Larger capacitors, additional capacitors, or bus bar extension (into an adjacent capacitor section) may not always be possible without proper planning and design of the bank.

• Although NEPSI attempts to utilize Silver-Plated Copper Bus for interconnecting medium-voltage components, other manufacturers may use less rigid medium voltage cable or bare wire.

Stack-Rack Capacitor Banks

Figure two shows a typical Stack-Rack Capacitor Bank. These banks are usually shipped from the supplier partially assembled. Typically, the elevating structure, individual capacitor stacks, PT’s, CT’s, Controls, Disconnect Switches, Grounding Switch, Reactors, and Neutral Sensing Device, require assembly and interconnection in the field. Just about any conceivable control or equipment feature can be provided in this style of a bank. It should be noted however, that sophisticated controls and operating features are not usually part of a standard equipment offering and are typically provided separately. Key features that are normally included with Stack-Rack Capacitor Banks and Harmonic Filter Banks are as follows:

Elevating Structure: Due to requirements of the NEC and NESC, Stack-Rack Capacitor banks, Harmonic Filter Banks, and associated components are typically mounted on an eight-foot (or higher) elevating structure.

Main Disconnect Switch: Depending upon the voltage level, these switches may be attached to the Stack-Rack or an adjacent elevating structure.  The switches may be of the type that are “Hot-Stick” operated or of the type that are operated by a rotary type handle.

Ground Switch: This switch grounds the load side terminals of the Main Disconnect Switch during maintenance. It also provides a means of discharging all trapped charge on the capacitors. This switch may, or may not be attached to the main Stack-Rack.

Vacuum Switches/Breaker: Vacuum Switches/Breakers allow the bank to be switched on and off in increments or as a whole.

Transient Inrush & Harmonic Filter Reactors: Transient Inrush reactors limit the magnitude and frequency of transients associated with capacitor bank switching.  Harmonic Filter Reactors tune capacitor banks to specific frequencies to lower harmonic distortion and improve power quality. The reactors may be mounted on the Stack-Rack capacitor bank or may be mounted on a separate elevating structure. Both reactors will be of the air-core type.

Individually Fused Capacitors: Stack-Rack Capacitor Banks are typically provided with double or single bushing capacitors that are protected from case rupture by expulsion style fuses. Current limiting fuses can be provided when requested.

Blown Fuse Detection: Blown fuse detection systems provide an external indication (typically by a strobe light or an externally located enunciator panel) of a blown fuse condition.

Figure 2 – Typical Stack Rack Capacitor Bank Showing Switches, Potential Transformers,

Transient Inrush Reactors, and Capacitor Bank Rack

Pros –

Stack-Rack Capacitor Banks

• Initial equipment cost is low.

• All electrical components are fully visible for viewing while bank is energized.

• The Stack-Rack Capacitor Bank is simple to expand by replacing smaller capacitors with larger capacitors or by specifying empty slots for future placement of capacitors.

• Expulsion style capacitor fuses are inexpensive to replace.

• Banks are available at the medium-voltage level (2.4kV through 34.5kV) and high-voltage level (voltages higher than 34.5kV).

• Stack-Racks are covered under ANSI and IEEE standards that all manufacturers should comply with.

Cons--

Stack-Rack Capacitor Banks

• Stack-Rack Banks provide a liability due to exposed live parts.

Installation cost may be higher for the following reasons:

1.      Installation time is high due to a high level of assembly.

2.      Installation by trained personnel is required due to assembly requirements.

3.      Bucket Trucks and Cranes are required due to assembly requirements and height of work.

• Stack-Rack Banks are exposed to air-born contaminants that can cause tracking and eventual insulator flashover.

• Stack-Rack Banks are exposed to wildlife that can cause phase and ground faults.

• Stack-Racks are difficult and more costly to maintain, as they can be as high as 25’ to 35’.

• Stack racks are not esthetically pleasing, as they are tall and built with structural steel (or aluminum) channel and angle. 

• Stack-Rack electrical components are exposed to the environment and are venerable to chemical splash, spillage, fumes, sunlight, and weather.

• Stack-Racks require a large footprint and clearance requirement, taking up valuable substation space.

• Stacks Racks are prone to installation error and require field-testing due to assembly requirements.

Pole-Top Capacitor Banks

Figure three shows a typical Pole-Top Capacitor Bank. These banks are normally shipped from the supplier assembled and ready for interconnection. The banks are typically placed near the top of the pole, under the phase conductors. Control voltage for the capacitors switches and other associated controls are obtained from a separate control power transformer (not shown in Figure 3). The majority of banks are supplied as single step automatic banks or fixed banks. The single step automatic banks are normally controlled based upon Voltage, Current, Var, Time, or Temperature. These banks are predominantly purchased by electric utilities. Key features that are normally supplied with a Pole-Top Capacitor Bank are as follows:

• Single-phase double bushing or single bushing capacitors (up to 9 in a single rack).

• Single-phase vacuum switches or oil switches.

• Single step controller when the bank is being applied as an automatic bank (normally mounted at the base of the pole).

• Single-phase cutouts for disconnecting and group protecting the capacitors from case rupture (normally mounted separately on a cross-arm).

• Single-phase control power transformer (normally mounted separately below the capacitor bank).

Figure 3 – Typical Pole-Top Capacitor Bank Showing Switches,

Capacitor Rack and Capacitors

Pros –

Pole-Top Capacitor Bank

• Low Equipment cost.

• Simple Design.

• Cutout fuse links are inexpensive to replace.

• Substation space is not required as capacitor bank is placed on pole.

Cons –

Pole-Top Capacitor Bank

• For industrial and commercial applications, the disconnecting means does not meet the National Electric Code.

• Pole-Top Banks are difficult to maintain because they can be as high as 35 feet and are normally placed in close proximity to an overhead medium voltage line.

• Pole-Top Banks are exposed to air-born contaminants that can cause tracking and eventual insulator flashover.

• Pole-Top Banks are exposed to wildlife that can cause phase and ground faults.

• The electrical components of Pole-Top Banks are exposed to the environment and are venerable to sunlight, weather and air-born settlements.

• Pole-Top banks are venerable to single phasing due to the widespread use of cutouts for protection.

Conclusion

This document has presented the Pros & Cons for the different types of medium voltage capacitor banks that are available in the market place.  Each type of bank has their Pros and Cons, and it is important that each capacitor bank application be evaluated based upon these Pros and Cons.