Electrical Capacitors in AC Circuits
In this month’s Tailgate, we will discuss the functions of a capacitor in an alternating current (AC) circuit, including charge and discharge, applications and connections in power circuits, and capacitor safety.
An electrical capacitor is an electrical device that stores up electricity or electrical energy and improves an AC circuit’s power factor. It has three essential parts. Two are usually metal plates separated and insulated by the third part, known as the dielectric. The capacitor’s charge is dependent upon the size and spacing of the conducting plates and the type of insulating or dielectric medium between the plates.
All capacitors, regardless of type, are designated by their charge capacity. For appliance circuits like motors and high-intensity discharge lighting, capacitors are designated by the farad, a unit of electrical capacitance named after British scientist Michael Faraday. In power distribution, capacitors are designated in kilovolt-amperes reactive, or kVARs, for simplicity of application. Demand meters measure power factor demand in kVARs. If a customer’s motor load places 700 kVARs inductive on the line, it can be corrected by connecting 700 kVARs capacitive on the line. It’s not that simple, but you get the idea.
Capacitor Charge and Discharge in an AC Circuit
A capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. You know that a battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons at the other. A capacitor is a much simpler device, and it cannot produce new electrons – it only stores them.
As an example, when you see lightning in the sky, you are seeing a huge capacitor. One plate is the cloud, the other plate is the ground, and the lightning is the charge releasing between these two plates. Obviously, in a capacitor that large, you can hold an enormous amount of charge.
Applications in Power Circuits
Capacitors must be carefully and properly applied. As previously stated, the purpose of a capacitor is to improve the power factor of the circuit. A capacitor only corrects the power factor from the capacitor back to the system. Capacitors have no power factor effect between the capacitor and the reactive load causing the power factor. Improperly applied capacitors can supply more reactive current than the load requires, resulting in a leading power factor and an increase in losses instead of a decrease. That is why capacitors are located after careful system study by qualified engineers.
Connections in Power Circuits
The installation of a single capacitor or a bank of capacitors is a simple procedure once the correct size and installation location have been determined. In many respects, a capacitor is much simpler to install than a transformer because there are no secondary bushings and the capacitor is a sealed unit. Some units have two bushings while other units have one.
Distribution capacitors with two insulated bushings are usually connected between phases, but can be connected phase to ground. Capacitors with one insulated bushing are usually connected with the phase to the insulated bushing and the case to ground. In substation installations, distribution voltage capacitors are paralleled, dividing transmission phase voltages. They are banked on an insulated frame that is part of the interconnection operating at the transmission phase voltage. Anyone working on any capacitor should be well aware of how it is connected and at what voltage it operates.
You should also be aware of fixed capacitors and switched capacitors. These terms simply refer to the manner in which the capacitor is energized. If it is fixed, the bushing connected to the source goes directly to a fused cutout. This cutout is the only means of energizing or de-energizing the capacitor. In the case of a switched capacitor, the bushing connected to the source goes to a switching device connected in series between the fused cutout and the capacitor. The purpose of this switching device is to allow the capacitor to operate at times when it is most needed. There are many types of controls that determine if the capacitor needs to be on or off depending on the requirements of the system. Following are examples of controls and what they are most used for:
• Time control is used in areas where the load is known to occur at specific intervals either because of industrial use or residential requirements.
• Current control is used in areas where a customer’s load is intermittent and not always present at the same time of day.
• Temperature control is used in areas where seasonal changes will put more inductive load into the system (e.g., air conditioning). There are primarily two main temperature controls available: on at 85 degrees and off at 65 degrees, or on at 90 degrees and off at 70 degrees.
• Voltage control is used in areas where load causes a drop in system voltage that can be easily controlled. Voltage controls also are often used in conjunction with time, current and temperature controls.
• Manual control is used to more easily disconnect a switched bank that is often used for seasonal load.
Capacitor Safety
Although capacitors are simple devices, they are extremely dangerous after they have been disconnected from service because they can retain a charge. They are required to be built with bleed resistors that reduce their voltage to fewer than 50 volts after five minutes. However, never take anything for granted. It is imperative to wait five minutes after disconnect and then short across the bushings with a jumper wire using a shotgun stick. Before beginning work on any capacitor that is mounted on a pole, always follow these procedures because the capacitor can hold a charge without any indication that it is doing so.
When removing capacitors from service, the following steps cannot be emphasized enough:
• Disconnect from the source.
• Wait five minutes for the charge to drain off.
• Short-circuit the bushings or, in the case of single-bushing capacitors, place a short circuit between bushing and case.
• Keep the short-circuit connection in place until the capacitor is to be connected for service.
About the Author: John Morton, CUSP, began his career in the electrical industry in 1970 as a groundman for Houston Lighting and Power, now known as CenterPoint Energy. In 1997 he accepted a position at Texas A&M University’s extension service as a trainer for the electrical and communication industry, and in 2004 he assumed his current role as director of safety and training for Willbros T&D Services in Texas.