Few Things That Capacitors Do Perfectly

Capacitors provide tremendous benefits to distribution system performance. Most noticeably, capacitors reduce losses, free up capacity, and reduce voltage drop. Let’s go a little bit into details.

Losses and Capacity 

By canceling the reactive power to motors and other loads with low power factor, capacitors decrease the line current. Reduced current frees up capacity; the same circuit can serve more load. Reduced current also significantly lowers the I²R line losses.

Voltage drop 

Capacitors provide a voltage boost, which cancels part of the drop caused by system loads. Switched capacitors can regulate voltage on a circuit.

If applied properly and controlled, capacitors can significantly improve the performance of distribution circuits. But if not properly applied or controlled, the reactive power from capacitor banks can create losses and high voltages.

The greatest danger of overvoltages occurs under light load. Good planning helps ensure that capacitors are sited properly.

More sophisticated controllers (like two-way radios with monitoring) reduce the risk of improperly controlling capacitors, compared to simple controllers (like a time clock).

Capacitors work their magic by storing energy. Capacitors are simple devices: two metal plates sandwiched around an insulating dielectric. When charged to a given voltage, opposing charges fill the plates on either side of the dielectric. The strong attraction of the charges across the very short distance separating them makes a tank of energy.

Capacitors oppose changes in voltage. It takes time to fill up the plates with charge, and once charged, it takes time to discharge the voltage.

Circa 1963 vintage pole with a capacitor bank along with black porcelain insulators (photo credit: Astro Powerlines via Flickr)

On AC power systems, capacitors do not store their energy very long – just one-half cycle. Each half cycle, a capacitor charges up and then discharges its stored energy back into the system. The net real power transfer is zero.

Capacitors provide power just when reactive loads need it. Just when a motor with low power factor needs power from the system, the capacitor is there to provide it. Then in the next half cycle, the motor releases its excess energy, and the capacitor is there to absorb it.

Capacitors and reactive loads exchange this reactive power back and forth.

This benefits the system because that reactive power (and extra current) does not have to be transmitted from the generators all the way through many transformers and many miles of lines; the capacitors can provide the reactive power locally. This frees up the lines to carry real power,power that actually does work.

Elimination of penalties //

A high power factor eliminates penalty dollars imposed when operating with a low power factor. For many years, most utilities demanded a minimum of 85% power factor as an average for each monthly billing.

Now many of these same utilities are demanding 95%…or else pay a penalty!

The actual wording or formula in the utility rate contract might spell out the required power factor, or it might refer to KVA billing, or it might refer to KW demand billing with power factor adjustment multipliers. Have your utility representative explain the particular rate contract used in your monthly bill. This will insure you are taking the proper steps to obtain maximum money savings by maintaining a proper power factor.

Primary and Secondary Power Capacitors

Capacitors for power factor correction are usually connected in shunt across the power lines. They can be energized continuously or switched on and off depending on load changes.

Two kinds of capacitors perform power factor correction: secondary (low voltage) and primary (high voltage). These capacitors are rated in kilovars.

Secondary (low voltage) capacitors

Low-voltage capacitors with metallized polypropylene dielectrics are available with voltage ratingsfrom 240 to 600 V over the range of 2.5 to 100 kvar, three-phase. These capacitors are usually connected close to the lagging reactive loads on secondary lines. Low-voltage capacitors can either reduce the kVA requirements on nearby lines and transformers or allow a larger kilowatt load without requiring higher-rated lines or transformers.

Low-voltage capacitors (on photo: 150kVAr capacitor bank; credit: capacitor-banks.com)

Primary (high voltage) capacitors

High-voltage capacitors for primary high-voltage lines have all-film dielectrics and are available with 2.4- to 25-kV ratings over the range of 50 to 400 kvar. By connecting these capacitors in series and parallel arrangements, higher kvar ratings can be achieved. Because modern high-voltage capacitors consume lower watts per kvar than low-voltage capacitors, they can be operated more efficiently.

High-voltage capacitors for primary high-voltage lines (on photo: 115-kV Cap Bank; credit: ece.mtu.edu)

High-voltage capacitors for overhead distribution systems can be mounted on poles in banks of 300 to 3600 kvar at nearly any primary voltage up to 34.5 kV, phase-to-phase. Pad-mounted capacitors for raising the power factor in underground distribution systems are available in the same range of sizes and voltage ratings.

High-voltage capacitors for overhead distribution systems

The increasing use of motor-driven appliances and building service equipment has increased overall power loads as well as the inductive kvar on most power systems.

It is desirable to cancel them because:

• Substation and transformer load capacity can be taxed to full thermal limits.
• High inductive kilovar demands can cause excessive voltage drops.
• Local utilities charge power factor penalties.

The size of the power factor correction (number of kvar) that must be injected into the electric power system determines the method to be used. If the load is less than 500 kvar, capacitors can provide the capacitive reactance to cancel the inductive reactance, but if the load exceeds 500 kvar, a synchronous condenser is commonly installed.

Also, if there are large, rapid, and random swings in kvar demand during the day, a synchronous condenser is preferred. However, if the changes in kvar demand are small and can be corrected with capacitors, incremental capacitor banks provide a more practical solution.