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.
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.
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.
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.
Secondary (low voltage) capacitors
Low-voltage capacitors with metallized polypropylene dielectrics are available with voltage ratings from 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.
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 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.
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.
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.
- Electric Power Distribution Equipment and Systems – T.A. Short (Get it from Amazon)
- Handbook Of Electrical Design Details – Neil Sclater, John E. Traister (Get it from Amazon)
May I know how to calculate the require capacitor to improved the voltage at load end?
I have single phase 5HP electric motor. Supply voltage is 230 volts. when starting the motor.
the voltage drop to 180 volts. size of wire 8.0 mm square. 75 meters distance. Thank you.
We are in a container deport station where we do alot of lifting works with the heavy craines , our problem is we exprerience frequent damages with our UPSs, AVR and facility lifts, kindly help us with a good solution to this , can installation of a sizable capacitor bank at the input to the building DB help solve the problem if so what parameteres are we required to provide you with to provide us with a solution,
Hello every body
Any one guide me that we install a capacitor with our factory generator
which capacitor we should use for 80 KW load
Thinking about adding some capacitance reactive loads to my residence 200amp breaker panel. how should i go about this plan?
I recently discovered an open door on a Remote Capacitor Control box on a PGE power pole. Does exposure to weather cause damage to this unit? It has a rubber gasket which would provide a seal when closed. I have photos & attempted to contact PGE 3 days ago. It’s raining now.
WE HAVE FREQUENT PROBLEMS OF VOLTAGE DIP IN THE 11KV MEDIUM VOLTAGE SWITCH GEAR DUE TO THE SOURCE VOLTAGE BEHAVIOR WHICH VARIES FROM 10% TO 20% WHICH WE CAN’T AVOID. MY QUESTIONS ARE :
1. ANY SOLUTION TO INSTALL AT OUR SWITCH GEAR TO MINIMIZE THIS PROBLEM ?
2. WHETHER IF WE CAN INSTALL CAPACITOR BANKS IN THE MEDIUM VOLTAGE TO CORRECT THE SUDDEN VOLTAGE DROP ? IF SO THE NORMAL CAPACITOR BANKS ARE ENOUGH ?
In distribution network capacitor is not widely used, apart from the use for storing DC voltage for the circuit breakers.
we are highly interested in your capacitor bank we need 800kva capacitor bank kindly furnish us with the cost and other details
What if kvar demand is very low, what effect would a capacitor have on the system?
Thanks 4 your great efforts , your web is my best way 2 learn thank you very much .
keep doing, keep going
Referring to the capacitor activity, when using a diesel generator as a supply ac source feeding a load consist of several 3 phase motors connected to irrigation pumps . the load connected to a power factor correction unit to improve the load p.f i.e to reduce the current and total KVRR.
I am asking is there any risk to the generator AVR leed to stop the generator ? please ur technical advice thanks
iam highly thankfull to u for encouraging me to discover something new by getting deep knowledge through your nice ideas.
sir i have a doubt that can we do something for the utilisation of the high fault current in the power system , thru the help of capacitors.
As these fault current just go usseless thru protection devices.
IF it is possible than please provide me some idea with that so ican proceed my research on that.
( Btech pursuing)