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Power Factor
If you are concerned about power factor charges on your plant’s electricity bill. Call your utility. They can tell you how much you are being charged for your power factor, how much you stand to save by correcting it, and give you an estimate of how much it will cost you to do so
Electrical utilities often have a charge for low power factor on their monthly bills. This charge can be confusing unless you understand what power factor is. Power factor is the ratio of useful power to total power in an alternating current (AC) circuit. In alternating current produced by electric generators, voltage and current in the circuit rise and fall like waves, with the current wave typically lagging the voltage wave. As current flows in the wire, a magnetic field forms around the wire in proportion to the magnitude of the current and in a direction that depends on the direction of the current flow. As the current rises and falls, the magnetic field changes, inducing a counter-voltage in the circuit. This counter-voltage lowers the instantaneous voltage in the circuit and makes it appear that the voltage wave is leading the current wave.
The magnitude of the magnetic field surrounding a wire is generally small, however, if the wire is wrapped around an iron core such as in a motor or transformer, the magnetic field may be many times larger, resulting in greater changes in the magnetic field over the cycle. Items in a circuit that increase the magnetic field generated also increase the inductance of the circuit, causing the current to lag farther behind the voltage in the AC cycle.
The addition of capacitance in an AC circuit has the opposite effect of inductance, causing the current wave to approach, and in extreme cases even lead, the voltage wave.
At any point in the AC cycle, the instantaneous power in the circuit is the product of the voltage and the current. The resultant power is measured in watts. By measuring the instantaneous power at even time increments over the cycle and averaging the result, you could obtain the true power in the circuit. It is difficult to measure the instantaneous power, however, because the voltage and current are changing from positive to negative and back to positive 60 times a second. Instead, true power is calculated using the root-mean-square (RMS) values for current and voltage. The RMS voltage or current value is what you measure with a standard AC voltage or current meter.
Reactive Power in a circuit represents work done at the generator to compensate for power losses resulting form the inductive loads on the circuit. Reactive power does not represent useful work done by the circuit.
Apparent power is calculated as the product of the RMS current and voltage in a circuit and is often expressed as EI. Apparent power is the power the utility supplies at the electrical meter.
Power factor is the ratio of true power to apparent power in a circuit. In AC circuits, the true power is less than or equal to the apparent power, and so the power factor is always less than or equal to one.
Engineers often use the power triangle to describe the relationship between these quantities. In this triangle, the length of the hypotenuse of a right triangle represents the apparent power, one of the acute angles is used to represent the phase difference, the adjacent side to represents the true power, and the opposite side to represents the reactive power.
If E is in volts and I is in Amperes, power is calculated in Watts and reactive power is calculated in Volt-Amperes-Reactive or VARs.
Because utilities lose some of their generating capacity to the reactive power load, utilities try to keep the reactive power low (and the power factor high) in the loads they service.
Low power factor can also be caused by electronic equipment that generate harmonics of the power supply frequency and affect the current or voltage profile in the electrical circuit. In particular, electronic dimmers and electronic power supplies that rely on solid state diodes or silicon control rectifiers (SCRs) can develop these harmonics. Low power factor caused by these loads is generally not a problem unless one-half or more of the electrical load can be attributed to electronic loads.
Utilities charge for low power factor using several different methods. Often, utilities measure reactive power in commercial or industrial loads and have a charge based on the average amount of reactive power (in kilo Volt-Amperes-Reactive or kVAR) measured during a billing period. At other times, they may charge an additional billing cost if the average power factor falls below a certain value (generally between 0.8 and 0.9) in the billing period. The utility may simply charge on the basis of apparent power (in kilovolt-Amperes or kVA) instead of kilowatts (kW) used in a plant. At other times there may not be power factor charges at all. If there is a charge, it should be apparent on the bill. If the bill is unclear, call your utility and they should explain the power factor costs to you.
Repair of low power factor involves the addition of power factor correction capacitors at your site. The total amount of capacitance required must be sized specifically for your electrical load. In general, you would like to install enough capacitance to raise your power factor to 95%. The best source of information on how much capacitance you need and how much it can save on your electric bill is the local electrical utility.
Remember, if you are concerned about power factor costs on your electric bill, contact your local utility. They too have an interest in solving power factor problems at your site.
For further information
contact
Your local electrical utility,
Your Engineering Field Division, or
Energy & Utilities Department
Code 22
Naval Facilities Engineering Service Center
Port Hueneme, California 93043-4328
Telephone 805-982-3486
Autovon 551-3486
FAX 805-982-5388
