March† 1996††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††† Volume 2†† Issue 5
IMPLEMENTING POWER FACTOR CORRECTION
††† Correcting power factor in a production facility often reduces the electricity bill by thousands of dollars a year.† That was the message in the last issue of $mart Energy User.† This issue deals with actually doing it, with getting the technical advice you need, selecting the best correction option for your plant, and pricing out the job.
††† Letís assume that the Energy and Minerals Section has done an electrical energy audit of your facility.† We came, took measurements, and found that your plantís peak electrical demand was accompanied by a low power factor.† We then calculated what the annual electricity bill savings would be if the power factor was corrected.† We may also have come up with a rough cost estimate, based on the experience of others, and calculated an approximate payback figure.† You liked the preliminary numbers, and would now like to proceed.† Whatís the next step?
1. SELECT THE RIGHT POWER FACTOR CORRECTION APPROACH FOR YOUR SITUATION
Because production facilities differ greatly from each other, there is no single approach to power factor correction that is best in all situations.† Someone must analyze your situation and decide which approach is best for your plant.† Depending on plant complexity and other issues, that analysis might be done by
∑ a qualified in-house person,
∑ an electrical contractor who is experienced in power factor correction,
∑ a manufacturer of P.F. correction systems, or
∑ a professional electrical engineer.†
The following questions indicate what needs looking into, and might also help you decide who should do the looking.
∑ Correct P.F. at the service entrance, at individual motors, or both?
The electric utility is concerned with the plantís power factor at the point of metering, not whether the compensating capacitors are installed at the electrical service entrance or at individual motors.† Because installing one capacitor bank at the service entrance is less expensive than installing individual capacitors at each motor, this is the approach that is usually taken.† There are exceptions, however.† Perhaps the facility has only one or two large motors.† Or a large motor causes an excessive drop on the circuit that feeds it.† In these and some other situations, compensation at the motor can be more cost effective.
∑ Install capacitors only, or capacitors plus an automatic P.F. controller?
The electric utility is also concerned that any power factor of less than 100% be inductive, not capacitive.† A capacitive (leading) power factor can occur if an inductive (lagging) power factor has been overcompensated by putting too much capacitance across the line.† This can happen if, for instance, a fixed capacitor bank is installed at the service entrance and then some large motor loads are subsequently turned off.† Compensating individual motor loads avoids this problem.† So does installing a power factor controller.
††† A power factor controller continuously
monitors the service entrance power factor, and connects, at any given time,
only the amount of capacitance needed to accomplish the desired level of
correction.† These controllers often
come packaged together with a capacitor bank in a steel cabinet.† Here the engineering has been done by the
manufacturer, eliminating (in many cases) the need for on-site design by an
engineer, and eliminating that cost.
P.F.-correction capacitor banks can be supplied together with a switching controller in a metal cabinet.
∑ Does plant expansion need to be considered?
If plant expansion is likely at some point in the near or medium-term future, it makes sense to think about the relationship between expansion and power factor correction.† As mentioned in the last issue of $mart Energy User, correcting power factor can sometimes postpone the day when a higher capacity electrical service entrance will be required.† On the other hand, if the electrical room must be expanded now to house capacitor banks and a P.F. controller, perhaps it should be made large enough to allow for future additions.
∑ How will P.F. correction affect the supply voltage?
Power factor correction tends to increase voltage levels within the plant.† Where plant voltages are on the low side, this increase is welcome.† But if the voltage is already on the high side, it may not be.† Someone should estimate the magnitude of this increase, consider its possible effects, and contact the electric utility if it would be unacceptably high.
∑ Are power-line harmonics a factor?†
With our present monitoring equipment, the Energy and Minerals Section can monitor the overall harmonic level on your power circuits.† If the level we measure is sufficiently low, the designer of your power factor correction system can safely ignore harmonics.† Above a certain level, however, power line harmonics can cause capacitor heating and even failure.† Our meter readings are just a first-order indicator.† If they indicate that a problem may exist, it is important to have more detailed measurements made ó measurements that indicate the amplitude of specific harmonics such as the fifth, seventh, ninth, etc.† Some manufacturers of† P.F.-correction equipment will make these measurements for you.
∑ Where will the capacitor banks and the controller go?
Any changes or additions to a plantís electrical system must meet electrical code requirements.† There are usually costs associated with this, and they should be considered when deciding what approach to take.† For instance, if installing a capacitor bank and controller also means enlarging the electrical room, it might be less expensive to eliminate the controller and mount capacitors near individual motors.
2. SETTLE ON SPECIFIC HARDWARE, AND GET A FIRM PRICE ON THE† JOB
††† Once the best approach has been pinned down, the next step is to decide what specific hardware to purchase, and to get firm quotes.† Unless your organization happens to be blessed with a staff person experienced in specifying and installing power factor correction systems, or you have engaged an electrical engineer to analyze your situation and design your solution, you will probably want a competent electrical contractor to price out the job for you ó preferably a contractor who works regularly with suppliers of capacitors and P.F. control systems.