PF Correction Location
Considerations for Choosing the Correct Location
Your PF correction location is a fundamental part of the correction.
Poor power factor is most commonly corrected with the insertion of a capacitor or a capacitor bank (a grouping of capacitors) at the desired location to perform the correction. If you don't correct for power factor, your source will carry the excess current associated with the building and collapsing of magnetizing fields. This limits the capacity of all the components along the transmission path.
The capacitor provides current that Leads the voltage. This combines with the current associated with the magnetic fields that Lags the voltage and cancels it out, if sized correctly. Synchronous machines (either motors or generators) can also supply leading current, similar to a capacitor, by over-exciting their rotating field. Also, solid state devices are now available for supplying leading current, but for this discussion the capacitor will be used for correction.
Regarding PF correction location, as stated at the end of the Power Factor Capacitor section, if we connect a capacitor at the motor, the capacitor and the magnetizing field of the motor take turns feeding each other electric current. This is reactive load being supplied by the capacitor. When the capacitor discharges into the motor it builds the magnetic field. When the magnetic field collapses as a result of the voltage changing directions, the currrent is pushed out of the motor and into the capacitor and builds the charge.
Putting in too much capacitance can cause conditions of voltage spiking and instability. As a result, a common practice is to limit the capacitor's reactive load to 90%, or less, of the magnetizing load.
Because the capacitor supplies the magnetizing current (the Reactive Load), the cable coming from the source of power need only supply the load current (Real Load). The cable and other components upstream of the point where the capacitor ties in, deliver only load current to that motor, or the "corrected current" and therefore don't need to be sized to also handle the magnetizing current of that motor. "Corrected Current" has been emphasized because the current flow to the load has only been corrected upstream of the point in the system where you tie in the capacitor. There is no change to the current flow downstream of the tie-in point, which is still carrying the magnetizing current. Thus the PF correction location is key to accomplishing your goal.
Note: Choosing a PF correction location at a motor merely corrects the amperage demands on upstream equipment imposed by that motor. An upstream transformer may still be delivering disproportionately large amount of magnetizing current to other motors.
The real and reactive loads at an MCC, at a substation transformer or a feeder cable are simply the sum of all the real and reactive loads that it supplies. So if an MCC supplies 3 motors that each draw 2 kW and 0.3 kVARs, the feed to the MCC is delivering
2 + 2 + 2 = 6 kW
0.3 + 0.3 + 0.3 + 0.9 kVARs
A system will have several power factors. Every location in the system where the current changes, which is every branch, will have a different power factor.
Correcting Power Factor at the Motor
By applying the correction near the motors in your plant, which is the lowest level, you are helping everything upstream. This is the most beneficial PF correction location, but is expensive if not done during initial construction. An added benefit is that when the motor is turned off, the capacitor, which is not needed, comes out of the circuit.
Correcting Power Factor at the MCC
In the cases where we have several motors fed from a Motor Control Center (MCC), a convenient PF correction location is at this one point. This would be a larger capacitor or a capacitor bank (a group of capacitors) for all loads powered from that location.
What will this do? Again, it relieves the equipment upstream of that PF correction location from having to supply the VARs (magnetizing current) associated with these motors. This means less amperage on the components that deliver the power to the MCC. The situation downstream has not changed. The cables from the MCC to each of the motors still delivers the reactive power to the motors. This may not be a problem if the conductors are adequately sized such that the excess amperage does not cause a voltage drop to the loads.
One additional disadvantage is that a bank of capacitors may provide too much reactive load when some of the loads are turned off. Switching segments of capacitance into and out of the system may be required as the load changes. There are reliable systems available that automate this function. See my list of "Recommended Resources".
Correcting Power Factor at the Substation Transformer
If you need to reduce the kVA load of a substation transformer, you need to reduce its amps. Correcting power factor can help and possibly alleviate the need to re-feed loads from another source. One key point regarding PF correction location ....put the capacitor bank on the correct side of the transformer. Remember, you benefit the equipment upstream of the point where you tie-in the capacitor, so you tie it in on the secondary side.
* a 2 MVA transformer is operating at capacity and delivering power at a power factor of 0.7
* this means that the load that the transformer is supplying is
2MVA x 0.7 = 1.4 MWatts
*correcting to a power factor of 0.9 would mean that the 1.4MW load would now only draw
1.4 MW / 0.9 = 1.56 MVA
(that freed up 22% of the transformers capacity!)
Correcting Power Factor at the Utility Meter
If you are paying large reactive demand charges and you simply want to cut that down without correcting the power factor within your plant, your PF correction location can be to install a capacitor bank at your service entrance. Electrically, the Utility's lines receive the only benefit. But because they no longer need to deliver VARs to you, they have more current carrying capacity available on those lines to sell to others. The result: You don't pay a "Reactive Penalty".