power factor, inductive or capacitive??

[mriechert]

Hello

Why is a capacitive power factor undesirable?

I am currently working on a project and have managed to build a controller that corrects the power factor by generating a greater or lesser inductance on the system.

The system load is inductive but a fixed capacitor is used to make the load capacive and hence a variable inductor can be used to correct the power factor and it bring it to unity.

However I have been adviced to not bring it to unity but rather make it a little inductive. Becuase I should not risk a capacitive power factor.

Why is this the case?
Surely the same amount of current would be drawn from a load with the same power factor no matter if it is inductive or capacitive.. the only thing I can assume is that it has something to do with the fact that the current is leading the voltage when the PF is capacitive..

Can anyone enlighten me please...

Michael

 

[electricshock]

hello *chert
i feel capacitive (leading) p.f will increase the output voltage at no load conditions which can be undesirable. u can recall Ferraty Effect in long transmission lines!!! because of their high capacitive values, output voltage shoots up at no loads which is undesirable.
when u talk of unity p.f there is no capacitive or inducive current flowing but in case your power factor correction system fails u may get high voltages because of leading p.f

 

[electricshock]

let me correct myself, at unity p.f your inductive and capacitive currents are equal and resulting current flowing is in phase with voltage.

 

[Nemrod]

Usually it is recommended by many standards to keep PF at 0.92 instead of "Unity". In PF at unity it may easily slide to the capacitive "risky" side and that is the reason why 0.92 is considered a normal level.
By the way, why wouldn't you use a PF corrector with several low rated capacitors?

 

[dpc]

Traditionally power factor correction capacitors have been applied at induction motor terminals. If too much capacitance is used, there is a risk of the motor becoming "self-excited" by the capacitors when the motor is disconnected from its power source. This can lead to high voltage stresses on the motor.

So in most cases, we correct motor power factor such that the power factor at no load is just slightly lagging (0.98 or 0.97).

There is nothing inherently wrong with a leading power factor. You do have to watch out for high voltage conditions.

For the average utility customer, there is no benefit for operating at a leading power factor. In fact, under the new Bonneville Power Administration power factor penalty, you would be charged for leading power factor greater than 0.97, same as for lagging power factor.

 

[mriechert]

thankyou everyone for your responses,

so it would seem that the biggest problem with a leading power factor is with electric motors. Or no load scenarios where a higher than expected voltage may occur considering that the voltage leads the current when the load is capacitive.

 

[jbartos]

Suggestion: If nonlinearities exist, there may be a concern regarding harmonics and subharmonics. For example, the transmission lines that incorporate capacitances in series with the transmission line experience subharmonics that are detrimental to rotating machinery.

 

[infomaniac]

A voltage rise at no load conditions might be of concern but generally is not. The voltage rise is pretty much directly related to the amount of KVAR that is 'sitting' on the low voltage side of the customers feeding transformer when there is little or no load.

the approximate voltage rise at little or no load on the transformer with the capacitor connected would be as follows:

V rise = (KVAR * transformer %Z)/KVA of Transformer

Generally the voltage rise will be very small. Typically, if the calculation results in a 2-3% voltage rise, this is acceptable.

In regards to connection to the motor terminals, if the capacitor is oversized for the motor and it is connected at the motor terminals, I suggest to simply place a contactor in series with the capacitor so that the contactor will de-energize when the motor is stopped and will isolate the capacitor from the motor when in the off position.

 

[cuky2000]

I can no see any advantages from the energy consumption point of view to expend additional capital resources in larger rated equipment to compensate the PF above 100 %. For example, the KWH measure with a PF=110% and a PF=90% will be the same.

On the other hand, operating at PF=100% or near 100% there are a risk of resonant circuit creating overvoltages level that could damage the equipment insulation.

 

[infomaniac]

cuky2000, the resonant location has nothing to do directly with the PF %. You can easily resonate at a critical frequency when the power factor is 85%, 90%, 95%, or any other %. The resonant location is determined by a combination of the capacitance on the network and the shortcircuit value on the same bus. You are correct that correcting the PF to any value more or less will not change the KWH significantly. However, if there is a penalty for running below a certain power factor, then the payback needs to be calcualted based on the equipment cost needed to get to the non-penalty range. I have designed and recommended many systems in locations where the utility bills on KVA which essentially means the customer is penalized for anything below 100% power factor.