Utility penalizing customers for excessive reactance?

[LARGrizzly]

My company has been forced to pay a penalty because our reactive load on the utility is too high. I'm not sure what exactly this all means. I understand what reactance is but I don't understand where it is coming from. The rep from the utility said it was coming from our inductive motors.

Our electrical bill is based on a percentage of EITHER our KW usage or our KVA usage whichever is higher but I'm not sure why there is a difference between the two. The utility has suggested that we install capacitors on or near the service entrance in order to smooth out the demand.

Where would I start to look in order to reduce the 'reactance' or is it even something we can do anything about? I dont' trust the utilities as far as I can spit... and I drool!

 

[DanDel]

The reactive load they are talking about is called low Power Factor. Check out my company webpage for a quick explanation.

http://www.neete.com,

and click on the 'Power Factor/kW/kVA/kVAR Relationships' text in the left column. This is a very common problem and capacitor installation is the fix, but maybe where the utility says they should be installed is not the best for your company in terms of cost, efficiency, and reliability.

 

[jstickley]

LARGrizzly --

Sorry to hear that you don't trust your utility, but they do know what they're talking about (at least I hope so, considering they're generating your bill).

The penalties that you are being assessed are quite common. Without getting into details (I'll do that in a moment), most utilities impose a requirement on industrial customers to maintain a certain power factor. If that power factor is not met, penalties are charged. Believe it or not, there is a valid reason for doing so.

First off, reactive load / power factor. When a load is connected to an AC system, often times, depending on the characteristics of the load, there is a difference in the phase angle of the voltage applied to the load as compared to the current passed through the load. When the load is purely resistive, there is no difference in the phase of these waveforms. In a purely inductive (reactive) load, there is a phase angle difference of 90°, with the current lagging the voltage. In a purely capacitive load, there is also a phase angle difference of 90°, with the current leading the voltage. The power factor is basically a way of describing this relationship of the current and voltage waveforms. Mathematically, power factor is equal to the cosine of the angle difference between the two waveforms. If there is a phase angle difference of 20°, the power factor of the load is then 0.9397, or 93.97%.

The significance of this? When you have a purely resistive load, all current that is supplied to the load is actually used to do work. When you have capacitive or reactive load, the current supplied is stored by the electric and magnetic fields created by the capacitance and reactance. Because of this, if you have a load with a higher reactive component, it requires more current to supply the load than if your load is purely resistive.

The power factor is used to measure this. When you have a low power factor (say, 90%, for example), it requires 11.1% more current to supply the load than if you had a power factor of 100% (purely resistive load). This excess current requirement can mean several things: larger transformers to serve your load, higher power loss on the power system, lower voltages on the system, etc.

The good thing is that you can correct poor power factor. Remember above, where I said that capactive loads cause the current to lead the voltage? Adding capacitors to a reactive load basically brings the phase angle between current and voltage closer together, and thus reduces the overall current requirements of the load (and as such improves the power factor of the load).

In summary, what your utility representative told you was correct -- since you apparently have a poor power factor due to reactive load (as indicated by the ratio of your kWH usage to kVA usage, this is also a way to calculate power factor), you can add capacitors to improve your power factor.

Good luck....

 

[SidiropoulosM]

Any reactance in the electric circuit will cause the voltage and current to be out of phase. Motors have inductive reactance which causes a phase displacement such that the current does not peak at the same time as the voltage, causing a reduction of the real power delivered to the motor. A larger current is therefore required to deliver the power required by the duty of the motor. The usual remedy is to install a shunt capacitor which adds capacitive reactance to the circuit and brings the current back to a smaller phase difference with the voltage. These are called power factor correction capacitors. I have tried to describe this in the simplest possible terms, avoiding terminology such as "reactive power" and "phasors". The cost of the capacitor will depend on its power rating and the degree of power factor correction desired. You should consult an electrical engineer on this. Michael Sidiropoulos

 

[LARGrizzly]

Thank you. These were all extremely helpful and lucid replies. This helps clear it up.

Sincerely, thank you.

 

[busbar]

One caveat to that already posted—plain-vanilla power-factor-correction capacitors can be trouble {er, self destruct} if a significant part of plant load is DC or variable-frequency AC drives because of their tendency to sometimes generate harmonic currents and voltage. Be sure that is evaluated before ordering a palletload of capacitors.

Someone here may have time to reply with more specific suggestions, if you can post some typical billing numbers and a link to your utility’s applicable tariff.

 

[Acajun]

Here is BC Hydro's explanaition and Power Factor surcharges. This is probably as clear an explanation as you will find.
There are also some very interesting Technical Guides with tips on reducing your plants power consuption.

http://www.bchydro.com/business/guides/guide834.html

Hope this helped

Jp