Unity Power Factor

Power factor is a measure of the phase angles between the voltage and current in an AC circuit. Unity refers to the fact that they are in phase with each other (1.0) and act like a resistive load. That is the most efficient use of your power.

Please review the following link for more information. http://en.wikipedia.org/wiki/Power_factor

Does anyone knows websites containing power factor case studies? Not the definition but real situations of power factor improvement. On a electrical energy producer website I read that one of the methods of improving power factor is individual compensation. Is this only justifiable for very large inductive loads?

I work on a small factory where we don't have very big electrical motors like exist on heavy industry. Does individual compensation make sense on this situation?

What little I understand of power factor correction is that over compensation is just as bad as under compensation. So, individual compensation makes the most sense. The problem with individual compensation is cost of equipment if you have a lot of loads. On the other hand, if the loads are not spread out over a large area and run simultaneously, that is if your load is fairly constant, facility compensation can be very effective. In the US there are companies that will do an analysis and give a proposal for a low cost in an attempt to sell there PF correction products. You may want to see if there is such a service in you area. Also small handheld power analysers are becoming affordable for small users and could tell you a lot about where savings can be made. -- JHF

Yes individual motor compensation for each application does make sense, ad the compensated reactive load will result in reduced current in feeder conductors. Add capacitors (Run type only not start type) in parallel with motor supply conductors, monitor the current drawn by the motor/capacitor combination on the supply side of the combination of motor and capacitor, use various size (Micro farad) until minimum current is observed.

The benefit may not be realized in reduced cost of power unless the supplying utility imposes charges for poor power factor, because you pay only for the true power (Kilowatts), you do not pay for the reactive power (VARS) that are returned to the generator.

If you generate your own power as I do aboard ships, power compensation results in reduced fuel consumption.

Regards CEKM

Check earlier threads and discussion on precisely this subject.

Dear vanteru.madhu,

I don't know who told you that the US enjoys unity PF. We usually run around the same value you gave us for India. In fact those values look possibly better.

Unity power factor happens only in textbooks and fairy tales.

Some power factor regulators try to keep PF in the upper .90 region, but the process is far from perfect and has some negative effects on certain devices in the process, besides being fairly complicated to get correct. Can be quite expensive compared to the value received also.

As sb posted, detailed discussions abound on this site, simply do a search and you will be overwhelmed with stuff to read.

Regards, CJM

Dear Friend

In India, for industrial consumers, there is a penalty for operating on poor power factor ( read less than 0.8 ). Electronic meter installed for measuring electricity consumption also records power factor and so if it is less than additional amount gets added to the monthly bill.

While active pf compensation is expensive, so requires commercial feasibility also, generally pf improvement capacitors are installed or over excited synchronous motors are also used.

Thanks and regards

Ashok Toshniwal, Bangalore, India

Thanks ashoktoshniwal for the regional specifics, this is very helpful.

I have come to understand that in the case of India at least , there exist many smaller power grids, which are fed by Diesel Generators with grid load usage near or over capacity. I would expect that type of system to be very sensitive to the power factor, since power factor mainly affects the producer and supplier in fuel costs and line losses.

Line losses are much reduced if the conductors and equipment are over-sized for the expected loads, which is why domestic systems don't individually experience enough I²r (line resistance related) losses to make domestic PF correction cost-effective. Distribution level (whole neighborhood) correction can be cost-effective, but don't forget that everybody gets to share the cost by means of the power rates.

As you stated "In India, for industrial consumers, . . " so it is in most places, industry tends to use things that reverse-generate enough to affect the power factor. Since reverse-generation (CEMF) is developed in most devices that have coils of wire in their design, motors and transformers for instance, the industrial sized versions affect everyone on the grid more noticeably, and correction costs someone. Most power suppliers prefer not to have it cost them, and develop a tariff system to charge the major users with the greatest CEMF issues.

But realistically, any system running at or near peak load will affect every customer directly as the grid voltage becomes unstable. Distribution lines and equipment begin to develop heat losses when they are asked to carry greater currents than they were designed for. It only makes sense that the power producing entity seek to charge enough from the customers to cover the cost for the extra losses attributable to the oversizing of distribution equipment or the added losses otherwise.

All of the above words condense down to this;

Most electrical grid systems in the world are at or over capacity often enough, especially in industrial distribution areas, to require someone to cover the cost of poor power factor. Most power companies prefer that cost to be covered by the major contributors to the poor power factor.

Don't characterize this cost as a "penalty" but rather consider it a normal expense to large coil-based device users. They expect to pay for electricity to run their device, so why overlook the expense required to keep their device from affecting everyone else on the power grid? That must be factored into the cost of equipment and into the projected operating cost if those are estimated accurately.

Regards, CJM

Dear CJM

While the additional cost, is still called penalty & I agree to it, at least in Indian circumstances. Reason being, this "additional cost called penalty" "forces" companies to install atleast pf improvement capacitors or else they would not install even this!!! to save money in plant investment.

Utilities are compelled to add this "extra cost of electricity" which can be saved by user, if his/her system operates beyond the stipulated pf. Thus pf improvement capacitors or whatever methods, are installed in the plant to "save" on this extra cost.

In any case it is to the advantage of the consumer to operate at 0.8 pf or even higher, if possible, & irrespective of the type of load, so that atleast cable losses in his own system are reduced, forget about what is happening to utility ( do people really care about?).

Thanks and regards

Ashok Toshniwal, Bangalore, India

People care about that because the utility is going to charge for it's product, one way or the other. Why should a business customer expect to cause the utility an expense, and not have to cover the expense? Is that what you let your customers do with the product you make? Can they choose to pay you only for the material content in your product but not for the labor etc?

In order to be able to supply a customer with enough power to do his business, the utility must also supply the other unavoidable components of the power cycle. They don't come separately in the real world, and where I come from the customer base covers all the costs of manufacture, including the shipping and handling, (and hopefully some profit).

To expect the power supplier to give free to it's customers something it must pay for makes no sense to me, especially when that specific customer is personally causing the extra fuel burned and heat losses to a measurable degree.

It's not really a penalty because it is a very real cost of supplying you your power. Putting in equipment so that you do not cause the utility the extra expense let's you avoid them passing that extra expense on to you. It' simple science really.

regards, CJM

Unity power factor will occur when the inductive reactance and the capacitive reactance in the circuit are equal. While it is technically possible to achieve this, it tends to be avoided because it can create a resonance condition where large currents circulate between the (inductive) load and the capacitors. I have seen an example of this where a PFC controller was mis-adjusted so that on one of its steps it was just over-correcting past unity (~0.98 leading), and this was generating large enough circulating currents to nuisance-trip the motor overload relays.

The problem you mention (which happens more often than many believe because the compensation is usually fixed and far from the source of the problem), can be avoided if the compensation is at the motor and gets switched in when the motor gets switched on and off when its off.

This stops the large currents swirling along cables that might not be designed to take it when the inductance is situated far away from the compensation....

There are some other bad effects (if I remember correctly) that can be found on other CR4 blogs on the same subject.....

This begs the question "can amps flow 2 different directions at the same time in the same powerline?"

If not, then your understanding of how PF compensation works may need adjustment. Check the power triangle; there is no var amperage at unity PF, and very little at near-unity. What circulating current?

It's all about the timing. Think heterodyne. CJM

You appear to understand little about "Wattless power".....

Here is some uncomplicated reading for you:-

http://www.cip.ukcentre.com/Power%20Factor.htm

Andy, you do realize that when a utility places cap banks on feeders to a distribution system, the amps on that distribution system are reduced, not increased.

Harmonics are another story, and can cause the appearance of overload to some VFD's, which is why power quality issues must be resolved in those cases. This is one of the reasons PF correction is not a perfect solution without issues.

Regards CJM.

Naturally and completely self evident for anyone from electrical engineering I feel.....

There would hardly be a point in "INCREASING" the amps now would there......?

My plant ( a small 15mW CC Cogen) typically runs at .998 lagging. Power factor is also changed by the type of loads other users have placed on the grid. A synchronous motor might be the way for you to go for pf improvement.

Power Factor can be explained in different ways:

1. power factor is cosine of the phase angle between the phasor of voltage across any load and the phasor of the current through that load.
2. power factor is the ratio of Real (active) Power to input VI supplied to any load.
3. power factor depends on the type of load; for reactive load (lagging), capacitive load (leading)

BUT SIMPLIY POWER FACTOR REPRESENTS THAT HOW MUCH VI INPUT IS CONVERTED TO USEFUL ACTIVE POWER WHICH IS USED TO DO ANY WORK.

it means that the if power factor is unity (1.0) that total input power to a system or load is coverted to useful power. so unity power factor is preferred. but in actual practice no load is purely resistive so some input VI power is used as useless reactive power. all the motors are inductive in nature so the power factor is always remain below unity.

THE REACTIVE POWER IS THAT CONSUMED BY THE LOAD IT CAN BE TAKEN AS LOSS FROM THE INPUT TO OUTPUT. MORE IS THE REACTIVE POWER CONSUMPATION LESS WILL BE THE POWER FACTOR.

LOW POWER FACTOR directly effect the power supplier because user pays only for WATTS (REAL pOWER) NOT VI POWER. if the power factor is poor supplier has to supply more VI input (VOLT -AMP) FOR SAME WATTS IN COMPARISIN TO HIGH POWER FACTOR.

" unity power factor is an ideal condition which can not achived but near unity power factor is useful for power saving so recomaanded by utilites"

Unity power factor can be achieved, Synchronous motors can correct power factor to unity by varying the excitation. I Have sailed on a ship using Gas Turbine-Electric drive, the propulsion motor is a 75 MW Synchronous motor, we automatically vary the motor excitation to maintain unity power factor.

Unloaded Synchronous motors are employed to act as continuously variable rotary condensers for correction of power factor to unity.

Regards CEKM

When in doubt, correct the power factor at the supply point for the minimum average load. Add correction on your largest inductive loads individually ( at the contactor for the motor ). This isn't as precise, but it's usually the cheapest for best overall price-performance. Remember that switching power supplies such as variable speed drives, computers and electronic ballasted florescent lights do generate harmonics that can cause problems with low priced power factor correction capacitors. However, this type of load has to be a relatively large part of the loads to cause these problems. By the way, unity power factor is definitely ideal and the closer you get to it, the higher the efficiency of everything and the less the power bill.

In the USA we do not try for unity power factor (except in some products such as UPS) We strive for 95%. We always want to be lagging (inductive). We never want a leading power factor (capacitive).

The reason for lagging is the fact that most loads are inductive.

"Wanting" has nothing to do with it either way.....

Whether leading or lagging it does not matter, the angle defines the extra cost.....

As power factor increases,the cost of furthur correction increases,creating a diminishing return on investment for the last few percent.That is why you will seldom see a unity power factor in real life for an entire factory.It will vary as loads change.

Whe power factor is low, current lags voltage, and the reactive current actually produces no useable work, it is merely maintaining a magnetic field.However, the unused current must still be generated and transported by the supplier,and it produces heat, requiring larger conductors,transformers, etc. For this reason, a penalty is usually associated with low power factor.Industrial plants in the USA have 3 meters: 1 for KWH, one for PEAK DEMAND, and one for KVAR (power factor), and rates are set according to all three factors.Many factories have a computer to manage peak load, with minute by minute information on usage provided by the utility company.

With many small motors, the most efficient method is distributed capacitance at the motor starter, especially if they are to be started individually.I have seen operations with over 3000 motors that ran 24/7, and these were corrected by capacitor banks at the substation feeder.I have seen small machines with perhaps a dozen motors, and these were compensated at the starters.So there are many variables to consider.

-------- SSB-----

Good post SSB (whoever you are!)

How can you all keep stating that the "reactive current produces no usable work" and in the same sentence state that "the unused current must still be generated. . . and it produces heat. . "

Is heat not work? Is heat not watts? Are you trying to say that the var currents move in and out of the inductor and the capacitor in a circulating fashion but create no heat (watts) that can be measured by the kWh meter?

I state again after reading the "easy reading" link you kindly provided Andy, that after 23 years as an electric utility metering professional, I have a very good understanding of this subject, and I am concerned with the many almost correct explanations on a forum that is depended on for solid science.

In a perfect resistive circuit, the amps would flow exactly in time with the voltage and produce 100 ⁰/₀ work of some kind, either rotating a motor or producing heat, light etc.. In a perfect reactive circuit (inductive or capacitive) the current would flow 90 electrical degrees out of sync (timing) from the voltage and produce nothing (theory only, since any flow of current will produce some heat due to resistance of the medium).

This is the real world, and the net result of all the various forces results in one flow of current at a time. Electronic techs get the concept that the waves of current from the various forces heterodyne into one current waveform. This is the rms current that is referred to and measured as part of VA or kVA.

The reason that this one current interacting with the voltage that caused it does not produce 100 ⁰/₀ work has to do with the nature of alternating current. The "lagging" of the one current simply means that the current is still flowing in the 'negative' direction for a brief moment after the voltage has already begun rising in the 'positive' direction. During that brief moment when the voltage is positive but the current is still negative, the work produced is subtractive (reverse energy; reactive power).

Therefore, for the percentage of the cycle when the one current and it's voltage are both positive or both negative (neg X neg = pos), positive power is transferred through an inductor (coil or capacitor loads). We call this positive percentage "work".

The more time the current remains in the opposite direction from the voltage, the lower the positive percentage of work that volt/amp combination can accomplish. We call this positive percentage the power factor and includes the resistive "work" (heat etc) of the one current.

There is where the "excess heat" arises. Since the motor or other load will draw enough energy (V*Amps*PF) from the system to do its designed workload, and the voltage does not rise, the Amps must rise to compensate the reduced positive energy due to the PF.

More amperage is required to supply the demanded energy since a percentage of the amp flow is out of time, lagging the alternation of the voltage. The excess heat is not due to a separate current. It is this one slightly out of time current that is the net result of all the forces that would cause current to flow that makes all of the heat. If it is forced to rise, then there is more heat.

Please forgive the burden of such a lengthy post, but all of the formula explanations in the world can be technically correct, but the reality of what happens in common terms will remain cloudy and not serve those who look to us for understanding.

Regards, CJM

Interesting thread, if only because the OP has never again responded. probably scared to...

I'm curious to know where the original idea came from that the US is "taking Unity Power Factor" ? My suspicion is that he has read some of the lies being spread by people trying to sell "energy saviers" that are nothing more than capacitors. It's unfortunate that those people are allow to continue spewing that filth, and I think it is giving the wrong impression to the uninitiated masses.

Why did you post as "Off Topic"? You were right "On Topic!"

Keep posting please as its a pleasure to read your excellent and clear posts (showing an excellent background/knowledge and knowledge more than good enough to teach us with!!)

Many, many thanks.

You could easily be right on the money....

Its still an interesting thread in spite of the fact that the OOP is not very interested....

Andy,

I also suspect he is on target, and agree with the interesting, and will add enjoyable.

It would be ever so much more so if the asker of the original question would respond for further clarification or at least respond. But there we are.

Regards CJM

I wonder and sometimes I may collect the statistics _{(love that subject just for the fun of it, esp having been trained in SQC after the engineering)} Howmany times the OP has just posted one thread and disppeared.

In fact on quite a high percent of cases that was the one and the only post ever in CR4.

CR4 admin infact may think over to cancel subscription of the dormant beyond a certain time period.

I look at this way- if I don't use a certain facility, I will forget its user name and pass word in no time. So these users are going to come back and log in as new user.

(But it will give an inflated membership and hence ego to admin )

Dear Friend

I cannot comment on your statement that in USA they are taking unity power factor. First of all, there is nothing like "taking" unity power factor. As already explained in various replies in this thread, power factor depends on the type of load connected and therefore power "drawn".

Power for 3 ph. system = sq.rt 3 x V x I x p.f.

Voltage & sq.rt 3 are constant ( assuming fixed supply voltage ). Thus power is directly proportional to product of Current & pf. Hence if load pf is low, then current will increase & vice-versa, for same power drawn. Current drawn by load is delivered by the source which also has a max. current rating. Beyond this rating, the source will be overloaded.

Therefore if power factor is low, then power delivered to load will reduce. So also the total power available for utilisation will also go down. Simultaneously due to increased current, the losses in system will also increase ( power loss is in square proportion to the current )

Thus to have better utilisation of available power, operating on higher power factor is required.

Thanks and regards

Ashok Toshniwal, Bangalore.