Power factor correction 1

[amrh]

Dear all,

I would like to ask a question for you all and hope I would get a reply.
Would power factor correction lead to energy saving or reduce the GHG? Is there any studies in that matter?

Looking forward hearing from you soon.

Kind regards

--Amrh

 

[jghrist]

Power factor correction will reduce losses by decreasing the current in transformers and conductors that feed a reactive load.

There are a multitude of studies. You can probably find several with an internet search. One that I like in particular because of its simplicity and valid handling of load factor is "Calculation of Loss Reduction by Capacitors", Victor J. Farmer, Electrical World, October 29, 1956. Probably not available on the internet, however.

Optimization algorithms for the placement of capacitors on distribution systems seem to be a favorite topic in academia, based on the number of IEEE papers I've seen on the subject. In the real world, there is so little difference in losses with fairly large differences in capacitor location, that it hardly seems worth the effort to get too get too detailed.

 

[busbar]

Ehh, GHG?

 

[jwerthman]

I don't know what you mean by GHG. But jghrist is correct that the real energy savings from power factor correction almost never can justify the cost of the capacitors. However, if you have a power factor penalty or are charged for varhours in some manner, then it is quite possible that a reduction of these charges may justify the power factor correction. Sometimes the correction can be justified by purely technical merit to improve system voltage profiles and equipment/circuit loading.

 

[peebee]

I agree, energy savings are almost negligable. Let's say, for example, that you improve your PF from 80% to 100% (which is aggressive). This would reduce your total current by 20%. Now, assume that your distribution system has 3% losses. Your net energy savings is 3% of 20% = .6%. Not much bang for the buck, especially compared to other potential upgrades such as higher efficiency lighting, transformers, newer controls, etc., which might net 5% to 40% savings depending on how bad the existing installation was, and without all the headaches associated with PF equipment.

As jwerthman mentions, reduced utility charges can often be used to cost-justify the installation, though, depending on how the utility rate structure is set up, as this may help them to reduce the sizes of their feeders and distribution equipment. Similarly, on a large customer-owned distribution system, the reduction in kVA may permit reduced customer equipment sizes which might also help justify the cost of the correction.

 

[redtrumpet]

If by GHG you mean greenhouse gases, there is no reduction as capacitors do not reduce the real power required by the load, and real power is what a generator prime mover produces.

 

[RRaghunath]

When the system power factor is improved, the MVAR loading on the generators in the system comes down, which translates in to lesser excitation power to be fed to the generator rotor.

For a 200MW generator this could amount to some thing like 0.5MW, if the machine operates at UPF instead of 0.8 lagging PF.

 

[jghrist]

<<But jghrist is correct that the real energy savings from power factor correction almost never can justify the cost of the capacitors.>>

I didn't say that real energy savings almost never justify the cost of the capacitors. What I said was that there is very little difference in loss reduction with fairly significant differences in capacitor location on a distribution circuit. On utility overhead MV distribution circuits, you can usually justify the cost of fixed capacitors by loss reduction. I agree that this is not true in the typical industrial/commercial case, however, because of the shorter distances involved.

As far as greenhouse gases go, I am a global warming sceptic. Heated discussions of this topic will probably have more effect on global temperatures than will loss savings from capacitors.

 

[jwerthman]

I apologize jghrist for the misquote. Apparently, I didn't read you post very closely. It is difficult to justify based on loss reduction in an industrial facility, but you are more familiar with overhead distribution and I defer to your comments in that regard. Sorry again.

 

[amrh]

I believe that the cost savings comes from reduced Power Factor Penality or kVARh charges and kWh charges. The kWh reduction is very small. It is only the reduction
of losses from the fact that the kVARs are not being supplied by the utility. I thought the closer you put the capacitors to the loads that consume kVARs, the more savings on kWh you will recieve.

I have a question for you regarding the capacitor location whether its better on a distribution circuit or near the motor loads, and would like to know the pros and cons of that matter.And would the distance between distribution and the motor loads would really make a big different.



Thanks

 

[jghrist]

My response was based on application of capacitors on a long OH MV distribution line where the load is distributed along its length. If you have only load at the end, this is the best place for the capacitor from a loss perspective.

 

[MarkAthay]

The closer you place the capacitors to the load, the more overall loss reduction you can accomplish. Be careful in placing capacitors on the same contactor as a motor, you can create oscillation problems and actually create an overvoltage spike when the motor is turned off that can puncture the motor insulation. There are tables out there that show the maximum capacitor size based upon motor horsepower to avoid this problem.

There's an error above that needs to be addressed.

If the current is ropped 20% by correcting the power factor, the losses are reduced by the square of the current reduction. For instance, increasing the current 10% raises the resistive losses by 1.1 X 1.1 = 1.21 or a 21% increase in the losses.

You can see that fixing really poor power factors can quickly give some loss savings. This loss savings cover everything from the load to the energy source.

Most electric utilities have their rate structure set up so that a customer can recoup the cost of capacitors to fix their power factor between 9 months and 2 years. The usual disclaimers apply... Your performance may vary, yada yada, yada....

Mark in Utah

 

[peebee]

Thanks, Mark, I stand corrected

 

[amrh]

Dear All

Would the power factor really be of importance to reduce greenhouse gases. I gave you an example. If you have a 200 kW electrical motor with efficiency 90% the energy losses are 200*(100-90)/90 = 22 kW losses. Average European
electricity causes an emission of 516 kg CO2 emissions/MWh electricity.
This means an emission of 22*516/1000 = 11 kg/hour.

If the efficiency instead is 95% the emissionis 5,4 kg/hour.

Also it might be be argued that we might cause emissions of CO2 during the manufacturing of the motor and that this emission is higher for a high efficiency motor. A typical value id an emission of 25 kg CO2/kW during the mnufacturing. This means that for a life cycle of 15 years and with 6500 hours running/year the emission during usage of the machine is in thelater
example 5,4*6500*15/1000=530 tonnes while it in the manufacturing emitts only 25*200/1000= 5 tonnes. It is therefore to observe that the efficiency stands for 95% of all CO2 emission during the life cycle.

Other countlies have other values of emissions of generated electricity. USA has 737 kg/MWh, Sweden har 45 kg/MWh, Canada has 245 kg&MWh etc.

I would like you proffessional opinion with that.

Thanks

 

[jbartos]

Suggestions/Comments:
1. The nuclear power generation does not produce CO2. Perception of the nuclear power generation is changing with changing generations. The younger people see that the nuclear power will have to be used more and more. Built nuclear power plants produces electricity at about 2c/kWhr and fossil fuel power plants for about 3c/kWhr. Some articles are available in Nuclear News magazine.
2. The power factor reduction causes reduction of supply current which in turn decreases the voltage drop and increases voltages at load terminals. Then, loads consume more electricity. Therefore, there is hardly any savings in consumed electricity. However, the higher currents and lower voltages are somewhat detrimental to electrical hardware, performance and loads on the average.
Lower voltages cause:
1. Less light from lights.
2. Motors drawing higher currents.
3. Electronic ballasts out of range, e.g. +-5% or so
4. Heaters heat less
5. Etc.
Higher currents cause:
1. Bigger RI**2 losses in conductors, motors, coils, transformers, etc.
2. Bigger contact wear
3. Bigger insulation wear due to hotter conductors
4. Bigger damages by arcing
5. Etc.

 

[Marke]

The addition of power factor correction will reduce tha current drawn from the supply if the load is inductive in nature. This will not directly reduce the KW consumed by the load, but as has been correctly stated above, will reduce the kw losses in the supply system. Correcting the power factor will allow a greater utilization of the supply distribution system. For example, a 500KVA transformer can supply 500KW at a power factor of 1.0, but 400kw at a power factor of 0.8
In a typical installation where you do not own your own power supply reticulation transformers, do not expect to see a significant reduction in KWHr if any. There are a number of suppliers out ther promising ten per cent savings etc, but this can not be justified purely on the reduction on KWHr.
In order to minimise the cost of investment in plant, many supply authorities have power factor incentives to encourage the correction of poor power factor and load curves. It is common to apply a KVA Maximum demand tarif in addition to the KWHr charge. This will certainly be affected by a poor power factor. Another method, is to charge a penalty for poor power factor and there are many ways of doing this.
I believe that it is fair to say however, that domestic and very small commercial users only pay on KWHr and will achieve nothing by the use of power factor correction. Some of these suppliers are even getting to the home owner and suggesting that they will save by the use of power factor correction!!
The addition of power factor correction, can cause major problems on the supply, especially if the supply is very weak and inductive. (Long overhead lines in a rural environment) The capacitance combines with the inductance of the supply to form a resonant circuit. If the loading on the supply is low, then the 'Q' can be high and switching transients can excite the resonant circuit resulting in very high ringing voltages. You never get anything for nothing!!.
Best regards, Mark Empson

http://www.lmphotonics.com