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Power factor correction

Power factor correction

Power factor correction

(OP)
The calculated capacitor to be needed to improve PF is 60kvar. But the closest standard size is 50Kvar. the next size is 75kvar. I am thniking of adding some heaters in the pump house to help improve the P.F. Is it workable solution? Is there any other alternative? We don't want to spend money on an automatic capacitor or a custom designed one, which is more expensive. I appreciate any inputs.    

RE: Power factor correction

The heater will only improve your power factor by increasing the total load, certainly not an efficient solution.  Pick a capacitor size and use it, you will either have more than the ideal correction or less than the ideal correction, such is life in the real world.

RE: Power factor correction

You don't really want a PF of one... How far will you miss PF=1 by if you use the 50Kvar?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Power factor correction

What is the monthly consumption of KWHrs and KVARHrs on your power bill? When doing a survey for a customer i request at least the last 12 month billing and prefer 24 months or more.
Are you in North America and are you penalized for a power factor of less than 0.9 or are you in a country or area where the penalty is less forgiving?
What is the nature of your load? Do you have a few large motors, lots of small motors or a mix of motors.
How many hours a day does your plant operate? How many days a week does your plant operate? Is there different loading on different shifts?
What type of plant do you have?
Do you have your own transformers?
Do you have any motors that typically run 24 hours per day?
These are all questions that I consider before applying the "Art" of power factor correction.
The science of power factor correction often costs a little more than the art of PF correction, but not as much as fully automatic correction.
After assembling the data I first ask;
How many KVARHrs per month do I need for 100% correction?
How many KVARHrs per month do I need for 90% correction?
How Many KVARs may I safely connect 24/7 and how many do I still need?
How many large motors can I correct to 100% with permanently connected capacitors and how many KVARs do I still need?
Are there any motors that run 24/7 and how much capacity may I permanently connect to them?
If there are a large number of smaller motors, I will consider correcting them in groups. A PLC may monitor the motors and connect a capacitor bank with a contactor whenever there are enough motors running to absorb the KVARs.
If the motors typically run together on a machine, I would not use a PLC but just use auxiliary contacts on the motor starters to pull in the capacitor contactor when all of the motors are running.
There are a few other tricks to the "art" of power factor correction.
The first rule of the 'Art" is do not do more than required.
If your penalty starts at 90% then correct to 90%.
If you incur a small penalty one or two months out of the year after correction, step back and look at the numbers. You may decide not to spend a few thousand dollars on added correction to save $30 or $50 a year in penalties.
If you care to provide some numbers and a description of your plant, we may make more specific suggestions.
respectfully

RE: Power factor correction

Smoke posted "You don't really want a PF of one... How far will you miss PF=1 by if you use the 50Kvar? "

Question: Why not?

RE: Power factor correction

Yes, in some parts of the world you will be penalized for any power factor other than 1.0 (100%), lagging or leading.
respectfully

RE: Power factor correction

Really? Wow. that seems draconian. Here in the US, the convention is to be as close as possible to .95pf, penalties for being at .99 would seem like a way of indirect taxation!

RE: Power factor correction

The utility I deal with bills on KVA demand. So the closer to unity the better.

RE: Power factor correction

In the Northwest US, Bonneville Power's standard contract with local utilities includes a power factor penalty starting at 0.97, lagging OR leading (depending on time of day).  There is also a 12 month ratchet so penalty is based on the worst power factor of the preceding 12 months.

Most local utilities pass this along to their customers.  

RE: Power factor correction

Danged fine job waross.  A star for you.  I hope the OP gets back to you on this.  The only thing that I didn't see you ask him is if there were any applications in his plant where he could utilize a synchronous motor(s) as part of the solution.

rmw

RE: Power factor correction

Hey!

I think that Smoked has a very valid point.

If difference between 50 and 60 kvar compensation is important, then the total load seems to be 1. very small,  2. very constant throughout day, week and year and 3. has a constant PF.

Most automatic (switched) compensation equipment have larger steps than 10 or 15 kvar. But it surely is possible to build equipment with smaller steps. Using one fixed compensation is a mistake. It is very difficult to imagine a load that is constant in both power and reactive power, as described above. Unless it is a process plant running an identical process for years.

Hamid: What is your power consumption (low/high) and power factor (also low/high) over the year?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Power factor correction

(OP)
The power consumtion is 5kVA-15KVA and power factor is 0.90-0.99.The laods will be fed from 50KVA transformer, which is fed from 5MVA transformer. The 5MVA is almost unloaded.  

The required Kvar is 55kVar-65kvar to achieve power factor of 0.90-0.995. If we use 50Kvar capacitor, the Power factor is below 0.90. If we use 75 Kvar, the power factor will be leading.

Skogsgurra, maybe use a fixed capacitors with small steps, like 10-15kvars.

RE: Power factor correction

Wow, someone cares about a power factor with a load of only 15kVA behind a 5MVA transformer.

Surprises me greatly.

Is this your 5MVA transformer?

Does the utility have a meter on the 15kVA side?

Are you doing this because of utility charges/penalties?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Power factor correction

(OP)
The utility has a meter on 25KV side, which is on the primary side of 5MVA transformer. 15kVA load is fed from 50KVA transformer.  There would be a charge from Utility due to the low Power factor.  

RE: Power factor correction

Deal with the power factor between the 5MVA transformer and the 50kVA.  You also need to go back and check you numbers; there is no way that a 15kVA load could require 55-65kVAr to correct it back to a power factor between 0.9 and unity.  Just can't happen.

RE: Power factor correction

Current power                  - P
your current power factor        - k1
power factor you need - k2 (0.9)

Q=P*|{tg[arc(cosk1)] - tg[arc(cosk2)]}|

So recalculate like david said, because if you need 65Kvar for 15KV that means you have power factor around 0.2 now, and I don't know with what could you do that because it is almost pure inductive load. Must remind you that most of the engines have cosf around 0.8

RE: Power factor correction

Hamid,

I think that you should: A) either stop worrying or B) talk to someone that can explain this to you or C) dig out your text-books.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Power factor correction

(OP)
Remember that,when the 5MVA transformer is unloaded, the exciting current is 1.2%, which is almost purly inductive. That's why there is 65 kVars in the system, which will lead to lower Power factor of 0.20.   

RE: Power factor correction

If you need to correct the excitation current power factor, you need to do that at the voltage level between the 5MVA transformer and the 50kVA transformer.  Don't, DON'T, try to do that on the other side of the 50kA; you'll run into all sorts of voltage control issues.

RE: Power factor correction

(OP)
I am just trying to correct the power factor between 5MVA and 50kVA transformer,which is 4160V level.

RE: Power factor correction


Almost all of you charge is for energizing an unloaded transformer. IF the utility would agree, you might check into using a single phase 13.2 kv 25 kva transformer for
small load and disconnect the 5000 kva transformer or move it to where it can be used more efficiently.

RE: Power factor correction

(OP)
But the customer will like to keep the transformer in service just in case the other major loads could be on one day.   

RE: Power factor correction

Quote:

Remember that,when the 5MVA transformer is unloaded, the exciting current is 1.2%, which is almost purly inductive. That's why there is 65 kVars in the system, which will lead to lower Power factor of 0.20.
The no-load losses will make the power factor fairly high on the 25 kV side.  See thread238-192000: No load condition of energized tranformwer.  If you try to compensate for magnetizing current with capacitors on the low side, you will end up with a leading load power factor.  This could cause high voltage problems.  How much of a power factor penalty are you talking about on a 5-15 kVA load?  Probably less than the cost of your no-load losses that you don't seem to worry about.

RE: Power factor correction

(OP)
The power factor will probably be 0.20. If we measure the no-load KVA. This will be almost pure KVARs.  

RE: Power factor correction

This thread is an infinite loop.  I think we're wasting our time....

RE: Power factor correction

You got the formula to calculate capacitor banks needed to calculate capacitor banks, now You should consider one more thing. You should ask the power company to rectify and calculate your Power factor after transformer (what is an normal issue), that is in case that transformer is not in your property (they will not accept it than).

If it is, I believe that smaller transformer station would show as a cost effective solution. Remember that beside your load power spent you should pay for currents of magnetization and power factor correction equipment for some 120 - 160 KVar the cost of such capacitors (and what if you need to connect more p. load and rectify power factor for that also in future) and that might show up as a costly long time solution. If I was your boss we would have serious talk about engineering principles in planing and projecting.
What you should know is:
1. Who is the owner of that transformer (why would you pay for power company reactive power loses)
2. What is your load.
3. What is your expected load in near future

In case you have big transformer in your property you should  consider selling it and buying transformer of smaller power, I believe that counting on current cost of transformers you would not lose anything in its initial cost. Buy small power transformer that will cover your near future consumption, and consider purchasing one more in future that is able to work in parallel operation with this one. Create long term development project that would include more transformer fields with or without parallel operation. If you don't do that your costs will rise over time and costs for buying new capacitor banks for rectifying power factor before transformer load reaches over 70% will be so costly that my boss would fire the person that suggested buying such big transformer at first (5 days ago he wanted to cut from my salary difference between p-factor correction from 0.87 to 0.95 instead 0.93, I was calculating because junior engineer gave me wrong data and it was just talk and no capacitor banks were ordered).

RE: Power factor correction

Hamid,

I understand that the problem is the customer! aha! Why does the customer disagree taking out the 5MVA loaded at 1% of its rating! You don't have to worry about penalties if your transformer is sized to your needs - and that's the solution. You don't get charged for low PF, you don't have to buy capacitors, and most of all, your voltages don't spike!

Convince the customer to just de-activate the 5MVA, don't pull it out but install a 50kVA, 25kV primary(if line-line, or 13.8 kV if line-neutral loading).

My two cents!

RE: Power factor correction

This reminds me of a customer wanting advice on a power factor penalty of 90% (Penaly, not PF). It was a similar situation. A large unloaded transformer. The solution in that case was to de-energize the transformer.
In this case I would suggest adding enough capacity between the 5MVA xfrmr and the 50KVA xfrmr to equal the exciting KVARs of both transformers.
Don't correct to unity, just correct enough to eliminate the penalty charges. If standard sizes don't get you in the window, calculate the penalties payable for the smaller size and contact the utility for their policy on slightly leading power factors.
Then decide what size to go with.
respectfully

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