Capacitor Bank Placement

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cubdh

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Hi everyone. I have looked at the reactive loading profile of many feeders in our system and just intuitively decided that the right way to go about it, is to look at the low load case and place the amount of Kvars into the system = to Kvars needed at low load case.

Now i have come across a book (Electric Power Distribution Equiptment) By T.A Short which states " Use the average reactive loading profile to optimally size and place your capacitors for energy losses"

Now my concern with this is that at low load, there will be a leading power factor and i guess the voltage might raise too high. I can check the low load case.

Anyhow, i always thought that you used the low load case for fixed cap bank placement.

Can anyone shed light on which method is better. 1) cap banks during low load or 2) cap banks during avg reactive loading.
What is the negative effect of leading power factor, compared to lagging.

Thanks
 
Post more clear

Post more clear

Sorry if i was all over the place with my post.

Let me rephrase my question but first explaining what i am doing. I will use 1 hypotetical feeder.

From SCADA i have the reactive load profile for this feeder registered at the substation. Every hour for a month it gives the reactive load profile.

What i did was represent this reactive flow graphically. The graph has an area which can be considered a rectangle and then above that are the spikes in reactive flow. Lets say this rectangle which the upper limit of this rectangle would represent the low load case and lets say it is 300 Kvars lagging. I would place 2 x 150 kvars on the feeder based on 2/3rd rule or 1/2 rule.

Now i found a book stating that the cap banks must be placed based on the Average reactive load profile.

This would take those spikes into consideration and average them with the low load case. Now instead of needing 2 x 150 kvar banks for the 300 kvar lagging. I would need maybe 3 x 150 kvar banks for 450 kvar since the avg is greater than the low load case.

My question is which method is best. low reactive load or avg reactive load.

And what negative effects will the leading power factor have on the system.

I know leading reactive power will trace back to the substation.
 
T.A. Short says use average reactive loading profile because loss reduction depends on the average kvar loading. You might overcompensate during light load, but your average losses will be lower. You might want to check the voltage during light load to make sure that the overcompensation doesn't cause a high voltage condition. For typical utility distribution circuits, it won't because there is enough load at light load to reduce the voltage.
 
I understand why negating the average reactive load will give more savings and actually maximum savings for that matter, than looking at the low load case. But i think i read somewhere that you use the low load case for fixed capacitor banks and then the spikes so to speak for the switched cap banks. Well in our case the utility i work for does not have any switched cap banks except at a substation.

Thanks for the reply, i will look into using the average reactive profile.

But i still have a question regarding leading power factor, or leading vars in a system. Besides the extra current which will create more losses, are there negative effects of have leading vars?

I know some utilities penalize other utilities that they are connected with for exporting Vars. My utility is connected to another utility in a neighboring country. They dont like vars comming back on their system.
 
cubdh said:
But i still have a question regarding leading power factor, or leading vars in a system. Besides the extra current which will create more losses, are there negative effects of have leading vars?
Click to expand...
A negative effect is overvoltage.
Have you not considered automatic power factor correction to maintain about 0.95 lag?
 
You should not have to be concerned with high voltage at off peak times.
The regulator should back the voltage down so that the sub is putting out a lower voltage. If you are concerned, calculate the voltage rise at no load and see what the max VR would be. Then subtract the voltage Drop for the net result.
 
What I do is to use the next larger standard size bank from the minimum reactive load on a feeder. I use the average load to locate them. I locate them by adding them to the feeder about 2/3 out, calculate losses with average load. Then move them around, recalculating the losses until I get a minimum. Doesn't take long if the feeder is modelled on a distribution analysis program. The losses are not very dependent on location so an approximate minimum is OK. On most feeders, your possible locations are limited anyway by other junk on the poles like transformers and taps. You want a clean pole somewhere near the optimum location, but several spans one way or the other doesn't make a hill of beans difference in losses.
 
jghrist said:
I locate them by adding them to the feeder about 2/3 out, calculate losses with average load. Then move them around, recalculating the losses until I get a minimum.
Click to expand...

Please explain that.
 
jghrist said:
What I do is to use the next larger standard size bank from the minimum reactive load on a feeder. I use the average load to locate them. I locate them by adding them to the feeder about 2/3 out, calculate losses with average load. Then move them around, recalculating the losses until I get a minimum. Doesn't take long if the feeder is modelled on a distribution analysis program. The losses are not very dependent on location so an approximate minimum is OK. On most feeders, your possible locations are limited anyway by other junk on the poles like transformers and taps. You want a clean pole somewhere near the optimum location, but several spans one way or the other doesn't make a hill of beans difference in losses.
Click to expand...


Well yes, the optimal placement (location) of the capacitor banks can be found by "adding them to the feeder about 2/3 out, calculate losses with average load. Then move them around, recalculating the losses until I get a minimum. " But you can also model the system at low load and move the caps around till you get a minimum loss as well. Your system of using average load to calculate the losses and in turn savings makes sense. The way i did this thing is to find the optimal locations at low load, and at low load i try to use caps so that the power factor will be 1 at the begining of the feeder. then place those caps at the same locations at peak load, and record the losses with caps in place and losses without caps in place at peak load to get " Peak losses" multiply this by the Loss factor which is LsF = .15 ( LF) + .85 (LF)squared. This should give you the average savings or recovered losses.


The part i am not sure of is when it comes to the actual Number of Kvars to add to the system...... You say you use the next standard cap size from the minimum reactive load. I was under the impression that leading vars(during low load) are dangerous to equiptment. If the reason for this is Just because of overvoltages, i can easily test and see if overvoltage will be a problem which i highly doubt.

I read somewhere on the net that leading vars can cause resonance which can damage things. If this is an over exaggeration of some sorts. I want to redo my evaluation and use cap banks to cover Average reactive load. Or in other words make the power factor be 1 during average reactive load.

So in your expert opinion should i do that? By "that" i mean make the power factor be 1 during average reactive load?? of course by proper placement.


Thanks for everyones input.
 
Cubdh

I'm curious what your intent is in using these caps? Is it to reduce losses on the system only? If this is the case I've heard that using caps to reduce losses can only lead to a maximum 4% cost savings.

Or are you using these caps to provide more capacity on the system? If you wanted to add more load to a system I'm guessing you can use caps to free if some additional capacity if you are on the edge.

I understand the concept of overvoltage due to leading p.f. but can someone give me a brief explanation on why this overvoltage condition will persist in a system with leading pf?
 
Loss factor with capacitors is not the same as loss factor without capacitors. This is because losses are proportional to the current squared, while loss reduction from capacitor installation is proportional to the current (not squared). This is why average reactive load is used.

Possible high voltage at light load is normally the only problem with overcompensation.

You can get resonance at any level of compensation. This depends on the relationship between the capacitance and the system inductive reactance. Resonance at a particular frequency can cause high harmonic voltages if there are nonlinear loads on the circuit that create a harmonic current at the resonant frequency.
 
jghrist said:
Loss factor with capacitors is not the same as loss factor without capacitors. This is because losses are proportional to the current squared, while loss reduction from capacitor installation is proportional to the current (not squared).
Click to expand...

Why is that?
 
I want to thank everybody for their input on this. I have done a little more research and I am happy with using the Average reactive load as opposed to Low reactive load to place my capacitor banks.

Yes i am doing this to reduce losses on the lines. I know that the % loss savings will not be much. Using low load case i reduced the losses on the HV lines by 2.9 % only. I am guessing using the average load case it wil be in the low 3%. But if you do the math, considering the number of feeders, this comes out to some decent savings in money. Especially since our cost of power is sooo ridiculously high, it is worth it.

Again thank you for your input everybody...

It seems like if you are using a combo of fixed and switched then then u use low case for fixed and the rest for switched.
 
Do the savings in losses in a plant come from the fact that the utility no longer supplies some of the kVAR. The reason I ask is that we do not have a pf penalty therfore directly do not pay for kVAR or the losses associated with KVAR.

However I'm now seeing that although we dont pay for the kVAR losses directly, this kVAR is still adding to the real current and this total value is contributing to our overall losses. If the kVAR is coming from somewhere else then it is not contributing to the total current value in the overall losses therefore the less real power we have to use to make up for these total losses.

Does this sound right?
 
mull982 said:
Why is that?
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Calculation of loss reduction by capacitors can be simplified by realizing that the loss reduction depends only on the resistance, the capacitor current, and the reactive load current.

Watts loss reduction = WLR = 2?IC?R?IX - IC??R
where IC is the capacitor current and IX is the reactive load current.

Note that the load current term is not squared, so you can use average load current to get average loss reduction. The capacitor current does not change with the load. This is a large benefit to this approach when dealing with varying loads.

See Calculation of Loss Reduction by Capacitors, Victor J. Farmer, Electrical World, Oct. 29, 1956 for derivation of the above.
 
jghrist said:
See Calculation of Loss Reduction by Capacitors, Victor J. Farmer, Electrical World, Oct. 29, 1956 for derivation of the above.
Click to expand...


This is interesting. Do you have a link to this reference? Or is it a hard copy only?

My losses are automatically calculated with our distribution analysis program.
But i would like to see how it is done.

I have one last question, a hypotetical one.
I know the capacitor supplies vars or reactive current to specific load or loads
lets say 10 Amps at lagging p.f .95 Now at .95 leading p.f. does it still provide the same 10 Amps to the same loads? Which of course would add to losses in the later case? And because of this extra current in the system, ( the current provided by the capacitor ) is this the reason why we get a voltage rise?
 
cubdh said:
This is interesting. Do you have a link to this reference? Or is it a hard copy only?

My losses are automatically calculated with our distribution analysis program.
But i would like to see how it is done.

I have one last question, a hypotetical one.
I know the capacitor supplies vars or reactive current to specific load or loads
lets say 10 Amps at lagging p.f .95 Now at .95 leading p.f. does it still provide the same 10 Amps to the same loads? Which of course would add to losses in the later case? And because of this extra current in the system, ( the current provided by the capacitor ) is this the reason why we get a voltage rise?
Click to expand...
All I have is a rather poor photocopy of the original Farmer paper. Maybe you can get a reprint from Electrical World.

It's better to think of the capacitor drawing a leading current. The leading current will negate any lagging load current. If you think of the capacitor supplying reactive current to a load, what happens when the load is resistive? The resistive load will not draw reactive current, but there will still be a capacitor current.

If there is no lagging load current to negate, then the leading capacitor current will increase the total line current and losses will increase.

The reason we get a voltage rise is because the leading current flowing through the line reactance causes a negative voltage drop. Voltage drop is approximately:

VD = I?R?cos? + I?X?sin? where cos? is the power factor

If ? is negative (leading current), then the second term of the voltage drop will be negative. If the second term is greater than the first, then the voltage will rise.
 
jghrist said:
The reason we get a voltage rise is because the leading current flowing through the line reactance causes a negative voltage drop. Voltage drop is approximately:

VD = I?R?cos? + I?X?sin? where cos? is the power factor

If ? is negative (leading current), then the second term of the voltage drop will be negative. If the second term is greater than the first, then the voltage will rise.
Click to expand...
Don't know if this helps:
LeadingPFC.jpg

In the interests of clarity, I have simplified the representation of Vz as being an entirely reactive component.
And pply is a cropped version of supply......
 
It ain't that simple

It ain't that simple

Sorry to burst a bubble, but to optimize capacitor placement, you need to use computer software to examine the voltage profile along the feeder. You can select the total reactive support based on average reactive loading BUT need to look at the voltage profile along a feeder to determine the impact of installing fixed or switched capacitors at different locations. You also need to investigate the cost impacts. If the supplier does not have a cost penalty for going leading at light load, that will give a different answer than if they do penalize for leading. You also need to look at cost penalties for excessive lagging at peak loads. What about the impact on substation transformer capacity? Will you excessivly heat the substation transformer with leading reactive power at light loads? There may be a few rules of thumb to get you started, but to do capacitor sizing and placement correctly, so that there are no adverse unintended consequences, takes more than a rule of thumb approach.
 
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