PF Cap Install Question

[VTer]

We have a double ended 480V substation with a normally open tie. I would like to install 300kVAR of PF correction capacitors for this substation. The question I have is to either install 300kVAR cap on one side of the sub and only use one circuit breaker or install 150kVAR on both sides and use two breakers. The disadvantage of 300kVAR on one side would be VARs flowing through two transformers to the other bus, but I do not see that as a big concern. The sub is fed from the same source and the two transformers are fed from the same 25kV bus. The utility metering is upstream of the 25kV bus as well. See attached diagrams for option 1 and 2. Any thoughts would be appreciated.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[PHovnanian]

The price of two 150 kVAR banks plus breakers and misc. will most likely be higher than one 300 kVAR bank. That aside, the best solution will depend on where your loads are w.r.t. the bank(s). If the load (particularly the reactive load) is split equally on either side of the tie breaker, then the two bank option (Option 2) is preferable.

Keep the caps as close to the reactive load as possible.

 

[jghrist]

One purpose of the caps is to reduce the current through the transformers, and reduce losses.

If the 480V tie breaker is operated normally open, then you want to have half of the capacitors on each bus. Otherwise, there will be less reduction of current (or maybe an increase) through one transformer and no decrease in the other transformer (it will still carry all of the reactive load on that bus).

If the tie breaker is closed, then there is no difference in var flow between the two options.

 

[VTer]

Thank you for replies.

The load is split equally between the two sides, but I would like to maintain breaker space in the switchgear and only use one breaker if possible. This will also keep the cost down. This is why I like option 1.
I am also not concerned with the current flow through the transformers or VAR flow downstream of the utility meter. The load has significantly reduced over the years and the transformers have plenty of capacity. I am more interested in cost savings by increasing the PF seen by the utility meter. I just was not sure if there are any other technical or practical reasons why option 2 would be a better choice.


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[ghostbuster7]

Also be aware placing the larger cap value on one side will lower the natural electrical system harmonic resonant point and could now line up with a harmonic generated by your loads.

 

[Marmite]

Another thing to bear in mind is that for option 2 your switchgear will need to be rated for back to back capacitor bank switching if there is any possibility of closing a breaker and connecting a cap bank onto the busbar which already has another live cap bank connected to it. It may not have been type tested and assigned a rating for this switching duty if it is existing old gear.
Regards
Marmite

 

[waross]

If you are using a PF controller how will you connect your sensing?
If you are just connecting 300 KVAR you may have over voltage issues on transformer one if you lose the load on transformer two.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

If all you care about is the 25kV power factor why not just put the caps on the 25kV?

 

[LionelHutz]

Assuming you always need 300kVAR and you just care about improving the metered power factor, install the 300kVAR as you proposed. The actual connection point doesn't really matter.

 

[VTer]

Ghost, The larger cap will lower the resonant frequency to about 500Hz so we should still be good with the 300kVAR on the 60Hz system.

Marmite, I heard about this back to back switching and will definitely investigate this more if we peruse option 2.

Waross, This will be a fixed cap with manual switching via CB. My calculated voltage rise on 1500kVA xfmr was only 1.5% so this also will not be an issue.

David, 25kV option is not very practical for our system and not cost effective.

Lionel, thank you for your input. I have seen option 2 installed many times in the field so I was a little hesitant with option 1. I think for this case, option 1 will be less costly and also leave space in the switchgear for future loads. It will also provide better return on investment while achieving the same goal in improving the PF seen by the utility meter. Thank you all for your input.


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla