Two single phase, two bushings type 3.3 KV capacitors in series for 6.6 KV operation ? 3

[edison123]

Can two single phase, two bushings type 3.3 KV capacitors (like below) be connected in series for 6.6 KV operation?

If each 3.3 KV capacitor is rated for 10 amps at 3.3 KV (i.e 33 KVAR), will it be 2 times i.e. 66 KVAR at 6.6 KV when connected in series?

Muthu

http://www.edison.co.in

 

[edison123]

The proposed schematic for using six nos. single phase 3.3 KV caps for 6.6 KV Delta connection is below. Will it work?

 

[crshears]

It looks like it will indeed work...however the OP wanted to know the output of this configuration...

I have concerns about the three floating sections between each pair of cans; when off grid potential, the caps and hence the intermediate connections would eventually drain via the cans' internal discharge resistors, but while on potential might there not be a concern that each of these could develop an un-anchored "wild child" potential?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Palletjack]

I think I would go just a bit higher with my capacitors, maybe to a 4kV.
Your building a voltage divider.
Are the capacitance values exactly the same?
Measure the capacitance, it will vary.

 

[waross]

Are you going to insulate the cases?
While each 3.3 kV capacitor sees 3.3 kV, the case sees 6.6 kV on a 3.3 kV rated capacitor.

I worked on a 500 kV capacitor station.
The series capacitors (to be in series with the 500 kV line) were composed of individual 17 kV rated capacitor cans.
The cans were assembled 7 to a rack.
Three racks would be connected in series for a total of 21 individual cans in series parallel.
Each group of three racks was mounted on standoff insulators and the racks in a group were separated by insulators.
There were rows and rows of these three high racks for further series parallel connection.
They were mounted on a platform that was supported by 500 kV insulators.
There were two platforms for each phase for a total of six platforms.

Yes it will work if you can address the voltage to ground.
Each capacitor will see rated voltage and draw rated current. Each capacitor will still produce 33 KVARs.
33 KVARs x 2 in series = 66 KVARs per phase.
66 KVARs per phase times 3 phases = 198 KVARs Total.


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

 

[crshears]

More thoughts:

If a can shorts internally, the other one will be exposed to 6.6 kV, which would lead to explosive failure, so "outside" fuses in series with each pair of cans would be a necessity.

Our utility has no 500 kV cap banks, but we have quite a number of 230's, and they're constructed much as Bill described, but always in wye, never in delta.

One refinement most of ours use is to split each phase-to-neutral leg of series-parallel cans into two parallel paths, with a CT in the link between the two neutral points and grounded on one side; neutral unbalance alarms indicate an imbalance of capacitance, and we dispatch Electrical Maintenance to inspect the cap bank. "Grasshopper" fuses are generally used to facilitate easy detection in cases where an internal breakdown or failure has occurred. Open cans are a different matter, and in the absence of a bulging can or other visual evidence these are notoriously difficult to track down...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[edison123]

Thank you, Bill and crshears.

Good point about the cans withstanding the higher voltage especially when one of them shorts with the resultant disaster.

I did the math and found the current through the series caps will be 4 times since the impedance is halved when the caps are in series and the voltage is doubled from 3.3 KV to 6.6 KV. Is my math correct?



Muthu

http://www.edison.co.in

 

[waross]

The impedance will be doubled, just like resistors in series.

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

 

[edison123]

Thanks, Bill. Looks like I put the 2 in wrong place.

Muthu

http://www.edison.co.in