Fuse sizes for Power Factor Capacitors 1

[rockman7892]

What is the correct sizing practice or rule of thumb for sizing internal fuses on power factor capacitors (5kV)located on the load side of the motor contactor and overloads. I have heard everything from 135% -250% rated kVAR current but am not sure which is the correct rating and why.

Can someone give me some guidance?

 

[dpc]

Are you talking about fuses "Internal" to the capacitor can, or internal to the motor starter?

Consult capacitor manufacturer for recommendations. Also, see NEC article 460, but for caps over 600 V, the NEC does not provide any real specifics. The rms current can be higher than the rated fundamental current due to harmonic currents.

 

[rockman7892]

dpc

I am refering to the fuses that are "internal" to the capacitor can, not part of the motor starter.

Sec 460 of the NEC does not give much guidance as you mentioned, and the only thing it really mentions is that because of manufacturing tolerences and voltage fluculations fuses can be sized to 135% of the KVAr amp ratig.

I am awaiting a call back from the manufacturer but thought I'd check here to see if anybody knew any general guidelines or rule of thumb. From my understanding of these fuses, they are not necessarily used for overcurrent protection, but are used for short circuit protection, so that a short on the Caps does not trip out the motor overload protection and thus trip the motor.

 

[LionelHutz]

5kV capacitor fuses are typically sized at 175% to 225% or so. I know Fusetek has a chart to pick the fuse based on the capacitor current.

And you are correct, the fuses are mostly for short circuit protection to make sure the capacitor can does not rupture.

 

[rockman7892]

I checked out Fusetek website and there appears to be a pretty good chart as LionelHutz mentioned.

Two additional questions:

1) When calculating the capaciotr current do you use the following equation:

A= kVAR/kV/1.73

Therefore the current from an 150kVAR cap is:

A = 150kVAR / 4.16kV / 1.73 = 20.84A

2) If I have a voltage fluculation and the voltatge on the cap increases, thus causing the current from the cap to increase, do I not need to protect the cables from the cap to the motor as a result of this increase in current. For instance lets say my voltage increases to 4.4kV for a period of time thus causing the cap current to increase proportionally, do I need to provide cable overload protectin in this case?

 

[waross]

I tip that was given me by an old and wise master electrician.
When walking through your plant, pass your hand across every capacitor bank that you pass. If you notice any capacitors cooler than the others, check for failed capacitors or fuses.

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

 

[dpc]

That's a good tip, Bill. Almost as good as the one I got from an old electrician right after graduating from college and starting work at a power plant: "Don't touch anything".

 

[waross]

And from another old electrical supervisor
"I never wanted to be the smartest electrician. I always wanted to be the oldest electrician."


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

 

[rockman7892]

If I have two capacitors in parallel thus doubling the capacitance and KVAR do I need to double my fuse size to account for this additional current.

For instance if I had one set of cables coming into one capacitor which was fused and then jumpered over to the other cap to make them in parallel. I would therefore have one fuse for both caps and would think theat I needed to doubl the recommended fuse rating to account for this doubled current.

Is this correct?

 

[waross]

I believe that you are correct.
You are allowed to reduce the fuse increment for the second motor on a circuit if the motors do not start simultaneously but the capacitors are at full current all the time and an increase in voltage will cause an increase in the total current.
Fuse the capacitors based on the total connected kVARs.
There may be some adjustments if double does not fit a standard size or rating, but consider the load as double when selecting fuses.

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

 

[LionelHutz]

You would have to figure out the total current and apply fuses for that current if that is the route you are going.

However, the typical installation I see is fuses for each capacitor can. I personally won't fuse multiple cans with one set of fuses.

 

[rockman7892]

Waross and LionelHutz

I was finally able to get ahold of the fuse manufacturer and got a table with the recommended fuse sizes for a given capacitor kVAR rating as I mentioned.

I also discovered that I was wrong in thinking that there was only one set of fuses for two capacitors. I thought that there was one incoming set and the capacitors were then jumpered on the load side of these fuses. It turns out that there are fuses on each capacitor and the caps are jumpered on the line side of these fuses.


Now that it looks like I have all the right fuse information I can start concentrating on what is causing them to blow. Maybe a conversation for a different time.

 

[dpc]

If there is a VFD connected to the same bus as the caps, that is a very likely suspect.