Series resistors in PT circuit 5

[buzzp]

In an older design, they placed resistors (.1 ohm) in series with a fuse that feeds the primary of a PT for protection purposes (both leads are fused and have resistor in series). The secondary is also fused. I can not think of any reason to have the resistors in the circuit (sure they will limit the current but so will the fuse by opening). Anyone think of a reason for these resistors?

 

[busbar]

Note that for initiation of a resonant condition, there has to be a open phase resulting from something like a fuse operation—with hi-side capacitance in the right place. buzzp, what is the primary-bus voltage?

 

[buzzp]

The primary voltage is 480. The other PT I mentioned is wired the same only it is tied to the 7200V bus (actually 6900).

 

[busbar]

The primary resistors sound like a very misguided approach and a plain old fire hazard. Class-J 200- or 300-kA interrupting fuses are available by the ton.

If you really wanted to go crazy protecting the tap conductors above the fuses, strap on some Bussmann KQT cable limiters at the bus tap.

http://www.bussmann.com/library/bifs/1042.pdf

(Leave the fuse block as a handy isolation point.)

 

[buzzp]

I don't see the resistors being a fire hazard. What makes you say they are a fire hazard?

The IEEE standard points out that LV PT's are generally not an issue regarding ferroresonance. It does not rule it out. My only conclusion is they were placed there for that reason. Until more information is know, I am sticking with this conclusion. Nothing else makes sense.

 

[jghrist]

The resistors could be a fire hazard in the case of a high side fault on the PT. They may have been installed initially in a misguided attempt to limit fault current for high side PT faults. The current would be limited to I=V/R=277/0.1=2770A. But the power dissipation would be I²·R=767290W. Most resistors would become a fire hazard trying to dissipate this much power.

 

[TestBeforeTouch]

As another alternative to FR, I wonder if the resitors could have been installed to achieve the same benefits as S&C's fusistors since they are specifically designed to protect PT's. Check out:

http://www.sandc.com/products/fusistors/default.asp

 

[buzzp]

There is also fusing on the high side of the PT. The fault would have to be between the fusing and the resistor, highly unlikely, but certainly possible. If the PT itself faulted, we are protected. I can find many unprotected wires in any plant if we want to consider such scenarios. I mean the wire that goes to the primary has to be tapped in some where. Your not tying a fuse to the bus first, your tying a wire. So, technically, the wire from the bus to the first fuse is unprotected - fire hazard.

 

[jghrist]

Wires can carry fault current. If you have a high side PT fault, the fuse will clear the fault before the wire is damaged, but I doubt it would prevent damage to a resistor unless it were a current-limiting fuse. I'm thinking that this was somebody's way of enabling the use of a less expensive non current-limiting fuse.