PT FUSE IN STAR CONNECTION (GROUNDED) 1

[uts123]

Dear all,
I have few questions related to PT fuses in metalclad switchgear.

1. why fuse is rated for kV like 5A, 15kV. Does kV rating also affect the melting curve of a fuse?

2.As per my knowledge, kV rating of fuse is max. voltage that a fuse can withstand which appers across both end of fuse after fuse meltsdown and kV rating has nothing to do with melting curve of fuse. Am I right ?

3. For 25kV (L-L) system, can I use 15kV rated fuse for star connected (grounded) PT.
As fuse is never exposed to more than 15kV. I feel 15kv fuse is ok and do not need 25kV fuse.

Please give your opinion.

Thankx.

 

[alehman]

1. The fuse must be capable of interrupting fault current at whatever voltage is available. You can't use a 5kV fuse on a 13.8kV circuit. kV in and of itself does not affect the curve.

2. See #1.

3. No. If you have a phase-to-phase fault, the fuse could see more than 15kV.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[uts123]

Alehman,

Thank you for reply.
But still I have confusion in No 3.

When there is phase to phase fault, Suppose fault between phase A & B then fuses at phase A and phase B could see more than 15kV because Phase C appears to other end of fuse. Is this what you mean?

But I am thinking as star point is grounded, when two fuse blows in phase to phase falut. third phase also grounded through the neutral point and blows out third fuse as well.

In phase to phase fault, the fuses at phase A and B could see 25kV but it is for very short time and fuse might withstand that short time.


Please clarify.

 

[alehman]

Your last sentence is it, but a "very short time" is not acceptable. The under-rated fuse could fail to extinguish the arc and result in a fire.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[jghrist]

Under certain circumstances, a line-to-ground rated fuse can be applied. See Current-Limiting Fuse Voltage Selection Criteria - Analysis and Test.

http://www.cooperpower.com/Library/pdf/78018.pdf

There is no advantage to using line-to-ground rated fuses at 13.8 kV because 15 kV rated fuses are readily available. On a 34.5 kV system, availability can be a problem.

 

[uts123]

jghrist,


I think you are in same line with me.
Thanks for the link. I went through it and now I have developed a opinion that line to neutral rated fuse can be applied under certain circumstances.

Can you look the attach diagram where fuse is for grounded star/grounded star PT. Can you give your opinion whether I can go with line to neutral voltage rated fuse.

Thankx.

 

[jghrist]

Can you look the attach diagram where fuse is for grounded star/grounded star PT. Can you give your opinion whether I can go with line to neutral voltage rated fuse.

It all depends on how likely you think an ungrounded fault would be. I would use a 25.8 kV rated fuse since one is available. See Fusetek FCA22

http://www.fusetek.com/scatdata.cfm?showdata=yes&SELECTEDMID=6&SELECTEDSCATID=30&SCDATID=249

 

[alehman]

If there is no probability of a line-line fault and your system is solidly grounded, you could consider a lower voltage fuse I suppose. I don't have Autocad on this computer so I can't open your drawing right now.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[uts123]

Hi Alehman,

I have attached pdf file of the drawing.

It is a PT of metalclad switchgear so there is no probability of L-L fault. As well if there is line to line fault, third fuse will blow out in less than 0.2s.

I appreciate if you can see the drawing and give your opinion.

Thankx.

 

[waross]

You appear to have an ungrounded system. If any phase goes to ground anywhere in the system, the voltage to ground on the other two phases will be line to line voltage. This will possibly saturate your 14400:120V PTs. This may blow the fuses.
The probable result of a line to ground fault;
Step one, voltage as high as 208 volts sent to your 120 volt protection devices.
Probably saturation of the PTs so that the voltage is no longer an accurate reflection of the actual line conditions.
Probable blowing of the 15kV fuses with 25kV applied.
Loss of protection due to blown PT fuse during a fault when you need the protection to function.
You need 25kV fuses and PTs. Most protection devices of this class may be set to function properly with 69 Volts normally applied, and 120 Volts applied during some faults.

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

 

[uts123]

Waross,

The system is grounded. The PTs in drawing is also shown grounded.
If system is ungrounded, there is no fun of grounding PT.

Please clarify.

 

[waross]

Apologies, I missed the power flow arrow.
BUT, unless you are able to ground to the same ground grid as the source, there is a possibility that a line to ground fault close to your equipment may cause a voltage rise of your local ground which will cause possible overvoltages to ground on the unfaulted phases.

No probability of a line to line fault in metal clad switchgear???
I still remember watching a 480 Volt MCC go up one night.
We were in the control room when an arc lit up phase to phase inside an MCC. The lights went out immediately, and all the joint lines of the MCC were lit up brilliant white. It lasted several seconds until the primary fuses cleared. After that management agreed with engineering and allowed the expenditure of funds to install a fused switch ahead of the MCC instead of a direct feed.
I am not familiar with the situations that will cause the neutral voltage rise with a line to line fault,but I am aware of the effect.
WE don't want to pick on you or condemn your system out of hand. Your application may be perfectly safe but it may be a disaster waiting to happen. It depends on the system topography and that is not shown on your drawing.
What is the separation of your ground grid from the system ground grid, feet yards or miles? What is the impedance between your ground grid and the system ground grid? What fault currents may be expected and what voltage rise may be expected between the ground grids?
If the supply is close enough that the ground grids are interconnected, and the voltage rise due to a fault is negligible, you may be good as is.
If the supply is from a distant source and the ground conditions are questionable, then you need 25kV PTs and fuses. Between the two extremes, it depends.
Remember, if you use 15kV hardware and there is a voltage rise, the PT may saturate from the over voltage. The fuse may not clear. Then in addition to the direct damage caused by the initial fault, you will have compromised protection and a possible 25kV arc where a 15kV fuse used to be.

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

 

[VTer]

Well looking at your drawing, what are the chances that you get a phase to phase fault between the fuses and the source side of the PT’s? ( see attached). The voltage the fuse would see in this scenario would be line to line, and with a line to ground rated fuse that will be a problem. I am not sure what the probabilities are here, but maybe the PTs are connected wrong or some other abnormal condition. I am not sure since I am a new engineer but how do you guarantee that you won’t have problems in this area of your circuit. I would not risk it and would go with fuses rated for line to line voltages.

"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

 

[alehman]

As Bill said, how do you figure "no probably of L-L fault" because it's metal-clad gear? I don't understand.

I've never seen 15kV fuses on a 25kV system. I would only consider it on a single-phase circuit distant from the other phases. Even in that case, I would stay with 25kV unless there was some strong reason to do otherwise.

Alan
----
"It’s always fun to do the impossible." - Walt Disney