VT primary fuses and bus protection

[stevenal]

I have a possible answer to jghrist's question posted in thread thread238-37973 on Dec. 4th. Also a question.

jghrist: I agree that high side VT faults will be cleared instantly by bus protection, but how about secondary side faults? Secondary internal faults, and those located between the VT and secondary fusing could be cleared by a primary fuse if bus protection was set above this through fault value. Some unnecessary bus tripping could be avoided.

Now the question: What value of through fault current to use? I cannot seem to locate any impedance data for any of of our VTs. Anyone know where to find some typical values? I'm assuming there is sufficient fault current to burn a primary fuse, but I don't even know this much. Thanks.

 

[jbartos]

Suggestion: Contact VT manufacturers for VT parameters. They are hard to come by in literature.

 

[RRaghunath]

Stevenal,

One thing I know is that the primary fuses are generally not effective for VT secondary faults.

 

[jghrist]

I think you're worrying too much. Internal VT faults should be rare. Whatever impedance you find to calculate the fault current will depend on where you assume the internal fault is. An internal fault can always occur where the fault current is above your bus protection setting. So what you're trying to protect against is faults low enough in the winding that they are below the bus protection setting (without compromising sensitivity too much) and high enough that the primary fuse will clear the fault. In any case, it a subset of a low probability fault.

As faulty says, primary fuses are generally not effective for VT secondary faults. I have never tried to calculate the available secondary fault current on a VT, mainly because if it is above the bus protection level, what do you do about it? It probably won't be, but would you make the bus protection less sensitive because of this? Also because, as you stated, VT impedance data is not readily available.

 

[scottf]

Primary fusing for outdoor VTs is pretty rare, especially as you get above 15 kV.

Secondary fusing is common in substation applications and is normally done in the 3-phase juction box, or sometimes, the VTs (69 kV and above) can be purchased with secondary fuses installed in the terminal box. If you have secondary fusing, the only risk (on the secondary side of things) is for there to be a fault between the fuse and the VT secondary. I've worked for an instrument transformer manufacturer for a good while now, and I've never heard of that happening.

There is no reasonable way to clear secondary faults with primary fusing.

 

[stevenal]

Faulty,

How do you know this? References, experience?

jgrist,

I'm not sure the faults know they should be rare. I've seen shrapnel scattered pretty far from an unfused VT.

Alstom sent me some typical values for some 69kV VTs we've been using (These units came with a tag advising that primary fuses be used). It seems a bolted secondary fault would cause 2.6 amps to flow in the primary. A 1 amp fuse will clear this easy enough, but it would probably be below any differential settings.

 

[scottf]

Stevenal-

Most catastrophic VT failures are a result of breakdown in the HV insulation potentially caused by many factors.
Most are not caused by secondary shorts. When they are, they are usually the result of failure of connected instruments (meters or relays) and this can be easily guarded against with secondary fusing.

If there is an insulation failure, it is most likely not going to be cleared by primary fusing.

I don't believe you can generalize any primary fusing value, as the fault currents can vary widely with VT design.

 

[stevenal]

Scottf,

"Most catastrophic VT failures are a result of breakdown in the HV insulation"

And such events are not preceded by secondary insulation failure? And is it even possible to tell from afterward?

"If there is an insulation failure, it is most likely not going to be cleared by primary fusing"

I'm assuming you are speaking of primary insulation failure when bus protection is present.

"I don't believe you can generalize any primary fusing value, as the fault currents can vary widely with VT design."

One amp is the value. S&C makes such a link specifically for the application, up to 138kV. Whether it will clear through faults in all applications, I am not sure. The requirement for VTs to be accurate for all loadings up to the maximum burden would suggest that impedance values need to be kept low.

 

[scottf]

Stevenal-

If there is a primary insulation failure, you are not going to pick it up on the secondary. Obviously, there are a lot of ways a VT can fail, but I believe the above to be true generally.

Suprisingly, you can tell a good bit from the remains of a failed VT.

Actually, a VT's primary winding is a very high impedance, which obviously varies with design and voltage class. You can imagine a 230 kV VT with a primary winding consisting of approx 80,000 turns of very thin wire.

I personally don't believe a 1A fuse would clear a fault on a 138 kV VT's secondary.

For the sake of argument, let's say you have a 138 kV VT with a 700/1200:1 ratio, which is the norm in the US. A worst-case fault current on the 700:1 secondary winding would be around 600 A. That means that the primary current would be about 0.85 A. The above is based on the VT design that I have experience with and of course could be different for other designs.

 

[dpc]

I have had some direct experience with 13.8 kV PTs. A one amp primary fuse quickly cleared a short on the secondary side. I don't know how well they would work at higher voltages, probably less well.

I have seen a lot of installations done both ways - with fuses and without.

Unless you have bus differential protection, I don't think you can say that a PT primary fault will be cleared "instantly". It could take quite a while if you are relying on a backup OC relay.

Other than additional cost, what is the downside of having primary fuses?

 

[scottf]

dpc-

I don't think there is any downside, other than cost. As an employee of an instrument transformer manufacturer, I think it's great if everyone fused everything.

My only point above, was that VTs are commonly protected from secondary shorts by secondary fusing. I think it works as good (or better in some cases) than primary fusing and it's a heck of a lot easier and cheaper.

Just make sure you don't fuse CT secondaries!! (you would be surprised at what some folks do).

 

[jbartos]

Suggestion: Reference:
1. NFPA 70-2002 National Electrical Code, page 320:
requirements for such apparatus.
(C) Voltage Transformers. Voltage transformers installed
indoors or enclosed shall be protected with primary fuses.
FPN: For protection of instrument circuits including voltage transformers, see 408.32.

 

[Laplacian]

stevenal,

Could you provide the S&C model number for the PT primary protection link you mentioned. We have had 69KV PT failures that caused incoming bulk substations to our plants to shut down resulting in $M's of lost production. These substations have 2-4 independent lines coming in, but the PT explosion causes flash over on adjacent busses. I am interested to know what the true cost is for installing both primary and secondary PT protection up to 230KV if possible.

 

[stevenal]

446001R1 is the 1 A SMD1-A fuse unit for 69 kV. There are SMD-2B units to 138 kV. Don't see anything for 230 kV. I'm looking at bulletin 212-31.