Placement of VTs/PTs 3

[Mbrooke]

Is any one aware of, or sees an advantage in placing PTs within breaker bays (between the circuit breaker and disconnect) instead of on the outgoing line positions?

 

[Mbrooke]

Then there's the question of how do you lose a VT primary without tripping the bus or line it is connected to?

One of these

http://www.sandc.com/products/fusing-outdoor-transmission/smd-power-fuse.asp

I've seen it done at a few 66/69kv stations before. But outside of that I do agree with you, a VT fault is going to clear what ever its attached to.

I guess you might get an open, but anything nasty happening to one VT primary would render any other VT on the bus or line temporarily unavailable. It seems to me that a second, fully redundant, VT adds a lot of expense, space, and complexity for a fairly small gain in redundancy.

Two arguments Ive heard:

In double bus, single breaker applications with dynamic zones a VT tripping out any buss would not result in the remaining circuits loosing step distance voltages.

Second, any buss VT can be taken out of service as needed.

 

[scottf]

Davidbeach-

Thanks for your comments.

We lose VT fuses much more often than we lose VTs, so separate secondaries are far more redundant than one secondary daisy chained between relays.

I'm not following you here. Normally, each circuit would be fused separately. So, assuming you fuse the secondary circuits of VTs already, I don't see how creating a second fused circuit off of the same winding is any less redundant that 1 fused circuit from each secondary.

The only type of fault in which 2 separate secondary windings provides redundancy is an open-circuit in one of the secondary windings. In the 25 yrs I've been working for instrument transformer manufacturers, I've never seen that happen in the field. Primary turn-to-turn shorts, primary insulation break-down, and secondary turn-to-turns short are the types of failures I've seen and having 2 secondary windings isn't going to help in any of those situations.

Also, since we're discussing medium voltage VTs here, primary fusing is common and is present to clear a faulted VT from the line.

 

[davidbeach]

I thought we were discussing transmission level VTs; yes, we have primary fuses on our distribution VTs.

Typical, long standing transmission design, is to fuse the VT secondary circuits at the VT location and bring the fused secondary circuit into the control house.

As with many other aspects of substation design, there are myriad ways of getting to the same result; for better or worse an awful lot of it depends on decisions made decades ago. In our case it used to be one secondary in wye, the other in broken delta. We no longer have need for the broken delta to polarize the electromechanical directional ground overcurrent relays that we no longer have, so we have made that secondary available as a redundant voltage source.

Sure, I still see a significant common-mode failure risk of taking out both voltages; that's why we turn on overcurrent based protection during LOP conditions. Given that ability to provide alternate protection, I'd much rather push hard on dual batteries and dual trip coils than for a second VT. I know you'd like us to buy twice as many, but that do well enough that other non-redundancies are much more pressing.

 

[scottf]

Yes...for HV VTs, 2 secondary windings is standard.

The issue here is that we're seeing the requirement a lot now for MV VTs, where 1 secondary winding is the standard. In many cases, to go to 2 secondary windings requires a larger, more expensive VT. Hence the point about just having separate fused circuits being much more economical and providing about the same level of redundancy.

Yes..I would like to sell twice as many, but not what I'm pushing for here I'm actually pushing for folks to use the standard smaller, more cost effective MV VTs and spend an extra $10 on an additional fuse set-up.

 

[Mbrooke]

Sure, I still see a significant common-mode failure risk of taking out both voltages; that's why we turn on overcurrent based protection during LOP conditions. Given that ability to provide alternate protection, I'd much rather push hard on dual batteries and dual trip coils than for a second VT. I know you'd like us to buy twice as many, but that do well enough that other non-redundancies are much more pressing.

Question. Have you ever considered 3 phase VTs on the line for Relay A and 3 phase VTs on the bus for relay B? I know it sounds odd but as for late this is seriously being discussed for existing straight bus applications being overhauled.

 

[davidbeach]

No, we have not.

 

[Mbrooke]

Ok, just wondering. Thanks

 

[scottf]

What market are you in that 3-phase VTs are available? They are not typically used/available in North America.

 

[davidbeach]

Pretty sure that's just short hand for a group of three singles used for protection purposes, as opposed to one single phase VT used for synch check purposes. We regularly refer to a group of three as a three phase source. Just like referring to a bank of 3 single phase power transformers as a single three phase transformer.

 

[Mbrooke]

My mistake, I obfuscated my point. I wrote without thinking. By 3 phase I meant 3 single phase VTs used for step distance protection, and single phase VT meaning only one VT is present for sync check. I do not know of any actual 3 phase (as a unit) VTs.

 

[scottf]

Got it. There actually are/were 3-phase VTs, which is what I thought of when I read the post

 

[Mbrooke]

If anyone has a pic by chance, wouldn't mind seeing it. I myself have never seen a 3 phase unit VT. Learn something new everyday.