When to use 69 vs 115 volt PT secondary 4

[bacon4life]

Most of our 115 kV and 230 kV potential transformers (PTs) and 2 secondary windings, either of which can be 69 V or 115 V Line to Ground. We apparently typically connect relaying to one winding configured as 69 V and metering to another configured as 115 V. Is that typical industry practice, and if so, why?

As we move to microprocessor based relays and meters, would it be prudent to split the relaying between the two secondary windings?

Thanks,

 

[oldfieldguy]

Using the 69 volt windings in a wye connection gets you a 120-volt phase to phase secondary with the voltages in phase with the respective primaries, assuming your primaries are connected phase to ground as is the normal case.

Using the 115 volt windings you can do a three-phase delta with 115 volts phase to phase. then you get to deal with the phase shift from a wye primary to a delta secondary. In some cases that is what is needed.

Using the 115 volt windings in a wye configuration gives you a non-standard voltage of 200 volts. That's not something I've seen used.

old field guy

 

[scottf]

bacon4life-

As you stated, with modern electronic relays, it doesn't really matter any more and I'm seeing more and more start using the 115V winding for all applications.

As is said above, using the 69V tap (note it's sometimes 66.4 or 67V depending on the voltage class of the VT/CCVT) "mimics" the line-to-ground voltage and give 115kV line-to-line.

Electronic meters are the same story and most users just use the 115V output.

Strictly from the VT/CCVT's perspective, there is no advantage in using separate windings for metering and relaying, although a lot of utilities do it that way.

What many don't understand is that the secondary windings are wound around the same core. The voltage on one is the same/effected by the voltage on the other. If there is a wiring error (short) connected to one winding, the other windings output also goes to 0V. Same goes for an internal winding fault.

I normally recommend to utilities to use 1 winding for all normal relaying and metering, since modern burdens come no where close to approaching the accuracy limits of VTs or CCVTs. This leaves the 2nd winding available if zero-sequence/broken delta measurements are needed, or, in the case of VTs, if special ferro-resonance suppression is needed, normally in the form of a resistor placed across a broken delta.

 

[davidbeach]

NERC/FERC redundancy requirements will drive the change to using one secondary for the set 1 relays and the other secondary for the set 2 relays. To avoid having different secondary ohm reaches for both sets it is best to use the same voltage on both secondaries.

 

[scottf]

How is it redundant to use one set of relays on one winding and the second set on the other winding?

 

[slavag]

Hi.
It's redundancy on the secondary voltage wiring level.
Disconnection or trouble of one of winding isn't influence on the protection functionality of second relay.
Common practic for two distance protection ( for example).

For example: For large generator it's redandancy on the primary level: two sets of VT.
Best Regards.
Slava

 

[scottf]

slavaq-

In the US, it's common to run wiring for all 3 VTs to a marshalling/junction box on the structure of one of the VTs (normally the center phase). Wiring runs are then split out from there, with fuse blocks, etc... Redundant wiring can be (and normally is) made from that point.

1 winding can be used and then split out to redundant runs / fusing.

 

[davidbeach]

Scottf, it is redundant because the regulators say so. If using separate secondaries wasn't redundant, the redundancy requirements would require two sets of VTs and that has been seen as going too far, at least so far.

 

[stevenal]

David,

Can you give a reference for the regulation you speak of?

 

[scottf]

davidbeach....

Just so long as everyone realizes it's not really redundant, I don't care I guess, however I think it's pretty silly.

On the other hand, there are utilities that are forced into buying VTs/CCVTs with 3 secondary windings because metering wants their own secondary too. In some cases, that can add a lot of $$ to the units...all in the name of false security!

 

[slavag]

Hello.
Scott, this thread remaind me MCB/Fuse issue ( if you remember, PT/VT secondary protection)
Many times redundacy is maintanace issue too.
Before several mounths we provide some small job for big utilities: substation with 400kV, GIS ( I think with your's VT), for the several bays VT was with FIVE windings ( for the Main1 , Main2 protection, for the metering, backup metering, for the disturbance recorder, for the local sinchronazing system, for the teleprotection, for the remote
synchronazing system, etc.).
You wrote:
"In the US, it's common to run wiring for all 3 VTs to a marshalling/junction box on the structure of one of the VTs (normally the center phase). Wiring runs are then split out from there, with fuse blocks, etc... Redundant wiring can be (and normally is) made from that point".

Not only in US, it's common practic for most of systems, but
from 132/170 level start requerements for the two windings, and for the 220/330 and 420kV I think all utilities request two windings.
All MCB's in such big systems are 2-3A Z curves, that means
you haven't selectivity, and all MCB off up to main MCB near to VT ( it's really, not theoretically). System leave with some 400kV bay w/o any protection (voltage base) and possible for few hourses.

Before two weeks, we work on the retrofit of some 360MW block gen-trafo and check synchronazing with line, was some SC in voltage wiring, MCB's on the BB VT, gen VT and line VT was OFF and about two hours client tried found problem, but generator was with second set VT and line with second winding and system was still with all protection.
Regards.
Slava

 

[davidbeach]

Steve,

I'm still trying to come up to speed with all the regulations, and keeping track of what is final regulation and what is rules still in progress, so I'm not sure of chapter and verse at this point, but where redundancy is required, in WECC at least, the failure of a single VT secondary can not impact both relay sets. At this point there is little below 200kV that would be impacted by this.

 

[scottf]

Davidbeach....

My whole point is that a failure in one secondary DOES effect the other secondary, outside of maybe a connection falling off in the terminal box.

Since both secondaries are about the same core, a problem (short-circuit, partial short, etc...) effects both windings.

In other words, you can't have a short on 1 winding and everything be normal in the other. Both would have approx. 0 V.

 

[cranky108]

I've been in a substation where the relaying and metering are both from a 67v source. Finding replacment meters for 67v was a pain.
But they still had to bring one phase 115v from each bay for syncronizing.

I think the bigger issue is finding transmission relays that like 115v p-n, or finding meters that like 67v p-n. And forget finding a sync-scope that works on 67v.

If I recall correctly the secondarys of CCVT's are different from PT's and may not have much effect on each other.

 

[scottf]

cranky108-

Nope...same secondary issue on CCVTs as with VTs.