Why would there be a different relay settings on Each incoming phase?

[majesus]

I'm drawing the SLD for an existing wheat terminal facility that was designed in the early 70s and they have ABB CO-9 (Westinghouse) relays protecting the the incoming 12.5kV service feed.

There is three relays as each relay protects one phase.
However, I noticed this odd observation.
The middle relay is marked "Neutral." It has the following settings:
4 (pickup)
.5 (Time Delay)

The left and right relays have:
10 (pickup)
1 (Time Delay)

Any speculation as to why the middle relay is labeled netural and has different settings? Different CT ratio maybe?
I'm sorry, I don't have any other info, I don't even know what the CT ratio is, but the incoming line is measuring 28A and it is feeding a D-Y Step down Dry Type XFMR.

This might be a dead end as more information might be needed, but I would be curious to read what it could be.

 

[dpc]

Maybe because it is a ground relay and not a phase relay? It was common to provide only two phase relays and one ground relay in years past. Settings for a ground relay are typically much lower than the phase relays because the phase relays must be set above the maximum load current.

 

[davidbeach]

dpc nailed it completely.

 

[majesus]

I didn't know that.
I love you guys

That does make sense as all phases are protected anyhow. If it was a Phase to Gnd fault, then the gnd relay would protect the phases and if it was a phase to phase fault, then the other two relays would still provide protection. Why did they stop doing it this way?

Lack of redundancy?

 

[majesus]

Oh btw, the instanteous current setting on all relays are the same, so the Short CT rating remains consistent.

 

[majesus]

*I wish I could edit my posts...


Then this must also mean that they are protecting the Low Side as the H.V. service is connecting as Delta and assumed ungrounded.

 

[dpc]

Just because the transformer winding is delta doesn't mean the system is ungrounded.

If there is an analog ammeter, the range of ammeter is probably the same as the CT ratio.

 

[majesus]

There is an analog ammeter, 0 to 100A. Thanks, good rule of thumb to know

Yeah you are right, I didn't consider that it can be feed from a grounded substation upstreams some wheres. I wish we can edit our posts, but this option seems to be unavailable in the Eng forum.

 

[majesus]

WOW, that is amazing rule to know. I just checked a bunch of different SLD (from different projects) to see if the ammeter range is the same as its CT ratio. On all of them, if the ammeter has a range of X, then the CT ratio was X:5.

That is wicked! Thanks a lot DPC!

 

[waross]

Actually the ammeters are all 5 amp ammeters. You can order them with whatever faceplate you want to match the CTs. There are also specialized print shops who will print meter faceplates to order. For instance, the customer wanted to know the speed of his press in pages per minute. We had a new faceplate printed marked in pages per minute and installed it on the tachometer.
respectfully

 

[pwrengrds]

The middle relay could be a ground relay, but I would guess that it is wired as a residule neutral connection. It would serve the same function though. I have found the 5 amp for meters rule to be true almost all of the time, twice it was different though (0-2.5 amps)

 

[majesus]

"I have found the 5 amp for meters rule to be true almost all of the time, twice it was different though (0-2.5 amps)"

Agreed. When I said amazing rule to know... I meant to say "rule of thumb." Not always true, but most times.

 

[majesus]

I would like to ask some more details about this topic, just for clarification because there is are missing information unknown to me about this project. I shall post some photos and please tell me if I missed something.


I have another incoming service at the opposite side of the same factory. About 20ft up, the cable comes out of the wall (3 conductors only, no fourth conductor ie no gnd, no neutral) and it then becomes a bare HV 12.5kV bus bar.

http://i48.photobucket.com/albums/f223/majesus/DSC00843.jpg

(The Incoming 12.5kV service is red and shown in the background.)

The buses go into a CB switchgear, back out, two phases then go through CTs (one phase does not have high side CT) and then into a delta to delta 12.5kV/600V transformer.

The CB switchgear also houses these OLD ASEA RI relays.

http://i48.photobucket.com/albums/f223/majesus/aseaRI.jpg

The top relay is different than the other two, it has a faster time setting and a smaller current pickup. As we discussed, it is probably the gnd relay, and the other two are protecting the HS phases into the Step down XFMR. What I do not understand is how is the gnd relay setup? How can the system tap into the ground path? The incoming service is just three wires. Thus, the only way to provide gnd protection would have to be a zero sequence CT somewheres on the HS bus bar that I haven't seen. My reasoning is that even if the incoming service provides a gnd return conductor and there is a relay on it, it may not provide any protection. If one of the bus bars breaks and falls to the ground, the current can bypass the gnd relay as the current may not flow through the ground conductor but through the ground itself back to the substation upstream XFMR which is about 1.5km away. I've seen GND relays just before the GND point on upstream XFMR, but this location is so far from the feeding substation, thus a Zero sequence CT is the only possibility.

 

[slavag]

Hi Majesus.
I'm sure, that Dpc explanation is true.
( After some old BBP, that we see in the forum, I don't know, maybe you found for us new type of connection with intr CT into protection cubicle)
You cancelled two possible connection:
1. ZS CT on bus bar.
2. Connection to ZC CT on the neutral point of upstream xfr.
As I understand , you haven't design of protection cubicle.
You need open cubicle with ASEA relays and check connection of current wiring.
On this way possible found something in old configuration.
Regards.
Slava

 

[slavag]

Hi Majesus.
Today I found manual of this RI relay, may be it will be intresting for you (I looking for some old ASEA distance relay and found both of them).
Attached.

http://www.abb.com/product/us/9AAC7...date&displayVersion=islatest&content=external

Regards.
Slava