87 differential bus protection and CT Ratios
87 differential bus protection and CT Ratios
(OP)
Hello all,
new member here. My senior engineer seems unsure as to the answer to my question so i figured i'd try it here.
I have a client that wants to add a low impedance Bus differential relay for an expansion project adding a bus with a 138kV breaker.
The problem comes when trying to connect the TX's CT's for the zone scheme of the bus protection. The TX does not have any more free CT's for use. do I:
A) daisy chain off of an existing protection CT?
B) is it a problem that they have different CT ratios?
C) Can i use the TX's low side CT? what ratio should this be? can it be different than the other CTs used for the bus protection scheme?
new member here. My senior engineer seems unsure as to the answer to my question so i figured i'd try it here.
I have a client that wants to add a low impedance Bus differential relay for an expansion project adding a bus with a 138kV breaker.
The problem comes when trying to connect the TX's CT's for the zone scheme of the bus protection. The TX does not have any more free CT's for use. do I:
A) daisy chain off of an existing protection CT?
B) is it a problem that they have different CT ratios?
C) Can i use the TX's low side CT? what ratio should this be? can it be different than the other CTs used for the bus protection scheme?






RE: 87 differential bus protection and CT Ratios
Personally, because I do care, I would breakdown and do it right, no matter the cost.
If you are under a time crunch, do what you have to do.
If you have a limited budget, because you need to make a profit, do what you have to to keep in the budget.
The last part is the most important. Do you care. I have to live with the problems so I want it done right.
I like the higher CT ratios because there is less of a chance of CT saturation.
RE: 87 differential bus protection and CT Ratios
It's a less than ideal installation, but far from uncommon in older installations, it probably means you're stuck with circuit switchers for the transformer rather than breakers; if you had breakers you'd have plenty of CTs in the right places.
Trying to include a transformer in a bus diff zone is fraught with difficulties unless you're using a transformer diff that happens to have enough inputs; say a four position high side, one position on the low side and an SEL-487E. Straight bus diff would not deal well with the 30 degree phase shift of any sort of delta-wye and the ratio mismatch would be an issue.
More details would probably help get a better answer.
RE: 87 differential bus protection and CT Ratios
RE: 87 differential bus protection and CT Ratios
RE: 87 differential bus protection and CT Ratios
I've attached a small sketch of the TX in question. The revision clouds are my design. the one coming from the B87/T2 prot would be going to my A87/138B. Maybe i didnt want to completely extend the zone to cover all fo the TX but now that i think about it, there might be worth isolating A protection with A protection. the B protection is taking in the only TX spare CT.Reason i decided to go with the low side CT is because of the starpoint direction of the CT connected.
Thanks for weighing in Ooops11: The bus relays are (A-prot) GE-B30 and the B-prot is a Siemens 7TU365. You made a good point about isolating redundant protections. I might switch the CT to come from the A87/T2 instead since this goes to the A87/138B prot.
For Low impedance Relay, that was my understanding as well, that CT ratios is not a big deal.
Thoughts are appreciated.
RE: 87 differential bus protection and CT Ratios
RE: 87 differential bus protection and CT Ratios
Yes I have. I was told this has been an option used in the past. I haven't pitched it to the client yet. First I wanted to see if there were any other options as this is a fixed priced project and cost is certainly a constrain.
Do you have any experience with them? Good results?
RE: 87 differential bus protection and CT Ratios
RE: 87 differential bus protection and CT Ratios
RE: 87 differential bus protection and CT Ratios
Not to give a sales pitch, but they are normally relatively easy to install and a fairly simple product and have been in use for a long time from Ritz and other manufacturers with good results.
http://www.ritzusa.com/Resources/83.pdf
RE: 87 differential bus protection and CT Ratios
RE: 87 differential bus protection and CT Ratios
oh i misunderstood what you asked. this one is expanded.
http://files.engineering.com/getfile.aspx?folder=3...
@Scottf I think since we are not digging up the trench for the TX (originally) the client may not want to go for the slip-over CT.s I think they'd rather Scratch the redundant protection (B) before doing that. haha
RE: 87 differential bus protection and CT Ratios
On a bus where lines have breakers and transformers have circuit switchers and there's less than 4 CTs per bushing on the bus side of the transformer, it becomes necessary to share CTs between the bus and transformer zones. When sharing, the bus zone expects the set of CTs to be grounded toward the bus and the transformer zone expects the CTs to be grounded toward the transformer; obviously both can't happen simultaneously and still maintain the "grounded once and only once" rule. In the past we've solved that problem by having all the bus CTs grounded away from the bus; the expected sum of the currents leaving the bus being equal to the expected sum of currents entering the bus. But modern bus relays can accept, on a CT by CT basis, being told whether the CT has positive or negative polarity.
Transformer diff relays can also deal with a "backward" CT through the CT compensation settings, but I'd still rather keep them all going the right way. The relays we use can do the transformer diff with the CTs in either direction, but if the REF function is used, the CTs must be facing the "right" way.
RE: 87 differential bus protection and CT Ratios
The polarity issue needs care, however worth noting the position of CT ground is not the issue.
The ground is normally on the star point of the CT wiring, however if was grounded somewhere else instead it would not have an impact on polarity.
For many places normal convention is to connect the side of the CTs pointing towards the equipment in star and the other side of the CT is connected to the "polarity" input of the relay.
This convention is popular because it doesn't matter whether it is differential, directional or distance protection - this convention always works.
However it is not possible to follow this convention when sharing a CT for BBP and Trf protection (or feeder protection etc)
You can either connect all CT inputs for the BBP the non-conventional way (star point away from the bus and bus side of the CT to the polarity terminal of the relay for all CTs) - which is what we normally do, however you could also either reverse the connections on the BBP for the non conventional star point connection(s), or reverse the polarity in the relay settings if the relay allows this.
These are all possible, but need very special consideration and testing if attempted.
RE: 87 differential bus protection and CT Ratios
Again, thank you so much!
RE: 87 differential bus protection and CT Ratios
RE: 87 differential bus protection and CT Ratios
I actually remember reading one similar to this a few years back but it is always good to brush up these skills.
UPDATE: I just had a meeting with the client's P&C engineer, who no doubt had a conversation with the settings engineer. This is what was agreed/confirmed.
- We are to rewire and re-use the CT's starpoint closest to the bus, currently used by TX 'A' prot (SEL-387E), into the protection and daisy chain the new B87/138B1 protection from the FT switch. Settings change on the SEL-387E will compensate for this change.
- The only issue is with the 3rd TX. We are to do the same except apparently, we cant rewire this CT's starpoint (even though it is also wired to a SEL-387E). Instead he wanted me to make sure the protection being wired to this CT is the Siemens relay 7TU635 and compensate on it's settings for the CT's starpoint direction. I am guessing the GE-B30 cannot do this? Though i am sure the settings engineer had a good reason for this.
- He also didnt care that the Bus 'B' protection was going to be sharing the Transformer's 'A' protections CT.