87 differential bus protection and CT Ratios 2

[M1337]

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?

 

[cranky108]

What is the budget, timeline, and how much do you care?

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.

 

[davidbeach]

A oneline might help some, but assuming that the bus diff and the transformer diff abut, one consideration is that the shared CTs will be "backward" for one of the zones. For some relays that's a piece of cake, for others that's deal killer. Know your relays.

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.

 

[Ooops11]

Hi Typically with low impedance busbar differetial for example GE B30 or B90, CT ratio can be different. While configuring the relay you can enter each Different CT ratio. Secondly polarity of CTs are not an issue as well, above said numerical relays provides you option to define inside relay software, Ofcourse one can daisy chain this protection CT with other protetion for low impedance busbar provided your client is ready to compromise on redundancy of protection, one CT fails, multiple protections fails.. Last question was about utilizing secondary side CT of Transformer, I am not sure on this, this will create issue of corrections of turn ratio,I presume GE relays dont permit this.. not sure about others though

 

[Ooops11]

Lastly.. CT saturation will not be an issue, if low impedance busbar protection is being used, compared to stringent requirement by High Impedance Busbar prot.

 

[M1337]

Hi Davidbeach, thanks for your response.

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.

 

[scottf]

Have you considered adding a set of external slip-over CTs?

 

[M1337]

Hi Scottf,

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?

 

[davidbeach]

Where's the CTs on the breaker bushings?

 

[M1337]

@DavidBeach. Yes the CTs are on the breaker bushings.

 

[scottf]

Yes...I work for Ritz Instrument Transformers and we make them

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

 

[davidbeach]

The transformer protection should be using the CTs on the bus side of the breaker and those aren't shown in the drawing. That leaves the CTs on the transformer side of the breaker available for the bus protection.

 

[M1337]

@davidbeach,

oh i misunderstood what you asked. this one is expanded.

http://files.engineering.com/getfile.aspx?folder=38d05404-0b83-40a8-a17b-5387ca216fb4&file=txct.pdf

@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

 

[davidbeach]

On a bus with breakers connected to the bus; the CTs on the bus side of the breaker are used by the protection zone for the thing on the other side of the breaker; generally a line or a transformer. The CTs on the other side of the breaker are used for the bus protection. No need to share CTs between zones of protection.

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.

 

[DiscoP]

Sharing CTs for low impedance BBP and feeder/transformer protection has been done successfully at many of our substations. Different CT ratios is also not an issue for these relays.

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.

 

[M1337]

I see, i did not know you could have use a CT where the grounding was away from the bus. You guys have given me lots to think about and a few possible solutions. I thank you for your time and knowledge. I shall update you on the final decision.

Again, thank you so much!

 

[piterpol]

M1337, maybe you can get extra knowledge in reading this paper.

 

[M1337]

Love it, thank you piterpol.

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.