CT GORUNDING 1

[521AB]

Regarding the CT grounding issue:

Is it better to ground (connect to earth) the CT, serving a line protection for instance, towards the line or towards the busbar?
And for transformer protection (differential), should the CTs grounded towards the transformer or towards the feeders?

Any preference with some motivation?

 

[davidbeach]

Grounded toward the zone of protection.

 

[521AB]

From what I can see here in Europe, in the new installations the CTs are earthed towards the protected object, as you say davidbeach, and old installations are earthed the opposite.
What do you think: somebody has trown again the coin recently?

 

[stevenal]

davidbeach,

That's the conventional way, but what makes it better?

 

[davidbeach]

What makes it better is that it is convention, the relays use it, and the definitions of direction based on difference in phase angle between voltage and current are based on that convention. I think the last one is the strongest argument, but consistency is a good thing sometimes.

 

[slavag]

521AB, as usually you open intresting thread.
Its true, today, few protection relay mnf. recommended
grounded toward to protection object ( transformer , generator, busbar) That mens differntial protection. Im not sure if its same for line and feeders. For example at distance protection we have option for star point of CT or VT: bus side, line side. Now what we can do if I have some line-trafo direct connection unit:
for 87T ( trafo diff protection) we grounded towards to protection object : trafo, for line I need towards to line and what happend: in one case S1 is grounded in second S2 is grounded. Not so critical, but for maintanence not so good. In this cases we decide: or S1 grounded or S2 grounded on the all CTs on the site.
About grounded toward to protection object, once someone explained me why , but I don't remember now why and who explained me, next week I'll try found this materials.
Regards.
Slava

 

[521AB]

I have my thoughts of course.
My "interesting threads" are just the mirror of what I see around the world everyday. Mainly, I think, we should know why we do something, and not always rely on "because it has always been done like that". This is the only way to manage to do something new in the future.

Anyway, my comments on your posts:

DAVIDBEACH:
I don't think there is any convention actually. The relay designer is interested in knowing the primary quantities (for instance the angle between primary phase voltage and primary phase current if we consider a simple single phase system). In order to do that, it has to consider the grounding of the current transformer, because obviously for the same primary current I can have two secondary currents, shifted by 180 degrees, depending on CT grounding.
Numerical relays have a setting for this (CT Earthing, CT Grounding, etc). Other relays just need different physical connection of the input transformer to the main CT, depending on how the main CT is earthed.

SLAVAG:

Good idea the decision to always earth S1 or always S2, but it still depends on what comes first: S1 or S2? I have seen drawings with all possible combinations. More: most of teh countries do not even use S1-S2 P1-P2 notation. Sometimes the CT is one circle with one dot. Nice eh?
Anyway I think the best is to use the primary current as reference, and consider positive a primary current which enters the protected object. Then try to make the secondary current also entering the relay, for that reference primary current. Consequence: sometimes you earth S1 and sometimes S2, but you always know why.

As far as I can remember, the idea of earthing towards the protected object, comes from the attempt of protecting also the CT itself. I mean, you include the CT in the protection zone BUT.. is that really true? I have never seen one protection relay "saving" its own CT. That's why I have posted this question.

 

[slavag]

Yes, 521AB, I agree with you.
If we design system like to BBP or Generator/GSU/UAT protection, we always put P1-P2 as reference. It must.
But what is a problem: Numerical relays with so much options
for change direction. Many designers today not understand importance of right wiring for the differential protections.
One sied me: not important, you can change vector directions into relay.Our last commissioning of trafo diff. protection was crazy, becouse it.
Regards.
Slava

 

[521AB]

GOOD point. Connection of CTs MUST BE KNOWN, and CT settings in relay must be done according to the KNOWN connection (s). Specially in multifunction devices, you can get BIG surprises if you play with CT earth settings until DIFF is stable, and then surprise surprise, distance protection zone one trip in reverse direction when it is set in forward.. It HAS happened, it happens, it will happen.
BASIC knowledge first, the rest later.

 

[slavag]

Oh 521AB.
According to your OP. Actually, today with numerical relays isnt important. In all cases you design some overlaping of protection function and covered on this way faults at the CT/VT too.
Multifunction relays is half of problem. Problems start if you have some relays with 9,12,16,24 or more analog inputs and you divide connection of functions into relay according to sets CT/VT.
But its other issue and we can continue it at thread " TESTING OF NUMERICAL RELAY" or start new thread " WHAT IS A RIGHT COMMISSIONING OF NUMERICAL RELAYS" or "COMMISSIONING OF PROTECTION SYSTEMS".
Regards.
Slava

 

[DiscoP]

At our organisation, the standard is to have the ground towards the zone it is protecting.

It keeps it simple. Sometimes like in the case of non-directional o/c it has no importance at all, but we still insist that it how it is done to maintain the standard.

However we are organisation of merged smaller organisations. One of them had a convention of ground towards the direction of load flow. It causes problems with feeder diff schemes. We manage this on a case by case basis depending on what protection relays are used and what else is in the substation

It needs a bit of knowledge to manage properly. I liked that saying from 521AB - BASIC knowledge first, the rest later.

Another issue is where the same CT is used for the Feeder protection and Low impedance BBP. In these cases, the ground is towards the feeder/transformer and away from the bus zone on section breakers.

It can go wrong, so we spend a bit of time during the commissioning to ensure that the direction is correct.

 

[majesus]

"My "interesting threads" are just the mirror of what I see around the world everyday. Mainly, I think, we should know why we do something, and not always rely on "because it has always been done like that". This is the only way to manage to do something new in the future"

Very interesting and very well put. I am a strong believer on that methodolgy.

I have spent hours trying to diagnose and disect the most tiny electrical engineering detail just for my understanding on the concept, not just because that is the way it is done, because I want to know how it works... In the end, I actually save a lot more time, because the next time I see it as a problem presented to me, I can solve it quickly because I understand it.

Of course I got a long ways to go to be anywheres near some some of the members here... Some of the regulars on this forum, having amazing amounts of knowledge...

 

[slavag]

Maj!!!!.
Could you please explain this to my team.
Good Luck.
Slava

 

[alehman]

me too!