Earth fault protection

[haisa]

I want to know the basic difference between Neutral Earth fault and Stand-by Earth fault protections please

 

[alehman]

I don't know what is meant by "Stand-by" earth fault protection. Can you explain? If you can provide a little more detail, we could provide a better answer.

 

[slavag]

Hi Haisa.
Please search in the Forum, we had a lot of threads on the topic.
Shortly, I suggest: neutral EF protection, it's mean. residual connection of three phase CT to the additional CT input, and stand-by earth fault protection it's additional protection is connected to toroid CT on the trafo neutral.

Alehman, it are same 50N, 51N, 50G, 51G with lot of mix additionals, SEF-stand by earth fault or also used sensetive earth fault protection, BEF-back-up earth fault.
Once with toroid on the transformer neutral, once with toroid on the all 3-phases, once with residual/holmogren connection. Once it's toroid CT, once balanced core CT, once ZS CT.
So much terms about same :-(.
Best Regards.
Slava

 

[ScottyUK]

Hi Alehman,

It's definitely a UK term, possibly European too from what Slava has posted. In UK practice the standby E/F relay is usually a IDMT type, possibly with a high set, connected to a plain CT on the neutral earthing conductor. It backs up a faster relay which is often in a residual connection.


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If we learn from our mistakes I'm getting a great education!

 

[RRaghunath]

The Stand by Earth fault protection receives current signal from CT in the transformer neutral. Thus, in addition to being back up protection to Earth faults in the downstream outgoing feeders / incomers / bus bar, it can also detect faults in the Trafo windings / connected power cable.
Operation of Stand by Earth fault protection is wired to trip the upstream (Trafo primary) breaker in addition to the Trafo secondary breaker. Some times, Stage-1 element of the protection is also set and wired to open the bus coupler (in case of 'bus coupler closed' operation)

 

[haisa]

Hello Raghun.

Your explanation conform with what I know. But what of Neutral Earth fault?

 

[rbulsara]

Haisa:

I think the question is not correctly worded or its comparing apples to oranges.

A proper (answerable) question would be what is the difference between Neutral overcurrent (51N) and ground (earth) fault overcurrent (51G) protection.

OR you can ask what is the difference between a earth (ground) fault protection and a backup (standby, if it a real term) earth fault protection. You can deploy either kind of protection as primary or backup (earth or neutral) overcurrent protection. Only thing that a standby or back up protection will have higher (slower) setting than the primary settings so failing primary, the backup protection would come in picture.


As for 51N and 51G, the way modern relay manuals proposes, 51G actually measures the current in neutral using a CT (residual CTs), where as 51N is where current in neutral is calculated by vector sum of the 3 phase over currents by the relay.

 

[DiscoP]

In Australia, Neutral E/F is the same thing as term Standby E/F in the UK (i have worked in both countries).

ScottyUK has given a pretty good defintion. A CT on the Star point of the transformer will directly measure an earth fault, but it has no idea which feeder it is on, so it trips the transformer.

Therefore it is set slower than all other e/f protection devices, and hopefully something else should always operate first.

 

[rbulsara]

There is no such thing as neutral E/F or can't be. If it is it is a complete misnomer of something else. E/F is on the system and not of neutral. Neutral E/F suggests you are protecting neutral against ground fault???

There is neutral overcurrent and there is earth (ground) over current.

 

[DiscoP]

It is detecting an e/f using a CT in the Neutral (star point of the transformer).

You are correct, when you think about it, it is a confusing term, but in our organisation we all know what it means.

By the way, 51G can mean either measured e/f or calculated e/f, as can 51N.

For SEL 51G is the calculated e/f & 51N is the measured

For GE UR Relays 51G is the measured e/f, 51N is the calculated.

 

[RRaghunath]

rbulsara's explanation that the 51G measures the current in neutral is in line with the recommendation of ANSI C37.2. T

The code recommends use of suffix "G" when the CT is located in transformer / generator etc. neutral circuit.

 

[DiscoP]

What I was saying is that the relay manufacturers call it different things. SEL and GE for example are in complete contradiction.

Other relays have 51N and/or 51G elements that can be assigned to either a measured or calculated value (eg Basler)

It is not correct to make a general statement that 51N is a calculated value (or vice versa). It depends on the relay, or the person who has put it in the drawing/document.

But getting back to the orginal question , in our part of the world Neutral Earth Fault is a common term. It the same as Standby Earth Fault used in the UK.

 

[DiscoP]

Just re-reading these posts.

The moral of the story is that it doesn't matter what the term is, it is the application that counts.

I am sure most of us have seen different terms used for the same application, and also the same term used for different applications - I certainly have on many occaisions.

Haisa, could you please give us some more detail of your questions application or context.

 

[alehman]

Thanks for the explanations. Helps to get the terminology clear. 51G vs. 51N has always been confusing as well. Depends on who's definition you read.

I prefer the terms residual current and neutral current. No confusion. Either can be backup (standby?), depending on settings.

 

[jghrist]

I prefer the terms residual current and neutral current. No confusion.

Is the return wire in a wye CT circuit a neutral? Does it carry residual current? Or does neutral current come only from a CT in the neutral of a transformer? And then to add more confusion, you can have the relay calculate zero-sequence current from the phase quantities.

See thread238-212004

 

[rbulsara]

Please see page 4 and 5 of the linked pdf. My understanding coincides with that and our local utility co. protection engineers.

http://www.basler.com/downloads/transfguide.pdf

 

[DiscoP]

Hmmm - interesting. Basler call it 51G in their guide, but if you use the Basler BE1-CDS240 relay the target will be 51N (or 151N, 251N etc). There are no elements called 51G in this relay.

In this relay, the 51N element can be assigned to either the Ig CT input, or a vector sum of the phase CTs.

So you can make the 51G function (as defined in the Basler guide) using a 51N element. You can also make a 51N function (as defined in the guide) using the same 51N element of the relay depending on how you set it.

Thanks for helping me make my point ;-)

 

[rbulsara]

discoP:

Not sure what your point is. Plus I was not trying to make any point.

You can play with electrical wiring and schemes and make it work, that has nothing to do with accepted conventions per IEEE. I can write much longer repsonse as to where 51G and 51N designations come from but I have no apetite for it.

The flexibility of digital relay may have blurred some lines to some. But if you were to use only discrete or electromechanical relays, you would appreciate the difference.

I have seen and used residual CT connection in 51G input, but it still amounts to measured current.

 

[alehman]

jghrist - good point. In my mind, neutral current is measured from a CT on the neutral-ground connection. Residual is measured or calculated vector sum of multiple CT's. Neutral CT lead? Hmmm, well not sure what to call that. I guess that's residual too.

jensendrive had a pretty good explanation but maybe it would be best to forget semi-ambiguous terminology and stay with 3-line diagrams!

 

[DiscoP]

I guess my point is, the IEEE conventions have been twisted, mixed up & confused by protection people for a long time now.
Just by seeing 51N or 51G (or any other designation) written, it doesn't mean a great deal without looking into the detail.

As for 51N and 51G, the way modern relay manuals proposes, 51G actually measures the current in neutral using a CT (residual CTs), where as 51N is where current in neutral is calculated by vector sum of the 3 phase over currents by the relay.

I was trying to politely say this statement is not always true.