Selection of Ground Fault Detection method

[Pam60]

Hi gentleman,

I have done it many times but need a bit of scientific explanation to the approach I have taken.

I have a feeder entering a 2 MVA 13.8 kV transformer stepping it down to 4.16 kV Secondary which is resistance grounded with ground fault limited to 10 A NGR.


I need to have a ground fault protection on primary feeder from utility entering the transformer.I have two methods, either a residual scheme or a core balance CT.

My ground fault current from utility on primary side is 2000 A. My phase CT are 150: 5.

If I use a core balance approach, based on the fault as soon as it will pick up, relay will use TOC chr. What if it is a solid ground fault, a 2000 A ground fault will saturate a 150:5 zero seq. CT. Do you think it matters or it could be an issue.

I think It is the same issue in residual scheme, Could you guys suggest. Which one is better.

Thanks

 

[rbulsara]

Residual scheme is the most common as you already have the 3 phase CTs and the even the relay in most cases.

As for scientific, either scheme will measure the same current, the time delay depends on the relay settings.

As for the saturation, I would think the available 3 phase fault current is also in the range of 2000A. So if the CT are suitable to sense 2000A of phase fault currents, they would have no issue with a ground fault of the same magnitude.
The saturation issue is no different from when there is a phase to phase fault of 2000A.

Rafiq Bulsara

http://www.srengineersct.com

 

[pwrtran]

There is no enough to say your CT will saturate or not, it also depends on how much burden the CT has. Keep in mind to make CT sat is not the current but the secondary voltage. So if you have less than standard burden you could have more than 20 pu throughout current; however, it also depends on the fault angle at the incident moment and residual flux on the CT.

The other way you can detect the LG fault is to use open corner delta overvoltage detection.

 

[RRaghunath]

Pam60,
Earth fault current of 2000A could mean that the utility trafo NER is sized to the full load current of the trafo.
Such large currents will allow use of phase CTs for earth fault protection as well (through residual connection).
CBCT is superior to residual connction only when the earth fault current is limited to a low value (when compared to the phase CT primary rated current) which is not the case here.
With regard to the subject of CT saturation, you can estimate knowing the CT ALF, rated VA burden vis-a-vis the actual connected burden.
Generally speaking, the CT saturation is an issue with phase fault protection and in case of earth fault protection only if the system is solidly grounded.
Trust the above is helpful.

 

[Pam60]

Thanks Raghun
I guess you caught a nerve here that when currents are high residual scheme is the way to go and that is why I was seeing more and more residual schemes in the past on the primary side but could never got to see the actual explanation.

Your last comment
" speaking, the CT saturation is an issue with phase fault protection and in case of earth fault protection only if the system is solidly grounded."

How would a residual scheme CT saturation will play out in a solidly grounded system. Could you kindly explain.

Thanks

 

[RRaghunath]

Pam60,
Residual scheme, CT saturation will not be an issue presuming that the phase CTs are already adequately sized for the phase fault current magnitudes.
In other words, in solidly eathed systems with residually connected E/F protection, CT saturation can be an issue for both phase and earth faults if CTs are not adequately sized.
Trust I could make myself clear.

 

[alehman]

I think you see a lot of residual schemes because it doesn't require additional CT's and is built in to most modern relays.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[Pam60]

Thanks All. I also made an interesting observation recently.

I was looking at a relay file of SR750 from GE and I noticed that ground element was disabled. After I did my research I found that SR750 has two CT ground inputs G3 H3 and G10 and H10.

Neutral elements in the relay file calculates the residual current on its own and does not need any physical CT input and used phase CT ratio for calculation.

So Technically and physically there is no residual scheme and nothing is being physically implemented.

Is this the practice as well.

Thanks

 

[rbulsara]

Pam:

In SR750, terminals G3-H3 are for sensitive GF and G10-H10 is for regular GF scheme. This relay also has 51N function or neutral overcurrent and that is a calculated value (for GE SR relays). The separate ground CT input is for 51G function or a measured value(for GE again).

Just wiring alone is not sufficient, the relay also needs to be programmed correctly to function. I have also seen the neutral of the phase CTs connected to G10-H10 in a 3-wire system.

You would do well to involve someone familiar with SR750 and ground fault protection schemes. Also refer to the relay manual for more info.

Rafiq Bulsara

http://www.srengineersct.com

 

[alehman]

Pam - that's pretty much what I was trying to say. What you describe is accurate, but as others have said the relay must be programmed correctly to make it work. The manual is pretty good.


Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.