Sensitive earth fault protection - medium voltage 3

[RalphChristie]

I've got a protective issue question, I'll appreciate it if anyone can help me or point me in some direction.

With a high impedance grounding system (secondary of trsf grounded through resistors) in a medium voltage system, on what kind of earth fault protection does IEC insist? Is earth fault protection on the residual connection (3 CT's) enough, or is sensitive earth fault protection (Balancing core / ring CT) essential? The question is related to feeders of overhead lines, permanent cables and trailing cables.

Many thanks

Regards



Ralph

 

[peebee]

A complete guess here -- I'm not at all familiar with IEC.

But with high impedance grounding, I'd be very surprised if you had enough ground current to accurately relay using CT's, you might well need PT-based sensing instead.

 

[Bung]

I think your local regulations and supply authority requirements may be more relevant. IEC doesn't cover this kind of thing, to my knowledge. It sounds like a mining application or similar application if trailing cables are involved, so there may well also be some fairly stringent mines regulations to contend with (there are here in Australia).


Bung
Life is non-linear...

 

[sykimk]

As far as I know, there is no requirement on CT connection scheme for ground overcurrent protection relay.

I think that selection of grounding protection scheme from either residual connection or ZCT connection depends on type of system grounding.

For example, if solidly grounding has been applied to the system, as you are well aware, ground fault currents will be very high and close to 3PH fault current. For this system, residual connection is enough for relay to sense ground fault currents.

If the rated current of neutral grounding resistor is quite low (e.g. 10A), it will be quite hard to sense ground fault currents by use of residual connection scheme. Moreover, it is possible to cause nuisance tripping due to inrush currents

On the other word, if rated currents of NGR is 1000A, and CT's primary ratio on the feeders are lower than 1000A (e.g. if 200/*A CTs are used) I think that the residual connection scheme is enough to sense ground fault currents.

Therefore, you need to check CT ratios equipped into other feeders and rated currents of NGR in order to confirm which protection scheme is proper for your system.

 

[Bung]

We routinely use the residual of three phase CTs for sensitive earth fault protection down to 2% (CTs are normally 400/1, anything from 5P20 to 5P100). A definite time delay of 10 sec is applied. It works well in that we don't get any inappropriate trips due to CT issues. But we do have other problems, which is understandable when you are scratching around looking for signals close to the noise for extended periods.

But we have solid earthing, and some non-effectively earthed resistance earthing. We have no true high resistance earthing - perhaps zero sequence voltage sensing might be better for you?


Bung
Life is non-linear...

 

[RRaghunath]

Grounding the generator neutral through a distribution transformer with resistor connected on its secondary is quite common in large power stations.

The earth fault sensing is done through open delta connected VTs (residual voltage signal) and relay 59N, CT in the secondary of the distribution transformer connected to an earth fault relay (current based), 51N.

The protection in either case is slightly time delayed (say by a second) before trip command is issued.

Connecting 59N across the grounding resistor (on the secondary of grounding transformer) is also common.

The above applies to other than generators as well.

IEEE Buff book on electrical protection may be interesting in this regard.