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mokwadi (Electrical) (OP)
8 Aug 07 10:47
How are current transformers and the relay connected for a standby earth fault protection scheme?
slavag (Electrical)
8 Aug 07 11:41
Hi.
You have only two option for connection of EF relay:
1. Holmogren connection
2. Connection to toroid ( on all three phases).
Regards.
Slava
Helpful Member!  RalphChristie (Electrical)
8 Aug 07 12:13
The term "standby earth fault protection" I know, is related to a E/F relay that operates after all the other E/F-protection schemes fail to operate.

Normally this relay is connected to a CT which is located in the neutral/earthing point of a transformer. (never seen it been used on a solidly grounded transformer, always on a transformer grounded through a resistor or a NEC) It has to coordinate with downstream E/F protection and has a relative long time delay. (Either a definite time delay, 3-10 seconds, depending on the system, or a long-time inverse curve) I suppose the same would be true on generators.

The principle behind the idea is that, during earth faults, the current has to return via the earthing-point towards the source, and thus an ideal place to locate a E/F-relay.

Regards
Ralph

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

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RalphChristie (Electrical)
8 Aug 07 13:49
Also:

Just a few notes regarding E/F-protection (current operating) I am aware of:

Residual connection (holmogren): A CT on each one of the three phases, the relay connected in the residual position. Setting normally between 5% and 30% of full load rating. Commonly used.
Sensitive E/F connection: A ring CT around the three  phases. Can be set very sensitive, sometimes even lower than 1% of full load rating. Normally used on systems with a low earth fault current (High and low impedance/resistance earthed systems)
Restricted E/F protection: Differential (zone type) protection, used on generators or transformers. A CT in each of the three phases and one in the neutral/earth point. Very fast type of protection for internal earth faults in a transformer or a generator. Sometimes also seen as voltage operated, if used as a high impedance differential scheme.
Standby E/F protection: As described

There are also voltage operated E/F schemes, but I am not familiar with it. Maybe someone else can comment.

Regards
Ralph

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

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isquaredr (Electrical)
8 Aug 07 16:43
Standby earth fault SBEF as Ralphchristie says is the last line of defence once all other down stream devices have failed to operate. They are usually connected to a ct which is positioned between the star point and the earthing resistor. On a 2-transformer site with 2-transformer CB's and a bus section CB in between it is usefull to have a 2-stage SBEF relay. When a fault occurrs and all other devices fail to clear the SBEF stage-1 opens the bus section this leaves only 1-transformer feeding into the fault which trips on stage 2. By doing this you only have half the customers with no electricity.
mokwadi (Electrical) (OP)
9 Aug 07 1:54
Thanks a lot people for your invaluable information.

Please keep up the good work you are doing!
slavag (Electrical)
9 Aug 07 2:37
Hi .
Ralph, I'm not so agree with you in one point.
"Setting normally between 5% and 30% of full load rating"
From my excpirence 10%-15% it's min. Of course is depend on time setting.
And today with this 10% setting recommend use stabilization resistor.
Regards.
Slava
521AB (Electrical)
9 Aug 07 14:00
Sensitivity for earth-fault with Holmgreen connection:

considering that a 5P CT, can give  an error of 1% at rated current, if you are unlucky you get a 3% error only in CT measurement.
I don't suggest to set E/F with Holmgreen connection to less than 10% ... 15% , to avoid unwanted trip.

RalphChristie (Electrical)
11 Aug 07 14:55
Well:

This would depend on your system-parameters and configuration, but I have use a 5% setting one or two times without any problem. Normally I use a 10% setting, especially on downstream transformers, (on a radial feed) and that works fine with me.

Regards
Ralph

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veritas (Electrical)
14 Aug 07 21:09
RalphChristie

You mentioned the following:

Normally this relay is connected to a CT which is located in the neutral/earthing point of a transformer. (never seen it been used on a solidly grounded transformer, always on a transformer grounded through a resistor or a NEC)

Any reason, why it is not used on a solidly grounded transformer? I have an 11kV/415V delta/star trfr (solidly earthed) and am considering applying a CT in the 415V neutral feeding an LV EF relay which will be graded with the downstream protection. I am interested to know if this has any negative implications.

Thanks.
RalphChristie (Electrical)
15 Aug 07 3:01
Veritas:

Maybe someone is using it somewhere, but in my career I haven't seen it.

On transformers, earthed through a resistance/reactance, the fault-current is usually relative low. This low current can be of such a low magnitude, that it can't be seen from the HV-side. While earth-faults on a solidly-earthed transformer are normally very high. Remember, on a delta/star transformer, an earth-fault current on the secondary side is seen as a phase-phase fault on the primary side.
For earth faults on a resistance/reactance earthed system:
Faults are usually too low to be seen from the HV-side, even for earth-faults on the secondary near the transformer. If you have a breaker on the secondary side, and an earth-fault develop in front of that breaker, a standby-scheme will detect it, and trip the HV-breaker.
For earth faults on a solid earthed system:
Earth-faults are usually high in magnitude and for faults near the transformer it will usually be seen by the over-current protection on the HV-side.

Now lets think about it:

The purpose of a standby relay is to trip if all other earth-protection schemes fail to remove the fault. That actually means a standby-scheme should have a long time delay to ensure positive operation for downstream schemes. Normally, during earth-fault conditions, very high earth-fault currents will flow in a solidly-earthed transformer.
If the time-delay of the standby-scheme is too long, the transformer will be damaged before the standby-scheme can operate. Over-current protection on the HV-side might even operate for such a fault (before the standby-scheme can operate)
If the time-delay is too short, it will not coordinate properly with downstream devices.

Thus, in my opinion:
Such a scheme (on a solidly-earthed system) might be a so called "white elephant" and might add no value to your system.
If you use such a scheme on a solidly-earthed transformer, you should have just a few grading-levels downstream (one or two) and you should use a relatively "quick" delay-time or time-curve, say normal/standard inverse to be really effective.

This is just my opinion, someone else might add his/her comments. In the end such a choice would be dependent on the configuration of your system.

Regards
Ralph

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Marmite (Electrical)
15 Aug 07 4:22
In the UK Standby earth fault protection is widely applied to transformer neutral connections to provide back up to
bus zone protection, uncleared feeder earth faults, faults not within the REF protected zone, and
uncleared faults in the REF protected zone. The protection is applied to transformers with both solid and impedance neutral earthing arrangements.
In the Utilities I have worked for,the protection has two stages to avoid unnecessary interruption of healthy HV teed transformers.
The first stage will trip the LV circuit breaker. The second stage will trip the LV circuit breaker
and either intertrip to the remote HV source circuit breaker or trip the local HV circuit breaker.
Both stages have the same current setting but they are graded in time:-
• Where the same current transformers and relay are used for both stages the normal grading margin
is reduced from 400ms to 300ms for oil filled circuit breakers and from 400ms to
200ms for non-oil circuit breakers.
• Where there is a local HV circuit breaker present there should be no time delay between
stage 1 and stage 2.
The protection is usually standard inverse IDMT, graded with the highest setting on the outgoing feeders.
Regards
Marmite
raghun (Electrical)
15 Aug 07 6:02
I can add to what Marmite has already said,

The stage 1 is for uncleared faults in the outgoing feeders connected to the busbar downstream. The stage 2 for faults in the transformer incomer feeder or the windings and that is the reason it is wired to trip upstream as well.

Just to clarify RalphChristie, it may be mentioned that the HV side E/F relay fails to see the earth faults in the LV side until the fault turns in to a phase-to-phase fault or a bolted one.  
RalphChristie (Electrical)
15 Aug 07 16:37
Marmite, Raghun:

Thanks for the information. If I might ask, on the solidly earthed systems (with standby-schemes), how many downstream-levels do you have?

Thanks and Regards
Ralph

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Marmite (Electrical)
16 Aug 07 5:05
In my experience on the 11kV network, usually 1-3 more levels, beyond the outgoing feeder protection at the source substation.
Regards
Marmite
veritas (Electrical)
16 Aug 07 18:41
Gentlemen,

I thank you for your comments. On a 415V side of our 11kV/415V trfrs I have seldom found any protection other than the LV ACB. These are industrial applications where often not much thought has been given to the implications of EF protection. I take RalphChristie's comments on board with regards to the HV OC seeing the high LV EF currents on a solidly earthed system.

I see this as a back-up option though. On a 415V network the theoretical EF fault current is often in the region of 40-50kA. But it takes less than 1ohm fault impedance to bring this current right down to a few hundred amps - this will not be detected by the HV OC relay. In my example the HV OC relay will only detect a 415V EF of around 5kA and above.

I thus tend to strongly favour a NEF relay in the 415V neutral. I agree that it cannot be delayed too long as long clearing times are detrimental to the system. It has the important advantage of covering the blind spot from the 415V windings to the LV ACB - the ACB usually being the incomer to the LV distribution board. Therefore the LV NEF relay trips the HV breaker and not the LV one.

In fact I had a customer recently who asked that his 415 EF devices be set to trip instantaneously for faults above 20kA. "Rather overtrip and sacrifice selectivity for my plant is too critical to take the chance of being damaged due to non protection operation." was his reasoning.

Furthermore, Schneider now have a range of LV ACB's where discrimination between series breakers is maintained (so they say) even if all are set to the same hi-set threshold. Discrimination is achieved by means of energy let through. I am yet to delve into this a bit more and am interested to know if any of you have utilised this.

For utility applications I would take a different settings and tripping approach especially if REF, etc. have been applied.

Regards.
  
raghun (Electrical)
19 Aug 07 6:32
RalphChristie,
Why I said the HV side EF protection wouldn't be able to see the LV side fault is that the transformers are generally Delta/Star connected.
The zero sequence currents in the secondary are reflected on to primary but circulate within the delta.
Thus, my statement won't apply to Star/Star connected transformers or Auto transformers.
Trust this clarifies.
veritas (Electrical)
19 Aug 07 16:47
Raghun

With a delta/star trfr the HV side would be able to see the LV EF but not in terms of zero sequence but only positive and negative sequence currents (phase-to-phase condition). The same goes for a star/star trfr that only has the LV neutral earthed. Here the tank-delta effect comes into play if there is no third delta winding. Only if both star windings are earthed will the HV side see an LV earthfault in terms of zero sequence current.

Hope this helps.

Kind regards.
raghun (Electrical)
21 Aug 07 5:40
Veritas,
I agree with you. Thanks for the elaboration.
PUNGLU (Electrical)
21 Aug 07 6:46
In Kenya, the transformers are mainly of Delta-star and Sta-Star configurations. The earthing systen is solid. Since current flowing on the earthed neutral is appreciable, a CT is connected on the transformer neutral grounding. The relay is set to between 10% to 20 %. By grading using time the stand by relay is set to operate much later than the earth fault relay.
RalphChristie (Electrical)
22 Aug 07 7:03
I haven't seen standby schemes on solidly grounded transformers before, thanks for the correction. Lesson learned!

Raghun:

Quote (RalphChristie):

Remember, on a delta/star transformer, an earth-fault current on the secondary side is seen as a phase-phase fault on the primary side.

Thanks

Regards
Ralph

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

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