Sensitive earthfault settings 2

[veritas]

Gentlemen

I am interested to find out what other utilities set their SEF relays to and why? also what are their experiences with their settings. The utility I work for here in Australia applies SEF on its 11, 22 and 33kV feeders with typical pickup settings of 5A with trip times ranging from 5s to 10s.

It has always been my belief that the SEF time delay should allow for the high impedance fault to hopefully burn clear before the relay trips.

Is this correct? comments?

I thank you all in advance.

Veritas

 

[RRaghunath]

Firstly, SEF protection is adopted for overhead line feeders considering the fact that most of the faults in ovrhead lines involve earth (85% or better) and high resistance type yielding very little current.

The sensitive earth fault relay, as the name implies is set sensitive to detect a high resistance fault and is typically a few amps. As is the case always, sensitive protection increases the chances of spurious operation and the long time delay is meant to prevent such new nuisance trips.

CTUM 15 is Alstom's sensitive earth fault relay quite popular in distribution systems in India. This has setting range from 1 to 16% (of CT secondary) and time delay settable to many seconds (up to 60sec, if my memory is right).

Trust the above is useful.

 

[RalphChristie]

I agree with rraghunath's comments.

SEF-protection is used in rural areas, where if one of the conductors falls on the ground (with a high resistance) it remains energized because of the low leakage current and thus can cause danger to humans and animals. There are two ways (which I'm aware of) for this kind of scheme:

1. SEF relay used with a balancing core/ring CT.
2. SEF relay used in the residual connection of a O/C and E/F scheme. The SEF-element is in series with the normal E/F element.
Some of the new type of electronic relays have a SEF-function incorperated into the relay.

The current-setting is quite low with a long definite trip-time. If I remember correctly the settings on some of our local electricity provider's lines are between 1%-2% with a 6 second trip-time.

Regards
Ralph

 

[veritas]

Gentlemen

I appreciate the comment so far. Yes, I have noticed on some of our 11kV feeders that the standing zero sequence current (3*I0) is as much as 3A. I thus advocate a pick-up setting of no less than 5A.

Is the long definite time delay only to prevent spurious trips? I am interested to know what other reasons are put foward for the long time delay and what time delays are good ones.

Thanks again.

 

[RalphChristie]

Veritas

The current-setting of a SEF-scheme depends on the residual unbalanced capacitance and leakage currents that may flow in a healthy powerline, for which tripping is not required.
Because this schemes have a back-up function, supplementary to the main protection, they are given a time-delay of several seconds, long enough to ensure that they do not interfere with the normal discriminative protection. Although not suitable for grading with other forms of protection, sensitive earth-fault relays may be graded as an independent system.

Regards
Ralph

 

[dpc]

Just for my education, do these distribution circuits have phase-neutral loads? The SEF arrangement you are discussing would be difficult to implement (I think) in the US because we typically have a neutral conductor and serve many single-phase line-neutral transformers. The neutral is generally grounded in multiple locations on the circuit.

 

[RalphChristie]

Dpc:

Three phase loads - normally on overhead conductors in rural areas. I am familiar with 11kV lines, although it seems to be used on different MV-distribution voltage-levels.

And no, single phase loads will cause an unbalance and thus a trip.

Regards
Ralph

 

[Bung]

The main reason for the long time setting (5-10s) is to prevent spurious tripping on transient events. Unfortunately, the nature of low current earth faults is that they tend to be "pecking" faults, so the timer resets long before you ever get to time-out. I have actual recordings from a recloser demonstrating that fact - a >8A earth fault fizzed for over two minutes in bursts from a few cycles to several seconds before it was finally remote tripped by the operator. I've got some nice bits of fused sand on my desk as a memento. We now instal SEF relays with a delayed reset to try to minimise this effect.

Watch out too for single phase switching between feeders - this leads to false out of balance and hence trip of SEF relays. We always disable SEF tripping before doing any such cross-feeder switching.

Our typical setting is 2% on 400A CTs connected residually. The same CTs drive traditional overcurrent and earth fault relays. Used only on feeders that are predominantly overhead.

Bung
Life is non-linear...

 

[veritas]

Thank you Bung. We overcome the problem of false SEF tripping by using an SEF relay fed off a trfr neutral CT (or neutral point of an earthing trfr for a delta network) in series with the SEF output of the feeder relay. The series connection of their output ensures that both relays must trip before the breaker is tripped. In other words there must be earthfault current flowing up the trfr neutral.

The intermittant "pecking" fault you mention I have come across before. It has resulted in a feeder relay timing out and tripping before the downstream recloser did. Problem was fdr relay was electromechanical whilst the recloser (being numerical) had an instantaneous reset. Fdr relay would thus wind up and eventually trip before the recloser as the latter continues to reset completely. Solution is to set the recloser to a delayed resetif this is possible.

Regards.

Veritas

 

[Bung]

The SEF check may be fine if it too is timed, but there is still the (low) risk of inadvertent trip when the main relay picks up on normal ops (eg switching) and there is a genuine earth fault on another feeder. You could lose two feeders for the price of one.

I don't like the idea of protection picking up on normal operations, with only a station level restraint on it - but that is a personal preference. I guess it's just down to what you are used to!

The problem with pecking faults on most definite time relays is that there is normally no allowance for a delayed reset.

Bung
Life is non-linear...

 

[Rocketir]

Reminds me of a discussion we have had previously.

http://www.eng-tips.com/viewthread.cfm?qid=80163

HTH