Sensitive Earth Fault relay operation for faulted o/h lines 1

[michalakis]

In our electric utility we have been using Sensitive Earth Fault (SEF) relays for the protection of the 11 kV overhead distribution lines, and they are set for operation at 6 Amps and 5 seconds time delay.

Their purpose is to detect fallen conductors of o/h lines. We provide this protection additionally to normal Overcurrent and Earthfault relays.

When such a SEF relay operates, because of fear to create conditions for a possible death accident, we proceed very carefully and carry out a complete and detailed patrol of the faulted o/h line to locate the fault and repair it before attempting to reenergise the feeder. This process is, of course, both very time-consuming and costly. The consumers thus affected may remain without supply usually for more than 8 hours.

However, there are occasions where a SEF relay operates not because of a fallen conductor (whose handling is so difficult) but because of a fault on a pole-mounted distribution transformer whose 11kV-side fuses (20Amps, fast type)do not operate to isolate the fault.

Our 11 kV lines, being 3-phase, 3-wire, 50 Hz and usually very lengthy with many branches, are supplied from substation transformers with secondaries y-connected and their neutrals solidly earthed. The 11 kV lines supply 3-phase Dyn11, 11000/433 V distribution transformers ranging in capacity from 25 to 200 kVA each and with their neutral solidly earthed too. We operate the 11 kV feeders radially only.

We like to hear of experiences with SEF relays operation from utility engineers, or others regarding:
1. Settings adopted for such relays.
2. Whether they face similar SEF relays operation caused by reasons other than fallen line conductors. If yes, what procedure do they follow to restore the supply? Do they patrol the whole line before attempting to reenergise it, or what else?
3. Any suggestion to cope with the problem of SEF relay operation for reasons other than fallen conductors?

Thanks.

 

[Kiribanda]

Hi michalakis,

This is the first time I heard that a Sensitive earth fault protection (Zero seq. core balance ) scheme has been used for a 11 kV overhead distribution line system. Normally it is best suited for a cable system. If it is installed on a 11 kV overhead system with just 6 Amp setting, then even for a small leakage on the insulators or for a transient flash over due to lightening etc. my understanding is, that the system can lead to “Nuisance Tripping” .

So could any of our member(s) comment on this protection scheme? Is it practicable?

Regards!

Kiribanda :->

 

[hivoltage]

Hi Michalakis,
here is how we do in our country:
we have the distribution network based on 20kV 50Hz insulated neutral. Network is typically radial.
We normally use the following protections:
Max phase current, max residual current (threshold: typically 2A), max residual voltage (threshold: typically 5%).
For easy fault location we use to put along the overhead line some fault indicators.
These are three-phase models placed onto the O/H line poles about 6 metres below the line conductors.
They trip for residual current and short-circuit current, giving a radio signal that can be received by the linemen with a small handheld receiver.
They go by car along the line, and proceed as long as they receive the tripped signal. Where they find the first fault indicator not tripped, that's the faulty segment.
This makes fault location very fast.
Let me know if you need references for such devices.

Regards
Hivoltage

 

[Rocketir]

1. Settings adopted for such relays.
Where we have used them we set them to 10% of the Grd trip setting.

2. Whether they face similar SEF relays operation caused by reasons other than fallen line conductors. If yes, what procedure do they follow to restore the supply? Do they patrol the whole line before attempting to reenergise it, or what else?
We have had the relays operate for reasons other than fallen line conductor. These include blown lightning arresters, cracked/leaky insulators, self protected transformers that are starting to fail, and wires/jumpers laying on crossarms.

3. Any suggestion to cope with the problem of SEF relay operation for reasons other than fallen conductors?
Most of the time, the relay operated for a reason. However the reason is not always found on a patrol. Sometimes it has taken a infrared to find the problem and this has led to increased outage time.

The thing with the SGF setting is that it will see past fuses and our linemen have a hard time of understanding that. They don't see the reason that they should be patrolling past fuses. As a part of our procedures to locate the SGF, when a mainline patrol will not find a problem, we open all fuse tap lines on the mainline. Then start closing in the fuses. When the relay trips again for a SGF we have located the tap line.
In my opinion, it can be a very useful setting in the right circumstances. We mainly use this on our 12kV circuits serving rural customers. This setting also gets removed during storm season.
HTH
Jesse

 

[stevenal]

Just two suggestions:

Delay the SEF tripping so more faults will be cleared by the fuses. Unless it lands on somebody, downed conductor is not a safety hazard untill someone approaches it.

Overhead line fault indicators can be used to cut down the size of the patrolled area.

 

[Rocketir]

<<Delay the SEF tripping so more faults will be cleared by the fuses. Unless it lands on somebody, downed conductor is not a safety hazard untill someone approaches it.>>
I disagree. A conductor laying on the ground can cause fires. And if this results in a large forest fire it can result in a large monetary cost as well as endanger lives. Also Delaying the trip will not result in a fuse clearing. The downed conductor will still be seen as load.

<<Overhead line fault indicators can be used to cut down the size of the patrolled area.>>
Many times the fault indicators will not trip either, due to the low fault current

 

[stevenal]

&quot;Also Delaying the trip will not result in a fuse clearing. The downed conductor will still be seen as load.&quot;

The problem was that SEF was operating for distribution transformer faults. If these faults cannot produce enough current to blow the 20A fuses, not much can be done. If fault current levels are high enough, the problem is one of coordination between the fuses and the SEF protection.

On the fire issue, you need to consider all risks and response time. On one hand, a good bright fire will assist with fault locating at night.

As a separate but similar example, high impedance fault detection schemes use algorithms that can take minutes to decide if the harmonic signature of a downed line is present.

 

[asymptote]

In the UK, the largest Utility employed SEF(in conjunction with IDMT OC & EF and Auto Reclose) protection on many rural OH feeders and suffered for many years from nuisance tripping by SEF. With some trepidation the SEF was removed some 30 years ago without any real impact on on system faults or damage. Some 25 years ago the decision was made to make all drop-out-expulsion fuses into solid links ie no protection on PM Transformers and both these measures improved &quot;customers off supply&quot; times.

 

[jghrist]

I agree with stevenal. The 5 sec time delay setting for the SEF relays can be less than the clearing time for fuses with current at several times their rating. For instance, a 40K fuse takes more than 5 sec to clear for currents less than 150 A.

 

[jbartos]

Suggestion: Visit

http://search.netscape.com/ns/boomf...B+%22line%22&amp;clickedItemURN=http%3A%2F%2F

http://www.selinc.com%2Fsel-351.htm...&remove_url=http://www.selinc.com/sel-351.htm

for:
Application Note
Calculations
Relay type

http://stest.etnetera.cz/siemens/si...dici_systemy/data/7sa6xx_katalog_2002s_en.pdf

Etc.

 

[michalakis]

I would like to thank all of you who replied to this request and gave your valuable experiences, opinions, or other help, all of which are being considered in order to effect improvement to the existing situation.

 

[HiSet]

We to have similar SEF protection with similar settings but to make it more secure we employ current check feature.This check is issued by the neutral current in the supply tranformers in the substation.Also our distribution network service provider have recloser with SEF protection as well.