Underground cables discharging into upstream faults

[flickstar]

Hey Folks,

I am currrently looking at settings for fault indicators (FI) on our distribution network. On overhead feeders with considerable downstream underground cable installed, during a fault our FIs on the overhead lines often operate for currents discharged from the downstream cable back into the fault. Of course, this means that our fault indicators operate downstream of the fault and our operations personnel have little faith in the devices which should be making life easier for them.

I am keen to gain a better understanding of the currents discharged from the downstream cables and fed back into the fault. Can these currents be differentiated from normal load current (apart from direction)? Are they at different frequencies etc? Is there any way I can prevent our FIs from operating for these capacitive discharge currents without completely desensitising the devices? (FYI our FIs monitor di/dt and current magnitude only.)

Any assistance, whether it be advice based on previous experience or a link to relevant technical papers etc, is greatly appreciated.

Cheers,
flickstar

 

[Skogsgurra]

Hello flickstar,

This is really the kind of problem that makes you think.

I have never seen this before, but I can understand that it is a problem. I do not think that you can adjust the existing device to differentiate between the two fault currents.

The problem is, I think, that we do not know if the "discharge" is a discharge from an arbitrary DC level (downstream cables already disconnected) or if it is a capacitively coupled current (downstream still energised).

The former failure type should have a decaying DC component that would be fairly easy to detect while the latter failure probably has a deviating phase angle. If only one phase were involved, the current would lead by around 90 degrees. But, with three phases involved, you probably have a mix of currents coupled from all three phases to the fault - so the phase angle could be just about anything.

I will be talking to a distribution specialist this evening. Perhaps he understands your problem better. I will let you know.

Gunnar Englund

http://www.gke.org

 

[hdp]

If these FCI's are installed on a circuit with reclosing capability, there is a possibility that the false indications are due to inrush tripping, rather than stored energy from underground cable. I have seen this before...we had to utilize FCI's with a delayed trip curve to mitigate inrush tripping.

 

[dpc]

I'm a little doubtful that the capacitive discharge is really responsible for the false indication. How were you able to determine this? There are a lot of installation issues with the UG fault detectors. I'd suggest reviewing the installation instructions, especially with regards to cable shields, concentric neutrals and proximity to other cables.

But these little guys are used by the bucketful on underground systems, so capacitive discharge must be a pretty common thing that they have to deal with.

We specify the Joslyn (Fisher Pierce) Adaptive indicators and have good luck with them (mostly). Cooper also makes FCIs that they seem proud of, but I have no experience with them.

I agree with hdp that inrush is real possibility - the adaptive units are supposed to solve this.

Another possibility:

http://www.eosmfg.com/products/products.html

 

[flickstar]

Thanks skogsgurra & hdp,

The discharge current I refer to relates to stored energy in underground cables that have been de-energised when the feeder has tripped.

Also, our FIs have a time delay upon energisation to prevent the units operating for inrush current, and our problem with discharge currents flowing back into faults occurs even when there is no reclose.

In the past we have had problems with old auto-reclosers on our network operating for currents being discharged from downstream underground cables. The problem has been resolved by upgrading the ACRS to units with directional functionality - this is not really a valid solution for our FIs though.

Any further guidance/insight is welcomed.

Cheers,
flickstar

 

[flickstar]

Hey dpc,

Your post popped up just before I hit the 'submit' button.

The FIs I am talking about are for overhead conductors. They merely 'hang off' each phase conductor so I don't think installation is an issue. The underground cable creating the discharge current issue is installed downstream of where the FIs are located.

Thanks everyone for your advice. I look forward to further posts.

Cheers,
flickstar

 

[dpc]

The companies I referenced also make OH detectors, FWIW.

Are you reclosing into these faults?

 

[flickstar]

Yes we have reclose on our feeders.

 

[dpc]

Then I would just second what hdp already stated - it is quite possible that the inrush current alone is tripping the fault indicators.

Also, are these automatic resetting indicators or manual reset?

 

[flickstar]

I'm pretty sure inrush isn't the problem here. The FIs we have disable fault detection for at least three seconds after line energisation (whether this be after an outage or during a reclose cycle).

Once triggered the FIs remain flashing for a prescribed time (typically 8 hours) or until they are manually reset.

Cheers,
flickstar