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Topic Title: Fault Passage Indicators on cable joints
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Created On: 31 January 2014 12:08 PM
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 31 January 2014 12:08 PM
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JohnRRussell

Posts: 22
Joined: 02 December 2003

We will soon be installing a private 33kV cable circuit with an unbroken (one substation to the next) length of about 12km, over remote and frequently snow covered terrain. As an attempt to speed up location and repair of any cable faults that might occur in the winter (if only by starting to clear snow in the right place as soon as possible!) we are considering installing fault passage indicators in each joint bay. Has anybody any previous experience of trying this sort of monitoring?
 03 February 2014 09:22 AM
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ArthurHall

Posts: 736
Joined: 25 July 2008

Never fitted them in joint bays, but used them plenty of times in substations. Get the self resetting type, Bowthorpe used to do a good range among others.
I am not sure how you would fit them in a joint bay as you need to keep the cable armor out of the core, but I sure that could be over come.
Have you considered fitting fault locators? These calculate how far down the circuit the fault is based on the value of fault current.

Also, why do you think snow will be a problem for an underground cable?
 03 February 2014 11:51 AM
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JohnRRussell

Posts: 22
Joined: 02 December 2003

I'm not familiar with fitted fault distance locators - can you point me at references?

The issue with snow is simply access, the cable could be overlaid with six feet of snow for several months each year. With an immediate indication of where the fault is the estate can start to clear access ready for when the fault location test kit arrives. Otherwise the test kit gives an indication, then the guys have to sit around for a week while the snow is cleared before pinpointing can start.
 03 February 2014 10:11 PM
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JonathanHill

Posts: 225
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John, firstly I just wanted to offer a sanity check on the necessity for moving to such a sophisticated system. The major cause of cable faults is third-party interference (diggers in the road etc). If you're crossing open ground and using good installation techniques, the probability of this type of activity should be low, particularly if the route is well marked and clearly understood by the landowners. Cable failures due to manufacturing faults are very rare in my experience.

You could look at separable connectors such as are manufacturered by Coopers, Euromold, Nexans etc for your joints. They have several useful featurers in that they're submersible; the earth screens come outside the main joint which should make the installation of the EFIs easier; you can separate for sectioning the cable run to aid fault location. Speak to Langley Engineering - they are UK agents for Coopers.

For EFIs, suggest you also speak to Nortech - they offer communicating devices with some of their range. If mobile coverage along the route is problematic, you might be able to use a comms cable. I think you could also usefully check that your proposed application is feasible - I have slight misgivings in that, under fault conditions, with such a long feeder, the cable capacitance will feed the earth fault from both ends, thus causing inappropriate operation of the EFIs on the "load" side of the fault. Just a thought - it's a long time since I've worked with these devices, but I do recall that they do have limitations in application.

I think the Fault Distance Locators that Arthur mentioned, was referring to certain Overcurrent & Earth Fault relays that are able to give an approx. distance based on the cable data that your commissioning engineer can input. I think that all the major relay manufacturers (Schneider, ABB, Alstom etc) can supply these.

Hope this is helpful

Jonno

-------------------------
Jonno
 04 February 2014 02:13 PM
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JohnRRussell

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Thank you Jonno, that is all very helpful.

I'm fully on-board with your sanity check - use of FPIs is one of a range of options that we'll be discussing with the owner, to address his concerns that he'll be left with adequate tools to deal with his system when he's left alone with it.

The fault location function on the protection relay is one of the options, but I don't know how accurate this feature is. I was thinking that Arthur knew of a free-standing accurate device! Even if not accurate the relay feature should indicate which end to start clearing the snow from, and that might be enough.
 04 February 2014 02:58 PM
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ArthurHall

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The fault location device is was refering to is independant of the protection, they have a supply from the VT's and CT's and can calculate a distance to the fault, I think Hathaway is one maker but I am sure a look on google will help.

There is also the DTS system which uses a fibre pair in a multicore cable. The system looks down the fibre and can sense hot spots due to the change in refraction of the fibre sheath. This system is often used in the 132kV subsea export cables from offshore wind farms, however you need to have the cable specialy made.
 04 February 2014 03:35 PM
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JohnRRussell

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Thanks Arthur, I'll look into the Hathaway device.

The DTS system and purpose made cables is probably a step too far!
 04 February 2014 09:36 PM
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AdrianWint

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Joined: 25 May 2006

On something like an ALSTOM P442 distance protection relay, on a single feeder with no tappings or parallel circuits & with good info from the cable manufacturer (impedance, X/R ratio etc) you should be able to get an answer within a few 10's of metres without too much trouble - accuracy is all down to how well you set the thing up. I guess you'll be thinking of some kind of protection relay on the front end anyway so something like the P442 will also do over current & earth fault as well as distance protection & fault location.

Talk to the applications boys at ALSTOM.... (Uk based, Stafford) and see what they have to say.

PS. I don't work for ALSTOM or any related Company.
 05 February 2014 03:00 PM
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JohnRRussell

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Thanks Adrian,

That's very useful.
 13 February 2014 10:58 PM
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neilmcd84

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In my experience, 33kV cable comes in 750m drums. Are you suggesting that the customer will start clearing 750m of 2m deep snow from the cable based on the indications of a fault passage indicators? And all this will happen before the fault location equipment arrives to pinpoint the location of the fault? Are you planning for these devices to communcate back to a central point, have a visual indication on the surface, have the joints exposed inside some sort of inspection chamber, or have to dig up the ground to see if the FPI has flagged?
I think your resources would be better directed at two things - correct installation by making sure the excavation, backfill, record keeping, and especially jointing techniques are correct. Secondly, set up a decent fault response contract, paying a few extra £k per year to have a quick fault location and repair service. It will be a nightmare to install, maintain and replace all these FPIs in the kind of terrain you are talking about. You will have to replace them several times in the lifetime of the cable. Imagine the hassle caused by one malfunctioning FPI when a fault occurs.
 14 February 2014 09:58 AM
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JohnRRussell

Posts: 22
Joined: 02 December 2003

Thanks for your comments Neil.

Maybe I wasn't clear about the issue with snow. I'm not thinking about clearing snow from the cable route, I'm thinking about gaining access to the cable route - for fault pinpointing then repair. The substations at each end are near habitation, so near to tracks that will be kept passable. But the cable route is not - there are tracks crossing and there will be a 4WD track along the route but they are not used in the winter and in snow will not be passable without some clearing. If the approximate fault location was immediately known then the owner would make an immediate start on making the right track passable and from the right direction.

The need for good installation is a given.
 15 February 2014 01:00 AM
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alancapon

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Joined: 27 December 2005

In my opinion, you would be better off spending the money on a fault location trailer, or making arrangements for someone who already has one to come and locate the fault for you. With an HV cable fault (usually 11kV), we can locate the fault to within a few metres using the "Single Impulse Method" with a surge generator and filtered TDR. Following on from the initial location, we would then use the surge generator as a "thumper", and using a suitable listening device that can receive the magnetic and acoustic signals from the fault, we can usually locate the fault within 10cm, and it will be in the first hole we open.

Regards,

Alan.
 15 February 2014 02:32 PM
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JohnRRussell

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Joined: 02 December 2003

Thanks for that Alan.

Calling in somebody with the TDR and thumper location kit will always happen. But that could take a week to get to site. When the TDR identifies an approximate fault location it could then take another week to clear the access track to that approximate location before pinpointing can start. The guys are sitting around for that time. FPIs are one of a range of options for discussion to get an approximate location identified straight away, so that by the time the location kit arrives the site staff have already cleared access to the approximate location. We save a week of downtime, maybe more in a big snow.

As I've said, FPIs are one of a range of options for saving that week, not for avoiding the need to bring the location kit to site. It was never the only or even the preferred option. But I did think it was worth asking if anybody else had done anything similar. I guess the answer is 'no'!
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