Cross-bonding verification - reversing all links

[KangarooJoe]

Hi,

We are performing a cross-bonding verification on a 220KV cable circuit with 11 joint bays.

The links and SVLs at all locations (termination link box and joint link boxes) have been placed in the normal service position.

At the far end of the cable, we have connected the three cable conductors together with heavy leads bolted to the cable terminations.

At the near end of the cable, we are injecting a 3-phase supply by way of a generator with the AVR bypassed - the generator exciter windings are energised by a manually controlled DC power supply, turning the generator into a constant current supply. We have done this test many times before and we are confident of the method.

The measured sheath currents are too high unless ALL the links are reversed.

i.e. at JB1, we measure 10A, which is too high. We reverse the links at JB1 to fault the cross bonding system and measure 30A, which is expected. If we then reverse the links at JB2 WITHOUT un-reversing the links at JB1 (i.e. so the sheath rotation is counter-clockwise rather than clockwise) the measured current drops to 0.5A.

This is the same for the remainder of the circuit.

My understanding is that the direction of rotation should not matter, as long as for each section, the 'contiguously connected sheath (if you see what I mean)' shares an equal distance with all three conductor phases.

Does anyone have any ideas?

 

[waross]

The cables are mis-labeled. If the cables are in separate ducts, the ducts may have been transposed between pull points.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[KangarooJoe]

The main 220KV cables, or the concentric cables leading from the joint bays to the link boxes?

 

[waross]

Between transition points.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[KangarooJoe]

The client's since agreed the links are the wrong way around, but why does it actually matter?

The purpose of cross-bonding is to ensure sheath phase transposition (i.e. the 'contiguous' sheath spends an equal distance adjacent to red, blue and yellow phases) so why does the direction of rotation matter?

 

[ausphil]

Is the cable continuously cross bonded, or is it broken into major sections , with 3 equal length sections forming the minor sections (ie earth box, cross bonding box, cross bonding box then earth box). If it is broken into major sections (which is the typical design), then did you have the same problem when you changed the cross bonding links in say the 4th and 5th box, or did this next major section prove to be correct?

Did you check the phasing of each of the sheath sections prior to perfomring the crossbonding check? This will usually reveal if there has been an accidental "cross" in the bonding leads, which can lead to the problem that you have and that waross has described.

Usually you perform this check, along with loop resistance measurement of each section of sheath, IR and HV tests, to ensure that the phasing is correct. It can be confusing though to work out which phase is which, so you have got to follow it through from one end.

The only explaination of having the incorrect connections, then changing both link box connections (essentially reversing)to make it right is that the connections are the wrong ones in the first place (as waross described). If it was connected correctly in the first place, then if you had a clockwise or counter-clockwise rotation of the sheaths, then you would have the minimum current flowing as a result in the sheaths.

As a matter of interest, are you working to voltage limits at each of the cross bonding boxes when you are injecting? If so, what values, or are you just scaling up the test current values measured in the sheath sections to the rated load current? We work to 50V (with our test current scaled up to rated load current), bu that somes from a old british standard.


ausphil

 

[KangarooJoe]

The cross-bonding system uses major and minor sections as you've described, with solid earthing boxes at the major section boundaries. The problem was across all major sections - every single set of links had to be reversed.

We checked the phasing of the sheath, it was marked correctly at all link boxes.

There is definitely some issue related to having the correct rotation (because when we changed only one link box, the current went way up.)

The cross-bonding links can only be installed in two positions - either for clockwise rotation or counter-clockwise. For a minor section consisting of only 2 cross-bonding link boxes, there's only 4 total possible combinations:
1 and 2 clockwise
1 and 2 counter-clockwise
1 clockwise & 2 counter-clockwise
2 counter-clockwise and 1 clockwise.

Normally we calculate the max standing sheath voltage by scaling up the test current to the load current and present this to the client for evaluation; sometimes the client presents a specified maximum sheath current.

 

[ausphil]

KangarooJoe,

The only way that you can get these results is if (essentially) the bonding system is incorrect.

You have mentioned 4 situations, and that is correct, but if you have incorrect sheath connections you may not have all those situations with each connection of your links.

To me, what you have when you measure the original link position is each sheath running with 2 sections of the same conductor, and one section of another conductor - this is where you have measured 10A.

When you have changed one link, you have changed the configuration to each sheath running with 3 sections of the same conductor - this is where you measured 30A.

When you did a double change, you have hit the correct arrangement of each sheath running 1 section with each phase conductor - where you meaured 0.5A.

I'm not sure if you measured the other configuration (where you only changed the 2nd set of links) but I think this may also be a 10A measurement.

My guess is one of 3 situations
- that there has been either a rotation of phase conductors at one joint bay, and hence a possible double cross of sheaths
- an incorrect sheath connection at one joint bay
- a cross of 2 sheaths before they get into the link box

The way to check this would be to do a single phase injection into the one of the conductors, and measure the current in each of the sheath connections at the link box and check that they line up with the phase markings. Repeat this for injections into each other phase conductor. This should tell you exactly which sheath connection runs with which conductor over each of the sections. With this test, you will need to disconnect the sheaths at the cable sealing end on the 2 phases that you are not injecting into - this will prevent any current being injected into the other phase sheaths - but be aware that there will be induced voltage present on the other 2 phases at the sealing end and the link boxes.

If it is a common issue for each major section, maybe the design was incorrect in the first place, or maybe the design was correct, but the paperwork for the jointers was incorrect or not clear (or it wasn't the same way that the jointers have done it before).

Your easiest solution would be to re-label the links and sheath connections (after having verifying the exact cause from single phase testing.

best of luck.

ausphil.