When is cross-bonding needed? 1

[FEinTX]

I'm a degreed EE, but my background is electronics and I could use some help.

BACKGROUND: I'm working at a wind farm site w/ a 34.5 KV collection system. The individual circuits feed into a 34.5 - 345 KV substation. The circuits are typically between 25,000 and 50,000 feet in length, and each has 12-13 2-MW turbines. Cable sizes vary from 3/0 at end of a string to 1250 kcmil on the long runs to the substation and all cable is laid in a trefoil configuration.

ISSUE: Our drawings have the following note on the 1250 kcmil runs to the substation which are ~10,000 feet in length: "Bond cable screens together and to ground at quarter, midway, & three-quarter points along the length of the cable." (This means "cross-bonding", right?) Our developer is saying this is an old note that applied to the orignal plans which did not have the cable in trefoil, but rather, in a flat configuration. They claim the trefoil layout eliminates the need for any cross-bonding.

QUESTIONS:
(1) Am I being fed a line, or is it true that laying cable in trefoil can eliminate the need to do cross-bonding?

(2) What exactly is the purpose of this cross-bonding in this application? I'm guessing it has to do w/ minimizing losses and possibly protection in the event of arcing/lightning, but I'd sure like an educated answer here.

On a related note,
(3) Another issue is the bare ground wire. The drawings call for this to be at the bottom of a 4' trench w/ 6" of grated fill between it and the MV cables. In practice, this is sometimes difficult to achieve. I've seen cases where the ground has been pulled up for some reason and is virtually against the 3 MV cables. Is this something to be concerned about?

Thanks for any help you can offer!

KRCinTX

 

[itsmoked]

I cannot answer your questions but I will be giving you a star for your refreshingly clear well stated questions.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jghrist]

What you describe is not cross-bonding. Cross-bonding transposes the sheaths to avoid circulating currents in the sheaths. The sheaths would not be grounded at the cross-bonding points. Your drawings call for simple grounding of the sheaths. This will avoid a large buildup of voltage in the sheath, but there will be circulating currents that have to be accounted for in the ampacity calculations.

Laying the cables in trefoil would only eliminate the need for grounding if the cables had bare concentric neutrals that were in contact with each other and ground.

I don't know the reason for separating the bare ground wire from the cables. Perhaps to avoid damaging unfaulted portions of the cable during a heavy ground fault.

 

[stevenal]

Is this a multi-grounded neutral system? If so, NESC requires four grounds in any mile.

 

[waross]

Single conductor cables induce a voltage in the sheath or screen.
The longer the cable, the greater the induced voltage.
Long, shielded, single conductor cables may develop a dangerous voltage to ground on the sheath.
If the sheath, shield or screen is grounded at both ends, or at multiple points, then the induced voltage will cause a current to flow in the sheath. This current will cause some heating in addition to the heat produced in the core conductor by the load current.
Most codes have a clause requiring a cable to be de-rated when sheath currents are allowed to flow.

I suspect that the cables have been derated in the original design, and the multiple grounding is to limit induced voltages to a safe level.
An alternative is to break the sheaths and insulate them at each grounding point.
That is, the sheaths are grounded at the supply end. At the first grounding point, the sheaths are interupted and insulated. The outgoing sheaths are grounded. The result is that each section of sheath, (between grounding points) is grounded at one end (the supply end) and insulated at the other end.
This limits the magnitude of the induced voltage and does not provide a path for circulating currents, so no de-rating is required.
For this method to be utilized, the sheath must be insulated, or mounted on insulating supports (inside wiring).
If it is proposed to use this method it is well to megger the sheath to ground to verify the integrity of the cable jacket.
respectfully

 

[cuky2000]

QUESTIONS:

When is cross bonding needed?
To minimize the shield losses while drastically reduce the magnetic field generated by the unbalance current more obvious in long distance lines.
In the US, Canada and UK, the most prevalent practice has been the single-point grounded or sectionalized cross-bonded sheath.
For comparison with other shielding methods, check the enclose link.

http://cuky2000.250free.com/Shielding2.jpg

(0) _{..... "Bond cable screens together and to ground at quarter, midway, & three-quarter points along the length of the cable."} This means "cross-bonding", right? This could be also just a multi-grounded unless the specification is more explicit.
Cross bonding imply that the cable sheaths are electrically transposed at every splice as indicate in the enclose link.

http://catalog.tycoelectronics.com/AMP/GeneralInfo/4deppagi3a.jpg

(1) ….it true that laying cable in trefoil can eliminate the need to do cross-bonding? Not necessarily because unbalance generated circulated current could be large enough to created significant losses or unsafe conditions.

(2) What exactly is the purpose of this cross-bonding in this application? Your guessing is correct. The purpose of the shield grounded is to minimize the losses created by the circulation current through the shield and keep the generated induce voltage low enough to be safe.

(3 _{…… The drawings call for this to be at the bottom of a 4' trench w/ 6" of grated fill between it and the MV cables. In practice, this is sometimes difficult to achieve. I've seen cases where the ground has been pulled up for some reason and is virtually against the 3 MV cables.} Is this something to be concerned about? I can not see space constrains in 4 ft wide trench to locate the ground cable adjacent to the trefoil cable configuration. If cable pulling is a concern and alternative with cable trench and underground duct bank.

 

[jghrist]

Is this a multi-grounded neutral system? If so, NESC requires four grounds in any mile.

Grounding a 10,000 ft line three times only makes two grounds per mile. The bare ground wire negates the need for separate grounds. This serves as both a neutral and a grounding electrode. It's not exactly covered in the NESC, but the NESC does allow a bare concentric neutral (or jacketed with semi-con) to be the grounding electrode.

 

[stevenal]

I agree that the bare ground wire negates the need for driven rods per NESC. Still, the shields should be bonded to this ground wire four times in any mile to meet the intent of the code. Still assuming this is a multi-grounded neutral system. Also assuming that cables are jacketed with something other than semi-con.

 

[jghrist]

stevenal,

I think you have hit on a reason not to bury the bare ground wire 6" below the cables. You want any ground fault to have minimum impedance so that clearing is fast. Burying the ground wire below the cable makes it more likely that a dig-in will cut the cable without involving the ground wire. Fault current will be limited by the higher resistance of the sheath between the fault location and the nearest sheath ground bond. Even if you bond 4 times a mile instead of 2, the fault current could be restricted by over 1000' of high resistance sheath and clearing time increased.

You will also have more circulating current in the ground wire and less in the sheath if the ground wire is close to the cables. Less circulating current in the sheath will allow higher ampacity.

 

[matwong]

Some basic info on the following link.

http://www.energynetworks.org/cables/cable_bonding.asp

Further info may refer to IEEE 575 : Application Guide for sheath bonding on single core cable & calculation of induced voltage & sheath current.

 

[RAVWARD]

This is an interesting post but it confused me in my concepts and I called a application engineer in a cable company to clarify what I was misssing here. But he advised me to speak to in Medium Voltage cable specialist.
I guess, if some body is here in this forum might be able to put a word to my question.

Q#1 Are sheath and concentric neutral two different things ? I have only dealt with sheaths as in 600 V cables but I did a project for 8 KV distribution and came across the term concentric neutral. I guess it is just the voltage level terminilogy.
In canada at 600 V and 425 level, the sheath needs to be grounded at supply end. But I did not used the term concentric level at 600 V.

Regards,

 

[davidbeach]

Sheath, shield, and concentric neutral are at least two different things if not three.

Depending on where in the world, sheath and shield might be the same thing, what I would call the shield. To me, a sheath would be like the clear nylon around THHN.

A shield on MV cable is copper foil in a continuous spiral wrap around the cable. The shield is grounded at one or both ends (good reasons for either - different topic) and provides an equipotential cylinder around the cable to maintain the electrical stress at a low and uniform level. Not intended to carry current under normal conditions, although there will always be some leakage current.

A concentric neutral is comprised of individual strands in a spiral wrap around the cable, there may or may not be a shield also. The concentric neutral is intended to carry load current and is typically available in 33% and 100% ratings. The 33% percent is used in 3-phase circuits, the three in parallel having an ampacity equal to the phase conductors. The 100% is used for single phase circuits.

 

[ePOWEReng]

Cuky2000,

The trefoil configuration will eliminate the need for cross-bonding. In the trefoil configuration, the circulating current induced do to balanced three phase current is eliminated. Another way of eliminating this induced circulating current on a balanced three phase system is using cross-bonding, which will work when the phase configuration is not trefoil. In non-balanced systems, neither, trefoil or crossbonding will eliminate the induced current do to the unbalanced part of the current (or the zero sequence current).

 

[ePOWEReng]

KRCinTX,

It sounds like you misinterpertated the original statement of your drawings to mean cross-bonding, when it really means bonding to ground. In normal situations you should have balanced three phase current, since you are connecting a generation plant to a substation. What is the level of % current harmonic distortion that you can see? There could be high amounts of harmonic current flowing in the neutral/ground, depending on the system at the wind-farm, i.e. filters on output of inverters or not, size of inverters, fault duty at substation, etc.

 

[RAVWARD]

Thanks DavidBeach,

After reading your post, I called another application engineer and he gave me an answer but it was not that satisfying. I guess, he was close:

Here is what he told me,
Sheath are normally used in low volatage cables and concentric neutral is used in distribution cables rated at med. voltage and they normally dont have sheath but the system has to be three phase four wire.

He also mentioned, if there is a three phase three wire system then a copper take can be used. I could not get that point though.

Could you advise, When it comes to medium voltage cables, What option is better, A single conductor or three conductor ?

Thanks for the comments once again.

Regards,

 

[ePOWEReng]

RAVWARD,

The answer is, it depends. When installing cable for a load that is close to completely balanced three phase, three phase will reduce losses and the ampacity will generally be higher. I would recommend three phase cable for balanced three phase loads when possible. But there are also other physical considerations, like pulling distances, number of corners to pull through, etc. Three phase cable can get very large and heavy. For loads such as residential distribution MV single phase concentric cable is the most commmon where I am from. The biggest reason.....it is easy to work with.

 

[jghrist]

holds6448,

Circulating current will be induced in shields even if the cable is in trefoil configuration. In Rating of Electric Power Cables, by George J. Anders, IEEE Press, 1997, p. 156, the circulating current for cables in trefoil is given by:

Is = I·X/sqrt(Rs² + X²) where
X = [ω]·2·10^-7·ln(2·s/d)
Rs is the sheath resistance
s is the distance between conductor centers
d is the mean diameter of the sheath

 

[ePOWEReng]

jghrist,

You are correct. But if you look at the current induced on the sheath of conductor 'A', it is coupled almost equally by phases 'a', 'b', and 'c'. True, that this isn't perfect, since the sheath of 'A' is slightly closer to the phase 'a' than the phases 'b' and 'c'. But in balanced the phase systems it does reduce it drastically. Your equation is proof 's' almost equals 'd'.

The cross bonding method will have some error in it too since physical constraints will undoubtably cause the cross-bonding sections to not be exactly equal.

 

[FEinTX]

hold6448,

The max allowed THD for this collection system is 2.5%. Simulations from our turbine manufacturer project a THD of 2.15%.

As for my original reading of the drawing, you are correct, the designer was merely specifying that all neutrals were to be tied together and grounded at various points along the run.

Thanks to everyone who has responded to this post...I'm getting a good education here.

KRCinTX

 

[jghrist]

hold6448,

Even if s exactly equalled d, X would not go away; you would still have ln(2). The current does decrease as s decreases, but it would not go away unless s was half of d, which is not possible.