3 core cable vs. single core cable 2

[hmchi]

A utility engineer friend of mine is trying to gather arguments regarding the deployment of 3 conductor cables vs. single conductor cables in medium voltage distribution. It seems that the former is mostly used in Europe [and non-US markets] and the latter mostly in North America [and its spheres of influence, such as Korea and Taiwan]. It would be interesting to hear from both sides arguing its case. I will start this off ...

Favoring 3 conductor cables :

[1] Bringing conductors closer together lower losses [increasing fault current though]

Favoring single conductor cables :

[1] If one phase is faulted, the other 2 phases need not be replaced.

Guys, Please add to this list. Fire away ...

 

[dpc]

If you're talking about underground utility distribution, then I agree that US installations are typically single conductor cables (with jacketed concentric neutral) in three separate conduits. For industrial installations, this is almost never done.

US utilities use single conductor cable in individual conduits (PVC) to allow for easier replacement of a faulted cable and to allow for single-phase underground circuits. For a three-phase feeder, a fourth spare conduit is often installed.

The issue of the difference in impedance characteristics between the two configurations is insignificant and not a factor, as far I as know.

 

[tinfoil]

I always thought that single-conductor cable was favored by utilities because:

1) It allows stocking of a single conductor for all applications, rather than two.

2) When a fault occurs, the other two phases remain healthy. This is handy in underground residential dev't as 2/3 of the customers may stay energized, and you only need to repair one cable.

3) One heat-generating conductor usually allows for better heat dissipation, and a corresponding higher installed ampacity for a given wire size.

4)Can you imagine the weight and stiffness of >1000m of THREE-phase 1000MCM cable? This is a very common length for utility-types to install in one 'pull'.

5) They were developed first, and utilities like sticking with 'what works'

 

[dpc]

At the risk of pointing out the obvious - if individual conductors are run in separate conduit, metal conduit or elbows cannot be used, due to risk of induction heating of the metal conduit.

 

[busbar]

 
Aside to dpc’s suggestion — Even if non-metallic raceways are installed, specify that there be no steel-rebar “loops” betwwen individual-phase cables in raceways penetrating vault walls, or in concrete encasement for the circuit.

 

[mc5w]

The more modern utility practice in the U.S. is to use single conductor cables on circuits that are over 5,000 volt phase to phase.

Cleveland Electric Illuminating Company uses both kinds going from substations to overhead distribution. There are instances of radial circuits to 3 phase transformers where they use 4 single conductor cables and make up the fourth to reach any of the primary terminals and all 4 conductors are energized.

The primary problem with single conductor cables is that you will get circulating current in the shields which increases losses if the shields are grounded at both ends. If grounded at one end only the other end of the shield can have a nasty tingle voltage and needs to be isolated.

Essentially all of their 4,400 volt substation exit cables are 3 conductor because jacketed type MC 5,000 volt cable only needs the shield provided by the overall sheath which eliminates circulating current in individual shields. These are only 500 KCM copper at the most and only 100 feet to 1/4 mile long. A lot of the older substation exit cables at 7,620Y13,200 volts are 3 conductor and the newer ones use triplexed single conductor cables. I have seen some substations that have both types of exit cables because the second transformer was installed later.

On CEI's older 11,500 volt system the oil filled cables are 3 conductor with would you believe a splice every 300 feet! What CEI uses for replacing a section is to use single conductor solid dielectric cables and a very funny looking heat shrink tube to adapt.

For 5,000 volts and less and 240 amps and less the favorite practice is still 3 conductor type MC cable. For everything else the preference is for single conductor or triplexed single conductor.

Mike Cole, mc5w@earthlink.net

 

[alehman]

mc5w:
Does 3/C cable made of individual shielded conductors really eliminate circulating shield current?

regarding the "MC 5,000 volt cable only needs the shield provided by the overall sheath which eliminates circulating current in individual shields." Most 3/C cable is made of 3 individual shielded conductors. Is this accomplished by using unshielded individual conductors or by simply not grounding the shields?

 

[TMD]

I work for a company that supplies a "unique" part to a power system, so we sometimes need small quantities (<30 m) of cable. We use 3/c cable when possible, ie, when it's commercially available. But when it's somewhat special (large size, medium voltage, standards requirements, etc), we use single cores so we can buy 3x what we would have to buy if we used 3/c.

Just another, but I suspect rare, consideration.

 

[mc5w]

Go over to Pirelli's website to view their pages on different types of medium voltage cables. 5,000 volt MC cable is just like the 600 volt kind but with 5,000 volt unshielded conductors inside with insulation packed between the conductors and the sheath to keep out air and moisture and to keep down corona. The sheath is of the welded corrugated type. There is usually an overall PVC jacket that makes pulling through conduits easier and to help prevent electrolysis betwwen the metal sheath and metal or concrete conduits.

A 3 conductor cable with individual shields around each conductor is going to have problems with circulating current in the shields. There is also a problems with keeping the individual shields in contact with each other and in contact with the overall shield.

Another advantage of single conductor cables is that if the run is in equal pieces that are a multiple of 3 you can bond the shields together once every 3 splices - at the 2 splices in between you can transpose the shields so as to reduce induced voltage and circulating current to almost zero. At each transposition the ground wire from the left A phase shield is connected to the right B phase shield, the left B connected to the right C, and the left C connected to the right A.

 

[hmchi]

Thanks for your contributions, guys.

I guess we have heard from mostly North America, where the predominant utility practice is to use single conductor cable. I am very interested to hear the European points of view ... Or other places where the European IEC influences are strong ... Such as Singapore, China, or India, etc.

Where are you guys ? Don't tell me that this is an 'American' site !!! :=)

 

[sdlloyd]

Both single and three core cables are installed in the Uk and the preferance varies between utilities.

The days of paper cables below 132kV are just about dead and the lower the voltage the colder the corpse.

Looking at cost first, three core paper cables were cheaper than three single core cables. Paper cables were provided with a lead sheath as a radial water barrier. Three core cables can also be steel wire armoured as the steel losses are small and do not have such a great effect on rating. Three core cables only need one cable entry into a termination box. Only one joint containing three cables at each joint bay (splice pit) means a smaller joint bay and lower cost. Pulling cables only requires one cable pull to install all three conductors. One duct (raceway) can contain all three conductors. Short circuit magnetic bursting forces are contained within the cable sheath. Shaped conductors may be used to reduce cable size. LPOF 3 core cables can utilise feed joints to reduce transient heating/cooling pressure effects.

Disadvantages of three core cables are that the individually the cables are larger and heavier than a single core cable which limits the length of the cable that can be installed on one drum before it exceeds vehicle height and weight. Manufacturers also have a limit on their drum twisting capabilities to lay up 3 core cables. The maximum conductor size of three core cables is thus lower than single core counterparts requiring more joint bays on a long route. Three core cable joints are more complex than single core joints.

Thus you would find that for small conductor installations 3 core cables were used and for large conductor installation single core cables were used.

Single core cables will be seriously derated by steel armour wire althogh aluminium armour is common. Larger conductor sizes can be handled, manufactured and carried with single core cables. Bigger conductors allow larger cable rating (ampacity). Single core cables can be installed in flat formation with spacing to allow better heat disipation and again a better rating. Induced voltages and currents in cable sheaths are important but there are ways around these on single core cables. Longitudinal water blocking of single core cables is easier to acheive.


Thus for low current short length requirements (e.g. industry) three core cables are "better" whereas for high power (e.g. transmission) circuits single core installations are more cost effective.

The relative cost of cables has reduced dramatically since the early days and civil installation costs have become more significant in Europe. Polymerics do not necessarily require a metallic sheath and where necessary low cost light weight radial water barriers are available.

This change in dynamic plus the development of polymeric cables has altered the cost balance but some arguements persist. The weight and size of cables per unit length still affect three core cable conductor sizes and typically 630sqmm is about the commercial limit.

There are other factors that may change the choice such as, a corporate stocking policy, spares requirements and special applications where, for instance, steel armour is required or where a triplex design is preferred.

Hope this helps.

http://www.cableconsulting.net

 

[hmchi]

Hi, sdlloyd, great to hear from you ...

You have certainly given us a wealth of information of how various issues are considered and addressed in making this decision, and how historically decisions were made considering these issues. You also indicated that with modern technology, a lot of changes seem to be in the air ...

I would really like to have you comment on the single phase fault replacing 3 core or single core cable economics which seem to be a prevalent consideration in the other comments from our American friends.

Do you have an opinion on the claim that 3 core cables have lower losses and lower voltage drops than single core cables ? There seems to be a perception [could be misconception] that this is why in Europe current limiting fuses are much more prevalent than in North America --- due to the lower impedances causing higher fault clearing capability for equipment such as Ring-Main-Units [thus the use of CL fuses in RMUs]

Do we have other European [or IEC countries] engineers caring to add their comment ? Come on, guys ... no time to be shy ...

 

[sdlloyd]

hmchi,

I was really refering to large transmission cables.

For a quick comparison:

66kV cable 120sqmm Cu Pb Paper 3 core in air rating 294A
66kV cable 120sqmm Cu Pb Paper 1 core in air rating 483A

If single core cables are installed in ducts (raceway) and there is a cable fault in the duct then the likely prefered solution is to pull out the cable and replace it, especially at the MV/LV level. The reason for this is two fold, opening the road and breaking open a mass concrete duct encasement and constructing a manhole is a major cost exercise.

In the UK most cables are direct-buried and the preference will be to install a repair; typically a short length of cable and two joints. Most cable damage is caused by excavation and if a backhoe bucket is involved then all cables will likely be damaged as the cables are dragged.

In the USA it makes sense only to replace one conductor as the other conductors are "safe" in their own raceway. Thus in the USA a three core cable would be more expensive to repair than a single core cable.

In the UK, the chances are that more than one cable is damaged in which case a singe core cable system is more expensive.

In the end, like most things, the choice of cable will turn on the rating, environment and cost. There is no "right" answer, different strokes and horses for courses!

I don't think that the protection for LV RMU's turn on this.

http://www.cableconsulting.net