Method of laying of single core 11kV cable

[krisys]

We need to install single core (i.e; single conductor) 11kV cable with a circuit length of about one kM.
No. of cables per phase is six (6). The cables are aluminum wire armored. Since the facility is existing, the space available in the existing underground cable corridor (the corridor is the right of way assigned for the installation of direct buried cables) is limited.

The consultant has specified to install the cables in single layer, flat formation. This is good to improve the cable continuous current rating, as the heat dissipation is better than the trefoil formation. But by laying in the trefoil formation can accommodate the cable with in the allocated space for the cable.

Trefoil formation has the following advantages over flat formation.
- Reduces the overall cable reactance due to the reduced mutual reactance.
- Sheath voltage rise is less compared to laid in flat formation. Hence reduced circulating current (sheath current).
- For the relatively longer circuit length, the advantages gained with better heat dissipation in the flat formation, is offset due to sheath current. This is true if the Sheath Voltage Limiter (SVL) is not used.

So the method of laying is a trade off between the available space, continuous current rating, reactance etc.

Can anyone in the forum throw some light on this issue.

 

[Power0020]

Both sheath current and sheath voltage don't exist simultaneously, if the sheath is double point bonded, circulating sheath current exists with no voltage, if single point bonding is used, there is no circulating current but a standing sheath voltage appears.

If you go flat, you will be also in need to transpose cables twice along the route to compensate mutual unbalance. I personally don't think you need transposition for 6 trefoil groups distributed evenly.

You may avoid SVL with a double bonded sheath, the length isn't too much for that, you need to check the ampacity taking into consideration the circulating current (this is usually dependent on sheath CSA), this gives a reduction of about 4-7% in current carrying capacity of cables.

 

[krisys]

Thanks Power0020.

 

[ppedUK]

Your consultant has either selected very small cables, or you have one very large 11kV supply! I have not seen that many cables on a 60MVA 11kV Station TX. What is the rating of the feeder?

Trefoil formation will result in minimal circulating current, but it is not easy to install in that configuration direct buried. Buried single core cables in Trefoil formation are usually installed in ducts, but that results in large de-rating factors which is maybe why you are going the direct buried route. For direct burial, you will have to tie the cables in trefoil formation at regular intervals to ensure that they stay in that format when the trench is back-filled, with the spacing of the ties dependant on the system fault level. Bond the cables at both ends if in trefoil, but take into account that there may still be some circulating current.

The extra rating gained using flat configuration at 11kV does not usually result in large gains, as it would on a higher voltage system, but if your feeder is as large as implied, then flat configuration may be the only way the consultant can get the feeder rating using 6 cables per phase. You will have to re-calculate for trefoil.

Can you terminate 6 cables per phase in your switchgear?

 

[7anoter4]

A simple calculation could clarify more.
Let's say there are 6 single-phase 300 sqr.mm copper conductor 11 kV XLPE insulated AWA armored. Earth resistivity 90 w.cm/oC, 20oC temperature 0.8 m depth. No dry zone formation.
Single point grounded [20% eddy current in copper shield and aluminum armor].
Method1. 6 groups of trefoil- 3 cables bunched- no distance between.
Imax=240 A/cable S=SQRT(3)*6*240*11/1000=27.4 MVA
Built-up voltage [according IEEE 575/88 Fig.1] 30 V.
Required width=6*2.15*43=554.7 mm
Method 2.In a single row, one cable diameter apart, 36 cables. The ampacity will be 330 A. [330/240=137.5% ] S=SQRT(3)*6*330*11/1000=37.7 MVA.
If the cables are fully cross-bonded then a 60 V built-up voltage could be achieved.
Required width=36*86=3000 mm.
Method 3.Compromise:
6 groups of trefoil- 3 cables bunched- one bunch distance between.
Imax=275 A/cable [275/240=114.6%] S=SQRT(3)*6*275*11/1000=31.4 MVA
Built-up voltage [according IEEE 575/88 Fig.1] 30 V.
Required width=12*2.15*43=1110 mm

 

[7anoter4]

Sorry. One corrections:
120% shield and armor losses was considered[two ends grounded was considered].
If only eddy current is taken into consideration but also a partial drying-out of the soil then for
the first method ampacity will be 295 A, for second 400 and for the third 335 A.