Cable Selection for 44kv Line

hi!

Assume transformer %impedences - 5.5% ( actual values on name plates / test reports of Transformers )

neglecting impedence of the supply authority's line, as well as of cable,

Fault levels on 3750 kva , 44kv = 894 Amp max.

1500 kva , 44kv = 368 Amp.

As long as short circuit levels are concerned, while refering to manufacturer's data sheet, you can select the cable of 70 sqmm on wards.....

Now, max. line current at 3750 kva, 44kv = 49.2 Amp

& at 1500 kva = 19.68 Amp.

You can select cable with 70 sq mm on wards.

Now,

Length of cable = app. 210 Mtr ( 700 feet )

Volt drop of cables shall be negligible in volts as compared to the Supply KV.

So,

I recommend 3C * 120 sq mm UE grade cable for 44kv system ( 3phase 3 wire system)

for 3750 kva tr. ( to reduce the line losses - higher size selected for long cable)

& 3c * 70 sq mm cable for 1500 kva tr.

What type of cable can b used and what are losses that would occur on the line?

You can use any type you want (losses and application suitability will vary however).

You need to get a qualified electrical engineer, electrician or electrical consultancy group in to design and build your system. Ask them for the answer along with a quote, short cuts will likely result in loss of property and possibly death.

Harry Said "Now, max. line current at 3750 kva, 44kv = 49.2 Amp & at 1500 kva = 19.68 Amp. You can select cable with 70 sq mm on wards.

Length of cable = app. 210 Mtr ( 700 feet ). Volt drop of cables shall be negligible in volts as compared to the Supply KV.

I recommend 3C * 120 sq mm UE grade cable for 44kv system ( 3phase 3 wire system) for 3750 kva tr. ( to reduce the line losses - higher size selected for long cable) & 3c * 70 sq mm cable for 1500 kva tr.

Why did you increase the cable size. On one table I checked 70 mm is rated at 265 amps and 25mm rated at 120 amps. The rating for 44 kv cable may be slightly less.

Why such a lage increase?

Was fault current rating mentioned?

Selecting the correct normal current carrying capacity of the cable is not difficult.Often people forget about the withstanding KA rating of the cable selected for a period of say 1,2 or 3 seconds.

At 49A,a 35sqmm cu cable can carry the current without any problem.When there is a short circuit in or around the transformer,the small cable will blow like a fuse before the protection relay ( set at slower trip time ) could pick it up.

Regards,

Khor.

dear,

your suggestion for higher size cable is appreciated.

this is true when you deal with long cables > 100 mtr, to take care of fault energy and volt drops / losses.

3C x 70mm sq. is adequate at 700ft distance. Its up to you if you use the next higher commercial size. Your transformer is installed at fix distance of 700ft ( is the cable loop and riser included). Anyway, don't forget to consider the derating factor when cable is laid underground. NEC allows 82%(0.82) derating factor of max. current carying capacity of directly burried cable.(if you install it directly burried on the ground).

Sorry... i overlooked your supply voltage is 44kv. My experienced in Gulf states..it allows max of 13.80kv to installed underground above it will be overhead. Since you have your 44kv check some provision in NEC outdoor installations for HV/MV.