11 KV cable for Distant Guest House

[AbrarAhmad]

Dear All,
We have a guest house in our Paper Mill where a lot of variation of Voltages occur. So now we decided to place a transformer there which will have its input feeder from our existing Transformer room about 1.5 KM away from the Guest House. The cable will be armoured XLPE 11 KV. Kindly provide help whether in future the voltage drop will occur or not?

 

[ScottyUK]

Impossible to say. We have no information on the load, no information about the origin of the voltage disturbance, and we have none of the many other missing pieces of information. We are engineers, not magicians.


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If we learn from our mistakes I'm getting a great education!

 

[AbrarAhmad]

Dear Scotty,
The load of the guest house is approximately 100 A. The voltage used there is single phase 230Vac.The disturbance or the voltage variation is about 35Vac. It means the voltage normally in the guest house is 195Vac.Normally the lights and AC trip on this voltage level because they need 220-230 Vac.

Best Regards,

 

[edison123]

First, you need 3 phase, 415 V distribution at your guest house with the 100 A loads equally distributed in all 3 phases. Second, you should think of overhead line for such a long distance from cost and maintenance aspect.

 

[AbrarAhmad]

Dear Edison,
Thanx for your response.You mean that on 1 phase there should be 33A load.Also the budget for overhead line will be far more we have in our mill already 11 KV cable trench which is going from our Grid Station to the Main Distribution HT Room. The distance between Grid Station and The Main Distribution Room is about 2Km. We dont have any tradition in our mill for going Overhead. Also I was in the construction of that trench. That Trench is covered with concrete Slabs and There is a cable tray of 600 mm on which 3 main HT cables laid down. The trench is 3 ft. wide and about 4 ft deep.What do you suggest if we have another trench of the same kind for this guest house?

 

[ScottyUK]

Can we assume that the voltage at the 11kV bus is not the source of the voltage variation, and that the problem is only the load-dependent volt-drop in the 230V feeder cable?

If you are installing a new 11kV circuit then I agree with Edison: use a 3-phase supply. There's no reason why you can't use a buried or trenched cable, especially if you already have a route available. Maintain separation between the HV cable and the LV cables sharing the route.

You need to determine the acceptable volt-drop at the point of use and then work back from there to determine the acceptable transformer impedance and the minimum size of the feeder cable. It should be a fairly straightforward calculation.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

230V x 100 / 1000 = 23 KVA
23 KVA / 11000 V = 2.1 Amps
In North america we would be hard pressed to justify the added expense of three phase for a load that is only 2.1 amps single phae. A Minimun instalation would be a two wire 11 kV circuit and a single phase 11kV:230V, 25 KVA single phase transformer.
I don{t see any justification for three phase or an intermediate 415 Voltage. The service is now 230 volts single phase. The percentage voltage drop at 11kV will be negligible with single phase.
A 100 Amp single phase panel at 230 or 240 Volts fed from a single phase 25 KVA transformer is a common installation in North America.

Bill
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"Why not the best?"
Jimmy Carter

 

[AbrarAhmad]

Dear All,
We have another option and that is to place 100 m away from guest house a transformer of 250 KVA already purchased and we have this transformer in our inventory. Then LV cable of 400 V from the MCC to be laid down upto the primary of Trafo and then on the secondary side we can get 230 Vac from its taps.This Trafo will also supply load to the nearby departments of the mill including Main Gate Office and Road Lighting Poles and Vehicle Stand Points.
Dear Waross I am in Pakistan near Lahore and the North American standards are not followed here but European standards are used.

 

[alehman]

I'm with waross. Regardless of standards, it seems like the added cost of 3-phase for this small load would be hard to justify, unless there is some particular need for the 3-phase power.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[ScottyUK]

The added cost of three phase probably isn't all that significant, primarily being the cost of a three-core cable over a pair of singles. Two-core cable certainly isn't common, if it exists at all. The source will almost certainly be equipped for 3-phase, even if only two are used. The cost of future conversion to three phase would be of the same order as this initial installation: I'd rather take the incremental cost and install it from the outset.

The latest option looks like it may be a reasonable idea from an economic and technical perspective uness there are a few more bits of information AbrarAhmad is going to drip feed to us...


Bill / Alan,

415V isn't an intermediate voltage, it's the line voltage with a corresponding phase-neutral voltage of 240V and is common in British-influenced places (except Britain, where it's now 400 / 230V to suit the Eurocracy ),

As this is in Pakistan I'm not sure how the costs of labour and materials weigh up: if materials are the majority of the overall cost then your suggestion has greater merit; if material and labour costs are of a similar order then I favour the approach from myself and Edison as it offers greater flexibility in future.


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If we learn from our mistakes I'm getting a great education!

 

[alehman]

In the U.S., we use concentric neutral cable for 1-phase circuits. We call it "URD" for underground residential distribution. Is this configuration available in Europe?


Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[davidbeach]

I agree, a URD cable is far and away the best choice for this application if available in that location. A single insulated 11kV core and a concentric neutral of equal ampacity for the grounded neutral. Works only on a solidly grounded, multi-point grounded system.

 

[edison123]

Ahmad

We have another option and that is to place 100 m away from guest house a transformer of 250 KVA already purchased and we have this transformer in our inventory. Then LV cable of 400 V from the MCC to be laid down upto the primary of Trafo and then on the secondary side we can get 230 Vac from its taps.This Trafo will also supply load to the nearby departments of the mill including Main Gate Office and Road Lighting Poles and Vehicle Stand Points.

I think you meant 250 KVA trafo with 11 KV/415 V rating. From the 415 V, 3 phase + neutral secondary of this transformer, run a 4 core, 50 sq. mm cable to your guesthouse. Distribute your loads (mostly air-cons right ?) in the guest house between all the 3 phases and neutral.

If your company has policy of using only cables, go for it.

 

[PHovnanian]

Contact your local utility regarding their line extension policies and rates. You might find it cheaper to have them run an 11 kV underground line, install a padmount transformer and a second service. If you do it yourself, you'll have to figure the capital costs of the installed equipment in addition to the expenses. Plus, you'll be on the hook for maintenence.

 

[AbrarAhmad]

Dear Edison,
U r right. We will do the same and I hope that it will work. Also only 35 mm2 is enough for this job.

Best Regards,