Generator Supply Volt Drop Problem

there is a thread on here like that but you will have to search this lot.

http://cr4.globalspec.com/forum/electrical-engineering

or use the search window on the right as has already been sugested

The easiest way to keep the voltage drop within the limit is to

(1) use the larger size of the cable or

(2) increase the number of cable (parallel cables of same size) or

(3) Both (use larger size of multiple cables of same size)

Do the voltage drop calculation for the option you select and make sure the voltage drop is within the specified limit. For option 2 and 3, current will be equally divided between the cables (that is why same size of cable is a requirement), so divide the total current by the number of cables and use this current for voltage drop calculation.

See the previous posting for voltage drop calculation formula: http://cr4.globalspec.com/thread/51446

See also the recent discussion about voltage drop, http://cr4.globalspec.com/thread/52788

- MS

supplying a load?

What load?

Why not shift the generator closer, after all they aren't that big and most are very well silenced. Re-fueling and servicing may be an issue but not one that can't be overcome.

Max permissible volt drop is dependant of standard that shall be full fill, as per which standard You shall make installation?

As per latest BS or IEE max volt drop deviation is from source to last point is +/-10% , but in moust countries and local distributions is adopted -6% as maximum.

Practise is that up to your Main distribution board Distribution network can have 2% drop, and you have limit of 4% from Your MDB onwards.

So in case of generator supply You shall consider that for 630KVA up to your MDB it can be max 2%.

If you can tune generator and run it on +2% Un than you can compensate drop and reduce cables.

Summary:

In case of Un=415V on 630KVA load with power factor 0.85 and on 150m run You will have:

2.61% (10.83V) drop with 3x4C240mm cable with current carry capacity 1146A (3x240 per phase)

1.96% (8.12V) drop with 4x4C240mm cable with current carry capacity 1528A (4x240 per phase)

So wee what You can do with gen set regulation, you can save one run.

If You are on 480V level with PF 0.85 You will have:

Lad current 891A Volt drop 9.36V or 1.95% on 3x4C240 cables

Volt drop 14.04V or 2.93% on 2x4C240 cables

See if You can run gen set on +2.5Un % level and go with double runs only

The obvious solution is to use bus ducts. This is generally more expensive initially but is a lot more reliable with lesser volt drop. Otherwise if the loads are inductive then capacitors are also applicable.

Hi Folks,

sorry, some more info to get the big picture. LV Voltage is 400v +/- 10% and the generator volt-drop is 1 - 1.5% as the rest of the 4% permissable volt-drop is from the LV Panel to the final sub circuits. The designed load is 504KW. It is an external installation and as stated the genset is approx 150m from LV Panel and cannot be located nearer - unfortunately!

My simple thinking is - can the generator produce a higher voltage that compensates the volt-drop arriving at the LV Panel within the stated 400v +/- 10% ? Or can a Voltage Compensation Panel be installed at the midpoint of the genset cabling to boost the supply voltage ? or is there some other alternative to avoid 12 no. cables per phase ?

thanks

GC05

Your load is 504KW or 504KVA? it is calculated load or diversified

Supply line shall be calculated for diversified load, usually Kd factor is 0.7

If it is 504KVA your current will be 856A for standard non compesat power factor 0.85, and drop will be 8.99V or 2.17% for 3x240 per phase.

If You calculate for diversified load your sizing shall be for 353KVA or 598A with 6.28V drop or 1.51%

You need 3x3C240 cables or 9x240 single core cables (3x240 per phase)

From where you get 12x240 per phase?

If I am running power long distance I always think of transformers you can do it the same way the utility company does it. Lets say your gen voltage is 120/208v you can set a transformer at the genset and boost it up to 480v and then re-transform it down at the load. Or if your genset voltage is the same as your load side power you (say its a 480V load) then install a 480V - 480V transformer at the load side and use the available taps built into the transformer to achieve the proper voltage. Or use a buck/boost autotransformer system to adjust for your voltage drop.

Remember, you will be able to eliminate your voltage drop, but you will transform it into a power drop. So make sure the generator is rated high enough to compensate the load.

Quote GC05 "LV Voltage is 400v +/- 10% and the generator volt-drop is 1 - 1.5% as the rest of the 4% permissible volt-drop is from the LV Panel to the final sub circuits. The designed load is 504KW. or is there some other alternative to avoid 12 no. cables per phase ?"

You do not need 12 cables per phase. You need to check you calculations. Have you read all of the posts or just ignored them.

Gc05 response is requested.

"or is there some other alternative to avoid 12 no. cables per phase ?"

permit me repeat lv busduct. pl check the ROI with attendant benefits.it may work out due to its simplicity and reliability.

Apologies, see my latest post

GC

Here is a voltage drop calculator. Do it yourself.

http://www.fyxm.net/british-standard-voltage-drop-75464.html

Greetings GC05,

Besides the viable solutions posted, have you considered a buck/boost transformer set at the end of the run prior to the load?

http://www.powertransformer.us/buckboosttransformers.htm

I don't know if these are compatable with the voltage you are using but I thought I would give it a shot.

GC05 & Friends,

Another approach is to have the generator's voltage regulator be sensing the voltage at the distribution panel 150m away from the generator. I see no design difficulty in doing this and it would allow the generator to automatically correct the voltage as the load changes. This may require a consultation with the generator's manufacturer to ensure that it is done correctly.

--JMM

As WAREAGLE said, "Check your calculations"!

12 cables of 240 square mm is ridiculous for the least and therefore you are missing thepoint somewhere in your calculations. There are tables for voltdrop on cables available from cable manufacturers if you want it easier...

Dear Wareagle & others,

apologies, I have been trying to get hold of the cable calcs from our Turkish Designers to verify. They have been promised today. So far their info has been by word of mouth and taken on trust. As soon as I receive I will review & maybe post for clarification.

I appreciate the various recommendations thus far.

thanks

GC

Hi Folks,

Apologies for the silence but a few serious things have been happening here. Needless to say I have not received the Electrical Designer's cable sizing calculations. But worse than that, the Project has been cancelled ! Looks like we're all out of a job and total staff affected is around 400 personnel.

Thanks again for the advice unfortunately could not implement it!

GC