Incoming feeder capacity for crane load ( Variable loads ) 1

[shersan]

Hello friends ,

There are total 6 Nos of 11kV , 3.2 MW cranes ( ship to shore ) to be fed from a 11kV substation. Total Connected load is equivalent is 19.2 MW . Maximum Loading for the incoming 11kV feeder is limited by the utility is 5.4 MW . To support 19.2 MW load ,then should have minimum of 4 Feeders . However one of our substation was running with 2 feeders for the same connected load wihtout any problem since 1 year . Our intention is to convince utility that we need only 2feeders ( or max of 3 ) instead of 4 feeders by saying the following .

1. Diversity of the crane itself is about 0.7 by knowing that the full load will not occur as this 3.2 MW consist of 4 individual operation motors which normally works only two at a time .

2. Second reason is that , it's very unlikely that the all 6 cranes will operate at full load at the same time . So the diversity applies ( approx .0.8 ) on the total connected load .

Any other resons ?????????????/

And why utility limited the one feeder ( 3c x 240 Sq mm XLPE , partly thrugh ducts and partly direct burieed , at a depth of 1 Meter down and soil resistivity is Middle east condition Etc ) is only 5.4 MW . No clues as by applying derating factor I'm getting about 7 MW .


By Giving more numbers of incoming feeders will constitute high cost and more switchgear , bigger size building .

Welcome some good suggestion to deal with this kind of issue .

 

[waross]

I would try to aquire copies of the power bills for the last year, or even two years if possible.
The bills are sure to include maximum demands.
When you are planning an instalation, you have to estimate the diversity but in an existing facility you have access to records.
On the one hand you have an estimate (Educated guess taken to 3 significant figures).
On the other hand you have actual documented history provided by the same utility that you have to convince.
Regardless of any calculated diversity factor, if the utility can point to a period of peak usage that exceeded your calculated value you have no argument.
On the other hand if you can use the utilities documentation from the power bills to show your maximum loading during the last 2 years you have a very strong case.
yours

 

[BJC]

Is this a new installation or an existing one? Have you measured the load of the cranes or of similar cranes? In my experence the load is proportional to what they are hoisting. Light lifts = light electrical loads.
IF it's a new installation who ever is supplying the cranes can give you a load vs power diagram. When they do you can get the average weights of Conex containers and forecast you load.
In the worse case ( when all the cranes are unloading heavy containers ) you could instrument the cranes so that each can see what the others are doing. At coal mines draglines are linked together so there not all loading the system at once.

 

[Simmers]

The NEC provides their recommendation for calculating motor loads for multiple cranes. Article 610. We use this exclusively.

 

[rcw retired EE]

Several years ago I put recording meters on 5 kV feeder to four container cranes at a North American port. We were studying exsitng loads to size new feeders for additional cranes. I remember being suprised at the small load compared to our estimates from the single line diagram.

The diversity of the cranes was such that not all would do the same thing at the same time as they went through the sequence of picking up a container, bringng it over the side and lowering it onto a waiting truck (or going the other way). What suprised me was the amount of dynamic braking that was occuring, putting power back into the system. The crane drive VFD's would generate power as the loads were lowered or slowed. This reduced the utility load.
On a crane, an individual load can't operate more than a few minutes. Either the crane is at the end of the rail or the load is all the way up, down, right or left. So the peak load can't occur for very long. Normal load is a series of long pulses.

All of thse factors should allow you to come up with a reasonable estimation of your required load.

Good Luck!