Step up transformer confusion

[warkan]

Hey guys,

I am pretty new to this field. We have a underground armored cable laid out for around 1.5KM -2KM from the plant(Power room) to a river pond. The job of the river pump is to pump water to our plant. There are two pumps rated around 60HP each. Only, one will be run at any time. The other is just for standby. Recently, we have been having a lot of problems with the cable such as shorting and burns. Temporary fixes are not helping the case. When we checked for voltage being received in the river panel, it was around 390V instead of 420-440V. So, I am wondering if the voltage drop is the reason for the cable problems. If it is, what rated step up transformer do I install in order to regulate the voltage back to normal ? What are the criteria I need to follow ? Any help is much appreciated.

Regards,
Warkan

 

[Knobhead]

Did it work before the cable failed?

If the cable is failing now a higher voltage isn’t going to improve things.

 

[Lucien Nunes]

That doesn't sound like the right explanation. If the voltage drop is as you say and the cable that long, I expect it will be of ample carrying capacity for the load. And the circuit protection should at least save it from catastrophic failures.

What size of cable is it and what kinds of cable faults are occurring exactly? Any pictures of them? Are the pumps running correctly?

 

[Bobster]

Sounds like a badly designed system if the circuit is capable of remaining energised whilst the cable suffers 'burning' damage.

 

[Darkwood]

Its Malaysia based OP so Im out, its ruddy hard enough trying to help the UK based members out on this stuff! :6:

 

[warkan]

Did it work before the cable failed?

If the cable is failing now a higher voltage isn’t going to improve things.

That doesn't sound like the right explanation. If the voltage drop is as you say and the cable that long, I expect it will be of ample carrying capacity for the load. And the circuit protection should at least save it from catastrophic failures.

What size of cable is it and what kinds of cable faults are occurring exactly? Any pictures of them? Are the pumps running correctly?

Sounds like a badly designed system if the circuit is capable of remaining energised whilst the cable suffers 'burning' damage.

Operating at a different voltage could possible overheat/damage the cable. Most of the problems in the recent times were cable burns and all three phase gets shorted because of that as seen in the attachment. The cable laid out is 4C * 50sqmm and when tested on load, the current is around 48Amps. The pumps are running fine but then obviously stops when the cable gets shorted. The distance certainly is not the reason for the lower volts right ? Would using a Boost transformer help in this case ?

 

[warkan]

Did it work before the cable failed?

If the cable is failing now a higher voltage isn’t going to improve things.

That doesn't sound like the right explanation. If the voltage drop is as you say and the cable that long, I expect it will be of ample carrying capacity for the load. And the circuit protection should at least save it from catastrophic failures.

What size of cable is it and what kinds of cable faults are occurring exactly? Any pictures of them? Are the pumps running correctly?

Sounds like a badly designed system if the circuit is capable of remaining energised whilst the cable suffers 'burning' damage.

Operating at a different voltage could possible overheat/damage the cable. Most of the problems in the recent times were cable burns and all three phase gets shorted because of that as seen in the attachment. The cable laid out is 4C * 50sqmm and when tested on load, the current is around 48Amps. The pumps are running fine but then obviously stops when the cable gets shorted. The distance certainly is not the reason for the lower volts right ? Would using a Boost transformer help in this case ?

 

[Engineer54]

Have you done ALL the calculations to confirm a SWA 50mm 4 core cable is suitable for this installation with a total length around 2 kilometers?? What other tests have you conducted on this cable?? Are you sure that the PVC outer sheath hasn't been penetrated at one or more locations.

Do you have any past test records on this installation, and if so how do they compare with the installation in it's present situation??

 

[Knobhead]

Oh Dear!
50mm 2Km @ the motors FLC, I’ve just roughly worked it out and fell about laughing. If you work out the VD for a motor starting DOL it’s hilarious.

I don’t believe the figures given for one minute, the motor would never have started.

I wouldn't be be all surprised if it isn't a SWA cable.

 

[Engineer54]

Oh Dear!
50mm 2Km @ the motors FLC, I’ve just roughly worked it out and fell about laughing. If you work out the VD for a motor starting DOL it’s hilarious.

I don’t believe the figures given for one minute, the motor would never have started.

I wouldn't be be all surprised if it isn't a SWA cable.

Exactly!! lol!! Though i doubt if a 60 HP motor will be DOL starting....

 

[warkan]

I am pretty new to this field. Sorry if my question was lame.. What sort of calculations do I need to do ? My electrician just told me 60HP motor should be okay with a 50sqmm 4 core SWA cable. The cable which is laid out is new and hence I doubt that there are any penetration through the outer sheath. And the motor wiring is star delta. In the past, the cable laid out was even smaller and was completely discarded due to several burns.

 

[Knobhead]

50mm would be fine for a short run of cable. But you haven’t taken in account the resistance of the cable which causes a voltage drop along its length.

You need to calculate the cable size by using volt drop tables that give mΩ per metre for the cable you intend to use. You also have to make an allowance for the starting load.

You need to go back to college.

I’m not working it out for you, you’ve got yourself in to this situation. It falls on you to do the calculations and find the correct cable size. It’s going to be a very expensive lesson.

 

[Engineer54]

What do you mean by, ....''My electrician just told me'' ?? Are you not an electrician??

How has this cable been installed, just laid on the ground or direct buried?? Sheath damage can and frequently does occur during installation. A good reason why a sheath tests should always be carried out during the backfilling of direct buried SWA cables....

To give you just a little help with your calculation, the mV volt drop / per ampere / per metre is derived from these values for a 50mm2 4 core armoured cable '' r 0.86 - x 0.135 - z 0.87 '' per metre.

Hopefully between you and your electrician you should be able to work out the overall volt drop on this 2 kilometre cable.

 

[Guest111]

50mm over 2 km oh dear,and your electrician says it,s fine? I think you need a new electrician because the one you have seems about as bight as a neon lamp.

 

[PEG]

I very much doubt that it is in a 2Km trench,it will be dragged across the wilderness,and in fairness to the OP,in territories such as his,cable selection comes down to what the supplier has,right or wrong. I have just calculated roughly that it is woefully inadequate for the task...options are 1) use cable as "control" for stand-alone engine driven pump.....2) use cable to provide mediocre lighting on site....so you can terminate the proper cable in the dark!