Voltage Drop/Selecting a Transformer

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excited_electrone

excited_electrone

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Canada
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Electrical E.I.T
Hello all,

I am facing a problem that relates to voltage drop for power cable and/or selecting a 4160/480V transformer!

Here is the scenario,

The team has given a project on mine hoist system! The mine goes down to 600m levels. We have to feed the power down to such 500m levels to operate few equipment rated at 480V(includes three 50HP motors for different jobs). There is a platform to work on, called Galloway! All the equipment on this platform works on 480/240V!

So, the problem is when I pulled down the cable at 480V, I end up surpassing the voltage drop limit (5%). Also, if I think of getting a transformer of 4160/480V on that platform, that definitely solve the problem of voltage drop but it weighs over 2 tons, moreover we are running out of space on that platform so providing 3m clearance would be a problem, too! Moreover, people working on that platform would be at risk having such high rated transformer nearby!

Is there any practical solution for such problem that I am missing out?

Thank You for your time!
 
You have a few options. Not enough detail so giving you a generic answer.

First off 5% VD is nothing. You can just jack up the taps on the primary transformer. NEMA calls for 10% maximum during normal running. Around 20% you will have starter problems but you can get around this with sag protection.

The first is look at your loads. If you can decrease the maximum load current then it’s no longer a problem. So if you have some larger loads (pumps and fans especially) use a soft start. This can often dramatically reduce the peak current. On a pump or fan you can easily cut starting current in half on those loads. Usually mines have a few very large loads and this works pretty good. Soft starts are very rugged and reliable if mounted correctly.

Second option is run larger cable or even better parallel cables. This reduces cable impedance a lot. I’m assuming all drops here are due to cable impedance. If it’s the primary transformer start there. Personally I like adding more parallel cables. If you have one conductor per phase and switch to two conductors per phase, you cut your voltage drop in half.

Third option is transformers. There are things you can do to reduce size/weight or %Z such as oil filled vs dry transformers but hands down the smallest and lightest transformer if you don’t mind the lack of isolation is an auto transformer. Copper coils vs aluminum also help with size and weight. Switching some of your larger motors to 4160 can also help since it no longer needed the load but you also have to add the switchgear. May also consider your switchgear layout but without seeing it I can’t offer a lot of suggestions.

Fourth option is power factor correction. Capacitor banks not only reduce current by cancelling some of the vars but also boost voltage. Just be very careful about resonance issues and use metal film or oil filled (not electrolytic!!) capacitors in mine applications. Usually you can get a 10% current decrease by doing this simple step and they don’t take up a lot of space.

You gave measurements in meters. All my mining experience, surface and underground, is North American based.
 
Shockerzz said:
Hello all,

I am facing a problem that relates to voltage drop for power cable and/or selecting a 4160/480V transformer!

Here is the scenario,

The team has given a project on mine hoist system! The mine goes down to 600m levels. We have to feed the power down to such 500m levels to operate few equipment rated at 480V(includes three 50HP motors for different jobs). There is a platform to work on, called Galloway! All the equipment on this platform works on 480/240V!

So, the problem is when I pulled down the cable at 480V, I end up surpassing the voltage drop limit (5%). Also, if I think of getting a transformer of 4160/480V on that platform, that definitely solve the problem of voltage drop but it weighs over 2 tons, moreover we are running out of space on that platform so providing 3m clearance would be a problem, too! Moreover, people working on that platform would be at risk having such high rated transformer nearby!

Is there any practical solution for such problem that I am missing out?

Thank You for your time!
Click to expand...
Seems to me your best solution is to make the 480 V cabling larger to reduce the voltage drop.

It seems to me that this kind of question is a design choice and better addressed to your supervising PE than to a bunch of random people on the Internet.
 
Shockerzz said:
Hello all,

I am facing a problem that relates to voltage drop for power cable and/or selecting a 4160/480V transformer!

Here is the scenario,

The team has given a project on mine hoist system! The mine goes down to 600m levels. We have to feed the power down to such 500m levels to operate few equipment rated at 480V(includes three 50HP motors for different jobs). There is a platform to work on, called Galloway! All the equipment on this platform works on 480/240V!

So, the problem is when I pulled down the cable at 480V, I end up surpassing the voltage drop limit (5%). Also, if I think of getting a transformer of 4160/480V on that platform, that definitely solve the problem of voltage drop but it weighs over 2 tons, moreover we are running out of space on that platform so providing 3m clearance would be a problem, too! Moreover, people working on that platform would be at risk having such high rated transformer nearby!

Is there any practical solution for such problem that I am missing out?

Thank You for your time!
Click to expand...

What about splitting the load into two smaller MV feeders each with their own smaller step down transformers? You can also tie/interlock the LV equipment between platform levels for redundancy.
 
paulengr said:
You have a few options. Not enough detail so giving you a generic answer.

First off 5% VD is nothing. You can just jack up the taps on the primary transformer. NEMA calls for 10% maximum during normal running. Around 20% you will have starter problems but you can get around this with sag protection.

The first is look at your loads. If you can decrease the maximum load current then it’s no longer a problem. So if you have some larger loads (pumps and fans especially) use a soft start. This can often dramatically reduce the peak current. On a pump or fan you can easily cut starting current in half on those loads. Usually mines have a few very large loads and this works pretty good. Soft starts are very rugged and reliable if mounted correctly.

Second option is run larger cable or even better parallel cables. This reduces cable impedance a lot. I’m assuming all drops here are due to cable impedance. If it’s the primary transformer start there. Personally I like adding more parallel cables. If you have one conductor per phase and switch to two conductors per phase, you cut your voltage drop in half.

Third option is transformers. There are things you can do to reduce size/weight or %Z such as oil filled vs dry transformers but hands down the smallest and lightest transformer if you don’t mind the lack of isolation is an auto transformer. Copper coils vs aluminum also help with size and weight. Switching some of your larger motors to 4160 can also help since it no longer needed the load but you also have to add the switchgear. May also consider your switchgear layout but without seeing it I can’t offer a lot of suggestions.

Fourth option is power factor correction. Capacitor banks not only reduce current by cancelling some of the vars but also boost voltage. Just be very careful about resonance issues and use metal film or oil filled (not electrolytic!!) capacitors in mine applications. Usually you can get a 10% current decrease by doing this simple step and they don’t take up a lot of space.

You gave measurements in meters. All my mining experience, surface and underground, is North American based.
Click to expand...
1) For the first point, there is no way I can decrease the max load current! Most of the loads are motors running almost all the time.

2) Running parallel cable seem to be a good idea!

3) At the moment I don't have more information that I can provide you for the best solution but I appreciate your help!

4) My supervisor advised not to use capacitors!

Thank you so much for your time, your answer helped me a lot making a good impression as an E.I.T. :p
 
petersonra said:
Seems to me your best solution is to make the 480 V cabling larger to reduce the voltage drop.

It seems to me that this kind of question is a design choice and better addressed to your supervising PE than to a bunch of random people on the Internet.
Click to expand...
Making the cable larger increase the overall weight! Yes, I am addressing my supervisor as well but getting a idea of the practical solution from whom have more experience than I do! I am totally new in this field just started my practice few weeks ago! I accidently found this forum where I can put my questions to gain more knowledge! (I am not gonna be the one who takes the final decision to implement the solution anyways!)
 
Is this an actual project, or an educational hypothetical?

You never say what current level you are operating at, I am guessing about 200A, which means you need a conductor resistance of about 0.07 ohm to achieve your 5% voltage drop requirement. To get this at 600 m length you will need about 350 kcmil copper conductors. Is this even available as a cable that you can hang in a hoistway?

-Jon
 
winnie said:
Is this an actual project, or an educational hypothetical?

You never say what current level you are operating at, I am guessing about 200A, which means you need a conductor resistance of about 0.07 ohm to achieve your 5% voltage drop requirement. To get this at 600 m length you will need about 350 kcmil copper conductors. Is this even available as a cable that you can hang in a hoistway?

-Jon
Click to expand...
If we take 480V down to the platform, it would be around 400A in total! It is an actual project in initial phase!
 
With the understanding that the below is brainstorming outside of my experience:
How much of the load could be via VFD?

My thought is that at 480V and 400A you would need about 300mm^2 conductors to meet a 5% drop at 600m, and this ignoring skin effect and inductance, both relevant at this scale.

Anyhow if you instead supply 700V DC you drop from 3 conductors to two, carrying 560A. By voltage drop you now only need 160 mm^2 per conductor, I think 3 parallel 1/0 conductors would work for + and - (assuming you can use the ampacity of flexible cords)

Anyhow just a random thought. :)

Jon
 
Second random thought: from my time in Canada, 600V systems were much more common than in the US. Motors are easily available at 600V. v VFDs are also reasonably available at 600V (though I have absolutely no experience with inverters operating at > 480V)

What about running your power at 600V, and only providing a transformer for item which _must_ operate at a lower voltage?

To deliver a given amount of power with acceptable voltage drop, cable size is inversely proportional to the square of the voltage. This is because current goes down inversely with voltage but allowed voltage drop goes up, and both effects help you. By taking voltage up 25% you reduce your required conductor cross section by 36%.

-Jon
 
Hey Jon,

TBH, I really have a little information on the loads! I am preparing initial phase documents for information only, which will be changed afterwards!

Well, Client is already using the last job equipment, and they don't have any 600V motors. So, here is the plan, I am gonna step down to 480 at the surface area and will run the cable down to the platform at 480V in the design, there will be voltage drop but client is okay with that!

There is a lot stuff I really don't understand with the project as I am a newbie! But trying to keep up with the stuff, also getting help from people like you and my colleagues!

This forum is really a big help for people like me!

Cheers to Mike!

Thank you to all who spend their time here! I've learned a lot of new things through your efforts.
 
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