Taking supply at HV vs. LV - Industrial Plant 2

[123MB]

Hi All

My client currently has two 400V points of supply and has requested a third point of supply from the distributor to cater for a planned expansion.

Due to retrospective changes in regulations my client is now required to change his assets to one point of supply only.

Supply at HV is being considered due to the relatively large (about 100m) seperation between major load concentrations on the site. Customer-owned HV distribution on site would reduce cable sizes considerably and therein the overall cost of the works.

I personally do not like the option and would prefer to see 1-off Low Voltage point of supply at a transformer sized to serve the entire site and have the customer pay for the extra cables.

If the HV supply option is taken the distributor will remove his existing transformers and the customer will have to install and maintain new ones. The cost benefits are only obtained if several smaller transformers are used. In addition a new HVMSB would be required in addition to the new LV MSB to cater for the expansion.

In a cursory sense it appears that the capital costs are about even.

There has been some talk about buying the existing xfmrs from the distributor for reuse.

The customer would have to pay for and maintain the new transformers and HV switchgear, for which his maintenance staff have no idea etc.

ATM the issue is pending advice from the distributor

Does anyone have experience in this sort of feasibility that could offer some advice. Cheers.

Michael

 

[itsmoked]

Been many discussions here on this very point recently. You can do a search in this forum.

You may confuse the people you are working with if you don't correct this.

I believe you actually do not mean HV as this would in no way be logical. You mean, perhaps, MV? In this area of power LV is generally anything below 1,000V MV starts here and goes way up. Most commonly you are probably talking something around 4,160V, a relatively common "plant" voltage.

I am not as experienced in 'power' as described here but I will pass on some observations to your situation.

1) MV is not to be trifled with. Anyone working on it should have written procedures to refer to and serious training.

2) Training requires time and money and assigned personnel.

3) Everything MV is seriously more expensive. Motors, circuit breakers, panels, motor starters, etc.

4) You basically will be trading cost of copper wire run across the plant verse copper wire wrapped around expensive magnetic iron in transformers in multiple places on site to distribute MV around a facility. Often this is not a very good trade-off.

5) MV makes more sense if you have several motors exceeding 1,000HP just for providing the required energy.

Others will have more to add - I'm sure.




Keith Cress
kcress -

http://www.flaminsystems.com

 

[FreddyNurk]

I agree with pretty much everything that itsmoked said, and can add the following considerations:
100m, depending on the load, can either be quite trivial, or a big issue in terms of distance. It really depends on whats in the plant, and how to get the required cabling to it.

Depending on how you'd restructure your distribution (and this goes both for MV or LV) you'd still need to look at all the issues. If the fault levels for LV end up large enough, you may have to look at going to MV regardless, and its impossible to say from the information provided. I'd assume that you're still up for costs for upgrading of the distribution to the site, as its not clear whether the existing utility transformers are up to the job, or where the best position is for your new main board.

Also, with LV / MV, you're often trading upfront capital costs for an ongoing maintenance expense for all the maintenance and operation of the new higher voltage equipment. Often, it won't make sense unless you've got some big motors that can be directly powered, and even then, the purchase price still isn't cheap.

Another consideration may be availability of service personnel. There are a lot more people that are available to work on LV than MV where I am. Might count for a lot if you happen to have a problem in the plant.

Best thing to do is to really analyse all the options before committing, including such issues as lead times, and go from there.

 

[123MB]

Yes everyone I meant HV to be 11 kV.

Thankyou to all that have replied.

The LV fault level will be OK. The plant could be supplied from 1-off 2.0 MVA T/F with max. fault 33 kA at 400V.

There are no HV motor loads. All loads are 400V AC motors, VSD controlled, approximately 200kW. The whole 2MVA load is made up of these motors.

 

[123MB]

Just reread again. Thankyou for valuable posts. Stars given.

Michael

 

[itsmoked]

Hey 123MB if you can describe your plant's major needs you may get some input like don't bother or some questions that would lead you closer to answer.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[andybro]

itsmoked.
I suspect that 123MB is working in the UK. Technically there is no such thing as MV (as you understand it) here. Everything above 1000V is HV. Historically MV was used for our 415V 3Ph systems. This does cause confusion for some of our North American suppliers and in practice many people use the term MV accordingly but the regulations are quite clear on the matter.
Andybr

 

[itsmoked]

Ah good point andybro. Thanks for the enlightenment.

And welcome to Eng-Tips!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[123MB]

Hello all and thankyou for your participation.

(close...)I am working in Australia where, exactly as andybro said, the term 'MV' is not used anymore... we use 'HV' to describe any operating voltage above 1kV.

I will use MV throughout the rest of this post to suit you guys.

The load is a water treatment plant. The requirements for the plant power supply are as follows:

--There are currently 2-off 400V points of supply on the site. To be consolidated into one point of supply.

--Capital costs are to be kept to a minimum.

--The existing switchboards are to be replaced. Fault level won't be a problem

--The load is comprised of AC drives, many 100 kW, totalling 2.0 MVA.

--Small generators will be installed at each of the (3-proposed) LV MDBs to power prob. 1.0 MVA (total across the site, about 300 kVA at each MDB).

 

[FreddyNurk]

I'd just left a project where they were running LV over at least the distances that you'd stated. Its not necessarily ideal, but at 200kW load for a motor, it doesn't make much sense at all to go to MV. Biggest LV motors that we had powered by VSDs (250kW) still had a cable run of about 150m. Certainly wasn't cheap in terms of copper, but your only option to reduce copper in that case is to run the transformer next to the motor, which isn't really feasible either, as you're just shifting the cost, and for very little benefit.

I've heard HV, MV and LV all used in Australia, though I am somewhat aware of the historic reasons for it. It appears to me that it depends on who you talk to as to whether its HV or MV. The distribution people at the local utility all seem to call it HV, but the transmission people don't.

Whereabouts is the plant?

 

[123MB]

Did I say 200 kW. I meant 100 kW. These loads are at the final distribution level. The copper costs are at the power distribution level (submains to MDB, see below.)

The plant is in WA

Yes, well, it seems in this case the only reason 11kV supply is being considered is to save money on copper.

Supply at LV would require the distributor to supply a 2MVA LV service and the customer to reticulate a 1000A submain 160m to a new LV MDB and a 700A submain 60m to a new LV MDB.

Supply at HV would require the customer to purchase and maintain 2-off T/F plus a HV MSB, in addition to costs for the LV MDBs. The saving, obviously being over those submains mentioned previously.

 

[123MB]

I note this subject has been discussed on this forum previously. Can someone help me with some tags... I can't get the right words...

 

[davidbeach]

Personally, I wouldn't consider higher than 400V for a load that small and that compact. 100m is not an unreasonable length and multiple small MV-LV transformers will undoubtedly be more expensive than doing it all at 400V.

 

[rockman7892]

This is a very interesting discussion. The one thing I don't understand which someone commented on was the fault ratings. Are fault ratings typically higher for LV supplys thus sometimes driving the need for an MV supply?

 

[davidbeach]

For a given source impedance the available fault current is inversely proportional to voltage. Typically lower voltage implies higher source impedance, but not enough to offset the inverse relationship. 20kA is a high fault current at 15kV class distribution.

 

[itsmoked]

123MB; Here are the links with pertinent info.

thread238-208639
thread238-206731 Discussion on voltage levels - perhaps somewhat useful.
thread238-203088
thread238-199460
thread238-193013
thread238-191385

Keith Cress
kcress -

http://www.flaminsystems.com

 

[123MB]

Thanks man