4160 vesus 480 volt

[tish53]

Does anyone have any guidance for what hp is the point to begin considering 4160 v versus 480v. I am designing a new plant with an 800 hp that will be 4160v. ( note all the advantages stated in other posts). We also will have opportunities to consider 4160v for motors rated 150,200, 250, 300,400 and 450 hp. Is there a minimal HP that is impractical because of motor availability?

Also, how would you size a transformer to handle starting a single 800 hp @ 4160 v. I am unsure of transformer's ability to handle starting the motor and if I need to oversize to handle this issue.

Thanks

 

[Jraef]

Transformer sizing does not change. We talk about starting CURRENT a lot, but that's just convenient, it's really about starting kVA.

There is no hard rule on when to change. I have done work at facilities that dictate 200HP and up is to be MV, but others have been 500HP and up. It all depends on someone's assessment of available replacements, installation cost (switchgear vs copper) as well as staffing. Remember, most jurisdictions require MV certification to be able to work on it. If you have 100 MV motors and only one electrician certified to work on them, that can be a problem.

I do think it might be difficult to find a 150HP 4160V motor off the shelf however, and the smallest MV motor starter is 200A, about 1250HP, so it gets pretty expensive to buy a 1250HP MV starter for a 150 or even 200HP motor, compared to a Size 5 480V motor starter.

 

[tish53]

thanks Jraef

That is where my head was at regarding hp beak point. Once we invest in the 4160 v infrastructure ( transformer and motor starter line-up) it seems to make sense to me to look for other appropriate motors. We are a stone aggregate company and this new plant will have a 800 hp primary crusher and a 300 hp conveyor and a 400 hp conveyor in close proximity. Also in another part of the plant we will have (2) 300 hp and (2) 450 hps crushers in the same area.

We will need and plan on doing MV training since we don't have any electrical techs with experience or certifications to date.

Any advice on the starting kva for an 800 hp motor. I can calculate 800hp x 746 watts/ hp/ 1000 resulting in 596.8 kw and assuming a .85 PF equaling 702 kva ( we will have PF correction at the plant to ensure .85 or better). Will the transformer handle the starting KVA for the short starting time and infrequent starts or do I need to figure starting kva somehow to determine transformer size.

Thanks again

 

[Phil Corso]

Tsh...

Every project I've been associated with had break-even studies for Hp vs voltage-levels. in 1964, a conducted a study that concluded a new refinery distribution @ 830V for 3-ph motors woud save a ton of money. But Electrical Xperts concluded it would be too danngerous to go above 480V!!!

Regards, Phil Corso

 

[Phil Corso]

Tsh...

Your question regarding motor-size vs voltage?

It depends on source capacity. One job had a 22,000Hp, 11kV motor that used DOL starting!

Regards, Phil Corso

 

[G._S._Ohm]

Tsh...

Every project I've been associated with had break-even studies for Hp vs voltage-levels. in 1964, a conducted a study that concluded a new refinery distribution @ 830V for 3-ph motors woud save a ton of money. But Electrical Xperts concluded it would be too danngerous to go above 480V!!!

Regards, Phil Corso

You can almost reverse-engineer this decision.

Today a human life is worth $200K to $4M so back 40 years it would be about 1/4th as much.

Voltages above 600 puncture the skin so the mortality should go up steeply above this point.

The last number is: How likely are we to be sued by the EC's survivors for wrongful death and how much will we have to pay out?

 

[Besoeker]

Any advice on the starting kva for an 800 hp motor. I can calculate 800hp x 746 watts/ hp/ 1000 resulting in 596.8 kw and assuming a .85 PF equaling 702 kva ( we will have PF correction at the plant to ensure .85 or better). Will the transformer handle the starting KVA for the short starting time and infrequent starts or do I need to figure starting kva somehow to determine transformer size.

There is no one size fits all solution.
The direct on line starting current is likely to be in the order of six to eight times running current. The application is a crusher so I'm guessing that the motor will start in an unloaded condition and have a fairly short run up time. On that basis, I'd suggest that thermal considerations for ratings can be put to one side.

Voltage drop is different matter. You haven't said what the voltage into your plant is but, given that you are considering transformer ratings in relation to the 800HP motor I would infer that it isn't 4160V. Obviously the starting current will cause a voltage drop at the transformer, both on the secondary and on the primary. If only the 800HP motor is connected to the secondary, the reduced voltage will affect the motor torque. The available torque from the motor varies approximately as the square of the voltage applied to it. So, for example, if the voltage drops to 90% of nominal, the torque will be around 80% of nominal. That likely wouldn't be a problem for an unloaded motor. It would take a bit longer to run up.

The voltage drop on the primary side may also need to be considered particularly if it is common to other customers. On one installation, we had to put a reduced voltage (autotransformer) starter because the motor starting current would have caused the supply side voltage to dip unacceptably. Which, in fact, it did when some bozo connected the autotransformer the wrong way round. It took out the POCO breakers leaving a lot of customers without electricity.

 

[JoeStillman]

The distance between the starters and motors has a lot to do with which voltage is best. I would price it out both ways for the 400 and 450 HP and see which is cheapest. My gut feeling is that anything smaller is probably overkill to go 4160.

When you size the transformers for all of these motors, you need to account for the number of starts-per-hour. If the transformer doesn't get a chance to cool off between starts, it can overheat, even though the steady-state load is within its kVA.

 

[mayanees]

arc flash ratings

arc flash ratings

One important consideration that hasn't been mentioned is that the incident energy levels for the MV system will be much lower than the comparable 480V system.
I would expect HRC 2 classifications for the 4160V system, and HRC Dangerous for the 480V system.
My experience has been that 400 hP and above are good MV applications; however, as suggested in the last post, field conditions should dictate the selection on a case-by-case basis.
John M

 

[iceworm]

... Remember, most jurisdictions require MV certification to be able to work on it. If you have 100 MV motors and only one electrician certified ....

J -
By "work on it", you mean maintenance involving the 4160V connections? If so:

What jurisdictions? What certifications? Do you know of any covering regulagation?

Zog occasionally comes up with a certifying agency. However every one I looked at is self certifying - they are the ones to certify because they say they are the ones.

I think the schools and training are a good idea - they are just not regulatory.

ice

 

[tish53]

thanks to all for the great responses

Most of the motors will be close to the MCC and transformer. There a 300 and 400 that will be around 1500 ft away and 4160 does offer some economic advantages.

we are meeting with the uitility company ( Co-op) in early Jan to discuss the impact these motors will have on thier side of the transformers

Also, starts per hour are not an issue. normal operation is once per day ( maybe twice)