Temporary power drop discussion 3

[itsmoked]

I have a big generator that's costing $8,100/mo to rent.
$150/mo fee for the CA air quality board.
$4,400/mo fuel.

Waiting for the new power drop from the POCO will take at least 8~9 months.

We're searching for a solution to the $12,650/mo pain.
The generator panel has never shown more than 50A @ 480V.

I have a power pole field drop with 200A 240V delta service barely used, a couple of lights.
One way distance is 750ft.
I have a 240/480 50kVA transformer.
2 AWG will cause a 2% drop carrying 50A.

My biggest concern is motor starting. I don't yet know what the largest motor is that needs to be supported. From memory I don't think there's probably one bigger than 15hp/480v but I'll have to check later today.

What do you think, doable?

If you think this can work, do you have any suggestions for 'cheap' cable or wire? This is all temporary.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

A couple of things to check.
The demand of the facility.-
In the Canadian code I would use Section 8- Circuit loading and demand factors, To verify that the installation is code compliant.
Even if you are a little under the code it may be well to know how much under.
Transformer.- Are the windings wye or delta? If you have a 240V wye:480V delta consider floating the primary wye point.
That solves the problem of what to do with the wye point on a delta feed and avoids some other nasty issues.
Cable: My first consideration would be Neutral supported cable run overhead. if your voltage drop is calculated on copper, you have three choices for aluminum cable;
1. two AWG sizes larger cable.
2. Parallel cables.
3. Compromise your 2% voltage drop.
You may be able to sell the neutral supported cable on E-bay when you no longer need it.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

BTW. For motor starting on a generator you want a minimum of 300% of the motor KVA.
For motor starting on a transformer you want a minimum of 125% of the motor KVA.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

For motor starting on a transformer you want a minimum of 125% of the motor KVA.

Oooooh. That's a juicy tidbit I'd not heard before.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Clarification:
Ampacity of a conductor for a single motor 125%.
Ampacity of a feeder, the sum of the motors plus 25% of the largest motor.
I have always assumed that the transformer capacity must support the feeder capacity.
My original comment was meant to illustrate that if the motors start on the generator then they should have no problem starting on a properly sized transformer.
Interestingly, I did a design of a shingle mill years ago.
At issue was the ownership of the main transformer.
If the customer owned the main transformer and used primary metering the size according to code was roughly 1 KVA per HP.
If the utility owned the transformer they were not bound by the code and for shingle mills used a demand factor of 1/2 KVA per HP.
If you have a large number of motors and are slipping your temporary installation under the code, this may give you some wiggle room on the transformer.
As for the 125% there are two factors to consider:
1. Transformer standard sizes versus calculated loads often dictate a transformer that is over 125% of capacity anyway.
2. Possibly more important, I may be wrong. Given the reality of factor 1, I can't remember when I needed to upsize a transformer to reach the 125% capacity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Ps. Feel free to red flag my previous post and take away the LPS if I have led you astray. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Oh no not at all. This is one of those "what have we have laying around that we can pull this off with" kind of installations. They're willing to deal with dimming lights kind of issues for a short time - as long as contactors and controls don't drop out.



They have three dry type 25kVA 240/480 transformers they just found so I might switch to those for the 50A/480 load (BTW it was found to be 40A today and not 50A.

They can then use the 50kVA for another building drawing only 25A/480. A chiller will stay on generator since it's a daily only 100A/480V.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

My table shows an Ampacity of 85 Amps for #6 AWG NS90 quadruplex (90C neutral supported cable, bare neutral and three insulated conductors)
My table shows an Ampacity of 70 Amps for #6 AWG NS75 quadruplex (75C neutral supported cable, bare neutral and three insulated conductors)
#6 AWG is the smallest size of NS cable listed.
40 Amps plus 25 Amps is 65 Amps.
That's about 54 KVA.
8% overload on a 50 KVA transformer.
Short term temporary installation.
If the two buildings are running at different PFs the total may be less than 50 KVA.
Your choice. You know the risk. I would use the 50 KVA tranny for the total load here.
The daily high temperature is around freezing here.
I suspect the ambient is a little higher in California. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Hi Bill!
6AWG?? Did you miss that this is a 750 foot distance?
We can't use NS cable as it has to be buried to prevent theft. It will have shoring plates piled on it the entire way too.

All on the 50KVA! I like that. Would it be better to use the three 25kVA dry types wired in delta for 75kVA and skip the 50kVA for a little more headroom?

This is delta so we don't need to drag the neutral 750 feet do we? I don't think there was a neutral from the original utility connection.

Heading over to size things up today. Anything to check you can think of?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I like the three 25 KVA solution. All else being equal, the regulation will be better and you are going to be bucking a voltage drop issue.
Do you need a neutral for the 480 Volt loads? 277 Volt lighting?
Will the permanent drop be wye or delta?
If you need a neutral/ground point at the building, the three 25 KVAs will make a good wye:delta artificial neutral.
That leaves you back at the whole load on the 50 KVA transformer.
What is the winding arrangement of the 50 KVA transformer.
Can you re-arrange the connections and make it a wye:delta? 480 Volt windings in wye, 240 Volt windings in delta.
You will be putting 277 Volts to the 480 Volt windings. no problem.
Nothing is connected to the delta so the delta voltage is unimportant.
Musing.... How do the transformers compare for regulation?
All else being equal, the underloaded 75 KVA bank will have better regulation than the overloaded 50 KVA transformer.
But, never assume. If the regulation is not readily available, compare the percent impedance voltages.
Regulation will be better than percent impedance but it will give a good indication of which will have the better regulation.
I would price out larger aluminum cables. I don't know the current comparative prices.

750 feet. This slick little rig will install direct buried cables in a hurry.This slick little rig will install direct buried cables in a hurry.

Spoiler

Gottcha. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Just got back. Wow, no traffic anywhere. Amazing.

This is getting crazy interesting on the power.

I found there are some 25 and one 30hp motor (480V)in the buildings we need to power. HOWEVER, it turns out they're ALL on VFD's and have docile speed ramps. A rare lucky break.

Funny you should mention "rail" (LOL touche') we're going to actually use rail. We will be using 4 private rail cars. We'll move them all onto one rail next to the facility and couple them together. They're all wired for HEP and we'll plug that together. HEP is 480VΔ except it's wired with multiple 4'O down each side that are common for 1,200A service! That will make for 400ft of pretty much - busbar. We'll place one of the transformer choices between the yard service pole and the first car (~50ft) and plug into the first car with an adapter cable from the transformer normally used in the yard to power an HEP car. Then, using another adapter cable out of the end car, tie it to new cable on the ground for 40 feet then into a hole in the building reducing exposure to the minimum. Any wire in the yard will be covered in pallets with shoring plates laid on it, (4ft x 10ft x 3/4" steel plates), 300ft thru that building to the same box in the rafters where the generator cable presently splices into the plant wiring. We'll hang onto the generator for a couple days until we're satisfied the expedient hack will work satisfactorily for a couple months.

The second building has underground conduit almost to the middle of the railcar string. We will tap off the rail cars other side cable string to feed that conduit off the HEP also the same way.

Should be pretty skoocum!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

They're lucky to have you.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Haven't checked against fuse curves, but make sure the 200A service fuses will allow you to initially energise that 50kVA transformer without blowing.

 

[waross]

Good catch, Scotty.
If you have a fuse problem, go to dual element fuses.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Oohh Yeah. Good point. And it will be 75kVA.

We have to gather the HEP cars together and it's a raining weekend so we don't want to do that using a CAT skip-loader in wet conditions. Don't want several hundred tons having its way with you.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Well %$@^, as I was talking to my contact he mentioned some of the lights in one building are...277V meaning there's a neutral lurking. Asked how this is possible and was told both 400kVA generators have large jumpers between "ground and neutral". So after all the planning, wire sizing, and re-configuring the rail yard that was done I decided to go lay eyes-on before anyone pulled out their wallets for hundreds of feet of 4AWG SO cord.

And low and behold..

A WYE secondary.

I dug around and found a 230Δ-480γ/277 100kVA which they can get, use, and re-sell in a couple of months since they're rare as hen's teeth. We'd now feed 240Δ down the HEP and after the train transform to Y.

We'll see what management wants to do Monday. The saga continues.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The transformer in the picture is probably a 120/240 Volt secondary. For use on a 120/240 Volt circuit the windings may be paralleled without going inside the case and into the oil to change to a 120 only configuration.
I have seen three bushing transformers fail shortly after being reconnected for 120/208 Volts.
Dirty tools contaminated the oil.
Two transformers failed within a day.
High voltage to 277 Volt distribution transformers are common and have two secondary bushings.

As for 277 Volt lighting.-
That is common for 277/480 volt wye systems, but ballasts tapped for 120V, 277V and 347V are common here.
The lighting may be 120 Volts with a 277 Volt tap. Verify yourself if possible.
I see only two primaries on that pig.
A three phase, 277/480 Volt pig will have three primary connections.
I may be wrong, but......

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

That may be 240/480V four wire delta.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

hundreds of feet of 4AWG SO cord.

Price out USE cable (Underground Service Entrance).
Also price types; RWU, TWU, ACWU90.
To my knowledge SO cable is not approved for direct burial.
The types that I have suggested are approved and may be cheaper.
or
Call your local cable supplier and ask what is available for direct burial.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Hi Bill,

That pig is a 21kV to 480/277. There are 3 primaries.

Here's maybe a better crop at it.

The guy has an insane in on cable. $1.25/ft for four conductor 4AWG SOJ type cord. I couldn't get THHN for better than about $1/foot for a single conductor!!

Keith Cress
kcress -

http://www.flaminsystems.com