How much overload can utility transformer do?

F

fastline

Senior Member
Location
midwest usa
Occupation
Engineer
I got a call but have not yet been onsite. Customer is on rural 1P. He requested 400A, which is broke down to 200A to a home and 200A to his shop. This is all being fed with a 50kva pad transformer. Complaint is when starting and running a big air compressor, it will struggle severely. When running, the current on the line is 130A. Obviously inrush has to be evaluated here but I am wondering how hard you can pull on that transformer before significant voltage drop occurs? I am sort of suspecting primary sag but again I have not been onsite yet. I do know after a struggle to start, the 200A main break has opened a couple times. To me that sounds like severe voltage drop where the load cannot accelerate, causing high current long enough to open. Just a hunch.

I've generally figured that 150% OL is fine for a transformer and I know POCOs push them extremely hard feeding homes.
 
How big is this compressor? Single phase or 3 phase?
As far as the transformer, there is a 50kVA pole mount in front of my house that feeds 10 houses, four with 200 amp services and rest with 100 amp services.
 
I have seen utilities feed 400A 120/240V 3 wire service with a dedicated 15kVA transformer for a machine shop.

And after 5 years I saw that transformer replaced with a 25kVA due to voltages issues. The customer had added more machine centers.
 
This is a 25hp, 3P compressor on a converter. Moving it to a VFD may be the right approach, but that still doesn't explain some other concerns. It does sound like the 50kva should eat this so I am sort of suspecting the long run of primary, which is much harder to fix. Hopefully I find something easier.
 
fastline said:
I got a call but have not yet been onsite. Customer is on rural 1P. He requested 400A, which is broke down to 200A to a home and 200A to his shop. This is all being fed with a 50kva pad transformer. Complaint is when starting and running a big air compressor, it will struggle severely. When running, the current on the line is 130A. Obviously inrush has to be evaluated here but I am wondering how hard you can pull on that transformer before significant voltage drop occurs? I am sort of suspecting primary sag but again I have not been onsite yet. I do know after a struggle to start, the 200A main break has opened a couple times. To me that sounds like severe voltage drop where the load cannot accelerate, causing high current long enough to open. Just a hunch.

I've generally figured that 150% OL is fine for a transformer and I know POCOs push them extremely hard feeding homes.
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The supply company provide a higher size transformer, issue of breaker tripping informed them
 
When I asked for a service upgrade, the poco asked two questions:
What is my service size in amps.
What is the largest motor locked rotor amps.

Im thinking in this case the LRA is the problem. I think the poco wants the transformer amp rating to equal or exceed the LRA.

A 50 KVA is probably fine for a 400A residential service. But if that compressor LRA is over 200A (assuming 240v single phase), you may need to go bigger or try a soft start kit.
 
Main 200a breaker tripping and motor branch breaker not tripping?
Can time delay 200a main breaker solve tripping?
 
What I would be asking is, is that compressor a positive displacement piston pump or a rotary screw type, and is it equipped with an automatic head pressure unloader system to re leave pressurized tank starts. Adding in the inefficiencies of a rotary phase converter can make it even more difficult to start the phase converter before every compressor start.

25HP = 68A at 3Φ 230V, converting that to 1Φ is 68A x1.732= 117.8A add in another 50% for the rotary phase converter and that puts you at 177A 1Φ. All the more reason to have unloaded starts whenever possible.

50KVA @ 240V 1Φ is rated at 208A, if you can have unloaded starts, should just be enough if voltage drop through the service and rotary phase converter is not too excessive.
 
Motor starting current matter not running current for vd at start, breaker tripping, check time delay breaker curves for same 200a , see it hold or not
 
What is the distance and wire size to the pad mount? If supplied by the poco, wire size is usually half of what we are required to run. How far is the HV line from the sub station? Lots of variables in play here.
 
MTW said:
What I would be asking is, is that compressor a positive displacement piston pump or a rotary screw type, and is it equipped with an automatic head pressure unloader system to re leave pressurized tank starts. Adding in the inefficiencies of a rotary phase converter can make it even more difficult to start the phase converter before every compressor start.

25HP = 68A at 3Φ 230V, converting that to 1Φ is 68A x1.732= 117.8A add in another 50% for the rotary phase converter and that puts you at 177A 1Φ. All the more reason to have unloaded starts whenever possible.

50KVA @ 240V 1Φ is rated at 208A, if you can have unloaded starts, should just be enough if voltage drop through the service and rotary phase converter is not too excessive.
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Plus you need to factor in whatever other load is running at the time this tries to start as well as conductor sizes and lengths. Marginal sized rotary converter will result in longer starting time as well.

This thing likely drawing well over 208 amps for a bit during starting. OP mentioned using VFD, possibly not a bad idea it would ramp up to speed instead of trying to give it full power instantly. Would need to be a 50 hp rated drive to do single phase input. But then the idler motor used on phase converter needed to be at least that big as well. Going to spend some $$ either way for phase conversion when larger motors are involved.
 
I know this guy and he is educated and competent enough to ensure wire sizing is correct, and likely oversized. That POCO only goes as far as the transformer so all secondary would be by an electrician. Someone asked about distance to a sub? Like, at how many miles do you stop counting? I have no idea, but these remote locations seem to rely on boost regulators just to keep going, but I don't think that plays well with big motors.

As for the converter, that is absolutely suspect! I actually build them and I know all too well that the generated leg will dip nasty low on a start like this. I might be able to tune it a bit though. The issue is the current seen on the 3rd leg is a function of fixed capacitance. It gets tricky.

Assuming he doesn't want to dive full into a VFD and wants to try just a soft start for now, what do you guys recommend? I've had questionable success with them in the past.
 
Here is where I'm at. After testing, I feel comfortable that the primary and transformer and performing admirably. Here is the first issue. Primary comes in and hits the dedicated 50kva xfmr and fed out on two line sets, one for house, one for shop, both with 200A mains right at the xfmr. That is an issue as the 200A breaker is getting trippy with big motor starts. The lineset is 4/0 Al and I need a bigger breaker on that! Can someone kick me a code compliant way to get this done? The panel in the shop also has a 200A breaker but has never tripped. Personally, I feel the breaker is not right but it also needs increased so that is not tripping. This is mostly an inrush issue, not FLA issue.
 
suemarkp said:
A 50 KVA is probably fine for a 400A residential service. But if that compressor LRA is over 200A (assuming 240v single phase), you may need to go bigger or try a soft start kit.
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A 15kVA would probably be fine on a 400A service.
Peak demand is what matters.
We pushed them to hit a report at 150%. With some handling 200+ percent depending on ambient temp
 
NE, in general has ignored and does not require disconnects at the meter. Some are supplied by POCOs, with some being a non-fused manual transfer switch. Feeds to buildings were sized per the needs of each. It is not unusual to see multiple conductors of various sizes and materials under the same lug on older installations. Many operational for decades. Newer installations would at least have a terminal box below the meter and/or the disconnect.
 
Like I posted in an earlier post it appears that utility companies think because a pole mounted transformer that is oil filled and has air circulation they can be continually overloaded by 250 to 300%. Case in point when I applied for a 800 amp service I told the utility company that this injection molding shop ran 24/6 with a load of over 620 amps. They connected the 800 amp service to an existing pole transformer that had a heavily loaded 600 amp service that ran 16/6 and a 200 amp service. All.on a 75 KVA transformer that is only rated for 180 amps. In other words penny pinching utility company thinks it okay to supply power from 180 amp transformer to over 1,000 amps 16 hours a day. The existing pole transformer burnt out within a month, replacement 75 KVA transformer burnt out in less then 6 months.
 
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