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.
 
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.
 
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