Inrush current for a 300 KVA Step up Transformer

[fourleaf]

I'm having trouble with a 300 KVA transformer. After hooking it up and ohming it out before turning it on
the transformer unloaded is tripping a 150 amp breaker.
I realize that 150 breaker cannot handle the transformer when it is fully loaded. I would have thought that the breaker could at least turn the transformer on unloaded.
I am feed a drilling rig power for lighting and cannot furnish them with no more than 150 amps. My question is Should that 150 amp breaker energize the transfomer, do I have a bad transfomer or would I have to get a bigger breaker? By the way the 150 breaker is a MCP breaker and I have turned the trip setting up to 900 amps. Could the Inrush current be tripping the breaker?

 

[mpparent]

Fourleaf,

First, I'll assume you have a 480-208/120V transformer. Inrush on a transformer is typically 9-12X the full load current, for a duration of .1s. With that, the inrush could be up to 4300A. I would suggest putting a larger breaker on the primary and choking your transformer down on the secondary to limit your load. You could try a 400A breaker on the primary, and a 350A breaker on the secondary. If you are saying that you are limited on the primary to 150A, you probably won't be able to turn the xfmr on. You'll have to downsize to a 112.5kVA.

 

[busbar]

Hate to bring up the obvious, but MCPs are not intended for the duty as described.

I believe the overcurrent multiples in article 450 are in place to address this problem.

Most would probably first do basic eliminations, like insulation resistance, ratio, excitation. In typical field practice, DC winding resistance seems like a fairy useless measurement.

 

[peebee]

Please note that using that MCP is a code violation due to the lack of overload protection on the transformer primary. You should have known better than to try to use that breaker.

You might find that installing extra primary cable increases the primary impedance enough to prevent the circuit breaker from tripping.

In general NEC allows sizing of up to 250% transformer namplate rating using and STANDARD (not MCP) breaker to address your very problem. A typical rule of thumb is to size primary overcurrent protection at 150% of nameplate, and secondary at 125%. Assuming a 480:208Y/120 volt 300kVA transformer, you'd have a primary rating of 360 amps, and use a breaker sized at 600 amps. Your secondary rated current would be 832 amps, 833 amps, and you'd use a breaker of about 1000 or 1200 amps. You'd need LOTS of extra primary-side cable to reduce the primary inrush current (probably not feasible).

A 150 amp breaker on a 112.5 kVA transformer would provide 120% of rated current and would LIKELY energize the transformer, but it would not surprise me if it still occasionally tripped on startup. Longer primary cables would likely take care of this situation, though. Alternately, you might find resetting the breaker a few times would allow the transformer to start OK.

 

[busbar]

Aside from code verbiage, an MCP seems like it would be a good application for low-voltage transformer-primary protection. Reasonable compromise could be using one that has an instantaneous-trip setting of >2kA for the 300kVA 480V unit, but that will probably only be found in a 250-400A-frame MCP.

One source of disagreement on the 600V supervised-location “250% primary” article 450-3b allowance is whether the primary conductors must match the overcurrent-device rating, or need they be sized only for the rated primary current?