Transformer inrush

Status
Not open for further replies.

hardworkingstiff

Senior Member
Location
Wilmington, NC
Looking at a marina that might have about at 1100' run from the PoCo transformer to a location on the floating dock where a distribution panel would be installed (it would not be the service disconnect because it's on the dock).

The PoCo around here typically will not deliver a 480-volt single-phase service. Because of something I saw years ago, I'm very leery of using 2 legs of a 3-phase service to power single-phase loads on docks (really don't want to get distracted with this).

I was thinking about stepping up a 120/240-volt service to 600-volts, then install a step down transformer on the dock to step it down to 120/240-volts.

If I fed a 100kVA single-phase step up transformer to feed a step down transformer, and protected it with a 400-amp breaker feeding the step up transformer, do you think I would have problems with inrush current tripping the breaker when the transformer is initially energized?

With GFP requirements for marinas, would there be capacitive coupling that might look like a current leakage to the GFP (max setting 100-ma) device?
 

templdl

Senior Member
Location
Wisconsin
I was thinking that the FLA of a 1ph 100kva is 417a @ 240v. As such I see no problem with a 600a breaker for primary protection if you also provide secondary OCPD to protect the 600v run.
You can expect on inrush of up to 4500a so a 400at breaker my be marginal as it has a max mag setting of 4000a +-20%.
But with 417 pri. fla are not you allowed 125% or 521a OCPD? As such you may consider a 550at breaker if that's a std. rating if not a 600at. But I would opt for a 500at because it has a 5000a max mag +- 20% which would be up to 6000a which is well above 4500a the max inrush that I would expect.
Being that your step up transformer would be a basic 240-600v transformer a sec (600v) OCPD would not be required and would be protected by the PRI OCPD.
Your thoughts?
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Reducing the current by going up in voltage is a good alternative to save cable. The selection of 600V though may be an issue regarding cable as the insulation rating on LV cable is 600V, but that gives you no operating margin if the utility voltage goes above nominal. Which, is a very real possibility.

You will need to use a higher voltage rated cable like the a PV-solar cable rated 2kV.

With that in mind, using the taps on the utility side transformer you can start with 630V (+5% tap) and then use the +5% tap on the 600-120/240V transformer as well, which allows you to drop nearly 10% over the 1100' run, thereby drastically reducing cable size down to I believe a single set of #2 AWG.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
And as far as the 400A breaker; even if you get a high magnetic trip, it will probably trip on inrush.

Even a 500A is probably going to give you trouble. A better fit is to use fuses. They are much better suited to protecting transformers due to the shape of the protection curve.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Reducing the current by going up in voltage is a good alternative to save cable. The selection of 600V though may be an issue regarding cable as the insulation rating on LV cable is 600V, but that gives you no operating margin if the utility voltage goes above nominal. Which, is a very real possibility.
Thank you, that's a good point. I use type W cable on the dock and it has a 2000V rating. I did not consider the land side conductors, I need to give that some more consideration.
 

templdl

Senior Member
Location
Wisconsin
And as far as the 400A breaker; even if you get a high magnetic trip, it will probably trip on inrush.

Even a 500A is probably going to give you trouble. A better fit is to use fuses. They are much better suited to protecting transformers due to the shape of the protection curve.
Interesting. Being an applications engineer as well as supporting dry type distribution transformers I was assigned by the marketing manager to do a study of the affect of transformer inrush on the primary OCPD. My study included the various transformer KVAs, primary voltages, 1ph and 3ph, std efficiencty, 80, 115, and 150 degC rise, K4, 9, and 13.
Where did you get the idea that a 500at PRI OCPD would "probably give you trouble." are you saying that a 100kva 1ph xfmr with a 240v primary would have more than a 4500a inrush max most likely slightly less than 4000a would trip a 500at breaker with the mag trip setting at the 5000a position calibrated +-20 allowed but are calibrated on the high side to 6000a would trip?
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
The inrush is going to depend on the multiplier for the particular transformer. This range can vary between 5 and 20 times the FLC of the transformer nominal rating, although values between 5 and 12 are probably more common, but unless this is known it pays to be more conservative in device selection.

Simply plot the transformer curves; including inrush (either point or curve) and determine required protection. Often it is difficult to find a breaker that will will have a curve that can protect the transformer from thermal damage, and at the same time not trip on inrush. Fuse curves are much more suited for this application for HV side protection.
 
Status
Not open for further replies.
Top