OCPD

McClary's ElectricalMcClary's Electrical Registered Users Posts: 21
This is a question posted on MH's forum. I'm curios to get advise from the true professionals. Her goes:






I have a xfmer that goes from 480 (pri) to 12470 (sec). Table 450.3 (A) is for xfmers 600 volts and above. Table 450.3(B) is for xfmers below 600 volts. Which table do I use to determine if I need OCP or not for this xfmer?
Here is the situation: I have a wind turbine feeding a 480 to 12470 xfmer. From this xfmer I have 15kV cables going to a 12470 to 208 xfmer. I am trying to determine if I need OCP in between the xfmers.
I think that we do need OCP in between the xfmers as I interpret the NEC. Others think that we don't need OCP in between the xfmers.
All xfmers are outside in supervised locations.
Any help would be, as always, greatly appreicated.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    Re: OCPD

    OCP (over current protection--fuses/breakers) are to protect wiring (and devices) from melting/catching fire.

    What is the output rating (and any OCP) of the wind turbine (and transformers) vs the current/power carrying capacity of the transformers and wiring?

    Also, large pole mounted transformers can/are designed to limit current (saturate core) to protect downstream loads (in the US, transformers that feed 120/240 VAC to homes are limited to 10,000 Amps maximum so that our fuses/breakers can interrupt any dead shorts up to 10,000 Amps safely)--Is the transformer set you have rated to regulate maximum current at all?

    At least that is how I would start.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: OCPD

    just kicking thoughts around a wind turbine will output more when presented with more wind and is usually limited by some means either physically or electrically or both from the turbine spinning faster than that preset area. this could be furling or turning away from the wind as well as excess power sent to an electronic braking device. when such fails more power can be realized as well as the turbine physically in danger of self-destruction.
    the transformer is down the line from that output and it could possibly exceed the limits of the transformer under some circumstances or at least overheat it leading to a premature demise. in this case i think the wires would be intact longer than the transformer due to the wiring of the transformer being enclosed, but the wiring is also subjected to overcurrent. this all also assumes the turbine can take giving this extra power as it too will strain. yes, i think it should be used even if the nec would say otherwise..
  • McClary's ElectricalMcClary's Electrical Registered Users Posts: 21
    Re: OCPD
    BB. wrote: »
    OCP (over current protection--fuses/breakers) are to protect wiring (and devices) from melting/catching fire.

    What is the output rating (and any OCP) of the wind turbine (and transformers) vs the current/power carrying capacity of the transformers and wiring?

    Also, large pole mounted transformers can/are designed to limit current (saturate core) to protect downstream loads (in the US, transformers that feed 120/240 VAC to homes are limited to 10,000 Amps maximum so that our fuses/breakers can interrupt any dead shorts up to 10,000 Amps safely)--Is the transformer set you have rated to regulate maximum current at all?

    At least that is how I would start.

    -Bill





    Is the 10,000 or less AIC for dwellings a guarantee? I've been told that it probably not good to count on that. Is that true?
  • McClary's ElectricalMcClary's Electrical Registered Users Posts: 21
    Re: OCPD
    niel wrote: »
    just kicking thoughts around a wind turbine will output more when presented with more wind and is usually limited by some means either physically or electrically or both from the turbine spinning faster than that preset area. this could be furling or turning away from the wind as well as excess power sent to an electronic braking device. when such fails more power can be realized as well as the turbine physically in danger of self-destruction.
    the transformer is down the line from that output and it could possibly exceed the limits of the transformer under some circumstances or at least overheat it leading to a premature demise. in this case i think the wires would be intact longer than the transformer due to the wiring of the transformer being enclosed, but the wiring is also subjected to overcurrent. this all also assumes the turbine can take giving this extra power as it too will strain. yes, i think it should be used even if the nec would say otherwise..




    Thanks, that makes perfect sense. Would you call the OCP a requirement? or a design issue?
  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    Re: OCPD

    The 10,000 amps is what (as I recall) all of the main breakers are rated at for home use...

    I know that for office parks and such out here--if your unit is some distance from the pole, they use breakers for the main service disconnect.. However, if the unit has the utility feed/drop right from the local transformer, it is required that they use special high AIC rated fuses because there is not enough impedance to ensure the XX,XXX current limit that breakers are rated for.

    In the end, what is the rating of the Wind Turbine and its own service disconnects. That should define what the "downstream" devices and their protection should be.

    The other possible issue is lightning protection--but I have no idea of what would be required/a good idea for a distribution system.

    Sorry--I am not an expert in the NEC...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: OCPD

    like bill, i'm not an nec expert either, but there is a drawback to a blown fuse, or even a fuse gone defective, to that xfmr in that the turbine would go loadless and would soon physically self destruct in a high wind. if this is better than protecting the xfmr or not is your guess as not all xfmrs would just open up as some could create a fire hazard under overload conditions.
    great choices huh?:grr
  • McClary's ElectricalMcClary's Electrical Registered Users Posts: 21
    Re: OCPD
    niel wrote: »
    like bill, i'm not an nec expert either, but there is a drawback to a blown fuse, or even a fuse gone defective, to that xfmr in that the turbine would go loadless and would soon physically self destruct in a high wind. if this is better than protecting the xfmr or not is your guess as not all xfmrs would just open up as some could create a fire hazard under overload conditions.
    great choices huh?:grr




    Wow, that gives us alot to ponder. It seems the wind mill itself needs to be mechanically limited against overspeed to prevent a catastrophic disaster.
  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    Re: OCPD

    Assuming that this is a standard horizontal axis wind turbine--yes it needs to control its RPM all the time (feather, turn from wind, brakes, electrical loads, etc.).

    You will need to work with the wind turbine vendor/engineer to make sure you meet its working requirements--but I would hope that it has "enough brains" to shut down if it looses the main utility feed (internal battery backup, mechanical safety brakes, etc).

    Also, many grid connected wind turbines have their 60 Hz (or whatever) power synced to the grid power. I have not heard of a lot of wind turbines that supply "wild AC frequency" for long distances to a local station for conversion to AC or DC power (as needed).

    Not to say there are not any--but it would be interesting to know the Mfg/model and your installation requirements. I believe this is off-grid, so how does the wind turbine interface with your power system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarvicsolarvic Solar Expert Posts: 1,053 ✭✭✭✭
    Re: OCPD

    I guess ocpd xfmr is an abreviation for an old common potato digger ex farmer, Which I am to a certain extent. :roll::roll:S:Dlarvic:confused:
Sign In or Register to comment.