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Thread: UTILITY TRANSFORMER DAMAGE CURVES

  1. #11
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    Quote Originally Posted by engineerintraining12 View Post
    My utility transformer is 750 kVA and my main size is 3000 amps. I know the transformer size seems oddly low for a 3000 amp main, but the utility has sized the transformer based on the demand load.
    You are not going to be able to coordinate; if utility has provided a smaller transformer (very typical). It is their transformer, they will provide protection for it.
    "Just because you're paranoid, doesn't mean they're not out to get you"

  2. #12
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    Quote Originally Posted by wbdvt View Post
    Your secondary protective device is not going to protect the transformer from any transformer internal faults or faults in the secondary conductors. The secondary may prevent a transformer overload but I think you would other issues if that is the case.

    Adjusting the main breaker settings to coordinate with the utility transformer primary fuse will most likely cause coordination issues with in your system. I have seen many systems designed and built to serve larger loads than they are presently doing and the utility has sized it's transformer for the existing loads not possible loading. This leads to the issue you are seeing.

    The area of mis-coordination is in the overload region which should not be a concern for a main breaker unless you have added significant loads and have not discussed this with the utility.
    No need to coordinate the primary and the secondary breakers there. They serve the same load! Either one tripping, you still lose the power to your loads, IMHO.

  3. #13
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    Quote Originally Posted by wbdvt View Post
    Your secondary protective device is not going to protect the transformer from any transformer internal faults or faults in the secondary conductors. The secondary may prevent a transformer overload but I think you would other issues if that is the case.

    Adjusting the main breaker settings to coordinate with the utility transformer primary fuse will most likely cause coordination issues with in your system. I have seen many systems designed and built to serve larger loads than they are presently doing and the utility has sized it's transformer for the existing loads not possible loading. This leads to the issue you are seeing.

    The area of mis-coordination is in the overload region which should not be a concern for a main breaker unless you have added significant loads and have not discussed this with the utility.
    Agreed, there is no way I would be able to get my 3000 amp main to coordinate with the transformer damage curve on this one. As you said, doing this would cause other coordination issues within the system and I do agree with you on the fact that there is no way we are ever going to overload the main unless they throw on a lot of future load. What I am somewhat concerned with is if the utility fuse will coordinate with anything down stream of the 3000 amp main breaker. The utility did not provide me with any information beyond the fact that we are using a SM-20 fuse at 12470 volts and it is being fed out of S&C pad mounted PME switchgear so I made an assumption on the utility fuse size and went with 50 amps for a 750 KVA transformer.
    Last edited by engineerintraining12; 01-14-19 at 09:40 PM.

  4. #14
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    Quote Originally Posted by engineerintraining12 View Post
    Agreed, there is no way I would be able to get my 3000 amp main to coordinate with the transformer damage curve on this one. As you said, doing this would cause other coordination issues within the system and I do agree with you on the fact that there is no way we are ever going to overload the main unless they throw on a lot of future load. What I am somewhat concerned with is if the utility fuse will coordinate with anything down stream of the 3000 amp main breaker. The utility did not provide me with any information beyond the fact that we are using a SM-20 fuse at 12470 volts and it is being fed out of S&C pad mounted PME switchgear so I made an assumption on the utility fuse size and went with 50 amps for a 750 KVA transformer.
    Maybe they meant an SM-4 with a 20 amp rating.

    However, SMU-20 or SM-4, I would think they might use a 30 amp fuse, maybe a 40 amp at the most.
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    Quote Originally Posted by engineerintraining12 View Post
    Agreed, there is no way I would be able to get my 3000 amp main to coordinate with the transformer damage curve on this one. As you said, doing this would cause other coordination issues within the system and I do agree with you on the fact that there is no way we are ever going to overload the main unless they throw on a lot of future load. What I am somewhat concerned with is if the utility fuse will coordinate with anything down stream of the 3000 amp main breaker. The utility did not provide me with any information beyond the fact that we are using a SM-20 fuse at 12470 volts and it is being fed out of S&C pad mounted PME switchgear so I made an assumption on the utility fuse size and went with 50 amps for a 750 KVA transformer.
    SM-20 fuse rated 50A will melt in about 100A. A fusing factor is about 100/34= 2.9!

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    Quote Originally Posted by topgone View Post
    SM-20 fuse rated 50A will melt in about 100A. A fusing factor is about 100/34= 2.9!
    and the transformer would not last very long.

    Fusing for cold load pick up of about 200% is normal in most cases. I doubt 300% would be a standard practice, for a 750 kVA anyway. You might be okay risking a smaller, cheaper transformer if you had a good reason.
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    One reason to coordinate the main bkr with the utility fuse is to aid in troubleshooting. If the fuse goes, then the issue could be the transformer or secondary cable if coordinated. If not the fuse could be blowing on an issue downstream of the secondary.

    The utility should be able to tell you the fuse size for certain. If they said SMU-20, that would be a fuse size but saying SM-20 that could referring to the fuse holder SMD-20.

    The utility I used to work for would use either a 140T or 100T cutout fusing, depending on mainline fusing, for a 12.5kV 500kVA padmount. This would be in conjunction with transformer bayonet CLF C12 fusing.

    Cooper's Distribution Book has either a 50A or 80A fusing for a 12.5kV 500kVA padmount.

    The SMU-20 seems a little small. Some utilities will put the fuse size on the riser pole.

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    Quote Originally Posted by wbdvt View Post
    The utility should be able to tell you the fuse size for certain. If they said SMU-20, that would be a fuse size but saying SM-20 that could referring to the fuse holder SMD-20.
    No. The SMD is the fuse assembly. The SMU-20 is a fuse cartridge that fits in the holder that comes in a range of sizes.

    Quote Originally Posted by wbdvt View Post
    The utility I used to work for would use either a 140T or 100T cutout fusing, depending on mainline fusing, for a 12.5kV 500kVA padmount. This would be in conjunction with transformer bayonet CLF C12 fusing.
    Different fuses are usually 150% or 200% rated so it matters what type fuse you are using.

    The SMU fuses are 200% rated.
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  9. #19
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    Quote Originally Posted by mivey View Post
    No. The SMD is the fuse assembly. The SMU-20 is a fuse cartridge that fits in the holder that comes in a range of sizes.
    I wasn't clear on the point I was trying to make. I am aware of the difference between SMD & SMU. However, the OP stated the utility just said SM-20 and the OP assumed that meant a 20A fuse. Without the D or U indicated, that assumption cannot be made.

    And my comment on fuse sizes was meant to illustrate that utilities usually have a book of standards they use for construction and fusing. The utility should be able to refer to that if they do not have the fuse size in their records or system model.

  10. #20
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    Quote Originally Posted by wbdvt View Post
    I wasn't clear on the point I was trying to make. I am aware of the difference between SMD & SMU. However, the OP stated the utility just said SM-20 and the OP assumed that meant a 20A fuse. Without the D or U indicated, that assumption cannot be made.

    And my comment on fuse sizes was meant to illustrate that utilities usually have a book of standards they use for construction and fusing. The utility should be able to refer to that if they do not have the fuse size in their records or system model.
    The OP assumed a 50 amp fuse. I just wondered if they meant a 20 amp fuse and the OP got the wrong info. 50 amp seems a little big to me.
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