HenryOhm
Electrical
- Jun 22, 2005
- 58
I am investigating possible arcing of connections on the primary side of a wye-delta transformer feeding a 6-pulse, three-phase SCR-based exciter. The arcing led to a small electrical fire so there isn't a lot of evidence left of where things might have started. The transformer is 18kVA and the wye neutral is floating. There is no center-tapped delta on the secondary as the only load is the three-phase exciter. If it relates, the exciter feeds a very large motor field about 10-40 amps depending on commanded DC motor speed, etc; the motor field is about 19H. Also, if it matters, the three-phase power system feeding the primary is floating, i.e. totally ungrounded.
My understanding is that the delta secondary could operate at a lower kVA rating going open delta and feeding all three-phase loads equally. My thought is a loose connection and series arc may have led to one of the wye primary connections briefly interrupting the current flow of one primary and then perhaps re-striking an arc with the aid of some measure of voltage spike. If the transition from three-phase delta to open delta and back is smooth enough maintaing its three-phase input, perhaps the digital exciter and all it's recorded sensing and alarm points might not offer any indication that a primary phase to the transformer was in an arcing meltdown mode.
I have two basic questions. First, links like the below suggest that a three-phase balanced load shouldn't lead to that much of a voltage spike across the opened primary connection. But, because the six-pulse SCR's are commutating constantly, could these extremely brief single-phase short circuits lead to higher voltage transient on an open primary connection than would be otherwise anticipated? The below paper does mention a single-phase short circuits o Pg. 4, Special Note.
More generally, are there any good references that discuss the types of transformers that should feed drives and what the advantages and disadvantages might be?
Thanks for any and all help!
My understanding is that the delta secondary could operate at a lower kVA rating going open delta and feeding all three-phase loads equally. My thought is a loose connection and series arc may have led to one of the wye primary connections briefly interrupting the current flow of one primary and then perhaps re-striking an arc with the aid of some measure of voltage spike. If the transition from three-phase delta to open delta and back is smooth enough maintaing its three-phase input, perhaps the digital exciter and all it's recorded sensing and alarm points might not offer any indication that a primary phase to the transformer was in an arcing meltdown mode.
I have two basic questions. First, links like the below suggest that a three-phase balanced load shouldn't lead to that much of a voltage spike across the opened primary connection. But, because the six-pulse SCR's are commutating constantly, could these extremely brief single-phase short circuits lead to higher voltage transient on an open primary connection than would be otherwise anticipated? The below paper does mention a single-phase short circuits o Pg. 4, Special Note.
More generally, are there any good references that discuss the types of transformers that should feed drives and what the advantages and disadvantages might be?
Thanks for any and all help!