DTR2011
Electrical
- Oct 12, 2006
- 682
I was recently doing a Power Quality analysis at a small utility. Internally, when they run equipment (UPS in particular), there are some issues. In particular, it is when they run a certain generator (4160V 10MW - Wye Connected, Reactance grounded). The utility parallels with a much larger utility via a 4160:12470 Xfmr (Wye-Gnd/Wye-Gnd/Delta). When the 10MW unit is running full load, I get 8-10% THD on voltage, with negligible current harmonics. With the 10MW unit off and only back feeding from the Larger Utility, there is less than 1% THD on voltage.
I made a few different recordings and what surprised me the most the the level of 3rd Voltage harmonic present in the generator. I am aware that the generator itself is a 3rd harmonic generator. I am also aware that there is a protection element (27TN), which responds to the loss (or lowering of) of the generator 3rd voltage.
I made several recordings. I measured from the PT's on the Gen side of breaker. Additionally, I measured a single phase outlet which fed the UPS. There is a 2400:120, single phase Xfmr between the Gen bus and the single phase panel. I observed a bit of dampening (~1%) for a given load.
It is my understanding that the harmonics are a function of the pitch of the windings, but that is where it ends. I have read through this
So how much is too much? Does too much 3rd Voltage indicate a gen problem? The gen is a 1960's vintage Westinghouse unit.
In my enclosed file, the current appears to be distorted, this is likely due to the fact that I had to use 3000A rope CT's on a CT secondary circuit - one of the pitfalls of renting equipment and not getting what you want. I wrapped the CT's 2x to get better resolution, but in any event, the actual current THD is negligible.
Of course, when I was there, we were unable to reproduce the exact problem (so it must not exist
)
I made a few different recordings and what surprised me the most the the level of 3rd Voltage harmonic present in the generator. I am aware that the generator itself is a 3rd harmonic generator. I am also aware that there is a protection element (27TN), which responds to the loss (or lowering of) of the generator 3rd voltage.
I made several recordings. I measured from the PT's on the Gen side of breaker. Additionally, I measured a single phase outlet which fed the UPS. There is a 2400:120, single phase Xfmr between the Gen bus and the single phase panel. I observed a bit of dampening (~1%) for a given load.
It is my understanding that the harmonics are a function of the pitch of the windings, but that is where it ends. I have read through this
So how much is too much? Does too much 3rd Voltage indicate a gen problem? The gen is a 1960's vintage Westinghouse unit.
In my enclosed file, the current appears to be distorted, this is likely due to the fact that I had to use 3000A rope CT's on a CT secondary circuit - one of the pitfalls of renting equipment and not getting what you want. I wrapped the CT's 2x to get better resolution, but in any event, the actual current THD is negligible.
Of course, when I was there, we were unable to reproduce the exact problem (so it must not exist
)