Negative Sequence Current In Plant Distribution System
Negative Sequence Current In Plant Distribution System
(OP)
I noticed on one of the distribution relays on our 4.16kV plant distribution system a negative sequence current reading. The current on the entire distribution system was running at about 2500A and I noticed that there was a negative sequence current of about 75A. I realize that this negative sequence current comes from unbalanced voltages, and the voltage unbalance voltage reading was at 0.4%.
What is a negative sequence current value that should deemed as unaccaptable or as a maximum. Is this negative sequence current strictly a functin of voltage unbalance or load unbalance. 75A out of 2500A is about 3% so this seems to line up with the voltage unbalance. If that is the case is 0.4% voltage unbalance cause for any alarm?
What is a negative sequence current value that should deemed as unaccaptable or as a maximum. Is this negative sequence current strictly a functin of voltage unbalance or load unbalance. 75A out of 2500A is about 3% so this seems to line up with the voltage unbalance. If that is the case is 0.4% voltage unbalance cause for any alarm?






RE: Negative Sequence Current In Plant Distribution System
RE: Negative Sequence Current In Plant Distribution System
Don't forget that you will find negative sequence current where you have single phase loads too.
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RE: Negative Sequence Current In Plant Distribution System
Some level of negative sequence current will be present due to unbalanced loads or asymmetrical capacitance or leakage from a phase to ground. The levels at which these will become a concern depend on the various fault modes and system configuration (neutral grounding resistance, source impedances, etc). The level of 'concern' should be that at which the resulting negative sequence currents are most likely produced by a fault rather than normal operating conditions. Also, given some maximum level of negative sequence voltage that can be tolerated by the load equipment, one can calculate the current that would produce this from the source impedance. This may give you a second (and probably lower) current threshold level at which to set some alarm or protection operation point.
Negative (and zero) sequence currents below these points are also monitored in some cases for changes over time as a sign of impending doom. So 75A might not be a problem unless it has grown to this level rather rapidly (with no innocent explanations like system or load modifications).
RE: Negative Sequence Current In Plant Distribution System
Our 4.16kV system is LRG limiting L-G fault current to 400A. This is a cement plant so most of the loads are transformers and induction motors.
I'm not sure if this I2 current of 75A is I2 or 3I2, I will have to look in the relay manual. The relay I am referencing is a Multilin 750 relay.
All our power is purchased from the utility.
PHovnanian
If there was signifigant asymmetrical capacitance or leakage from a phase to ground, then I would expect that we could see this on a relay monitoring the neutral of the secondary of the 4.16kV utility transformer?
RE: Negative Sequence Current In Plant Distribution System
RE: Negative Sequence Current In Plant Distribution System
Yes. You'd see it there as well. Unless you had some strange configuration like a corner grounded delta system.
RE: Negative Sequence Current In Plant Distribution System
RE: Negative Sequence Current In Plant Distribution System
Pwrengrds
Did you mean 3I2 and not 3I0 for a blown cap fuse?