Negative Sequence Current In Plant Distribution System 2

[rockman7892]

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?

 

[davidbeach]

Negative sequence current comes entirely from current imbalance. That current imbalance could be driven by voltage imbalance, load imbalance, or faults. Is that 75A of I2 or is it 75A of 3I2? Best to look at the relay specifications in detail before trying to answer that question. I certainly wouldn't worry about 25A of I2 if you have 2500A of I1. If you have any single phase loads of any consequence, I'd probably not worry about 75A of I2. Voltage phase imbalance (not exactly 120[°] between phases all the way around) can probably have as much or more affect than voltage magnitude imbalance. If you have phase imbalance it is probably coming from your source. If you are generating all of your own power I'd be more concerned about I2 than if you are buying all your power from the utility; generators don't like I2.

 

[ScottyUK]

Negative sequence voltage is a problem to rotating plant because it shows up as a double-line-frequency current circulating in the rotor forging where it causes abnormal and excessive heating. How much of a problem is very dependent on the machine design and certainly for larger machines the OEM will issue some information regarding negative sequence relay settings. Small machines under a few MW are fairly forgiving unless you have an outrageous imbalance. What type of loads do you have?

Don't forget that you will find negative sequence current where you have single phase loads too.


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If we learn from our mistakes I'm getting a great education!

 

[PHovnanian]

What is a negative sequence current value that should deemed as unacceptable or as a maximum.

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).

 

[rockman7892]

Thanks for the responses guys.

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?

 

[jghrist]

This is a cement plant so most of the loads are transformers and induction motors.

If there are 1Ø transformers connected Ø-Ø and the phases are not balanced, there will be negative-sequence currents without zero-sequence currents.

 

[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?

Yes. You'd see it there as well. Unless you had some strange configuration like a corner grounded delta system.

 

[pwrengrds]

For the source of 31O I would guess a blown cap fuse given a cement plant with lots of motors.

 

[rockman7892]

I looked in the Multilin 750 manual but could not find anything saying weather or not the negative sequence current is I2 or 3I2. I may try to call them to get an explanation.

Pwrengrds

Did you mean 3I2 and not 3I0 for a blown cap fuse?