Negative Sequence Voltage 3

[rockman7892]

I understand that positive and negative sequence currents are represented by the following equations:

Pos - I1 = Ia + aIb + a^2Ic
Neg - I2 = Ia + a^2Ib + aIc

Can these same equation sets be used for calculating negative sequence voltages? If so I am assuming that the voltage magnitudes and angles are used in the equations just like those for currents are used.

We had an event last week where due to a fault on the utility transmission line, we lost our 4.16kV power to our plant. In the process of power being interrupted to our plant our main breaker tripped on a negative sequence voltage. I was trying to figure out why this was, and I am guessing that it has something to do with the way the voltage or current was interrupted during the utility circuit switcher opening? During the pickup of the event the current phasors all reversed their angles for a complete opposite rotation.

As I scrolled through the event history of the relay I found two other instances where the relay picked up on negative sequence voltages but then droped out or returned to normal before a trip occured. Looking at the phasors for these events I notice that the voltage magnitudes are below nominal voltage and appear to be about 5% unbalanced however have close to a 120deg displacement. The currents during these events appear to be either unbalanced or have a phase displacement of larger than 120deg. Are these negative sequence voltages usually a result of something occuring on the utility side of things such as a voltage drop of something else?

I have attached the phasors that were recorded during these events for viewing. The pickup for the Neg Sequence Voltage is .05VT (VT=4.16kV - 120V). Our system normally has pretty balanced voltage and has a pf of .69LAG(new plant slowly being brought online)

 

[rockman7892]

After reading several of the above responses I think it would make sense to eliminate this setting from our main breker. There are seveal other important loads that would shut off and cause a nusiance that would not really be effected by neg sequence voltage. I'm still going to investigate with the utility why we are seeing these neg seq voltages at our main breaker on several occasions.

I have read here and several other places that negative sequence currents in a motor can be very damaging. I'm trying to figure out weather the neg sequence currents in a motor are a result of unequal currents being supplied to the motor or unequal voltages at the motor terminal.

1)Ifthe motor is drawing an unbalanced load then the voltage at the motor termianls would be different due to different voltage drops and would therefore cause neg seq currents in motor.

2) If the motor is drawing a balanced load but either the source voltage is unbalanced or the circuit impedances are unbalanced then there would be unbalanced current pulled to the motor as a result of impdeance and thus unbalanced voltage at the motor terminals due to voltage drop and would cause neg seq current.

Does this sum up the two scenarios of how neq seq current can occur in a motor.