Negative sequence current

[tom]

Hello

I frequently see reference to negative sequence current. What does this mean??

How is it measured??

[merajdab]

Hi

 

When there is unbalance in the voltage supply due to any reason, the unbalance supply can be resolved into three balanced quantites called possitive sequence, negative sequence and zero sequence voltages. These sequence voltages produce their sequence currents as well.Thus negative sequence current is produced. Under normal balanced condition , what ever voltage and current is present is possitive sequence. The negative sequence comes into picture only when unbalance exists in the supply voltage.

 

Regarding measurement of negative sequence, it is measured by the negative sequence filters within the relays. For more details, refer to PRAG- ALSTOM. However it can always be calculated for a given set of unbalanced voltages. To calculate the negative sequence current, the negative sequence impedance of the machine should be known. The calculation proceedure for the sequence voltages and currents can be obtained from any standard text book of symmetrical components.