NEGATIVE PHASE SEQUENCE RELAY

Function of Negative Phase Sequence Relay

A relay which protect the electrical system from negative sequence component is called negative sequence relay.

A negative sequence relay protects the generator and motor from the unbalanced load which mainly ocurs because the phase to phase fault.

A current or voltage unbalanced between phases in magnitude or phase angle gives rise to negative and zero sequence component.

The negative sequence component has a rotation opposite that of the power system.

The zero sequence component has represent an unbalance that causes current flow in the neutral

Operation of Negative phase sequence relay

what is negative sequence, negative sequence is a balance 3 phase system with the opposite phase sequence which means the set of 3 phase are equal in magnitude spaced 120 degree apart from each other and having the phase sequence opposite to that of the original phases.

Under the Balance condition the motor gives equal current in all the 3 phases either star connected or delta connected load.

here the vector sum of the negative sequence current is zero hence net unbalance current is zero here.

The unbalance condition create due to the some fault create in the system.

negative phase sequence relay ANSI code

is 46.

working of Negative Sequence Relay

When fault create in the system then negative sequence current start flowing through the stator winding.

which creates additional flux in the motor

which rotates in the opposite direction to that of the rotor synchronous speed.

This flux induced current in the rotor body.

Due to this the rotor body temperature is increases.

But if the magnitude of these negative phase sequence current is high then it will create problem in connected device with the motor.

The current from phase R at junction A is equally devided into two branches as I1 and I4 but I4 will lag behind I1 by 60 degree.

similarly the current from phase B is equally devided into two branches as I3 and I2 but I2 will lag behind 60degree by I3

I2=I3=IB/√3

I1 leads IR by 30 degree I4 lags behind IR by 30 degree

similarly I2 lags behind IB by 30 degree and I3 leads IB by 30 degree

The current trough relay operatiIng coil at junction B will be equal to phases sum of I1 AND I2 AND IY

so relay I1+l2+lY

so IR/√3 leading IR by 30 degree+ IB/ √3  lagging IB by 30 degree+IY

fig represents the phaser diagram when the load is balance or when there is no sequence current.

since the current through the relay is I1+I2+IY=0

SO I1+I2=-IY