Protection of Alternator

  There are no of protections for Alternator. in that Negative phase sequence Relay is one. A brief information about Negative phase sequence Relay .

Negative Phase sequence Relay (46):-

Purpose:-  protection Against UN BALANCING LOAD

Causes for unbalancing  :-  

Ø  Asymmetries'(un transposed lines)

Ø  Un balanced loads on Alternator

Ø  Asymmetric faults & Open Ckt

Effect on Alternator by Negative Phase Sequence  Currents:-

Ø  it's induces double frequency currents in the surface of rotor, Retaining rings & Slot wedges

Ø  A small degree in the field winding.

                  Negative Phase Sequence is Phenomena generally found in motors,

the formula for positive phase sequence is : Ip= Ir + a * Iy + a^2*Ib

the formula for negative phase sequence is : In= Ir + a ^2* Iy + a*Ib

            As the name suggest the sequence of current or the mmf(magnetic flux) is in reverse order, a reverse order current in stator will produce a reverse order mmf, now this reverse order mmf is rotating in direction opposite to the rotor of the motor,  So the stator mmf  to rotate at twice the System frequency & induces Eddy currents in to the rotor  .  

            This Eddy current in your rotor leads to overheating and finally burning of  winding.

                             Due to these reasons High & Possibility dangerous temp. on rotor winding in a short time.

Technical Requirements:-

Ø  Time lag over current relay with negative sequence filter  

Type of Relays:-

Ø  Electro mechanical relay (induction disc type) time over current  with Extremely inverse characteristics.

Ø  Static relays with time over currents characteristics  which matches   I^2 *t capability curves for generator.

Note:- This relay always compare the three phases currents.

Inputs  for relay:- Phase protection CT's(CT Core Class 3P,5P..)