Over Fluxing(volts/hertz) protection (24):-

Purpose:- To detect Voltage-Regulator malfunction.& The relay measures the ratio of voltage to frequency (V/Hz) as this ratio is proportional to the flux-density level of the electromagnetic machine.

Causes:- It mainly occur in start up, shut down & as a result of remote load rejection

Ø  Over Fluxing (Over Excitation) may be occur when a generator is on an isolated operation with the AVR in service and the generator speed decreasing from the synchronous speed.

Ø  Over Fluxing may also be occurring when the utility system frequency decreases to a low value by any reason.

Types of  Relays used:-  There are two sets of the V/Hz relays are provided for Generator Protection:-
A) Relay 24G (#2) for alarm
A definite time element #2 of the relay will be used to provide an alarm.

B) Relay 24G (#1 & INV) for trip
Operation of the relay 24G will trip the switchyard breaker.
An inverse time element of the relay will be used to provide a trip contact and a definite time element #1 is also used to further shape the protection curves.

Effect on Alternator:-  It will cause over heating of cores of generator, main T/F& aux T/F  associated

Data for setting calculations

Short-time V/Hz withstand level at full load -----120%
Continuous V/Hz withstand level at full load-----105%
AVR V/Hz limiter setting -----------------------------105%

Setting of generator V/Hz alarm relay 24G (#2)
1)V/Hz alarm setting
Setting < continuous V/Hz withstand level at full load = 105 %

2)V/Hz alarm delay
Setting = 30 cycles (0.6 Sec.)

Setting of generator V/Hz alarm relay 24G (INV)
1) V/Hz trip setting for inverse time element
Setting = 107 %
2) V/Hz Reset rate for inverse time element
Reset rate = Practical recommendation is minimum setpoint of the relay
Setting = 1 Sec.

NOTE!! “If the generator is synchronized immediately after tripping by V/Hz relay operation, the generator will be damaged under V/Hz condition before limitation value. “

Setting of generator V/Hz alarm relay 24G (#1)
1)V/Hz alarm setting
Setting < short V/Hz withstand level at full load = 120 %

2)V/Hz alarm delay
Setting = 50 cycles (1 Sec.)

Technical Requirements:-

                                 it need over Exaction sensing unit which has liner volts per hertz pickup characteristics & time unit provided a definite time before initiating alarms & trip

Note:- the settings of V/Hz shall be coordinated with exaction system of AVR because it acts as a primary protection in case of voltage regulator fails.

In put for relay:- protection PT

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..)