As the field is operated ungrounded, a single fault does not cause any flow of current or affect the operation of the electric generator. However, a single rotor earth fault increases the stress to the ground in the field

1. Rotor Earth Fault Protection
As the field is operated ungrounded, a single fault does not cause any flow of current or
affect the operation of the electric generator. However, a single rotor earth fault
increases the stress to the ground in the field, when the stator transients induce an
extravoltage in the field windings. Thus, the probability of the occurrence of the
second ground fault is increased.
In case, a second ground fault occurs, a part of the field winding is bypassed, thereby
increasing the current through the remaining portion of the field winding. This causes an
unbalance in the air gap fluxes thereby creating an unbalance in the magnetic forces on
opposite sides of the rotor. This unbalancing in magnetic forces makes the rotor shaft
eccentric and also causes vibrations.
A high resistance is connected across the rotor circuit. The centre point of this is connected
to earth through a sensitive relay. The relay detects the rotor earth faults in most of the
rotor circuit except the centre point of the rotor.
Rotor Earth Fault Protection by DC Injection Method
The rotor windings may be damaged by earth faults. The figure below shows the modern
method of rotor earth fault protection.

2. A DC voltage bias is impressed between the field circuit and earth through a polarized
moving iron relay. When an earth fault occurs in the field winding, the DC bias causes the
current to flow through the relay “R” and it operates, since the circuit is completed through
the DC bias relay and the earth fault. It is not necessary to trip the generator, when a single
field earth fault occurs, usually an alarm is sounded. Then immediate steps are taken to
transfer the load from the faulty generator and shut it down as quickly as possible to avoid
further damages.
Rotor Temperature Alarm
This type of protection of electric generator is employed for large sets. As it is not
practicable to embed thermocouples in rotor winding since the slip ring connections would
be complicated, resistance measurement is adopted. The rotor voltage and current are
compared by a moving coil relay. The voltage coil of the relay is connected across the slip
ring brushes. The current coil is connected across the shunt in the field circuit. The
operating coil is voltage coil and the current coil is the restraining coil. The relay measures
the ratio V/I=R, which gives the measure of rotor temperature, since the resistance
increases with temperature.

3. Loss of Excitation (Field Failure) Protection
This protection uses MHO relay or off set impedance relay. The figure shows the loss of
excitation characteristics.
The two distinct effects of loss of excitation are:
1. The machine starts drawing large magnetizing current from the system. It runs
as an induction generator.

4. 2. The slip frequency emf is induced in the rotor.
Both cause over heating of the rotor.
The loss of excitation can be detected by measuring the reactive component of the stator
current. An excessive VAR import indicates the loss of synchronism. The “mal-operation” of
the relay due to system transients (which causes momentary reversal of VAR component) is
over come by incorporating a time delay of 1 to 5 secs in the tripping sequence of the relay.
An undercurrent relay in the field circuit may also be used except in large generator where
the excitation varies widely. But under current relay fails to operate when the field excitation
fails. A fast acting under current relay will malfunctioning due to AC induced during
synchronizing and external faults.
Offset Impedance Relay Characteristics
An alternative solution is to use an offset impedance or Mho relay at the generator
terminals.
The Mho relay operating characteristic is arranged as shown in the figure (loss of
excitation). During extremely low excitation or complete loss of excitation, the equivalent
generator impedance falls within the tripping zone of the relay. The above protection
scheme is also applicable for protection of rotor open circuit.

5. Negative Sequence Protection of Electric Generators against
Unbalanced Loads
The unbalanced 3-phase stator cause double frequency currents induced in the rotor.
Consequently, heating of rotor resulting in its damage can occur. Under unbalance
conditions, the phase currents have negative sequence components, rotating at
synchronous speed in a direction opposite to the direction of rotation of rotor. Therefore,
double frequency currents flow is induced in the rotor.
To save the rotor getting heated up under such conditions, Negative sequence Relay is
used, which has a characteristic I2
2t = K or t α (1/I2
2), where t is the time of operation of relay
and I2 is the negative sequence current component and K is a constant, value 7 for turbo-
gen set with direct cooling and 60 for salient pole hydro generator set.
The relay will trip the generator main breaker.
Rotor overheating can also be due to unbalanced external fault not cleared quickly, open
circuiting in a phase or failure of one phase contact of the circuit breaker.