Answer / kannan

STATOR EARTH FAULT PROTECTION :
Setting : 70% Time - 5 Sec.
Normally the generator stator neutral operates at a
potential close to ground. If a faulty phase winding
connected to ground, the normal low neutral voltage could
rise as high as line-to-neutral voltage depending on the
fault location. Although a single ground fault will not
necessarily cause immediate damage, the presence of one
increases the probability of a second. A second fault even
if detected by differential relay, may cause serious damage.
The usual method of detection fault is by measuring the
voltage across the secondary of neutral grounding
transformer (NGT). Here are two over lapping zones to detect
stator ground faults in a high impedance grounded generator
system, the two zones are put together cover 100% stator
winding for earth faults. A fundamental frequency neutral
over voltage relay covers about 0-95% of the stator zonal
winding for all faults except those near the neutral.
Another third harmonic neutral under voltage relay covers
remaining 96-100% of the stator zone 2 winding on neutral side.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.


ROTOR EARTH FAULT PROTECTION (64R) :
Settings : Less than 80K ohm
Any rotor field winding of the generator is electrically
isolated from the ground. Therefore the existence of one
ground fault in the field winding will usually not damage
the rotor. However the presence of two or more ground faults
in the winding will cause magnetic and thermal imbalance
plus localized heating and damage to the rotor metallic
parts. The rotor earth fault may be caused due to insulation
failure of winding or inter-turn fault followed by localized
heat.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.

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