• Partial or total generator loss of field
• Large dc current flow through rotor magnetic circuit
• Rotor vibration
• Rotor displacement sufficient to cause stator mechanical
damage
Therefore practically all bigger generators have some dedicated
protection which is capable to detect the first earth-fault in the
rotor circuit and then, depending on the fault resistance, either
just to give an alarm to the operating personnel or actually to
give stop command to the machine. An external injection unit is
required for rotor earth fault protection RXTTE4 and an external
protective resistor on plate for correct operation. Either
SDEPSDE or EF4PTOC function can be used in conjunction
with RXTTE4 as rotor earth-fault protection.
7. Voltage protection
Two step undervoltage protection UV2PTUV
Undervoltages can occur in the power system during faults or
abnormal conditions. Two step undervoltage protection
(UV2PTUV) function can be used to open circuit breakers to
prepare for system restoration at power outages or as long-
time delayed back-up to primary protection.
UV2PTUV has two voltage steps, where step 1 is settable as
inverse or definite time delayed. Step 2 is always definite time
delayed.
UV2PTUV has a high reset ratio to allow settings close to
system service voltage.
Two step overvoltage protection OV2PTOV
Overvoltages may occur in the power system during abnormal
conditions such as sudden power loss, tap changer regulating
failures, and open line ends on long lines.
OV2PTOV has two voltage steps, where step 1 can be set as
inverse or definite time delayed. Step 2 is always definite time
delayed.
OV2PTOV has a high reset ratio to allow settings close to
system service voltage.
Two step residual overvoltage protection ROV2PTOV
Residual voltages may occur in the power system during earth
faults.
Two step residual overvoltage protection ROV2PTOV function
calculates the residual voltage from the three-phase voltage
input transformers or measures it from a single voltage input
transformer fed from an open delta or neutral point voltage
transformer.
ROV2PTOV has two voltage steps, where step 1 can be set as
inverse or definite time delayed. Step 2 is always definite time
delayed.
Overexcitation protection OEXPVPH
When the laminated core of a power transformer or generator is
subjected to a magnetic flux density beyond its design limits,
stray flux will flow into non-laminated components that are not
designed to carry flux. This will cause eddy currents to flow.
These eddy currents can cause excessive heating and severe
damage to insulation and adjacent parts in a relatively short
time. The function has settable inverse operating curves and
independent alarm stages.
95% and 100% Stator earth fault protection based on 3rd
harmonic STEFPHIZ
Stator earth fault is a fault type having relatively high fault rate.
The generator systems normally have high impedance earthing,
that is, earthing via a neutral point resistor. This resistor is
normally dimensioned to give an earth fault current in the range
3 – 15 A at a solid earth-fault directly at the generator high
voltage terminal. The relatively small earth fault currents give
much less thermal and mechanical stress on the generator,
compared to the short circuit case, which is between
conductors of two phases. Anyhow, the earth faults in the
generator have to be detected and the generator has to be
tripped, even if longer fault time compared to internal short
circuits, can be allowed.
In normal non-faulted operation of the generating unit the
neutral point voltage is close to zero, and there is no zero
sequence current flow in the generator. When a phase-to-earth
fault occurs the neutral point voltage will increase and there will
be a current flow through the neutral point resistor.
To detect an earth fault on the windings of a generating unit one
may use a neutral point overvoltage protection, a neutral point
overcurrent protection, a zero sequence overvoltage protection
or a residual differential protection. These protections are
simple and have served well during many years. However, at
best these simple schemes protect only 95% of the stator
winding. They leave 5% close to the neutral end unprotected.
Under unfavorable conditions the blind zone may extend up to
20% from the neutral.
The 95% stator earth fault protection measures the
fundamental frequency voltage component in the generator
star point and it operates when the fundamental frequency
voltage exceeds the preset value. By applying this principle
approximately 95% of the stator winding can be protected. In
order to protect the last 5% of the stator winding close to the
neutral end the 3rd harmonic voltage measurement can be
performed. In 100% Stator E/F 3rd harmonic protection either
the 3rd harmonic voltage differential principle, the neutral point
3rd harmonic undervoltage principle or the terminal side 3rd
harmonic overvoltage principle can be applied. However,
differential principle is strongly recommended. Combination of
these two measuring principles provides coverage for entire
stator winding against earth faults.
Generator protection REG650
1MRK 502 050-BEN B
Product version: 1.3
ABB
17