Instruction manual
–
AQ G3x7 Generator protection IED
88 (211
)
3.2.15
H
ARMONIC UNDER VOLTAGE
(64H)
The definite time third harmonic undervoltage protection function can be applied to extend
the stator earth fault protection system for a generator to 100% stator earth fault protection.
Other protection functions, based on network frequency quantities, cannot detect the stator
earth-faults near to the neutral point of the generator. This is due to the low value of the
generated voltage in this range of the stator coil. These functions operate only if the earth-
fault is relatively far from the neutral point.
The basic principle of extending the protected zone to the area near to the neutral point is the
third harmonic voltage detection. It can be applied if a generator is connected to the unit
transformer, the connection group of which isolates the generator form the network,
regarding the zero sequence voltage and current.
Along the stator windings of the phases, due to the construction of a generator, a third
harmonic voltage component is generated, which is of zero sequence nature. This zero
sequence third harmonic voltage is divided between the distributed capacitances of the
system (generator and transformer earth capacitance, etc.). As a consequence, in normal,
symmetric operation a certain amount of third harmonic voltage can be measured in the
neutral of the generator.
In case of a single phase-to-ground fault near to the neutral point of the generator, this
voltage decreases, and the third harmonic undervoltage protection function detects the earth
fault.
The function generates start signal if the third harmonic voltage component is below the
setting value.
The function generates a trip command only if the time delay has expired.
The function can be disabled via binary input if e.g. the basic harmonic voltage is low,
indicating a not excited state of the generator. This needs the application also of a network
frequency undervoltage function.
