External loss detection
The module checks the status information of the excitation system. It is activated
when the
External Los Det Ena setting is set to "Enable". The total loss of excitation
current or a failure in the excitation system is indicated by connecting the external
binary signal to the
EXT_LOS_DET
input. The Timer is enabled immediately when the
EXT_LOS_DET
input is activated.
Timer
Once activated, the Timer activates the
START
output. The time characteristic is
according to DT. When the duration of the underexcitation exceeds the set definite
Operate delay time, the
OPERATE
output is activated. If the impedance locus moves
out of the offset-mho operating characteristics before the module operates, the
reset timer is activated. If the reset timer reaches the value set by
Reset delay time,
the operating timer resets and the
START
output is deactivated.
The Timer calculates the start duration value START_DUR, which indicates the
percentage ratio of the pickup situation and the set operating time (DT). The value
is available in the Monitored data view.
Blocking logic
There are three operation modes in the blocking functionality. The operation
modes are controlled by the
BLOCK
input and the global setting Configuration >
System > Blocking mode which selects the blocking mode. The
BLOCK
input can be
controlled by a binary input, a horizontal communication input or an internal signal
of the protection relay's program. The influence of the
BLOCK
signal activation is
preselected with the global setting
Blocking mode.
The
Blocking mode setting has three blocking methods. In the "Freeze timers"
mode, the operate timer is frozen to the prevailing value. In the "Block all" mode, the
whole function is blocked and the timers are reset. In the "Block OPERATE output"
mode, the function operates normally but the
OPERATE
output is not activated.
4.7.1.5
Application
There are limits for the underexcitation of a synchronous machine. A reduction of
the excitation current weakens the coupling between the rotor and the external
power system. The machine may lose the synchronism and start to operate like an
induction machine, which increases the consumption of the reactive power. Even if
the machine does not lose synchronism, it is not recommended to operate in this
state. The underexcitation causes excessive heating in the end region of the stator
winding. This can damage the insulation of the stator winding and even the iron
core.
The underexcitation also causes the generator to operate in the asynchronous
mode. This increases the rotor speed, which causes heating in the rotor iron and
damps the windings. A high intake of the reactive power from the network during
underexcitation causes problems in the network, for example voltage dip, stability
and power swings. Power swings stress the prime mover, causing for example
turbine blade cavitation and mechanical stress in the gearbox.
The capability curve of a synchronous generator describes the underexcitation
capability of the machine. An excessive capacitive load on the synchronous machine
causes it to drop out-of-step. The reason is the steady-state stability limit as
defined by the load angle being 90°, which can only be reached when the unit is
underexcited.
1MRS757644 H
Protection functions
620 series
Technical Manual
723
