ABB RELION 620 Series Technical Manual page 523

1MAC504801-IB E
4.7.3.5
620 series ANSI
Technical Manual
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 PICKUP output is deactivated.
The Timer calculates the start duration value PICKUP_DUR, which indicates the
percentage ratio of the start situation and the set operating time (DT). The value is
available in the Monitored data view.
Blocking logic
The activation of the BLOCK input deactivates all binary outputs and resets internal timers.
The binary input BLK_PICKUP can be used to block the PICKUP signal. The binary
input BLK_TRIP can be used to block the TRIP signal. The operation timer counting can
be frozen on the prevailing value by activation of the FR_TIMER input.
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 lose
synchronization with the power grid. 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.
Section 4
Protection functions
517