Application - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.7.3.6
936
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 start 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.

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.
1MRS759142 C
REX640
Technical Manual