Application - ABB 615 series Technical Manual

Section 4
Protection functions
4.7.2.5
700
Timer
Once activated, the Timer activates the PICKUP output. The time characteristic is
according to DT. When the duration of the underexcitation exceeds the set definite Trip
delay time, the TRIP 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
PICKUP output is deactivated.
The Timer calculates the pickup 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
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 TRIP output" mode, the function operates
normally but the TRIP 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
1MAC059074-MB A
615 series ANSI
Technical Manual