thanks every one.
I found this in SEL website.
Out-of-Step Tripping and Blocking Functions
Out-of-step tripping schemes are designed to protect the power system during unstable
conditions, isolating unstable generators or larger power system areas from each other with the
formation of system islands, in order to maintain stability within each island by balancing the
generation resources with the area load.
To accomplish this, OOS tripping systems must be applied at preselected network locations,
typically near the network electrical center, and network separation must take place at such points
to preserve a close balance between load and generation. However, as discussed earlier, many
relay systems are prone to operate at different locations in the power system during an OOS
condition and cause undesired tripping. Therefore, OST systems must be complemented with outof-
step blocking functions to prevent undesired relay system operations, prevent equipment
damage and shutdown of major portions of the power system, and achieve a controlled system
separation.
In addition, OOS blocking must be used at other locations in the network to prevent system
separation in an indiscriminate manner. Where a load-generation balance cannot be achieved,
some means of shedding nonessential load or generation will have to take place to avoid a
complete shutdown of the area.
Typically, the location of OST relay systems determines the location where system islanding
takes place during loss of synchronism. However, in some systems it may be necessary to
separate the network at a location other than the one where OST is installed. This is accomplished
with the application of a transfer tripping type of scheme. Another important aspect of OOS
tripping is to avoid tripping a line when the angle between systems is near 180 degrees. Tripping
during this condition imposes high stresses on the breaker and can cause restrikes and breaker
damage.
