Application - ABB RELION REX640 Technical Manual

Section 4
Protection functions
4.7.2.6
928
Blocking logic
There are three operation modes in the blocking function. The operation modes are
controlled by the BLOCK input and the global setting in 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 operation timer is frozen to the prevailing value, but the OPERATE output is not
deactivated when blocking is activated. 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

Out of step protection functions detect stable power swings and out of step conditions
based on the fact that the voltage/current variation during a power swing is slow
compared to the step change during a fault. Both faults and power swings may cause
the measured impedance to enter into the operating characteristic of a distance relay
element. The apparent impedance moves from the pre-fault value to a fault value in a
very short time, a few milliseconds, during a fault condition. However, the rate of
change of the impedance is much slower during a power swing or out of step condition
than during a fault depending on the slip frequency of the out of step. The impedance
measurement should not be used by itself to distinguish between a fault condition and
an out of step condition from a phase fault. The fundamental method for
discriminating between faults and power swings is to track the rate of change of the
measured impedance.
The function measures the rate of change of the impedance using two impedance
measurement elements known as blinders together with a timing device. If the
measured impedance stays between the blinders for a predetermined time, the
function declares a power swing condition and asserts an output that can be used to
block the distance protection. However, if the impedance passes the inner blinder and
exits on the other side of the mho characteristics (that is, the resistive component of
impedance has opposite sign as at the time of point of entry) an out of step operate is
issued by the function. Figure 503 gives an example of out of step detection.
1MRS759142 C
REX640
Technical Manual