ABB Relion 670 series, Technical Manual

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The first method
The circuit breaker is only allowed to break the current when the rotor angle has become less than
the set value
TripAngle, on its way to 0 electrical degrees. A recommended value for the setting
TripAngle is 90 degrees or less, for example 60 degrees. Figure
205 illustrates the case with
TripAngle = 90 degrees. The offset Mho circle represents loci of the complex impedance Z(R, X) for
which the rotor (power) angle is 90 degrees. If the circuit breaker must not open before the rotor
angle has reached 90 degrees on its way towards 0 degrees, then it is clear that the circle delimits
the R – X plane into a “no trip” and a “trip” region. For
TripAngle = 90 degrees, the trip command
will be issued at point 3 when the complex impedance Z(R, X) exits the circle. By that time the relay
logic had already ascertained the loss of step, and the general decision to trip the generator has
already been taken.
The second method
This method is more exact. If the break-time of the circuit breaker is known, (and specified as the
setting tBreaker) than it is possible to initiate a trip (break) command almost exactly tBreaker
milliseconds before the rotor (power) angle reaches 0 degrees, where the currents are at their
minimum possible values. The breaker contacts open at almost exactly 0 degrees, as illustrated in
Figure 206 for
tBreaker = 0.060 s. The point in time when the breaker opening process must be
initiated is estimated by solving on-line the so called “synchronizer” differential equation. Note
that if
tBreaker is left on the initial (default) value, which is zero (0), then the alternative setting
TripAngle decides when the trip command is given. If specified tBreaker > 0, for example tBreaker
= 0.040 second, then automatically, the
TripAngle is ignored and the second, more exact method
applied.
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
-0.4
-0.2
0
0.2
0.4
0.6
Real part (R) of Z in Ohms →
Im
ag
ina
ry
pa
rt
(X
) o
f Z
in
O
hm
s
→
R[Ohm]
no trip
region
loci of Z(R, X)
no trip
region
rotor angle
= ±180°
X[Ohm]
RE - Receiving End (infinite bus)
←
this circle
is loci of
the rotor
angle = 90°
2
relay
SE - Sending End (generator)
no trip
region
inside
circle
outside the
circle is the trip
region for
TripAngle <= 90°
3
here rotor
angle
is -90°
here
rotor angle
is +90°
1
trip
region
← Z - line connects
points SE & RE
IEC10000114-1-en.vsd
IEC10000114 V1 EN-US
Figure 205: The imaginary offset Mho circle represents loci of the impedance Z(R, X) for
which the rotor angle is 90 degrees
1MRK 502 066-UUS B Section 7
Impedance protection
385
Technical manual