ABB RELION 670 SERIES Applications Manual page 478

Section 8
Impedance protection
472
• For the synchronous machines as the generator in Table 32, the transient reactance
Xd' shall be used. This due to the relatively slow electromechanical oscillations under
out-of-step conditions.
• Sometimes the equivalent resistance of the generator is difficult to get. A good
estimate is 1 percent of transient reactance Xd'. No great error is done if this resistance
is set to zero (0).
• Inclination of the Z-line, connecting points SE and RE, against the real (R) axis can
be calculated as arctan ((ReverseX + ForwardX) / (ReverseR + ForwardR)), and is for
the case in Table 32
Other settings:
• ReachZ1: Determines the reach of the zone 1 in the forward direction. Determines the
position of the X-line which delimits zone 1 from zone 2. Set in % of ForwardX. In
the case shown in Table 32, where the reactance of the step-up power transformer is
11.32 % of the total ForwardX, the setting ReachZ1 should be set to ReachZ1 = 12 %.
This means that the generator – step-up transformer unit would be in the zone 1. In
other words, if the centre of oscillation would be found to be within the zone 1, only
a very limited number of pole-slips would be allowed, usually only one.
• pick up Angle: Angle between the two equivalent rotors induced voltages (that is, the
angle between the two internal induced voltages E1 and E2 in an equivalent
simplified two-machine system) to get the pickup signal, in degrees. The width of the
lens characteristic is determined by the value of this setting. Whenever the complex
impedance Z(R, X) enters the lens, this is a sign of instability. The angle
recommended is 110 or 120 degrees, because it is at this rotor angle where problems
with dynamic stability usually begin. Power angle 120 degrees is sometimes called
"the angle of no return" because if this angle is reached under generator swings, the
generator is most likely to lose synchronism. When the complex impedance Z(R, X)
enters the lens the start output signal (PICKUP) is set to 1 (TRUE).
• TripAngle: The setting TripAngle specifies the value of the rotor angle where the trip
command is sent to the circuit breaker in order to minimize the stress to which the
breaker is exposed when breaking the currents. The range of this value is from 15° to
90°, with higher values suitable for longer breaker opening times. If a breaker
opening is initiated at for example 60°, then the circuit breaker opens its contacts
closer to 0°, where the currents are smaller. If the breaker opening time tBreaker is
known, then it is possible to calculate more exactly when opening must be initiated
in order to open the circuit breaker contacts as close as possible to 0°, where the
currents are smallest. If the breaker opening time tBreaker is specified (that is, higher
than the default 0.0 s, where 0.0 s means that tBreaker is unknown), then this
alternative way to determine the moment when a command to open the breaker is
sent, is automatically chosen instead of the more approximate method, based on the
TripAngle.
• tReset: Interval of time since the last pole-slip detected, when the Out-of-step
protection is reset. If there is no more pole slips detected under the time interval
equal to 84.55 degrees, which is a typical value.
1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual