Hitachi Relion 670 Series Applications Manual page 113

1MRK511407-UUS Rev. N
one line is in service after the fault on one power line has been cleared by line protections. The
settings ForwardX, ForwardR must therefore take into account the reactance and resistance of only
one power line.
• All the reactances and resistances must be referred to the voltage level where the Out-of-step relay
is installed; for the example case shown in Table 13, this is the generator nominal voltage VBase =
13.8 kV. This affects all the forward reactances and resistances in Table 13.
• All reactances and resistances must be finally expressed in percent of ZBase, where ZBase is for
the example shown in Table
that the power transformer's base impedance is different, ZBase = 0.6348 Ω. Observe that this latter
power transformer ZBase = 0.6348 Ω must be used when the power transformer reactance and
resistance are transformed.
• For the synchronous machines as the generator in Table 13, 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
84.55 degrees, which is a typical value.
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 13,
where the reactance of the unit 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 – unit 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 precisely 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 specified by tReset since the previous
one, the function is reset. All outputs are set to 0 (FALSE). If no pole slip at all is detected under
interval of time specified by tReset since the pickup signal has been set (for example a stable case
with synchronism retained), the function is as well reset, which includes the pickup output signal
(PICKUP), which is reset to 0 (FALSE) after tReset interval of time has elapsed. However, the
measurements of analogue quantities such as R, X, P, Q, and so on continue without interruptions.
Recommended setting of tReset is in the range of 6 to 12 seconds.
Phasor measurement unit RES670
Application manual
13 the base impedance of the generator, ZBase = 0.9522 Ω. Observe
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Section 7
Impedance protection
13 equal to
107