ABB REG670 Applications Manual page 285

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
• A precondition in order to be able to use the Out-of-step protection and construct
a suitable lens characteristic is that the power system in which the Out-of-step
protection is installed, is modeled as a two-machine equivalent system, or as a
single machine – infinite bus equivalent power system. Then the impedances
from the position of the Out-of-step protection in the direction of the normal load
flow can be taken as forward.
• The settings ForwardX, ForwardR, ReverseX and ReverseR must, if possible,
take into account, the post-disturbance configuration of the simplified power
system. This is not always easy, in particular with islanding. But for the two
machine model as in Table 29, the most probable scenario is that only 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 (ForwardX, ForwardR, ReverseX and
ReverseR) must be referred to the voltage level where the Out-of-step relay is
installed; for the example case shown in Table 29, this is the generator nominal
voltage UBase = 13.8 kV. This affects all the forward reactances and resistances
in Table 29.
• All reactances and resistances must be finally expressed in percent of ZBase,
where ZBase is for the example shown in Table
generator, ZBase = 0.9522 Ω. Observe 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 29, 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
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 29, 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.
• StartAngle: 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 start signal, in degrees. The width of
the lens characteristic is determined by the value of this setting. Whenever the
Impedance protection
29
29 equal to 84.55 degrees, which is a typical value.
Section 8
the base impedance of the
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