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Section 8
Impedance protection
440
Settings ForwardR, ForwardX, ReverseR, and ReverseX.
•
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 32, 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 32, this is the generator nominal
voltage UBase = 13.8 kV. This affects all the forward reactances and
resistances in Table 32.
•
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 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
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 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
1MRK 506 369-UEN B
32
the base impedance of the
32
equal to 84.55 degrees, which is a
Line distance protection REL670 2.2 IEC
Application manual
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