ABB Relion REG670, Applications Manual

Page: 285 / 780

• 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
29 the base impedance of the
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 29 equal to 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 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
1MRK 502 071-UEN -
Section 8
Impedance protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 279
Application manual