CHAPTER 4: SETPOINTS PROTECTION
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–235
The element is set to use the single blinder characteristic with a supervisory mho as
illustrated below.
The FlexLogic output operands for the out-of-step element are described as follows:
• The OOS Lft Bld PKP, OOS Rgt Bld PKP, and OOS Timer PKP FlexLogic operands are
auxil
iary operands that can be used to facilitate testing and special applications.
• The OOS OP FlexLogic operand can be used to trip the circuit breaker to isolate the
loss-of
-synchronism motor.
Follow these steps for a typical setting procedure of the out-of-step element:
1. Carry out detailed transient stability studies for the overall system.
2.
Determine the values of the motor transient reactance (X'
d
), and system impedance
(Z
S
). The total impedance is given by Z
total
= Z
S
+ j * X’
d
3. Set MHO REVERSE REACH for 0.05 to 0.15 times the motor transient reactance. Set
MHO FORWARD REACH to twice the motor transient reactance in the motor direction.
4. Set BLINDERS RCA to the angle of Z
total
, θ.
5. Using the results of transient stability analysis, find the critical angle δ
c
between the
motor and the system, beyond which the system begins to become unstable. If a
stability study is not available, this angle is typically set at 120°. Then, 10 degrees are
normally added in order to increase relay operation security, δ = δ
c
+10.
6. Determine the blinder distance, d, from the following equation:
7. Set RIGHT BLINDER and LEFT BLINDER to d.
8. Using the results of the transient stability analysis, find the PICKUP DELAY, which is
shorter than the time required for the impedance locus to travel between the left and
right blinders during the fastest expected out-of-step.
9. Validate the settings with the transient stability study cases.
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