Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

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4.84
SEL-700G Relay Instruction Manual Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
The inner resistance blinder must be inside the mho circle while the outer
resistance blinder should be outside the mho circle for the logic to operate
correctly.
Settings Calculation.
Collect the following information to calculate the
out-of-step protection settings.
➤
Generator transient reactance, X'
d
in secondary ohms.
➤
Generator step-up transformer impedance in secondary ohms.
➤
Impedance of line or lines beyond the generator step-up
transformers, if necessary.
Convert all impedances to generator base kV.
Recommendations.
Figure 4.51 shows the elements set according to the
following recommendations.
The out-of-step protection zone, which is limited by mho element 78Z1,
should extend from the generator neutral to the high-side bushings of the
generator step-up transformer. Normally, the forward reach 78FWD of the
mho element is set at 2–3 times the generator transient reactance X'
d
and its
reverse reach 78REV at 1.5–2 times the transformer reactance, XT. These
settings for the forward and the reverse reach provide adequate coverage plus
some margin. Refer to Figure 4.51 for details.
Set the inner blinder 78R2 to detect all out-of-step conditions. To do this, set
the inner blinder so that the equivalent machine angle, shown in Figure 4.51,
is approximately 120 degrees. A separation angle of 120 degrees or greater
between two sources generally results in loss of synchronism.
The outer blinder 78R1 and out-of-step timer 78D should be set to satisfy the
following:
➤
The outer blinder should not assert on maximum load.
➤
The outer blinder should lie outside the mho circle, to satisfy the
relay logic.
➤
The outer blinder should separate from the inner blinder far
enough to ensure that the 78D timer accurately times the out-of-
step slip cycle.
The SEL-700G processes the out-of-step logic every half cycle of the system
frequency. To ensure that the relay times the out-of-step slip frequency
accurately, the outer and inner blinders must be separated appropriately. For
example, assume that the highest out-of-step frequency encountered is five
slip cycles per second, which translates to 30 degrees per cycle (60 Hz). Set
the blinders with a 70-degree separation. This separation translates to a
positive-sequence impedance travel time of 2.3 cycles between the two
blinders, which should provide adequate timing accuracy. Set the 78D timer at
approximately 0.034 seconds (two cycles), which ensures that 78D will
pickup for swings traveling at 30 degrees per cycle or less.
The out-of-step slip frequency is a system-specific value. A transient stability
study normally determines this variable and, therefore, the double-blinder
settings.