Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

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4.80
SEL-700G Relay Instruction Manual Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
The 78 element torque control SEL
OGIC
control equation OOSTC has a
default setting of one. If this value is left at one, the out-of-step element is not
controlled by any other conditions external to the element. However, users can
block the operation of the 78 element for certain conditions, such as the
presence of excessive negative-sequence currents, by setting OOSTC to NOT
46Q1. Refer to Logic Settings (SET L Command) on page 4.193 for a detailed
discussion of SEL
OGIC
control equations.
The trip delay timer also has an adjustable dropout delay 78TDURD (Trip
Duration). The 78TDURD should be set appropriately if the Relay Word bit
OOST is configured to operate an output contact directly. The default setting
for 78TDURD is zero because the Relay Word bit OOST is configured to trip
the generator breaker with default trip logic TRX (which includes an identical
timer TDURD). You must change the settings (trip logic and/or 78TDURD) if
your application requires a different action.
The scheme includes positive-sequence current supervision setting 50ABC,
which has a setting range of 0.25–30.00 A for 5 A relays and 0.05–6.00 A for
1 A relays. Normally, a setting of 0.25 A for 5 A relays is adequate for most
applications; however, a higher setting can be applied based on minimum
expected swing currents. Note that the positive-sequence current levels below
the 50ABC setting block the out-of-step function.
Both 78R1 and 78R2 must be within the mho circle.
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 4.48 shows the elements set according to the
following recommendations.
A transient stability study normally provides adequate data for setting the
elements and timers properly. 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, set forward
reach 78FWD at 2–3 times the generator transient reactance, X'
d
, and set
reverse reach 78REV at 1.5–2.0 times the transformer reactance, XT, to
provide adequate coverage with a margin of error.
Set the left and right blinders to detect all out-of-step conditions. To do this,
the right and left blinders are set so that the equivalent machine angles
and

are approximately 120 degrees, as shown in Figure 4.48. Separation angles
of 120 degrees or greater between the two sources usually results in loss of
synchronism.
Make sure that the mho element and the blinders do not include the maximum
possible generator load, to avoid assertion of 78Z1, 78R1, and 78R2 under
normal system operation.
