Relay Element Settings
2-79
SEL-300G Instruction Manual
Date Code 20060731
O
UT
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OF
-S
TEP
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LEMENT
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INGLE
B
LINDER
S
CHEME
Element Description
Functional Description
The SEL-300G contains an out-of-step element to detect out-of-step conditions between two
electrical sources. Two interconnected systems can experience an out-of-step condition for
several reasons. For example, loss of excitation can cause a generator to lose synchronism with
the rest of the system. Similarly, delayed tripping of a generator breaker to isolate a fault can
cause the generator to go out-of-step with the rest of the system.
It is imperative to detect and isolate an out-of-step condition as early as possible because the
resulting high peak currents, winding stresses, and high shaft torques can be very damaging to
the generator and the associated generator step-up transformer.
The SEL-300G implements two out-of-step tripping schemes: single blinder and double blinder.
Users can select whichever scheme suits their application or can disable out-of-step protection.
The single blinder scheme, shown in Figure 2.48, consists of mho element 78Z1, right blinder
78R1, and left blinder 78R2.
B
X
R
78REV
78FWD
78Z1
78R2
Left Blinder
DWG: M300G245
78R2
78R1
78R1
Right Blinder
A
C
78R2
Pickup
78R1
Pickup
Note:
Diameter of 78Z1=
(78REV+78FWD)
Figure 2.48: Single Blinder Scheme Operating Characteristics
Summary of Contents for SEL-300G
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