4.111
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
The slip frequency in this example is negative, indicating that voltage VS is
not “slipping” behind voltage VP, but in fact “slipping” ahead of voltage VP.
In a time period of one second, the angular distance between voltage VP and
voltage VS changes by 0.10 slip cycles, which translates into:
0.10 slip cycles/second • (360°/slip cycle) • 1 second = 36°
Thus, in a time period of one second, the angular distance between voltage VP
and voltage VS changes by 36 degrees.
The absolute value of the Slip Frequency output is run through a comparator
and if the slip frequency is less than the maximum slip frequency setting,
25SF, Relay Word bit SF asserts to logical 1.
Angle Difference Calculator
The synchronism-check element Angle Difference Calculator in Figure 4.68
runs if the slip frequency is less than the maximum slip frequency setting
25SF (Relay Word bit SF is asserted).
Voltages VP and VS Are “Static”. Refer to top of Figure 4.68. If the slip
frequency is less than or equal to 0.005 Hz, the Angle Difference Calculator
does not take into account breaker close time—it presumes voltages VP and
VS are “static” (not “slipping” with respect to one another). This would usually be
the case for an open breaker with voltages VP and VS that are paralleled via
some other electric path in the power system. The Angle Difference Calcula-
tor calculates the angle difference between voltages VP and VS:
Angle Difference = |(VP – VS)|
For example, if SYNCPH := 90 (indicating VS constantly lags VP = VA by
90 degrees), but VS actually lags VA by 100 angular degrees on the power
system at a given instant, the Angle Difference Calculator automatically
accounts for the 90 degrees and:
Angle Difference = |( VP – VS)| = 10°
Also, if breaker close time setting TCLOSD := OFF, the Angle Difference
Calculator does not take into account breaker close time, even if the voltages
VP and VS are “slipping” with respect to one another. Thus, synchronism-
check elements 25A1 or 25A2 assert to logical 1 if the Angle Difference is
less than corresponding maximum angle setting 25ANG1 or 25ANG2.
Voltages VP and VS Are “Slipping”. Refer to bottom of Figure 4.68. If the
slip frequency is greater than 0.005 Hz and breaker close time setting
TCLOSD OFF, the Angle Difference Calculator takes the breaker close time into
account with breaker close time setting TCLOSD (set in ms; see Figure 4.69).
The Angle Difference Calculator calculates the Angle Difference between
voltages VP and VS, compensated with the breaker close time:
Angle Difference = |(VP – VS) + [(fP – fS) • TCLOSD • (1 / 1000) •
(360°/slip cycle)]|
Angle Difference Example (Voltages VP and VS are “Slipping”). Refer
to bottom of Figure 4.68. For example, if the breaker close time is 100 ms, set
TCLOSD := 100. Presume the slip frequency is the example slip frequency
calculated previously. The Angle Difference Calculator calculates the angle
difference between voltages VP and VS, compensated with the breaker close
time:
Angle Difference = |(VP – VS) + [(fP – fS) • TCLOSD • (1 / 1000) •
(360°/slip cycle)]|
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