4.38
SEL-751A Relay
Instruction Manual
Date Code 20100129
Protection and Logic Functions
Voltage-Based Protection
For example, in
, if voltage VP has a frequency of 59.95 Hz and
voltage VS has a frequency of 60.05 Hz, the difference between them is the
slip frequency:
Slip Frequency = 59.95 Hz – 60.05 Hz = –0.10 Hz = –0.10 slip cycles/
second
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
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”.
. 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 Calculator 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.
Содержание 751A
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