4.110
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
59VS indicates that voltage VS is within voltage window setting limits
25VLO and 25VHI
Other Uses for Voltage Window Elements
If voltage limits 25VLO and 25VHI are applicable to other control schemes,
you can use Relay Word bits 59VP and 59VS in other logic at the same time
that you use them in the synchronism-check logic.
If you are not using synchronism check, you can still use Relay Word bits
59VP and 59VS in other logic, with voltage limit settings 25VLO and 25VHI
set as desired. Enable the synchronism-check logic (setting E25 := Y) and
make settings 25LO, 25HI, and 25RCF. Apply Relay Word bits 59VP and
59VS in the logic scheme you want, using SEL
OGIC control equations. Even
though synchronism-check logic is enabled, you do not need to use the syn-
chronism-check logic outputs (Relay Word bits SF, 25A1, and 25A2).
Block Synchronism-Check Conditions
Refer to Figure 4.67. The synchronism-check element slip frequency calcula-
tor runs if both voltages VP and VS are healthy (59VP and 59VS asserted to
logical 1) and the SEL
OGIC control equation setting BSYNCH (Block Syn-
chronism Check) is deasserted (= logical 0). Setting BSYNCH is most com-
monly set to block synchronism-check operation when the circuit breaker is
closed (synchronism check is only necessary when the circuit breaker is
open):
BSYNCH := 52A (see Figure 4.80)
In addition, you can block synchronism-check operation when the relay is
tripping:
BSYNCH := … OR TRIP
Slip Frequency Calculator
Refer to Figure 4.67. The synchronism-check element Slip Frequency Calcu-
lator in Figure 4.67 runs if voltages VP and VS are healthy (59VP and 59VS
asserted to logical 1) and the SEL
OGIC control equation setting BSYNCH
(Block Synchronism Check) is deasserted (= logical 0). The Slip Frequency
Calculator output is:
Slip Frequency = fP – fS (in units of Hz = slip cycles/second)
fP = frequency of voltage VP (in units of Hz = cycles/second)
fS = frequency of voltage VS (in units of Hz = cycles/second)
A complete slip cycle is one single 360-degree revolution of one voltage (e.g.,
VS) by another voltage (e.g., VP). Both voltages are thought of as revolving
phasor-wise, so the “slipping” of VS past VP is the relative revolving of VS
past VP.
For example, in Figure 4.67, 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
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