Schweitzer Engineering Laboratories SEL-751 - Figure 4.78 81 RF Fast Rate-Of-Change-Of-Frequency Logic

4.128
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
Figure 4.78 81RF Fast Rate-of-Change-of-Frequency Logic
Trip/Close Logic
The SEL-751 tripping logic is designed to trip the circuit breakers. The relay
logic lets you define the conditions that cause a trip, the conditions that
unlatch the trip, and the performance of the relay output contact. Figure 4.79
illustrates the tripping logic.
0.2 Hz/s
0.1 Hz
—0.2 Hz/s
—0.1 Hz
81RFPU
81RFDO
0.1 sec
0 sec
DF3C
DF
81RFSLP
81RFRP
FREQTRK
81RFT
Relay
Word
Bits
0
81RFBLDO
|VPPmin|
81RFIBLK
|IC|
|IA|
81RFVBLK
|IB|
81RFBL
Relay Word bit FREQTRK = 1, means relay is tracking frequency.
81RFRP, 81RFDFP, 81RFPU, 81RFDO, 81RFVBLK, 81RFIBLK are settings.
DF3C is df/dt calculated over 3-cycle window (Hz/s).
DF is the difference (FREQ–FNOM) in Hz.
81RFSLP = - (81RFRP/81RFDFP).
|IA|, |IB|, |IC| are A-, B-, and C-phase current magnitudes.
VP** = |VPP| min
When 81RFIBLK := OFF, the input is held at a high value such that 81RFBLK is logical 0.
When 81RFVBLK := OFF, the input is held to zero such that 81RFBLK is logical 0.
1
2
3
4
5
6
7
8
SELOGIC
Setting
81RFP
81RFI
81RFBL
81RFBLK
6
2
8
2
7
2
5
4
3
1
2
2
9
9