Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

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SEL-700G Relay Instruction Manual Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
The settings for 25ANG1X, 25ANG2X, and CANGLE depend on the
requirements of the application.
Set TCLOSDX equal to the circuit breaker closing time in milliseconds. The
relay uses this value to calculate the slip-compensated phase-angle difference
between the generator and system voltages, as described previously. If there
are interposing relays between the SEL-700G CLOSEX output and the circuit
breaker close coil, add the operating time of these components to the breaker’s
own closing time to calculate the TCLOSDX setting.
Large generators can sustain serious damage if the generator circuit breaker
closes while there is a large phase-angle difference between the generator
voltage and the system voltage. Synchronism-check relays help prevent this
occurrence. However, if the circuit breaker is slow to close, the generator slip
frequency can cause the generator voltage to rotate away from the system
voltage, increasing the phase-angle difference to dangerous levels. At this
point it is not possible to trip the slowly closing breaker; but if the breaker
does close eventually, it could badly damage the generator or reduce its life.
To prevent this, you can use a breaker failure lockout signal to clear the
generator bus. This removes system voltage from the outboard side of the
circuit breaker so that if the breaker does close, it only energizes a dead bus.
The SEL-700G synchronism-check function includes breaker slow close
detection logic. If the relay initiates a circuit breaker close, the breaker close
failure logic is armed. If the breaker closes, the 3POX Relay Word bit
deasserts and the logic is disarmed. If the breaker does not close and the
generator voltage rotates to greater than the close failure angle, CFANGLE,
the relay asserts the BKRCF circuit breaker close failure Relay Word bit. This
Relay Word bit would be applied to trip the bus lockout relay. If some type of
control failure occurs that prevents the generator breaker from closing, the
breaker close failure logic is disarmed automatically after 99 seconds, as long
as the generator voltage does not enter the close failure region.
If breaker close failure protection is not necessary, set CFANGLE := Off. If
breaker close failure protection is necessary, first determine the phase angle
difference at which generator or prime mover damage can occur.
Damage Angle = ________°
Next, calculate the phase angle through which the generator rotates while the
generator bus is being cleared if a close failure occurs at maximum slip. Use
the following equation:
Angle Rotation = Bus clearing time • 25Sxx • 360 degrees
Set CFANGLE less than or equal to:
CFANGLE = Damage Angle° – Angle Rotation°
CFANGLE := _______°
where
Bus clearing time = time in seconds for all breakers connected to the
generator bus to open in the event of a breaker failure
lockout relay operation
25Sxx = maximum acceptable slip frequency. This value will
be equal to the absolute value of 25SHI or 25SLO,
whichever is larger.
Angle Rotation = _______°