ABB RELION RET670 Applications Manual page 686

Section 15
Control
680
• Live line and live bus.
• Voltage level difference.
• Frequency difference (slip). The bus and line frequency must also be within a
range of ±5 Hz from rated frequency.
• Phase angle difference.
A time delay is available to ensure that the conditions are fulfilled for a minimum
period of time.
In very stable power systems the frequency difference is insignificant or zero for
manually initiated closing or closing by automatic restoration. In steady conditions a
bigger phase angle difference can be allowed as this is sometimes the case in a long
and loaded parallel power line. For this application we accept a synchronism check
with a long operation time and high sensitivity regarding the frequency difference. The
phase angle difference setting can be set for steady state conditions.
Another example is the operation of a power network that is disturbed by a fault event:
after the fault clearance a highspeed auto-reclosing takes place. This can cause a power
swing in the net and the phase angle difference may begin to oscillate. Generally, the
frequency difference is the time derivative of the phase angle difference and will,
typically oscillate between positive and negative values. When the circuit breaker
needs to be closed by auto-reclosing after fault-clearance some frequency difference
should be tolerated, to a greater extent than in the steady condition mentioned in the
case above. But if a big phase angle difference is allowed at the same time, there is
some risk that auto-reclosing will take place when the phase angle difference is big and
increasing. In this case it should be safer to close when the phase angle difference is
smaller.
To fulfill the above requirements the synchronism check function is provided with
duplicate settings, one for steady (Manual) conditions and one for operation under
disturbed conditions (Auto).
1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual