• 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).
Section 15 1MRK 506 369-UUS -
Control
636 Line distance protection REL670 2.2 ANSI
Application manual
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