ABB Relion 670 Series Applications Manual page 261

1MRK 511 358-UUS A
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ANSI04000179 V1 EN-US
Figure 101: Two interconnected power systems
Figure 101
can be further away, that is, a weak connection through other stations. The need for a check of
synchronization increases if the meshed system decreases since the risk of the two networks
being out of synchronization at manual or automatic closing is greater.
The synchronism check function measures the conditions across the circuit breaker and compares
them to set limits. Output is generated only when all measured conditions are within their set
limits simultaneously. The check consists of:
• 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).
Application manual
shows two interconnected power systems. The cloud means that the interconnection
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en04000179_ansi.vsd
Section 13
Control
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