ABB Relion 670 Series Applications Manual page 271

1MRK 505 370-UUS Rev. K
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ANSI04000179 V1 EN-US
Figure 123: Two interconnected power systems
Figure 123
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).
Busbar protection REB670
Application manual
shows two interconnected power systems. The cloud means that the interconnection can be
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en04000179_ansi.vsd
Section 12
Control
265