ABB Relion 670 Series Applications Manual page 321

1MRK 511 401-UEN Rev. K
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IEC04000179 V1 EN-US
Figure 138: Two interconnected power systems
Figure 138
interconnection 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 synchrocheck 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 synchrocheck 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 synchrocheck function is provided with duplicate
settings, one for steady (Manual) conditions and one for operation under disturbed conditions
(Auto).
Bay control REC670
Application manual
shows two interconnected power systems. The cloud means that the
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Section 15
Control
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