ABB RET670 Applications Manual page 587

1MRK504116-UUS C
Application manual
Synchronism check
The main purpose of the synchronism check function is to provide control over the
closing of circuit breakers in power networks in order to prevent closing if conditions
for synchronism are not detected. It is also used to prevent the re-connection of two
systems, which are divided after islanding and after a three pole reclosing.
Single pole auto-reclosing does not require any synchronism check
since the system is tied together by two phases.
SESRSYN (25) function block includes both the synchronism check function and the
energizing function to allow closing when one side of the breaker is dead. SESRSYN
(25) function also includes a built in voltage selection scheme which allows simple
application in busbar arrangements.
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Figure 222: Two interconnected power systems
Figure 222 shows two interconnected power systems. The cloud means that the
interconnection can be further away, that is, a weak connection through other stations.
The need for a check of synchronization increases as 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
Section 3
IED application
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