ABB REL650 Applications Manual page 295

1MRK 506 334-UUS A
Application manual
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ANSI04000179 V1 EN
Figure 128: Two interconnected power systems
Figure 128 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 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-
Section 11
Control
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