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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).
1MRK 505 291-UEN A
Section 10
Control
185
Application manual
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