Siemens 7SR157 Argus, Manual

Page: 171 / 180

Chapter 7 - 7SR157 Applications Guide
© 2016 Siemens Protection Devices Limited Chapter 7 - Page 7 of 14
2.6 Check Synchronising Settings
The check synchronising operation of the relay is used mainly in switching operations which link two parts of a
system which are weakly tied via other paths elsewhere in the system. In this synchronous system there should
be no frequency difference across the breaker but significant differences in phase angle and voltage magnitude
may exist due to the transmission line characteristics such as its length and type of loading.
For 25 Check Sync operation the relay should be set to the maximum phase angle and maximum voltage
difference which still permit the circuit breaker to close without causing large disturbances to the system. For most
systems the
25 Check Sync Angle can be set between 20° and 30°. There should not be any slip frequency but
a setting of 50mHz is typically applied as a check against loss of synchronism. Table 2 shows some possible
check synchronising settings when using the
25 Check Sync Angle plus 25 Check Sync Timer method. This
shows a range of 25 Check Sync Angle and the required 25 Check Sync Timer settings to achieve a slip
frequency limit of 50mHz. Note that due to the step resolution of the timer, an exact 50mHz slip limit is not always
achievable when using this method.
25 Check Sync
Angle
(θ°)
25 Check Sync
Timer settings (sec)
Slip Frequency
Limit (mHz)
± 10° 1.11 50.05
± 15° 1.66 50.20
± 20° 2.22 50.05
± 25° 2.77 50.14
± 30° 3.33 50.05
± 35° 3.88 50.11
± 40° 4.44 50.05
± 45° 5.00 50.00
Table 2 - Typical Check Synchronising Settings
Alternatively, if the 25 Check Sync Slip Freq detector is Enabled and the 25 Check Sync Timer set to 0.00s , a
setting of 50mHz could be applied to the 25 Check Sync Slip directly to achieve the same ends.
Note: in check synchronising mode the valid phase difference window for closing is actually twice the 25 Check
Sync Angle setting value because the valid Check Sync close can be given when the phase angle is either
decreasing or increasing. It should also be noted that the Check Sync Close output will stay on for a minimum of
100ms and for the whole duration of the time that the close parameters are met.
2.7 System Synchronising Settings
The system synchronising operation of the relay can automatically start if the two systems become asynchronous
i.e. there are no ties between the two systems and one system is effectively ‘islanded’. If this situation occurs the
frequencies will slip past each other and may cause the phase angle to come into the system split limits. The
system split detector can be set anywhere from 90° to 175° and is typically set to 170°. This will start system
synchronising automatically if
System Sync Enable has been set to AUTO.
When there are high rates of slip between the two systems greater care is needed when closing the breaker and
for this reason the
25 system Sync mode has independent settings from the 25 Check Sync mode. The
allowable phase angle close window is usually set much narrower than for check synchronising operation. Also,
the close decision from the relay is only given in the case of the phase angle decreasing. It will not issue a close if
the phase angle is increasing in value. Typically the slip frequency will be set to a limit of 250mHz or less and the
phase angle to 10° or 15°. Table 3 shows some possible system synchronising settings for limits of 100mHz and
250mHz using the timer method. Note that due to the step resolution of the timer, an exact 100mHz or 250mHz
slip limit is not always achievable.
25 System Sync
Angle (θ°)
25 System Sync
Timer settings (sec)
Slip Frequency
Limit (mHz)
± 10° 0.27 102.88
± 15° 0.42 99.21
± 10° 0.11 252.53
± 15° 0.17 245.10
Table 3 - Typical System Synchronising Settings