Check Synchronising Settings; System Synchronising Settings - Siemens 7SR224 Technical Manual

4.2.7

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 check synchronising operation the relay should be set to the maximum phase angle and maximum voltage
differences which still permit the circuit breaker to close without causing large disturbances to the system. For
most systems the phase 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 due to tripping of all parallel
interconnections. Table 2 shows some possible check synchronising settings when using the phase detector plus
time delay method. This shows a range of phase angles and the required slip 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.
CS Phase Angle Setting ( °)
± 10°
± 15°
± 20°
± 25°
± 30°
± 35°
± 40°
± 45°
Table 4-1 Typical Check Synchronising Settings
Alternatively, if the slip frequency detector is used and the slip timer turned OFF, a setting of 50mHz could be
applied to the slip frequency detector directly to achieve the same ends.
Note : in check synchronising mode the valid phase difference window for closing is actually twice the phase
angle setting value because the valid Check Sync close can be given when the phase angle is either decreasing
or increasing.
4.2.8

System Synchronising Settings

The changeover to system synchronising operation will occur automatically if set, 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 to operate on a differential angle anywhere from 90° to 175° and
is typically set to 170°. Alternatively, the change to System Sync can be made based on Slip frequency using a
high value of typically 125mHz or more.
When there are high rates of slip between the two systems greater care is needed when closing the breaker and
for this reason the system synchronising mode has independent settings from the check synchronising 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 4-2 shows some possible system synchronising settings for limits of 100mHz
and 250mHz. Note that due to the step resolution of the timer, an exact 100mHz or 250mHz slip limit is not
always achievable.
SS Phase Angle Setting
( °
± 10°
± 15°
± 10°
©2010 Siemens Protection Devices Limited
CS Slip Timer Setting
(sec)
1.1
1.7
2.2
2.8
3.3
3.9
4.4
5.0
SS Slip Timer
Setting
(sec)
0.3
0.4
0.1
7SR224 Argus Applications
Slip Frequency
Limit
(mHz)
50.51
49.02
50.51
49.60
50.51
49.86
50.51
50.00
Slip Frequency Limit
(mHz)
92.59
104.17
277.78
Chapter 7 Page 34 of 50