Example Setting Calculations For Slip Timer - Siemens 7SR224 Technical Manual

± 15°
Table 4-2 Typical System Synchronising Settings
Alternatively, if the slip frequency detector is used and the slip timer turned OFF, settings of 100mHz or 250mHz
could be applied to the slip frequency detector directly to achieve the same ends.
The relay will automatically revert to Check Synchronising mode when zero slip is applied.
4.2.9

Example Setting Calculations For Slip Timer

In Check Synchronising operation the relay will issue a Check Sync close if the system conditions are such that
the phase angle and slip frequency are within limits. There is a possibility, however, that a Check Sync close
could be issued at a point where the phase angle is approaching the angular limits, say + 20°, and the slip
frequency is at the maximum allowable value. The consequence of this is that due to the inherent closing time of
the CB the actual CB close occurs outside of the phase angle limits. The angle overshoot being dependent on the
actual slip frequency and the total CB closing delay.
The total delays involved in this process include the main software timing loop which issues the close command,
the output relay time to pick up and the actual breaker closing time delays. To reduce the risk of a late closure it is
common practice to set the slip timer setting (Td) to typically 10x the CB closing time. This will ensure that the CB
will close no later than 1.2x the actual phase angle setting of the relay e.g. ± 24° for a ± 20° setting.
e.g. :-
The change in phase angle between two waveforms is directly related to the frequency difference, or slip,
between them. The change in phase angle
θ
Δ
Change in phase angle
The distance the phasor can travel during the breaker close time can therefore be given by,
( )
θ
Δ = × ×
Slip 360 t
CB
Using the equation given in section 2.7 for check synchronising,
θ
1
= ×
Slip
°
Td
180
and substituting this into
θ
1
θ
Δ = × × °
360
°
Td
180
It was stated that the slip timer setting Td should be set to 10x the breaker closing time t
Substituting for this in the above equation gives,
( )
θ
×
2
θ
Δ =
θ
Δ =
10
or
Thus for a slip timer setting (Td) of 10x breaker closing time (t
the phase angle setting. The maximum closing angle will be 120% of phase angle setting.
In practice, however, the relay operating times need to be taken into consideration. A typical example now follows
:
- Maximum allowed phase angle for closure = 30°.
- Circuit breaker closure time = 150ms.
Maximum relay delays : Software timing loop + Output relay delays = 5ms + 7ms = 12ms.
Therefore slip timer time delay should be set to 10x (150ms + 12ms) = 1.62sec. In practice this will have to be set
to 1.6sec due to the resolution of the slip timer.
The phase angle setting should be set to 80% of the maximum allowable closing angle, which is 24°.
If the relay was to issue a close right on the boundary of 24 then the breaker will not close outside of 30°.
With an angle of 24° and a slip timer delay (Td) of 1.6sec, using the equation from section 2.7, the slip is
therefore,
©2010 Siemens Protection Devices Limited
0.2
θ
Δ
for a system with 1Hz slip is 360º in 1 second. Thus,
( )
= Slip ×
360
°/sec.
- where t is the breaker close time in seconds.
CB
(
θ
Δ =
Slip
θ
× Δ =
t
CB
which gives
θ
×
0 2 .
7SR224 Argus Applications
208.33
)
× ×
360 t
CB
gives the following,
t
θ
× ×
CB
2
Td
) the actual change in phase angle will be 20% of
CB
.
CB
Chapter 7 Page 35 of 50