Transformer Application Example - ABB RELION 650 Series Applications Manual

Section 7
Current protection
ActivationSOTF : This setting will select the signal to activate SOTF function; CB position
open/CB position closed/CB close command .
tSOTF : Time delay for operation of the SOTF function. The setting range is 0.000 - 60.000 s in
step of 0.001 s. The default setting is 0.100 s
StepForSOTF : If this parameter is set on the step 3 start signal will be used as current set level.
If set off step 2 start signal will be used as current set level.
t4U : Time interval when the SOTF function is active after breaker closing. The setting range is
0.000 - 60.000 s in step of 0.001 s. The default setting is 1.000 s.
ActUnderTime : Describes the mode to activate the sensitive undertime function. The function
can be activated by Circuit breaker position (change) or Circuit breaker command.
tUnderTime : Time delay for operation of the sensitive undertime function. The setting range is
0.000 - 60.000 s in step of 0.001 s. The default setting is 0.300 s
7.4.3.6

Transformer application example

Two main cases are of interest when residual overcurrent protection is used for a power
transformer, namely if residual current can be fed from the protected transformer winding or
not.
The protected winding will feed earth-fault (residual) current to earth faults in the connected
power system. The residual current fed from the transformer at external phase-to-earth faults
is highly dependent on the total positive and zero-sequence source impedances. It is also
dependent on the residual current distribution between the network zero-sequence
impedance and the transformer zero-sequence impedance. An example of this application is
shown in Figure 50.
IEC05000490 V1 EN-US
Figure 50: Residual overcurrent protection application on a directly earthed transformer
In this case the protection has two different tasks:
• Detection of earth faults on the transformer winding.
• Detection of earth faults in the power system.
108
Three phase CT
summated
IN>
winding
YN/D or YN/Y
transformer
alt
Single CT
en05000490.vsd
1MRK 504 158-UEN A
SEMOD55591-4 v7
Application manual