Transformer Application Example - ABB REG670 Applications Manual

1MRK 502 071-UEN -
9.4.3.6
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
tPRCrvx, tTRCrvx, tCRCrvx: Parameters for user programmable of inverse reset time
characteristic curve. Further description can be found in the technical reference
manual.

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 183.
IEC05000490 V1 EN
Figure 183: Residual overcurrent protection application on a directly earthed
transformer winding
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.
It can be suitable to use a residual overcurrent protection with at least two steps. Step
1 shall have a short definite time delay and a relatively high current setting, in order
to detect and clear high current earth faults in the transformer winding or in the power
system close to the transformer. Step 2 shall have a longer time delay (definite or
inverse time delay) and a lower current operation level. Step 2 shall detect and clear
YN/D or YN/Y
transformer
Three phase CT
summated
alt
Single CT
IN>
en05000490.vsd
Section 9
Current protection
347