ABB REG670 Applications Manual page 136

Section 7
Differential protection
130
differential current to appear. Differential protection can overcome this problem if the
bias is obtained separately from each set of current transformer circuits. It is therefore
important to avoid paralleling of two or more current transformers for connection to
a single restraint input. Each current connected to the IED is available for biasing the
differential protection function.
The unrestrained operation level has a default value of IdUnre = 10pu, which is
typically acceptable for most of the standard power transformer applications. In the
following case, this setting need to be changed accordingly:
• For differential applications on HV shunt reactors, due to the fact that there is no
heavy through-fault condition, the unrestrained differential operation level can
be set to IdUnre = 1.75pu
The overall operating characteristic of the transformer differential protection is shown
in figure 53.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
Application manual