windings or the current transformers will be in breakers that are part of the bus, such as
a breaker-and-a-half or a ring bus scheme. For current transformers with primaries in
series with the power transformer winding, the current transformer primary current for
external faults will be limited by the transformer impedance. When the current
transformers are part of the bus scheme, as in the breaker-and-a-half or the ring bus
scheme, the current transformer primary current is not limited by the power
transformer impedance. High primary currents may be expected. In either case, any
deficiency of current output caused by saturation of one current transformer that is not
matched by a similar deficiency of another current transformer will cause a false
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:
•
When CT from "T-connection" are connected to IED, as in the breaker-and-a-half
or the ring bus scheme, special care shall be taken in order to prevent unwanted
operation of transformer differential IED for through-faults due to different CT
saturation of "T-connected" CTs. Thus if such uneven saturation is a possibility it
is typically required to increase unrestrained operational level to
IdUnre
=
20-25pu
•
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
.
Section 7
1MRK 504 163-UUS A
Differential protection
138
Transformer protection RET670 2.2 ANSI
Application manual
Summary of Contents for RELION RET670
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