ABB RELION 650 Series Applications Manual page 71

1MRK 504 158-UEN A
Transformers can be connected to buses in such ways that the current transformers used for
the differential protection will be either in series with the power transformer 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
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
• 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
1.75pu
=
The overall operating characteristic of the transformer differential protection is shown in
figure 27.
Application manual
IdUnre = 10pu , which is typically
IdUnre = 20-25pu
Section 6
Differential protection
IdUnre
65