ABB RET650 Applications Manual page 84

Section 6
Differential protection
78
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 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 30.
1MRK 504 169-UEN A
Transformer protection RET650 2.2 IEC
Application manual