ABB RET670 Applications Manual page 106

Section 3
IED application
100
transformers have difficult operating conditions. This bias quantity gives the best
stability against an unwanted operation during external faults.
The usual practice for transformer protection is to set the bias characteristic to a value
of at least twice the value of the expected spill current under through faults conditions.
These criteria can vary considerably from application to application and are often a
matter of judgment. The second slope is increased to ensure stability under heavy
through fault conditions which could lead to increased differential current due to
saturation of current transformers. Default settings for the operating characteristic with
IdMin = 0.3pu of the power transformer rated current can be recommended as a default
setting in normal applications. If the conditions are known more in detail, higher or
lower sensitivity can be chosen. The selection of suitable characteristic should in such
cases be based on the knowledge of the class of the current transformers, availability of
information on the load tap changer position, short circuit power of the systems, and so
on.
The second section of the restrain characteristic has an increased slope in order to deal
with increased differential current due to additional power transformer losses during
heavy loading of the transformer and external fault currents. The third section of the
restrain characteristic decreases the sensitivity of the restrained differential function
further in order to cope with CT saturation and transformer losses during heavy
through faults. A default setting for the operating characteristic with IdMin = 0.3 *
IBase is recommended in normal applications. If the conditions are known in more
detail, higher or lower sensitivity can be chosen. The selection of suitable characteristic
should in such cases be based on the knowledge of the class of the current
transformers, availability of information on the tap changer position, short circuit
power of the systems, and so on.
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.
1MRK504116-UUS C
Application manual