ABB ret650, Applications Manual

Page: 83 / 392

percentage differential current related to the current through the transformer. This
stabilizes the protection under through fault conditions while still permitting the
system to have good basic sensitivity. The bias current can be defined in many
different ways. One classical way of defining the bias current has been Ibias = (I1 +
I2) / 2, where I1 is the magnitude of the power transformer primary current, and I2
the magnitude of the power transformer secondary current. However, it has been
found that if the bias current is defined as the highest power transformer current
this will reflect the difficulties met by the current transformers much better. The
differential protection function uses the highest current of all restrain inputs as bias
current. For applications where the power transformer rated current and the CT
primary rated current can differ considerably, (applications with T-connections),
measured currents in the T connections are converted to pu value using the rated
primary current of the CT, but one additional "measuring" point is introduced as
sum of this two T currents. This summed current is converted to pu value using the
power transformer winding rated currents. After that the highest pu value is taken
as bias current in pu. In this way the best possible combination between sensitivity
and security for differential protection function with T connection is obtained. The
main philosophy behind the principle with the operate bias characteristic is to
increase the pickup level when the current 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
1MRK 504 169-UEN A
Section 6
Differential protection
Transformer protection RET650 2.2 IEC 77
Application manual