ABB Relion REQ650, Applications Manual

Page: 68 / 340

current flows. Although the phase angle difference between the
different phases is firm, the whole system appears to be rotating when
the measurement functions are observed.
4.2.2 Relationships between setting parameter Base Current, CT
rated primary current and minimum pickup of a protection
IED
Note that for all line protection applications the parameter Base Current (i.e. IBase
setting in the IED) used by the relevant protection function, shall always be set equal
to the largest rated CT primary current among all CTs involved in the protection
scheme. The rated CT primary current value is set as parameter
CTPrim under the IED
TRM settings.
For all other protection applications (e.g. transformer protection) it is typically
desirable to set
IBase parameter equal to the rated current of the protected object.
However this is only recommended to do if the rated current of the protected object is
within the range of 40% to 120% of the selected CT rated primary current. If for any
reason (e.g. high maximum short circuit current) the rated current of the protected
object is less than 40% of the rated CT primary current, it is strongly recommended to
set the parameter
IBase in the IED to be equal to the largest rated CT primary current
among all CTs involved in the protection scheme and installed on the same voltage
level. This will effectively make the protection scheme less sensitive; however, such
measures are necessary in order to avoid possible problems with loss of the
measurement accuracy in the IED.
Regardless of the applied relationship between the IBase parameter and the rated CT
primary current, the corresponding minimum pickup of the function on the CT
secondary side must always be verified. It is strongly recommended that the minimum
pickup of any instantaneous protection function (e.g. differential, restricted earth
fault, distance, instantaneous overcurrent, etc.) shall under no circumstances be less
than 4% of the used IED CT input rating. This corresponds to 40mA secondary for
main CTs with 1A rating and to 200mA secondary for main CTs with 5A rating. This
shall be individually verified for all current inputs involved in the protection scheme.
4.2.3 Setting of current channels
The direction of a current depends on the connection of the CT. Unless indicated
otherwise, the main CTs are supposed to be star connected. The IED can be connected
with its earthing point towards the object or away from the object. This information
must be set in the IED via the parameter
CTStarPoint , which can be changed between
FromObject and ToObject . Internally in the IED algorithms and IED functions, the
convention of the directionality is defined as follows:
Section 4
1MRK 505 291-UEN A
Analog inputs
62
Application manual