4.2.3 Relationships between setting parameter Base Current, CT
rated primary current and minimum pickup of a protection IED
Note that for all line protection applications (e.g. distance protection or line differential
protection) 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. generator, shunt reactor, shunt capacitor and
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 (i.e. 1A or 5A). This corresponds to 40mA secondary for IED 1A
rated inputs and to 200mA secondary for IED 5A rated inputs used by the function. This
shall be individually verified for all current inputs involved in the protection scheme.
Note that exceptions from the above 4% rule may be acceptable for very special
applications (e.g. when Multipurpose filter SMAIHPAC is involved in the protection
scheme).
4.2.4 Setting of voltage channels
As the IED uses primary system quantities, the main VT ratios must be known to the IED.
This is done by setting the two parameters VTsec and VTprim for each voltage channel.
The phase-to-phase value can be used even if each channel is connected to a phase-to-
ground voltage from the VT.
4.2.4.1 Example
Consider a VT with the following data:
Section 4 1MRK 506 369-UUS -
Analog inputs
84 Line distance protection REL670 2.2 ANSI
Application manual
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