4.2.3
Relationships between setting parameter Base Current, CT
rated primary current and minimum pickup of a protection IED
GUID-8EB19363-9178-4F04-A6AC-AF0C2F99C5AB v1
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
SEMOD55055-44 v4
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.
Section 4
1MRK 505 370-UUS A
Analog inputs
84
Busbar protection REB670 2.2 ANSI
Application manual
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