ABB RED650, Applications Manual

Page: 61 / 454

4.2.3 Relationships between setting parameter Base Current, CT
rated primary current and minimum pickup of a protection
IED
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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
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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-earth voltage from the VT.
4.2.4.1 Example
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Consider a VT with the following data:
1MRK 505 393-UEN B
Section 4
Analog inputs
Line differential protection RED650 2.2 IEC 55
Application manual