ABB RELION 670 Series Applications Manual page 103

1MRK 505 337-UUS A
Consequently, all these phenomena have to be considered during the design stage of a busbar
differential IED in order to prevent the unwanted operation of the IED during external fault
conditions.
The analog generation of the busbar differential IEDs (that is, KA2, 87B, RADHA, RADSS, REB 103)
generally solves all these problems caused by the CT non-linear characteristics by using the
galvanic connection between the secondary circuits of all CTs connected to the protected zone.
These IEDs are designed in such a way that the current distribution through the IED differential
branch during all transient conditions caused by non-linearity of the CTs will not cause the
unwanted operation of the differential IED. In order to obtain the required secondary CT current
distribution, the resistive burden in the individual CT secondary circuits must be kept below the
pre-calculated value in order to guaranty the stability of the IED.
In new numerical protection IEDs, all CT and VT inputs are galvanically separated from each other.
All analog input quantities are sampled with a constant sampling rate and these discreet values
are then transferred to corresponding numerical values (that is, AD conversion). After these
conversions, only the numbers are used in the protection algorithms. Therefore, for the modern
numerical differential IEDs the secondary CT circuit resistance might not be a decisive factor any
more.
The important factor for the numerical differential IED is the time available to the IED to make the
measurements before the CT saturation, which will enable the IED to take the necessary corrective
actions. This practically means that the IED has to be able to make the measurement and the
decision during the short period of time, within each power system cycle, when the CTs are not
saturated. From the practical experience, obtained from heavy current testing, this time, even
under extremely heavy CT saturation, is for practical CTs around two milliseconds. Because of this,
it was decided to take this time as the design criterion in REB 670 IED, for the minimum
acceptable time before saturation of a practical magnetic core CT. Thus, the CT requirements for
REB 670 IED are kept to an absolute minimum. Refer to section
requirements"
However, if the necessary preventive action has to be taken for every single CT input connected to
the differential IED, the IED algorithm would be quite complex. Thus, it was decided to re-use the
ABB excellent experience from the analog percentage restrained differential protection IED (that
is, RADSS and REB 103), and use only the following three quantities:
1. incoming current (that is, sum of all currents which are entering the protection zone)
2. outgoing current (that is, sum of all currents which are leaving the protection zone)
3. differential current (that is, sum of all currents connected to the protection zone)
as inputs into the differential algorithm in the numerical IED design.
These three quantities can be easily calculated numerically from the raw sample values (that is,
twenty times within each power system cycle in the IED) from all analog CT inputs connected to
the differential zone. At the same time, they have extremely valuable physical meaning, which
clearly describes the condition of the protected zone during all operating conditions.
By using the properties of only these three quantities, a new patented differential algorithm has
been formed in the IED. This differential algorithm is completely stable for all external faults. All
problems caused by the non-linearity of the CTs are solved in an innovative numerical way. In the
same time, very fast tripping time, down to 10 ms, can be commonly obtained for heavy internal
faults.
Please refer to the technical reference manual for more details about the working principles of the
Differential Function algorithm.
Application manual
for more details.
Differential protection
"Rated equivalent secondary e.m.f.
Section 6
97