ABB REB670 Applications Manual page 112

Section 6
Differential protection
106
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
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.
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Application manual