ABB Relion 670 Series Applications Manual page 101

1MRK 505 370-UUS Rev. K
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 ( 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
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 Hitachi
Power grids excellent experience from the analog percentage restrained differential protection IED (that
is, RADSS and REB 103), and use only the following three quantities as the inputs into the differential
algorithm in the numerical IED design:
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)
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. Meanwhile, very fast
tripping time, typically 11 ms, can be commonly obtained for heavy internal faults.
Discriminating zones in the IED includes a sensitive operational level. This sensitive operational level is
designed to be able to detect busbar ground faults in low impedance edgrounded power systems (that is,
power systems where the ground-fault current is limited to a certain level, typically between 300 A and
2000 A by neutral point reactor or resistor) or for some other special applications where increased
sensitivity is required. Operation and operating characteristic of the sensitive differential protection can
be set independently from the operating characteristic of the main differential protection. The sensitive
differential level is blocked as soon as the total incoming current exceeds the pre-set level. By
appropriate setting then it can be insured that this sensitive level is blocked for external phase-to-phase
Busbar protection REB670
Application manual
"Rated equivalent secondary e.m.f. requirements"
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Section 6
Differential protection
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