ABB Relion 670 Series Applications Manual page 235

1MRK 511 401-UEN Rev. K
The same is valid for the SCB zero sequence quantities if the SCB is earthed. However, if the
failure is within the SCB, then such relationship is not valid.
The following table summarizes the possible current/voltage pairs which can be used for the
voltage restrained principle in order to detect the SCB cascading failure.
Table 32:
Current signal I
SCB Neg. Seq. Current
SCB Zero Seq. Current
Operation characteristic
13.0(VDepFact2: 0.100)
8 (VDepFact1: 0.064)
IEC19000106 V1 EN-US
Figure 107: Voltage restrained characteristic for SCB protection
The voltage restrained characteristic in
continuous (that is, solid black line) line defines the relationship between the magnitudes of
the measured pair of current I and voltage U (see
impedance for all the external unbalance conditions. The easiest method to draw this
characteristic is in percent with the SCB rating as a base. In this case, the characteristic has a
slope of 45° and it passes through the points (0,0) and (100,100).
The sensitivity and the speed of operation is handled with two voltage restrained over-current
stages. The first stage is set above (for example, +5%) the external fault characteristic. It has
sensitivity, and hence uses a certain time delay (for example, between 100 ms and 200 ms) to
guarantee the stability under all operating conditions of the SCB.
The second stage is set 10% - 40% above the external fault characteristic. It is less sensitive
and allows to trip much faster (for example, with 30 ms - 50 ms delay) for more severe
cascading failures.
Both the steps will operate when the operating point defined by the magnitude of the
measured current and voltage pair is above the set characteristic (red line).
Bay control REC670
Application manual
Possible pairs of measurement quantities for cascading failure detection
Voltage signal U
%IBase
I>>:130
I>:125
120
100
3
3
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SCB Neg. Seq. Voltage
SCB Zero Seq. Voltage
120
100
Figure 107 is used as cascading failure protection. The
Equation
Unbalance protection
Can be used on any SCB
Can only be used on directly
earthed SCB
Second fast step
First sensitive step
Characteristic for
external faults
defined by X
SCB
% UBase
IEC19000106-1-en.vsdx
102) as defined by the SCB
Section 10
229