GE b30, Instruction Manual, Page: 546 / 632

9-6 B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
DIFFERENTIAL PRINCIPLE CHAPTER 9: THEORY OF OPERATION
9
• A smooth transition from the bias of
LOW SLOPE
to
HIGH SLOPE
between the breakpoints
9.3.2 Differential and restraining currents
The differential current is produced as a sum of the phasors of the input currents of a differential bus zone taking into
account the status signals of the currents, for example applying the dynamic bus replica of the protected zone. The
differential current is scaled to the maximum rated primary current. The scaling must be taken into account when setting
the
PICKUP
value of the biased differential characteristic and the
HIGH SET
operating point of the unbiased differential
function.
The restraining current is produced as a maximum of the magnitudes of the phasors of the zone input currents taking into
account the status signals of the currents, for example applying the dynamic bus replica of the protected bus zone. The
restraining current is scaled to the maximum rated primary current. The scaling must be taken into account when setting
the breakpoints of the biased differential characteristic.
The “maximum of” definition of the restraining signal biases the relay toward dependability without jeopardizing security
as the relay uses additional means to cope with CT saturation on external faults. An additional benefit of this approach is
that the restraining signal always represents a physical — compared to an “average” or “sum of” — current flowing through
the CT that is most likely to saturate during given external fault. This brings more meaning to the breakpoint settings of the
operating characteristic.
The following example is provided with respect to the breakpoint settings.
9.3.2.1 Example 3
Proceed with the previous example and assume that taking into account the relevant factors such as properties of the CTs
themselves, resistance of the leads, and burden of the CTs, the following primary currents are guaranteed to be
transformed without significant saturation:
•
1A CT: 6.0 kA
• 1B CT: 7.5 kA
• 1C CT: 5.0 kA
• 1D CT: 13.0 kA
• 1E CT: 8.0 kA
• 1F CT: 9.0 kA
As having the lowest primary current guaranteeing operation without saturation, the CT associated with the 1C input is
most exposed to saturation. During an external fault on the 1C circuit, the 1C CT carries the fault current contributed by
potentially all the remaining circuits. The fault current is higher than any contributing current, and therefore, the current of
the 1C CT becomes the restraining signal for the biased differential characteristic for external faults on the 1C circuit.
Consequently, the higher breakpoint of the differential characteristic (
HIGH BPNT
) needs to be set no higher than 5000 A,
where 1000 A = 5 pu (1000 A is the base unit as outlined in the previous the example).
The same approach applies to the setting of the lower breakpoint,
LOW BPNT
.
9.3.3 Enhanced security
To enhance the performance of the B30, the differential characteristic is divided into two regions having diverse operating
modes, as shown in following figure.
The first region applies to comparatively low differential currents and has been introduced to deal with CT saturation on
low-current external faults. Certain distant external faults can cause CT saturation due to extremely long time constants of
the DC component or multiple autoreclosure shots. The saturation, however, is difficult to detect in such cases. Additional
security via the “directional check” is permanently applied to this region without regard to the saturation detector.