Differential And Restraining Currents - GE B90 Instruction Manual

DIFFERENTIAL PRINCIPLE
The higher slope used by the B90 acts as an actual percentage bias regardless of the value of the restraining signal. This is
so because the boundary of the operating characteristic in the higher slope region is a straight line intersecting the origin
of the 'differential - restraining' plane. The advantage of having a constant bias specified by the
an obstacle of a discontinuity between the first and second slopes. This is overcome by using a smooth approximation
(cubic spline) of the characteristic between the lower and higher breakpoints. Consequently, the characteristic ensures
• A constant percentage bias of
• A constant percentage bias of
• A smooth transition from the bias of

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 value of the biased differential characteristic and the
PICKUP
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
9
operating characteristic.
The following example is provided with respect to the breakpoint settings.
9.3.2.1 Example 2
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:
9-4
Figure 9-2: Biased operating characteristic
for restraining currents below the lower breakpoint of
LOW SLOPE
for restraining currents above the higher breakpoint of
HIGH SLOPE
to
LOW SLOPE HIGH SLOPE
B90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
CHAPTER 9: THEORY OF OPERATION
between the breakpoints
operating point of the unbiased differential
HIGH SET
setting creates
HIGH SLOPE
,
LOW BPNT
, and
HIGH BPNT