8-4 B30 Bus Differential System GE Multilin
8.3 DIFFERENTIAL PRINCIPLE 8 THEORY OF OPERATION
8
8.3DIFFERENTIAL PRINCIPLE 8.3.1 BIASED DIFFERENTIAL CHARACTERISTIC
The B30 uses a dual-slope dual-breakpoint operating characteristic as shown in the figure below.
The PICKUP setting is provided to cope with spurious differential signals when the bus carries a light load and there is no
effective restraining signal.
The first breakpoint (
LOW BPNT) is provided to specify the limit of guaranteed linear operation of the CTs in the most unfa-
vorable conditions such as high residual magnetism left in the magnetic cores or multiple autoreclosure shots. This point
defines the upper limit for the application of the first slope (
LOW SLOPE).
The second breakpoint (
HIGH BPNT) is provided to specify the limits of operation of the CTs without any substantial satura-
tion. This point defines the lower limit for the application of the second slope (
HIGH SLOPE).
Figure 8–3: BIASED OPERATING CHARACTERISTIC
The higher slope used by the B30 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
HIGH SLOPE setting creates 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
LOW SLOPE for restraining currents below the lower breakpoint of LOW BPNT,
• a constant percentage bias of HIGH SLOPE for restraining currents above the higher breakpoint of HIGH BPNT, and
• a smooth transition from the bias of
LOW SLOPE to HIGH SLOPE between the breakpoints.
differential
restraining
LOW
SLOPE
OPERATE
BLOCK
Ir
|Id|
HIGH
SLOPE
LOW BPNT
HIGH BPNT
PICKUP
836720A1.CDR
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