Differential Principle - GE B30 Instruction Manual

CHAPTER 9: THEORY OF OPERATION
Note that independently from the ratios and rated secondary currents, the per unit values of the differential current retain
their original meaning regardless of the distribution of the differential current between individual circuits. Assume, for
example that the differential current is fed by the inputs 1A and 1B exclusively, and consider two situations:
• The 1A input supplies 1kA primary, and the 1B input supplies 2kA primary. The currents are in phase. The pu current of
the 1A source is 1000 A : (600:5) : 5A/pu = 1.67 pu. The pu current of the 1B source is 2000 A : (500:1) : 1A/pu = 4.00 pu.
The pu differential current is (1000A + 2000A) : 1000A = 3.00 pu.
• The 1A input supplies 2kA primary, and the 1B input supplies 1kA primary. The currents are in phase. The pu current of
the 1A source is 2000 A : (600:5) : 5A/pu = 3.33 pu. The pu current of the 1B source is 1000 A : (500:1) : 1A/pu = 2.00 pu.
The pu differential current is (1000A + 2000A) : 1000A = 3.00 pu.

9.3 Differential principle

9.3.1 Biased differential characteristic
The B30 uses a dual-slope dual-breakpoint operating characteristic as shown in the following figure.
The setting is provided to cope with spurious differential signals when the bus carries a light load and there is no
PICKUP
effective restraining signal.
The first breakpoint (
LOW BPNT
unfavorable 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 (
The second breakpoint (
HIGH BPNT
saturation. This point defines the lower limit for the application of the second slope (
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
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
B30 BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
) is provided to specify the limit of guaranteed linear operation of the CTs in the most
) is provided to specify the limits of operation of the CTs without any substantial
Figure 9-3: Biased operating characteristic
for restraining currents below the lower breakpoint of
LOW SLOPE
for restraining currents above the higher breakpoint of
HIGH SLOPE
).
LOW SLOPE
HIGH SLOPE
DIFFERENTIAL PRINCIPLE
).
setting creates
HIGH SLOPE
,
LOW BPNT
, and
HIGH BPNT
9-5
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