BE1-46N Functional Description
3-1
SECTION 3 • FUNCTIONAL DESCRIPTION
GENERAL
The following discussion is referenced to the Functional Block Diagram, Figure 3-1.
Figure 3-1. Functional Block Diagram
INPUT SENSING
Three-phase currents are applied to the negative sequence filter network which removes the zero and
positive sequence components of sensed line currents.
Resolved negative sequence currents (I
2
) are scaled by the TAP ADJUST switch. The TAP ADJUST switch
selects resistive loading to establish per unit (pu) current values. Switch positions, A through J, select tap
values from 3.2 amperes to 5.0 amperes in increments of 0.2 ampere. The CAL potentiometer is a vernier
control for selecting tap values between the settings of the TAP ADJUST switch.
The output from the filter is applied to the analog to digital (A/D) conversion network and to the buffer
amplifier to drive the external meter (optional) for I
2
level monitoring.
MEASURING I
2
The input from the filter network is converted to an absolute value and applied to a comparator and a times
four amplifier. The amplified output is also sent to a comparator. Both the direct and amplified values are
measured by the microprocessor. When I
2
values are small, the amplified output is used. This improves
resolution and accuracy.
Successive approximation measuring techniques allow the microprocessor to measure the level of I
2
. A
digital number with only the most significant digit set high is sent from the microprocessor to the digital to
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