Forming the Positive-Sequence Polarizing Voltage with Memory
The block diagram in Figure 2.8 shows how a relay can determine positive-sequence memory
voltage polarizing quantities from the three-phase voltages. The following description
assumes a digital implementation.
Figure 2.8: Positive-Sequence Polarizing Voltage Block Diagram
The relay filters and samples each of the voltages VA, VB, and VC every 90° (or four times
per power cycle). A digital filter removes dc offset. The result is a phasor for each of the
three voltages.
Next, the computer calculates the positive-sequence voltage, referred to phase A, from the
voltage phasors.
The block diagram in Figure 2.8 completes the set of polarizing voltages by rotating VA1M
by ±60° and inverting the two results to obtain VB1 and VC1. As our table for the mho
elements showed earlier, VA1M, VB1M, and VC1M polarize the AG, BG, and CG elements.
The polarization voltages for the BC, CA, and AB elements are the same as those for the AG,
BG, and CG elements rotated -90°. Rotations back 90° are simple:
For example, if (x,y) is the phasor for polarizing the AG element, then (y,-x) is the phasor
for polarizing the BC element.
Time-Overcurrent Elements and Curves
The 51N time-overcurrent element provides directional forward or nondirectional ground fault
protection as enabled. You can program its pickup (51NP) and trip (51NT) states into any
mask. The 51NP bit appears in the Relay Word to provide a means of determining residual
overcurrent element pickup.
The setting procedure includes time dial and curve shape selections. Four curve shapes are
available: moderately inverse, inverse, very inverse, and extremely inverse. The curves and
their equations are shown near the end of this section.
The relay forms the time-overcurrent characteristics by calculating a recursive sum of the
magnitude or magnitude-squared of the phase or residual current adjusted by the appropriate
pickup setting.
Date Code 920508
Specifications
SEL-221F, -1 Instruction Manual
2-35
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