A valid voltage condition starts the voltage-condition timer (VCT). When it times out, the
VSC bit is set in the Relay Word. It remains set until the voltage condition is no longer
valid.
Section 5: APPLICATIONS provides a discussion of the applications and setting selections
for this logic.
Signal Processing
The relay low-pass filters all analog input channels to remove high frequency components.
Next it samples each channel four times per power system cycle. After low-pass filtering, the
relay digitally filters each sample with the CAL digital filter method. The CAL filter
eliminates dc offset and reduces the decaying exponential offset that may be present on the
input signal following a fault.
The digital filter has the properties of a double differentiator smoother and requires only
addition and subtraction of data samples. Let the latest four samples of one channel be X1,
X2, X3, and X4. Then the digital filter is defined:
This filter eliminates dc offsets. When all samples are set to the same value, the filter output
is zero. It also eliminates ramps, which you may verify by setting the samples equal to 1, 2,
3, and 4. Again, the output is zero.
Every quarter-cycle, the relay computes a new value of P for each input. The current value
of P combines with the previous value (renamed Q) to form a Cartesian coordinate pair. This
pair represents the input signal as a phasor (P, Q). The relay processes these phasor
representations of the input signals.
Event Reporting
The relay retains an eleven-cycle data record for each of the last twelve events. The record
includes input currents and voltages, element states, input contacts, and output contacts. The
relay saves a report when any of the following occur:
The relay trips.
Certain protective elements, inputs, or outputs assert.
User executes the TRIGGER command.
Protective elements which trigger event reports when they pick up are:
Zones 1, 2, and 3 phase and ground distance elements.
Ground overcurrent elements.
High-set nondirectional phase overcurrent element.
2-26
Specifications
SEL-221F, -1 Instruction Manual
Date Code 920508
Summary of Contents for SEL-221F
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