Schweitzer Engineering Laboratories SEL-167D, Instruction Manual

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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:
P = X1 - X2 - X3 + X4.
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.
INPUTS AND OUTPUTS
Serial Interfaces
The SEL-267D relay is equipped with two EIA RS-232-C serial communication ports.
PORT 2 has 9-pin connectors on both the front and rear panels, designated PORT 2F and
PORT 2R, respectively.
PORT 2R, located on the relay rear panel, is typically used with an SEL-DTA Display/-
Transducer Adapter, SEL-RD Relay Display, or local printer. PORT 2F is always available
for short term local communications with a portable computer or printing terminal. Simply
plug the device into the front panel port. The relay automatically discontinues
communications with PORT 2R and addresses PORT 2F. When testing or data retrieval is
complete, unplug the temporary device from PORT 2F. The relay automatically resumes
communication with the device connected to PORT 2R.
Serial communications PORT 1 and the Auxiliary Input for demodulated IRIG-B time code
input remain on the relay rear panel.
2-6
Specifications
SEL-267D/167D Instruction Manual
Date Code 920326