701 Motor Protection Relay
Date Code 20010719
SEL
OGIC®
Control Equations & Relay Logic
Relay Functional Overview
B.2
Relay Functional Overview
Understanding the relay’s internal processes makes it easier to understand the
relay’s programmable logic capabilities.
illustrates these processes.
Data Acquisition & Filtering
The SEL-701 Relay passes ac current and (if included) voltage signals through
low-pass filters to remove signals above the eighth harmonic. Then the relay digitally
samples the signals 16 times per power system cycle. The relay automatically tracks
system frequency over the range of 20 to 70 Hz to ensure that exactly 16 samples are
taken every cycle.
Four times per power system cycle, the relay processes the sampled data using a
digital filter that further removes unwanted signals, leaving the desired fundamental
frequency data for protection calculations. At this time, the relay also checks the
contact inputs to determine if any are asserted.
Element Processing
Having acquired and filtered the ac signals, the relay performs the enabled
protection algorithms to determine if protection elements are picked up or dropped out.
Relay Word
Each relay element is represented internally as a logic point, called a Relay Word
bit. Each Relay Word bit exists in one of two states: true or false, picked up or dropped
out. The relay uses these results to evaluate fixed logic and the programmable SEL
OGIC
control equations defined in the relay settings. For further details see
SEL
OGIC
Control Equations
After evaluating the fixed logic, the relay evaluates the SEL
OGIC
control
equations, then controls the relay output contacts. See
for a full discussion.
Background Tasks
By design, there is always processing time left after the relay controls its output
contacts and before it begins digitally filtering acquired signals again. During this free
time, the relay performs background tasks such as:
Self-testing.
Monitoring RTD temperature.
Storing start report, event report, and SER records.
Responding to serial port and front-panel commands.
Содержание SEL-701
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