1.4
SEL-351A Relay
Instruction Manual
Date Code 20080213
Introduction and Specifications
SEL-351A Models
“Wire-Alike” Rear
Panel Terminal
Marking
Dual terminal labels are shown in
(hardware connection features)
,
Referring to
, and
:
➤
The top set of labels consists of a letter and a two-digit number.
These are consistent with the SEL-351 family.
➤
The bottom set of labels consists of a number only. These are
consistent with the SEL-251 and SEL-267 relays.
These two choices allow the SEL-351A to be wired into existing designs that
use SEL-251 or SEL-267 terminal numbers. See
251 and SEL-267 Relay Settings in the SEL-351A Relay
for detailed
discussion about using settings from the SEL-251 or SEL-267 relays in the
SEL-351A.
CT Saturation
Protection
The SEL-351A phase instantaneous overcurrent elements normally operate
using the output of a cosine filter algorithm. During heavy fault currents when
the relay detects severe CT saturation, the overcurrent elements can operate on
the adaptive current algorithm.
The adaptive current algorithm is only used for phase instantaneous
overcurrent elements if and only if the corresponding pickup setting is greater
than eight times the nominal phase current. For example, if 50P1P = 45 A (in a
5 Amp nominal phase current relay), then the 50P1, 50A1, 50B1, and 50C1
elements operate on the adaptive current algorithm. However, if 50P1P =
35 A, then the 50P1, 50A1, 50B1, and 50C1 elements operate on the output of
a cosine filter algorithm. No other overcurrent elements use the adaptive
current algorithm.
Based on the level of a “harmonic distortion index,” the adaptive current is
either the output of the cosine filter or the output of the bipolar peak detector.
When the harmonic distortion index exceeds the fixed threshold that indicates
severe CT saturation, the adaptive current is the output of the bipolar peak
detector. When the harmonic distortion index is below the fixed threshold, the
adaptive current is the output of the cosine filter.
The cosine filter provides excellent performance in removing dc offset and
harmonics. However, the bipolar peak detector has the best performance in
situations of severe CT saturation when the cosine filter magnitude estimation
is significantly degraded. Combining the two filters provides an elegant
solution for ensuring dependable phase instantaneous overcurrent element
operation.
Potential Transformer
Connections
Firmware revisions prior to R104 (inclusive) operate with wye-connected
(line-to-neutral) voltages only. The
VER
command (available at Access
Level 2) identifies these relays as “wye-connected.” In these earlier firmware
Number of residual-ground time overcurrent
elements
1
2
1
Number of frequency elements
6
6
3
Directional (32, 67) elements
•
•
Sensitive Earth Fault elements
•
•
Table 1.3
Major Differences—SEL-351, SEL-351A, and SEL-351A-1
(Sheet 2 of 2)
SEL-351
SEL-351A
SEL-351A-1
Summary of Contents for SEL-351A
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