Schweitzer Engineering Laboratories, Inc. SEL-451-6 Data Sheet
5
Protection Features
Overcurrent Elements
The SEL-451 includes four phase, four negative-
sequence, and four ground instantaneous overcurrent ele-
ments. The SEL-451 also includes six selectable operat-
ing quantity inverse-time overcurrent elements. You can
select the operating quantities from the following:
IA, IB, IC, MAX(IA, IB, IC), I1, 3I2, IG
where IA, IB, IC can be fundamental or rms quantities
from either circuit breaker or combined currents.
The time-overcurrent curves (listed in Table 1)have two
reset characteristic choices for each time-overcurrent
element. One choice resets the elements if current drops
below pickup for one cycle. The other choice emulates
the reset characteristic of an electromechanical induction
disc relay.
Directional Elements
The SEL-451 includes a number of directional elements
for supervision of overcurrent elements. The negative-
sequence directional element uses the same patented
principle proven in the SEL-351. This directional element
can be applied in virtually any application, regardless of
the amount of negative-sequence voltage available at the
relay location.
The following three directional elements working together
provide directional control for the ground overcurrent
elements:
➤Negative-sequence voltage-polarized directional
element
➤Zero-sequence voltage-polarized directional element
➤Zero-sequence current-polarized directional element
Our patented Best Choice Ground Directional Element
selects the best ground directional element for the system
conditions and simplifies directional element settings.
(You can override this automatic setting feature for spe-
cial applications.)
Communications-Assisted
Tripping Schemes
Use communications to improve tripping time for better
customer service. The SEL-451 is the ideal relay for use
in pilot-based tripping schemes. Enhanced MIRRORED
BITS communications with SEL fiber-optic transceivers
provide 3–6 ms relay-to-relay transmission time. Among
the schemes supported are the following:
➤Permissive overreaching transfer tripping (POTT)
➤Directional comparison unblocking (DCUB)
➤Directional comparison blocking (DCB)
Use the SELOGIC control equation TRCOMM to program
specific elements, combinations of elements, inputs, etc.,
to perform communications scheme tripping and other
scheme functions. The logic readily accommodates the
following conditions:
➤Current reversals
➤Breaker open at one terminal
➤Weak-infeed conditions at one terminal
➤Switch-onto-fault conditions
LOP Logic Supervises Directional
Elements
The SEL-451 includes logic to detect an LOP caused by
failures such as blown fuses, which can cause an incor-
rect operation in directional elements. Simple settings
configure the LOP logic to either block or force forward
ground and phase directional elements under these con-
ditions. The logic checks for a sudden change in positive-
sequence voltage without a corresponding change in pos-
itive- or zero-sequence current. Tests and field experi-
ence show that this principle is very secure and is faster
than the tripping elements.
HIF Detection
HIFs are short-circuit faults with fault currents smaller than
what a traditional overcurrent protective relay can detect.
The SEL-451 includes logic used to detect HIF signatures
without being affected by loads and other system operation
conditions. A running average provides a stable pre-fault
reference and adaptive tuning learns and tunes out feeder
ambient noise conditions. Decision logic differentiates an
HIF condition from other system conditions such as switch-
ing operations and noisy loads. As much as 40 minutes of
HIF activity is stored in high-resolution COMTRADE
Table 1 Time-Overcurrent Curves
U.S. IEC
Moderately Inverse Standard Inverse
Inverse Very Inverse
Very Inverse Extremely Inverse
Extremely Inverse Long-Time Inverse
Short-Time Inverse Short-Time Inverse
