Schweitzer Engineering Laboratories, Inc.
SEL-451 Data Sheet
5
Ground overcurrent elements are directionally controlled
by three directional elements working together:
➤
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 M
IRRORED
B
ITS
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 SEL
OGIC
control equation TRCOMM to pro-
gram specific elements, combinations of elements,
inputs, etc., to perform communications scheme tripping
and other scheme functions. The logic readily accommo-
dates the following conditions:
➤
Current reversals
➤
Breaker open at one terminal
➤
Weak-infeed conditions at one terminal
➤
Switch-onto-fault conditions
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
)
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.
Breaker Failure Protection
Incorporated into the SEL-451 is a full function breaker
failure system. Current can be individually monitored in
two breakers. High-speed open-pole detection logic
allows you to set the pickup current below minimum load
for sensitivity without sacrificing high-speed dropout.
Even in cases with delayed current zero in the secondary
of the CT caused by trapped flux, high-speed detection
of circuit breaker opening is achieved. This feature is
essential if breaker failure is initiated on all circuit
breaker trips. A reset of less than one cycle reduces coor-
dination times, improving stability.
Thermal Overload Protection
The SEL-451 supports three independent thermal ele-
ments that conform to the IEC 60255-149 standard. Use
these elements to activate a control action or issue an
alarm or trip when your equipment overheats as a result
of adverse operating conditions.
The SEL-2600 RTD Module provides ambient tempera-
ture measurements for the thermal model.
Loss-of-Potential (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.
High-Impedance Fault Detection
High-impedance faults are short-circuit faults with fault
currents smaller than what a traditional overcurrent pro-
tective relay can detect. The SEL-451 includes logic used
to detect HIF signatures without being affected by loads
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
