P.3.43
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
Also for this application, you would typically need extra differential
protection coordinated with the short-circuit protection of the tapped and
unmeasured load. For this purpose, the relay allows its selectable time-
overcurrent elements to work with the 87L differential current.
Overall, the following approach is taken in such partial line current differential
applications.
➤ The 87L elements provide instantaneous protection for line
faults, intentionally desensitized however, to prevent operation
for faults in the tapped load.
➤ The differential time-overcurrent protection provides sensitive
but time-coordinated protection for low-current line faults and
possible internal faults in the tapped transformer, as well as
remote backup for short-circuit protection in the load network.
Use the selectable time-overcurrent elements, 51S, to configure differential
time-overcurrent protection. Selectable Time-Overcurrent Elements (51)
describes the 51S elements in detail. Use these elements to select the input
current (operating quantity), pickup threshold, type of time-current curve
(both ANSI and IEC curves supported), and time dial. The elements provide
also for torque control and the ability to emulate the reset of
electromechanical relays.
There are as many as 10 available 51S elements, labeled 51S01–51S10. These
elements are single-phase elements. Use three 51S elements to provide phase
time-overcurrent differential protection, and use two more for negative-
sequence and zero-sequence differential time-overcurrent protection as
necessary.
The operating quantities 51O01–51O10 include differential current
measurements per Table 3.12. Enter the pickup threshold settings,
51P01–51P10, in per unit values of the 87L function when you select the
elements to work with the differential current as the operating quantity.
Note that the differential operating quantities are filtered fundamental
frequency components, so the 51S elements coordinate naturally with
microprocessor-based relays protecting the tapped load network. Coordination
with fuses and electromechanical relays may be less accurate. Fuses and
electromechanical relays respond to dc components in the short-circuit
currents, so they time out faster if decaying dc components are present in the
currents than do the upstream 51S elements in the relay that use filtered
quantities. As a result, the application yields an extra time margin but does not
miscoordinate with fuses or electromechanical relays.
The 51S elements assert the 51T01–51T10 Relay Word bits upon time-out.
Use these bits for tripping as necessary.
Note that only relays operating in the master mode calculate the differential
current and use the 51S elements effectively for differential time-overcurrent
protection. The direct transfer tripping functionality embedded in the 87L
function allows the 51S elements to send a direct trip request to the outstation
relays. The E51DTT enable setting controls this operation, as the 87DTT
Direct Transfer Tripping Logic explains.
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