Sel 411L - Table 3.12 51 S Operating Quantities Related to the Differential Current

P.3.44
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
Stub Bus Condition
Stub bus configuration refers to a condition where the protected line is
separated from the CT (or CTs) terminating its zone of protection by an
opened line disconnect switch (see 87L Theory of Operation for more
information). In general, we address the stub bus configuration by ensuring
dependable protection for the line and for bus work between the line CT (or
CTs) and the disconnect switch.
The 87L function of the relay accepts the stub bus status the ESTUB SEL
OGIC
control equation provides and uses this status to ensure secure and dependable
protection for the line and for bus work at the line terminal which the stub bus
is in effect.
Effectively, the 87L function splits the differential zone of protection into two
differential zones if the ESTUB SEL
OGIC control equation is asserted.
Specifically, the following takes place.
➤ The relay substitutes the local currents it transmits to remote
relay peers with zeroes, reflecting that the line disconnect
switch is opened and that local current into the line zone is
zero. This operation causes the remote 87L function (or
functions in three-terminal and four-terminal applications) to
operate as expected and its zone of protection to contract to the
opened disconnect switch at the line terminal with stub bus
configuration.
➤ The relay substitutes currents it receives from remote relay
peers with zeroes, reflecting that the line disconnect switch is
opened and that the remote current into the bus work between
the opened line disconnect switch and the line CT (or CTs) is
zero. This operation causes the local 87L function to operate as
expected and its zone of protection to extend from the line CT
(or CTs) to the opened line disconnect switch.
➤ The local relay becomes a master in the stub bus condition,
even if this relay were previously an outstation. In the stub bus
configuration, the 87L function requires no remote currents
and becomes a local differential function protecting the bus
work between the line CT (or CTs) and the opened line
disconnect switch.
➤ If the local relay detects an external fault, the external fault-
detected (EFD) bit asserts and enhances security of the local
87L function. The local relay does not send the bit to the
remote peers, and local external faults have no effect on the
Table 3.12 51S Operating Quantities Related to the Differential Current
Name Description Units
87IAFM Differential current magnitude, A-phase, full-cycle cosine-filtered pu
87IBFM Differential current magnitude, B-phase, full-cycle cosine-filtered pu
87ICFM Differential current magnitude, C-phase, full-cycle cosine-filtered pu
87I1FM Positive-sequence differential current (I1) magnitude, full-cycle
cosine-filtered
pu
87IQFM Negative-sequence differential current (3I2) magnitude, full-cycle
cosine-filtered
pu
87IGFM Zero-sequence differential current (3I0) magnitude, full-cycle
cosine-filtered
pu