Sel 411L - Figure 3.10 a Combined Transformer and Line Zone Protected with a Single Line Differential Relay

P.3.16
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Theory of Operation
sources become unreliable, the relay responds in an adaptive fashion. Some
relays in the 87L scheme may suspend their compensation because of the open
pole or loss of potential conditions, but other relays in the scheme with healthy
voltage will continue compensating. The effect of compensation is global and
applies to all relays, including these with no voltage. The 87L function is
therefore not exposed to problems with voltage inputs, as long as there is at
least one healthy voltage it can use for compensation. A fallback mode in the
relay causes the function to switch to alternate settings in case of problems
with compensation.
In-Line Transformers
The 87L function accommodates in-line power transformers as in
Figure 3.10 (a). We might encounter such an application typically where there
is no circuit breaker (CB) present to separate the line and the transformer
under fault conditions; the differential function protects both the transformer
and the line. The 87L function performs vector group, ratio, and zero-
sequence compensation per the art of transformer protection. It provides for
overexcitation inhibit as well as harmonic blocking and/or restraint for
magnetizing inrush conditions.
This application does not provide a full set of transformer protection features
such as would be the case for a dedicated transformer protection relay shown
in Figure 3.10 (b). This application provides better protection for the
transformer, improves operational experience, and allows autoreclosing by
identifying faults in the transformer and line, with accurate fault location for
the latter. You can use the line differential relay to execute direct transfer
tripping (DTT) of the remote CB(s) from the transformer relay.
As Figure 3.10 (c) illustrates, you can use the combined transformer and line
application to provide backup or to protect transformers in very large
substations with very long CT leads between the control enclosure and the
breaker CTs. A full-featured transformer relay protects the transformer using
the bushing CTs.
Figure 3.10 A Combined Transformer and Line Zone Protected With a Single
Line Differential Relay Capable of Handling In-Line Transformers (a), With
Dedicated Transformer and Line Relays (b), and With the Primary Protection
21
DTT
21
87
L+T
87
L+T
87
L+T
87
L+T
87L Communications channel
(c)
(b)
(a)
87L Communications channel
21 Communications
channel
87
T
87
T
87
L
87
L
DTT
87L Communications
channel