P.3.77
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
The CT ratio is the maximum ratio of the local current inputs to the 87L
function, as we have configured these inputs with the 87CTWL and 87CTXL
settings (see Configuration of the 87L Current Inputs for more details).
Note that, because of potentially different PT and CT ratios at various relay
terminals of the same line, the 87CCB1 and 87CCB0 settings may differ
between relays protecting the same line.
EXAMPLE 3.1
Assume that a dual breaker application (87CTWL = 1, 87CTXL = 1)
uses the Y voltage terminal for compensation (87LINEV = Y). The
instrument transformer ratios are as follows: PTRY = 2165,
CTRW = 400, CTRX = 320. The primary line susceptance is 2.00 mS.
The secondary value we need as a setting is 87CCB1 = 2.00 •
2165/maximum (400, 320) = 10.83 mS.
87CCN
This setting specifies the number of relays in the 87L scheme that you
configured to perform the line charging current compensation. The relay
monitors how many relays in the scheme actually compensate for the charging
current, and it compares this number with the value this setting specifies.
Based on the number of relays actually providing compensation, the 87CCB,
87CCD, and 87CCU Relay Word bits assert as necessary to indicate the
quality of compensation.
87TAPCC
This read-only setting is the tap value the relay uses to scale its local
calculated charging current to per unit values for use in the 87L function.
The relay calculates the tap value as follows (in applications without in-line
transformers).
87TAPCC = CTBASE/CTR_LOC
Equation 3.35
EXAMPLE 3.2
Refer to Example 3.1 and assume a two-terminal application. The
remote terminal is a single breaker terminal with a CT ratio of 600.
All CTs in the scheme are 5 A nominal.
At the local relay, you should enter 87CTP1R = 600 • 5 A = 3000 A
(maximum CT primary of the remote relay).
The local relay determines the following:
➤ 1 pu CTBASE as maximum (400 • 5 A, 320 • 5 A, 3000 A) =
3000 A (1 pu = 3000 A primary),
➤ CTR_LOC as maximum (400, 320) = 400, and
➤ 87TAPCC as 3000/400 = 7.5.
With PTRY = 2165 and a secondary susceptance defined by
PTRY = 2165, CTRW = 400, and CTRX = 320, the relay calculates
charging current from the secondary voltage. Before the relay can
use this current in the 87L function, however, it must first divide it by
7. 5 .
where: CTBASE defines the 1 pu of the 87L zone as
Configuration of the 87L Current Inputs explains
CTR_LOC is the greater of the local current terminal CT
ratios that the 87L function uses.
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