P.3.324
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
Protection Application Examples
We would have the following at Terminal R:
Equation 3.112
The relay will generate the following values.
We would have the following at Relay S:
87LTAPW := 6.25 Tap Value of Terminal W (5.00–50)
87LTAPX := 6.25 Tap Value of Terminal X (5.00–50)
We would have the following at Relay R:
87LTAPW := 5.00 Tap Value of Terminal W (5.00–50)
87LTAPX := 5.00 Tap Value of Terminal X (5.00–50)
CT Performance for External Faults
The steps for determining CT performance are as follows:
➤ Calculate the current (IF) for an external fault. Include both
phase faults and ground faults.
➤ Calculate the system X/R ratio.
➤ Use Equation 3.115 to determine whether you can expect CT
saturation.
➤ In the case that saturation is possible and the CTs comprising
the zone have different characteristics, ratios, or burdens, use
simulation tools to evaluate the CT performance.
For this example, assume an external ground fault at S. This location will
produce the largest secondary fault current. Determine the secondary
current at R from the following:
Equation 3.113
where ZT = (3 • ZR + 2 • Z1L _ Z0L).
We can calculate the system X/R ratio as follows:
Equation 3.114
We can use Equation 3.115 to determine CT performance. Saturation will not
occur if Equation 3.115 is satisfied.
Equation 3.115
where,
Z
burden
is the total loop impedance of the CT secondary circuit for the
particular fault type.
V
S
is the saturation voltage of the CT.
TAP
CTP
BASE
CTR
LOCAL
-----------------------------
2000
400
------------ 5.00===
I
1F
3V
LN–
•
ZT
-------------------------
1
CTRR
----------------
• 10.2 A secondary==
XR
Im ZR Z1L+
Re ZR Z1L+
------------------------------------- 28==
Z
burden
V
S
I
F
XR 1+•
--------------------------------------
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