Schweitzer Engineering Laboratories SEL-587Z, Instruction Manual

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3.5
Date Code 20020903
SEL-587Z Instruction Manual
Relay Elements
High-Impedance Protection
Figure 3.6 CT Characteristic.
This is a very pessimistic value, because the current transformer completely
saturates only under extreme conditions. Comparing the calculated value with
the CT knee-point voltage,
Figure 3.6 shows that 178 V is well below the
knee-point voltage of 432 V.
Set the voltage elements of the high-impedance relay for the maximum
possible voltage developed across the elements for an external fault as
follows:
Equation 3.3
where
V
s
= voltage setting
K = a safety factor, representing the necessary security
level and CT performance
For this example, choose K as 150 percent. Follow your company practices for
setting this safety factor in your particular application.
Equation 3.4
Set the high-impedance elements 87A1P = 87B1P = 87C1P = 267 V.
Minimum
Internal Fault
Operating Current
Use the following equation to calculate the minimum primary current required
to operate the relay for an internal fault:
Equation 3.5
0.01 0.10 1.00 10.00 100.00
10
100
1000
Se
co
nda
ry
R
M
S
Ex
citing
Vo
lts
Secondary RMS Exciting Amperes
V
s
K V
r
• =
V
s
1.5 V
r
• =
1.5 178 • =
267 V =
I
min
n I
e
I
r
I
m
+ + •
( )
N • =