Schweitzer Engineering Laboratories SEL-587Z - Equation 3.6

3.6
SEL-587Z Instruction Manual Date Code 20020903
Relay Elements
High-Impedance Protection
where
I
min
= minimum current
n = number of current transformers in parallel with the
relay, per single phase
I
e
= current transformer exciting current at relay setting
voltage = V
r
I
r
= current through the relay at relay setting voltage = V
r
I
m
= current through the MOV at relay setting voltage = V
r
To achieve better sensitivity for the minimum internal fault current, use CTs
with the highest possible knee voltage to assure small CT exciting current (I
e
).
The SEL-587Z has a built-in high-value resistor (2000 ohm) that provides
good sensitivity with a small relay current (I
r
).
To assure a small leakage current (I
m
) through the MOV at the relay setting
voltage, the relay uses a high-quality device, selected to the highest possible
clamping voltage within the rated insulation parameters.
Exciting Current (I
e
)
The exciting (I
e
) current is ±110 mA as read from Figure 3.6 on page 3.5 and
for a voltage value of 267 V (V
s
).
Current Through the Relay (I
r
)
Use Equation 3.6 to calculate the current through the relay.
Equation 3.6
Current Through MOV (I
m
)
The current through the MOV (I
m
) is 0 A as read from Figure 3.7 on page 3.7
and for a voltage value of 267 V (V
s
).
Thus, we can ignore the current through the MOV.
Minimum Operating Current Calculation
Equation 3.7 provides the total minimum primary current required to operate
the relay.
Equation 3.7
I
r
V
S
R
2000
-------------=
267
2000
------------=
134 mA=
I
min
nI
e
I
r
0++• ()N• =
5 0.11 0.134+•
()400• =
274 A=