3.33
Date Code 20020903
SEL-587Z Instruction Manual
Relay Elements
Demand Ammetering
Equation 3.32
where:
DMTC = time constant
ln
= natural logarithm
After five time constants, the results from both equations are equal:
Equation 3.33
Calculate the time when PDEMP will assert for a 15 percent (1.25 A)
overload, starting with zero element register values and a full-load
step-function current flowing:
Equation 3.34
PDEMP
I
FL
1
e
10
( )
t
•
ln
–
DMTC
----------------------------
–
è
ø
ç
÷
æ
ö
•
=
1.09
1
e
10
120
•
ln
–
120
------------------------------
–
è
ø
ç
÷
æ
ö
•
=
0.98 A (90 percent of 1.09 A)
=
PDEMP
θ
=
CHAR
1.2
1
e
600
–
120
------------
–
è
ø
ç
÷
æ
ö
•
=
1.2
1
e
10
600
•
ln
–
120
------------------------------
–
è
ø
ç
÷
æ
ö
•
=
1.2 A
=
t
DMTC
–
10
( )
ln
---------------------
1
PDEMP
I
FL
---------------------
–
è
ø
æ
ö
•
=
120
–
10
( )
ln
----------------
1
1.2
1.25
----------
–
è
ø
æ
ö
ln
•
=
168 min (or 1.4 time constants)
=
Summary of Contents for SEL-587Z
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