Schweitzer Engineering Laboratories SEL-587Z - Table 3.5 Temperature Parameters (IEEE C57.91: 1995)

3.32
SEL-587Z Instruction Manual Date Code 20020903
Relay Elements
Demand Ammetering
Calculate the transformer HV full-load secondary current.
Equation 3.28
where:
I
FL
= Full-load current
V
HV
= High-voltage side line-to-line voltage
CTR
HV
= High-voltage side current transformer ratio
MVA = Transformer rating
Calculate the PDEMP setting.
Assume continuous overload conditions may not exceed 10 percent,
and thus set the threshold to 10 percent higher than full load:
Equation 3.29
Consider the following numerical example.
From the IEEE temperature curve
Equation 3.30
where:
θ
CHAR
= current at time t
I
FIN
= final current value (full load)
θ = time constant (120 minutes)
t = time of measurement
Calculating the current value after 120 minutes (1 time constant), with
full-load current applied, yields the value in Equation 3.31.
Equation 3.31
The thermal demand element is calculated as shown in Equation 3.32
on page 3.33.
Table 3.5 Temperature Parameters (IEEE C57.91: 1995)
Setting
THwr = 55°C THwr = 65°C
CS = 1 CS = 2 CS = 3 CS = 1 CS = 2 CS = 3
OTR (hours) 3.0 2.0 1.25 3.0 2.0 1.25
I
FL
MVA 10
3
•
3V
HV
CTR
HV
• •
----------------------------------------------------=
50 10
3
•
3 132 200• •
--------------------------------------=
1.09 A=
PDEMP 1.1 I
FL
• =
PDEMP 1.1 1.09• =
PDEMP 1.2 A=
θ
CHAR
I
FIN
1e
t–
τ
----
–
èø
ç÷
æö
• =
θ
CHAR
1.09 1 e
120–
120
------------
–
èø
ç÷
æö
• =
0.69 A=