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3 Operation Theory
47
50/51P4.C
48
50/51P4.Alpha
49
50/51P5.I_Set
50
50/51P5.t_Op
51
50/51P5.En
52
50/51P6.I_Set
53
50/51P6.t_Op
54
50/51P6.En
55
50/51P.I_CCE
56
50/51P.En_CCE
3.4 Thermal Overload Protection
3.4.1 Thermal Overload Protection Theory
The relay incorporates a current based thermal replica, using load current to model heating and
cooling of the protected plant.
The heat generated within an item of the plant, such as a cable or a transformer, is the resistive
2
loss (Ι
R×t). Thus, heating is directly proportional to current squared. The thermal time
characteristic used in the relay is therefore based on current squared, integrated over ti me. The
relay automatically uses the largest phase current for input to the thermal model.
Equipment is designed to operate continuously at a temperature corresponding to its full load
rating, where heat generated is balanced with heat dissipated by radiation etc.
This relay provides a thermal overload model which is based on the IEC60255-8 standard. The
thermal overload formulas are shown as below.
Criterion of cooling start characteristic:
Criterion of hot start characteristic:
3-16
Constant "C" for the stage 4 IDMT overcurrent
protection, see Section 3.3.2
Constant "α" for the stage 4 IDMT o vercurrent
protection, see Section 3.3.2
The current s etting of the stage 5 overcurrent
protection
The time setting of the stage 5 overcurrent
protection
The logic setting of the stage 5 overcurrent
protection
The current s etting of the stage 6 overcurrent
protection
The time setting of the stage 6 overcurrent
protection
The logic setting of the stage 6 overcurrent
protection
The current setting of the heavy current blocking
element for the overcurrent protection
The logic setting of the heavy current blocking
element for the overcurrent protection
T
ln
2
I
Date: 2014-04-08
2
I
2
(
k
I
)
B
0.00~1.00
0.0001
0.01~3.00
0.0001
0.05In~30In
0.001A
0~100s
0.001s
0~1
1
0.05In~30In
0.001A
0~100s
0.001s
0~1
1
0.05In~30In
0.001A
0~1
1
PCS-9611 Feeder Relay
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