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2.8 Thermal overload protection T>
(49)
2 Protection functions
Technical description
68
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VM265.EN011
Recorded values of the undirectional earth fault stages (8
latest faults) I
0
>, I
0
>>, I
0
>>>, I
0
>>>> (50N/51N)
Parameter
Value
Unit Description
yyyy-mm-dd
Time stamp of the recording, date
hh:mm:ss.ms
Time stamp, time of day
Flt
pu
Maximum earth fault current
EDly
%
Elapsed time of the operating time
setting. 100% = trip
SetGrp
1
2
Active setting group during fault
2.8.
Thermal overload protection T> (49)
The thermal overload function protects the transformer against
excessive temperatures.
Thermal model
The temperature is calculated using rms values of phase
currents and a thermal model according IEC 60255-8. The rms
values is calculated using harmonic components up to the 15
th
.
Trip time:
2
2
2
2
ln
a
I
I
I
t
P
Alarm:
alarm
I
k
k
a
e
mod
(Alarm 60% = 0.6)
Trip:
e
I
k
k
a
mod
Release time:
2
2
2
ln
I
a
I
C
t
P
Trip release:
e
I
k
a
mod
95
.
0
Start release:
alarm
I
k
a
e
mod
95
.
0
(Alarm 60% = 0.6)
T
= Operation time
= Thermal time constant tau (Setting value)
ln
= Natural logarithm function
I
= Measured rms phase current (the max. value of
three phase currents)
I
p
= Preload current,
n
P
I
k
I
(If temperature
rise is 120%
2
.
1
). This parameter is the
memory of the algorithm and corresponds to the
actual temperature rise.
k
= Overload factor (Maximum continuous current), i.e.
service factor. (Setting value)
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