2.9 Thermal Overload Protection (ANSI 49)
63
7UM61 Manual
C53000-G1176-C127-3
Example: Generator and current transformer with the following data:
Permissible Continuous Current I
max prim
= 1.15 · I
N, Machine
Generator Nominal Current
I
N Machine
= 483 A
Current Transformer
500 A / 1 A
Time Constant
τ
Overload protection tracks overtemperature progression, employing a thermal differ-
ential equation whose steady state solution is an exponential function. The
TIME
CONSTANT
τ
(address
1603
) is used in the calculation to determine the threshold of
excessive temperature and thus, the tripping temperature.
If the overload characteristic of the generator to be protected is pre-determined, the
user must select the protection trip characteristic so that it largely corresponds the
overload characteristic, at least for small overloads.
This is also the case if the admissible power-up time corresponding to a certain over-
load value is indicated.
Alarm Stages
By setting the thermal alarm level
Θ
ALARM
(address
1604
), a alarm message can be
issued before the tripping temperature is reached, thus avoiding tripping by promptly
reducing load. This alarm level simultaneously represents the dropout level for the trip-
ping signal. The tripping signal is interrupted only when this threshold value is again
undershot.
The thermal alarm level is given in % of the tripping overtemperature level.
Note:
With the typical value of
K-FACTOR
= 1.1, on application of nominal machine
current and adapted primary transformer current, the following final tripping overtem-
perature results
of the tripping temperature. Consequently, the alarm stage should be set between the
final overtemperature with the nominal current (in this case 83 %) and the tripping
overtemperature (100 %).
In the present example, the thermal memory reaches the following value if the nominal
current is applied:
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