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Example: Generator and current transformer with the following data:
Permissible Continuous Current I
Generator Nominal Current
Current Transformer
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.
By setting the thermal alarm level Θ ALARM (address 1604), a alarm message can be
Alarm Stages
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:
7UM61 Manual
C53000-G1176-C127-3
2.9 Thermal Overload Protection (ANSI 49)
= 1.15 · I
max prim
N, Machine
I
= 483 A
N Machine
500 A / 1 A
63
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