MarelliMotori MEC-100 Manuel De L'utilisateur page 68

V
V
NOM
0
3.7.2. Overexcitation limiter
Maximum
Current
Maximum
Continuative
Current
The MEC-100 is capable of reducing the excitation current when this last reaches such a value as to cause the overheating of
the exciter field. When this function is active (the activation results from the relevant authorization) and a field over-current takes
place, the field current value is decreased to a safety value, within a pre-established time interval, which can be derived from the
curve shown in Fig. 3.7.2.a. This characteristic curve is calculated starting from the determination of a maximum current level
which can never be exceeded (with value included between 0 and 25A, minimum increase of 0.1A), a minimum operation time
value (0 to 10s, minimum increase of 0.1s) and a maximum field current value which the MEC-100 can support continuously
without the activation of the relevant protection (0 to 15A, minimum increase of 0,1A). When the field current value exceeds the
maximum continuative current value, the field over-current limitation is activated after a time interval depending on the field
current value which has been reached, according to the curve of Fig. 3.7.2.a. Higher the over-current, shorter the operation
time. The activation of this limitation consists in reducing the field current until the maximum continuative current value is
reached. This current value is maintained until both the following conditions are simultaneously satisfied:
•
Enough time is passed to eliminate the generator overheating.
•
The operating conditions set the excitation current value required to the MEC-100 below the maximum continuous current
value.
The activation of this limitation is accompanied by a visual warning in the MEC-100 Interface System (Overexcitation Limiter
item flashing, see Par. 5.8.3), and optionally can be associated to one of the two programmable output relays. This function can
be enabled/disabled:
•
If enabled, it can operate in all working Modes.
•
Even if disabled, the MEC-100 limits the maximum excitation current which can be supplied to the maximum allowed set
value.
963857137_M
f f
ZV C
Generator Voltage Setpoint in Under-frequency Conditions
I
Time
Delay
Overexcitation Limiter Curve
f f
NOM
Fig. 3.7.1.a
Fig. 3.7.2.a
MEC-100 – Marelli Motori
Rated Voltage
V
NOM
Zero Volt Frequency
f
ZV
f Corner Frequency
C
f Rated Frequency
NOM
t
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