MarelliMotori MEC-100 Manuel De L'utilisateur page 67

3.6.7. Diode Failure Monitoring (P.N. M71FA320A)
The MEC-100 is capable of sensing abnormal excitation currents due to the damaging of one or more diodes in the generator
rectifier bridge (interrupted or shorted diode). These currents can lead to the exciter and/or regulator damaging. MEC-100
measures the excitation current (average value) and the width of its ripple.
If a diode is damaged, the excitation current ripple is higher than the one present in correct behaviour conditions, as showed in
Fig. 3.6.7.a. MEC-100 offers two types of protection threshold or failure level: Low Level and High Level of failure. The two
thresholds can be chosen in such a way to discern from a light failure (i.e. interrupted diode) to a heavy failure (i.e. shorted
diode). For example, the two levels can be chosen with the following method:
•
If excitation current ripple is lower than the Low Level of failure, the rectifier bridge can be considered properly working.
•
If excitation current ripple is higher than the Low Level of failure (for a period longer than the set delay) but lower than the
High Level of failure, the Low Level allarm occures. This situation could be associated for example to a light failure (i.e.
interrupted diode) which can not damage the generator in a short period of time, but has to be anyway solved.
•
If excitation current ripple is higher than the High Level of failure (for a period longer than the set delay), the High Level
allarm occures. This situation could be associated for example to a heavy failure (i.e. shorted diode) which can damage the
generator in a short period of time, and has to be solved as soon as possible.
The activation of this protection is accompanied by a visual warning in the MEC-100 Interface System: when Low Level is
reached, Diode Monitoring - Low Level item starts flashing (see Par. 5.8.3) and optionally can be associated to one of the two
programmable output relays. When High Level is reached, Diode Monitoring - High Level item starts flashing (see Par. 5.8.3)
and optionally can be associated to one of the two programmable output relays or to the Shutdown option. This function can be
enabled/disabled.
3.7. LIMITATION FUNCTIONS
3.7.1. Under-frequency Limiter
The MEC-100 reduces the excitation current any time the generator is used at low speed to avoid damages to the generator
excitation system: in particular the voltage setpoint is automatically modified and reduced as soon as the generator frequency
falls below a set value, according to the curve shown in Fig. 3.7.1.a.
The parameters which determine the curve and, in particular, its slope are the following:
•
The Corner Frequency f
which the MEC-100 decreases the voltage setpoint.
•
The Zero Volt Frequency f
the point where the voltage setpoint is zeroed.
The activation of this limitation function is accompanied by a visual warning in the MEC-100 Interface System (Underfrequency
Limiter item flashing, Par. 5.8.3). This function is always enabled and operates in AVR Mode.
963857137_M
Interrupted diode
Time
Excitation current when diode fails.
, which can be set from 40 to 60Hz with increases of 0.1Hz: represents the frequency value below
C
, which can be set from 0 to 40Hz with increases of 0.1Hz: represents the frequency relative to
ZV
Diode works fine
Time
Ripple
Fig. 3.6.7.a
MEC-100 – Marelli Motori
Ripple
Shorted diode
Ripple
Time
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