Limiting Functions - MarelliMotori D-Vo Light Manuel D'utilisation

5.6.

LIMITING FUNCTIONS

D-Vo provides three functions which limit the field current of the exciter, in order to protect the generators against
any operating conditions that can prejudice its reliability.
LIMITER
Underfrequency (UF)
Overexcitation (OEL)
9638571A0_C
DESCRIPTION
D-Vo reduces the excitation current every time the generator is used at low speed,
to prevent any damages to the generator excitation system: in particular, the
voltage setpoint is automatically modified and decreased as soon as the
generator frequency drops below a set value, according to the curve shown in the
figure.
V
NOM
f
ZV
The parameters determining the curve, and in particular its slope, are:
•
the Corner Frequency, f
•
the Zero Volt Frequency, f
where the setpoint becomes zero.
The switching of the limitation function to active state is signalled by a visual
warning in D-Vo Dashboard.
The function operates in AVR Mode and is always enabled.
D-Vo can limit the exciter field current when it reaches a value that would cause
the exciter to overheat. When this function is active and an exciter field
overcurrent occurs, the exciter field current value is brought back to a safe value
within a preset time, which can be inferred from the curve shown in the following
figure.
The characteristic shown in the figure is calculated starting from the setting of a
maximum allowed field current level, I
tripping time value, T , and of a maximum current value, I
D
continuously withstand with no protection tripping. The higher the overcurrent, the
lower the tripping time.
The tripping consists of decreasing the field current to the maximum continuous
value, which is maintained until both the following conditions are met:
•
The operating conditions bring the excitation current value requested from the
D-Vo below the maximum continuous current value.
•
Enough time has elapsed for the generator overheating to have ceased.
The switching of the limitation to active state is signalled by a visual warning in D-
Vo Dashboard, and can be optionally associated with one of the three
programmable output relays.
When the limiter exits the active state in accordance with the conditions listed
above, the limitation, the relevant alarm and the associated relay, if any, shall be
automatically reset.
D-Vo Light – Marelli Motori
Underfrequency limiter
f
C
.
C
, which represents the frequency at the point
ZV
, which can never be exceeded, of a
EXC_MAX
f
frequency
NOM
, which D-Vo can
EXC_C
EN 32