SECTION 2
PRINCIPLE OF OPERATION
The following description makes reference to the block diagram
shown in Fig 2 ( Principle of Operation ).
Signals representing generator voltage and current are fed into
the PFC3 via isolation transformers and are used for various
tasks within the unit.
Two measuring circuits (the Active Current Transducer and the
Reactive Current Transducer) use both voltage and current
signals to produce internal dc references proportional to
generator kW and kVAr. The voltage input is also used to provide
internal power supplies and further internal circuits measure the
incoming voltage for voltage matching purposes.
The PFC3 is capable of operating in a variety of modes with
internal switching circuits routing the necessary control signals to
the main comparator/amplifier.
2.1 Power Factor Control Mode
In the 'Power Factor' mode of operation, the measured reactive
current is compared to a proportion of the measured active
current as set on the VAr/PF control potentiometer. If the
measured reactive current rises above the 'kW' signal, then the
PFC3 acts to reduce the internal set-point of the AVR (via
terminals A1 and A2) and brings the generated VAr (and hence
power factor) to the correct controlled level. If the measured
reactive current is lower than required then the reverse action
takes place. Under these conditions, closed-loop control of
generator power factor is achieved.
2.2 VAr Control Mode
In the 'VAr' mode of operation, the measured reactive current is
compared to a reference level derived from the setting on the
PF/VAR control potentiometer. If the measured reactive current
rises above this set-point, then the PFC3 acts to reduce the
internal set-point of the AVR (via terminals A1 and A2) and brings
the generated VAr to a controlled level. If the measured VAr is
lower than the demand then the reverse action takes place.
Under these conditions, closed-loop control of generated reactive
current is achieved.
The PFC3 incorporates circuits to provide a level of protection to
the generator not normally encountered in small/medium sized
installations.
2.3 Generator Current Limiting
Under normal circumstances the sizing of the generator set will
be such that the kW transfer to the mains (and hence the
generated VAr) will be limited by the engine and its fuel system.
In applications using the generator as power factor 'correction' of
the incoming mains this limiting does not apply and the level of
VAr demanded is a function of the feeder load. This can be many
times the rating of the generator set (particularly under fault
conditions) and must be protected against.
The PFC3 incorporates the necessary circuits to implement
generator current limiting. The output of a separate reactive
current measuring transducer is fed to the input of the main
comparator/amplifier in such a way as to override normal PF or
VAr control should excess current be detected. With this feature
connected, normal power factor (or VAr) control is provided
UNTIL the set current limit is reached, at which point the output to
the AVR restricts further increases in exported (or imported)
generator current. The level at which current limiting applies is set
using the appropriate circuit board control.
2.4 Low Excitation Limiting
In applications requiring the generator to operate at leading
power factor, the level of VAr imported may cause the generator
to reach an unstable region due to self excitation. This can result
in the loss of synchronisation and the associated large circulating
currents may cause circuit breakers to trip or even equipment
damage.
The PFC3 incorporates the necessary circuits to implement low
excitation limiting. The output of a separate excitation voltage
measuring transducer is fed to the input of the main comparator/
amplifier in such a way as to override normal PF or VAr control
should low excitation be detected. With this feature connected,
normal power factor (or VAr) control is provided UNTIL the set
low excitation limit is reached, at which point the output to the
AVR restricts further increases in imported generator current. The
level at which low excitation limiting applies is set using the
appropriate circuit board control. The circuit is automatically
disabled if left unconnected.
The PFC3 incorporates two more features which provide the user
with the choice of additional operating modes not normally
encountered in small/medium sized installations.
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