DESING DETAILS
The main functions of the AVR are:
Sensing Resistors take a proportion of the generator output
voltage and attenuate it. This input chain of resistors includes
the range potentiometer and hand trimmer which adjust the
generator voltage. An isolating transformer allows connection
to windings of different polarity and phase. An operational
precision rectifier converts the ac sensing voltage into dc for
further processing.
Quadrature droop circuit converts the current input into a
voltage, which is phase mixed with the sensing voltage. The
result is a net increase in the output from the sensing network
as the power factor lags, causing the reduction in excitation
needed for reactive load sharing of parallelled generators. A
trimmer allows control over the amount of droop signal.
Sensing loss detector is an electronic changeover switch
which normally connects the main comparator/amplifier to the
"input sensing" network, and automatically changes over to
the "power sensing" network when the normal sensing voltage
is lost.
DC Mixer provides an interface between the AVR and
accessories and allows the generators excitation to be
controlled by adding or subtracting the accessory dc output
voltage to the AVR rectifier sensing voltage.
Main Comparator/Amplifier compares the sensing voltage to
the "reference voltage" and amplifies the difference (error) to
provide a controlling signal for power devices. The "pedestal
and ramp" circuit and "level detector and driver" provide the
means to infinitely control the conduction period of the output
device over each half cycle (phase control), and provide the
exciter with the required power to maintain the generator
voltage within the specified limits.
The "Stability circuit" provides adjustable negative ac
feedback to ensure good steady state and transient
performance of the control system.
Low Hz detector measures the period of each electrical cycle
and causes the reference voltage to be reduced
approximately linearly with speed below a presettable
threshold. A light emitting diode gives indication of
underspeed running.
Synchronising circuit provides a short pulse at the zero
crossing of each cycle and is used to synchronise the
underspeed and pedestal and ramp circuit to the generator
waveform. The circuit is preceded by a "low pass filter" to
prevent false zero crossing pulses caused by severely
distorted waveforms.
Power Devices are configured as half-wave thyristor and
freewheel diode to vary the amount of exciter field current in
response to the error signal produced by the main
comparator.
Suppression components are included to prevent sub cycle
voltage spikes damaging the AVR components and also to
reduce the amount of AVR thyristor noise on the main
terminals of the generator.
Power Supply components consist of zener diodes with
dropper resistor and smoothing to provide the required
voltages for the integrated circuits and reference voltage.
The AVR is fully encapsulated to ensure long trouble-free
operation. It is usually fitted on a panel of the terminal box. It
can also be separately fitted in a switchboard.
STABILITY
CIRCUIT
REFERENCE
VOLTAGE
LOW
Hz DETECTOR
SYNCHRONISING
CIRCUIT
POWER
SUPPLY
POWER
CONTROL
DEVICES
SUPPRESSION
EXCITER
STATOR
GENERATOR
SENSING
RESISTORS
PEDESTAL AND
RAMP CIRCUIT
LEVEL DETECTOR
AND DRIVER
MAIN COMPARATOR
AM PLIFIER
LOW PASS
FILTER
VOLTAGE
SENSE
VOLTAGE
TRIM
POWER
SENSING
CURRENT
INPUT
DC MIXER
DROOP
ACCESSORY
INPUT
SEN SING
LO SS D E TE C TO R
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