c) Phase Rotation [optional]
The AVS can detect a phase rotation error of the three phase mains supply. Detection of parameters
c) and d) above is not standard, but are obtained by an optional plug-in board. On this board, it is
possible to select by a d.i.l. switch whether abnormality is indicated only, or it causes disconnection
also.
4.3.2 Principle of Operation
The frequency and phase rotation detection circuits are explained in a separate section. The detailed
operation of the AVS in detecting the other parameters is given under CIRCUIT DESCRIPTION below.
Basically, however, the AVS compares the peak of the mains AC sinusoid of each phase with two
references, one corresponding to the lower or undervoltage limit of the window, and the other to the
upper or over-voltage limit. If the mains is normal, so that the peaks lie between the two limits and
also within a time not exceeding T/3 (T is the period of one cycle), a monostable is triggered which,
after the wait time, switches the power to the equipment. If any one or more of the peaks are below
the lower limit, above the upper limit or the separation between two consecutive peaks exceeds T/3,
the AVS is reset to disconnect the equipment.
4.3.3 Checks and adjustments
a) Window Limits
P1 and P2 are adjusted to equalise the three phases, so that P1 adjusts the peak at the junction of
P1 and R12, and P2 at the junction of P2 and R20 to make them equal to the peak at the junction of
R2 and R3. For measurement, an ordinary multi-meter or digital multi-meter may be used on the
AC range, since these give readings proportional to peak.
P3 and P4 adjust the limits of the window. Start with these around the centre of their travel.
Connect the normal three phase supply to the AVS with one phase via a Variac and monitor voltage
with voltmeter. Adjust Variac to the under-voltage limit. Adjust P4 so that indication goes from
Red to Amber. Adjust Variac to over-voltage limit. Adjust P3 so that indication fluctuates between
Amber and Red.
If the Variac is set so that the voltage is within the window, with Amber indicating, after the wait
time (nominal 1 minute) Green will indicate and the contactor is energised.
For a complete check, three Variacs should be used, one on each phase, and the various
combinations of under- and over-voltage on each phase with the others tested.
b) Wait Time
The wait time is given by 0.7xR37xC6. With R37 = 820K and C6 = 100uF, the wait time is around 60
sec. to within the tolerance of the components.
4.4 -HA Option
This option is available on all ratings of the AVR (Automatic Voltage Regulator) three phase units larger
than 21kVA.
The standard Three Phase AVR provides an output which is stable to within + 4% given an input
voltage variation of + 27% from a defined nominal. Although it is likely that voltage stability of + 4%
will meet most customers’ requirements, higher accuracy can be provided by incorporating a further
‘fine’ resolution stage beyond the standard AVR system.
The standard AVR incorporates a fully electronic (static) 7-tap changing system providing an output
regulated to + 4%. This is fed to the -HA option which utilises a further 7 taps, again fully electronic, to
achieve an output stability of + 2.0%.
THREE PHASE AUTOMATIC VOLTAGE REGULATOR Issue: July 2021 ENGLISHENGLISH THREE PHASE AUTOMATIC VOLTAGE REGULATOR Issue: July 2021
A watchdog function is implemented in the micro controller. This independently monitors the operation
of the micro-controller and its software. If it detects a malfunction, it will reset the micro and
re-initialise the control system.
The low voltage DC supply to the control circuit is also protected by a fuse.
Additionally, a hardware reset circuit is included which monitors the supply rail for the control circuit.
If the mains is so low that the control circuit will not function correctly, the monitor circuit will put the
micro-controller into the reset state and turn off all Triacs.
When the mains supply increases to a usable level, the monitor circuit will restart the micro and the
system will re-initialise. This ensures an orderly and controlled restart from a brownout or blackout
condition. The circuit is designed with a large hysteresis so that the unit will not attempt to turn
on again until the supply voltage is sufficient to withstand possible starting surges. This avoids the
possibility of such a surge of current causing the supply to dip sufficiently to turn the unit off again.
Additional protection is provided by temperature sensors fitted to each transformer. If the AVR is
used at full load and either the ambient temperature is excessively high or the ventilation grills have
been obstructed, the temperature of the transformer may increase beyond reasonable limits. In such
an event, the temperature sensor will disconnect the supply to the corresponding control board and
thereby turn the output off. When the transformer has cooled sufficiently, the sensor will restart the
AVR.
When restarting after the above condition the AVR may cause equipment to begin to operate suddenly.
Steps should be taken to ensure that this does not expose persons to risk.
4.3 AVS Function [Optional - has to be ordered separately at time of purchase]
4.3.1 General Description
The Automatic Voltage Switcher (AVS) is a device for the protection of electrical equipment against
fluctuations, interruptions and other abnormalities in the electricity mains supply.
The Three Phase AVS monitors various parameters of the mains supply, and keeps it connected to
the equipment so long as all the parameters are within defined acceptable limits. This is the normal
condition and it is indicated by a Green LED (light emitting diode). If the mains voltage goes outside
these limits, the AVS disconnects the equipment from the mains and this is indicated by the Red LED
(In some options, it is possible to select indication only without disconnection's.) When the mains
supply returns within the acceptable limits, indicated by an Amber LED, the mains remain disconnect
from the equipment during the wait time, set to a nominal 1 minute by factory selected components. If
during the wait time the mains again goes outside the limits, the wait time starts from the beginning
At the end of the wait time, when the mains supply has been continuously within the limits for its
duration, normal condition returns indicated by the Green LED, and the equipment is re-connected to
the mains.
The parameters monitored by the Three Phase AVS are:
a) Value of the Mains Voltage
The normal condition is when the values of the mains voltage of all the phases are within certain
preset limits referred to as the "window". The AVS detects when the voltage of any one or more
phases goes outside the window, either over- or undervoltage.
b) Phase Relationship (timing)
The AVS monitors the phase relationship between the three phases of the supply. The normal
condition is when the phase difference between the three phases is 120 degrees, corresponding to
T/3 where T is the period of one cycle.
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