Page 290 - Introduction to Marine Engineering
P. 290
264 Electrical equipment
Alternating current supply
Three-phase alternators arranged for parallel operation require a
considerable amount of instrumentation. This will include ammeters,
wattmeter, voltmeter, frequency meter and a synchronising device. Most
of these instruments will use transformers to reduce the actual values
taken to the instrument. This also enables switching, for instance,
between phases or an incoming machine and the bus-bars, so that one
instrument can display one of a number of values. The wattmeter
measures the power being used in a circuit, which, because of the power
factor aspect of alternating current load, will be less than the product of
the volts and amps. Reverse power protection is provided to alternators
since reverse current protection cannot be used. Alternatively various
trips may be provided in the event of prime mover failure to ensure that
the alternator does not act as a motor.
The operation of paralleling two alternators requires the voltages to
be equal and also in phase. The alternating current output of any
machine is always changing, so for two machines to operate together
their voltages must be changing at the same rate or frequency and be
reaching their maximum (or any other value) together. They are then
said to be 'in phase'. Use is nowadays made of a synchroscope when
paralleling two a.c. machines. The synchroscope has two windings which
are connected one to each side of the paralleling switch. A pointer is free
to rotate and is moved by the magnetic effect of the two windings. When
the two voltage supplies are in phase the pointer is stationary in the 12
o'clock position. If the pointer is rotating then a frequency difference
exists and the dial is marked for clockwise rotation FAST and
anti-clockwise rotation SLOW, the reference being to the incoming
machine frequency.
To parallel an incoming machine to a running machine therefore it is
necessary to ensure firstly that both voltages are equal Voltmeters are
provided for this purpose. Secondly the frequencies must be brought
into phase. In practice the synchroscope usually moves slowly in the FAST
direction and the paralleling switch is closed as the pointer reaches the
11 o'clock position. This results in the incoming machine immediately
accepting a small amount of load.
A set of three lamps may also be provided to enable synchronising.
The sequence method of lamp connection has a key lamp connected
across one phase with the two other lamps cross connected over the
other two phases. If the frequencies of the machines are different the
lamps will brighten and darken in rotation, depending upon the
incoming frequency being FAST or SLOW. The correct moment for
synchronising is when the key lamp is dark and the other two are equally
bright.