In an alternating current electric power system,
synchronization is the process of matching the speed and frequency of a
generator or other source to a running network. An AC generator cannot deliver
power to an electrical grid unless it is running at the same frequency as the
network. If two segments of a grid are disconnected, they cannot exchange AC
power again until they are brought back into exact synchronization.
A DC generator can be connected to a power network by
adjusting its open-circuit terminal voltage to match the network voltage by
either adjusting its speed or its field excitation; the exact engine speed is
not critical. However, an AC machine must match both the amplitude and the
timing of the network voltage, which requires both speed and excitation to be
systematically controlled for synchronization. This extra complexity was one of
the arguments against AC operation during the War of Currents in the 1880s. In
modern systems, synchronization of generators is carried out by automatic systems.
Conditions
There are five conditions that must be met before the
synchronization process takes place. The source (generator or sub-network) must
have equal line voltage, frequency, phase sequence, phase angle, and waveform
to that of the system to which it is being synchronized.
Waveform and phase sequence are fixed by the construction of
the generator and its connections to the system. During installation of a
generator, careful checks are made to ensure the generator terminals and all
control wiring are correct so that the order of phases (phase sequence) matches
the system. Connecting a generator with the wrong phase sequence will result in
a short circuit as the system voltages are opposite to those of the generator
terminal voltages.
The voltage, frequency and phase angle must be controlled
each time a generator is to be connected to a grid.
Generating units for connection to a power grid have an
inherent droop speed control that allows them to share load proportional to
their rating. Some generator units, especially in isolated systems, operate
with isochronous frequency control, maintaining constant system frequency
independent of load.
Process
The sequence of events is similar for manual or automatic
synchronization. The generator is brought up to approximate synchronous speed
by supplying more energy to its shaft - for example, opening the valves on a
steam turbine, opening the gates on a hydraulic turbine, or increasing the fuel
rack setting on a diesel engine. The field of the generator is energized and
the voltage at the terminals of the generator is observed and compared with the
system. The voltage magnitude must be the same as the system voltage.
If one machine is slightly out of phase it will pull into
step with the others but, if the phase difference is large, there will be heavy
cross-currents which can cause voltage fluctuations and, in extreme cases,
damage to the machines.
From top to bottom: synchroscope, voltmeter, frequency
meter. When the two systems are synchronized, the pointer on the synchrosope is
stationary and points straight up.
Synchronizing lamps
Formerly, three light bulbs were connected between the
generator terminals and the system terminals (or more generally, to the
terminals of instrument transformers connected to generator and system). As the
generator speed changes, the lights will flicker at the beat frequency
proportional to the difference between generator frequency and system
frequency. When the voltage at the generator is opposite to the system voltage
(either ahead or behind in phase), the lamps will be bright. When the voltage
at the generator matches the system voltage, the lights will be dark. At that
instant, the circuit breaker connecting the generator to the system may be
closed and the generator will then stay in synchronism with the system.
An alternative technique used a similar scheme to the above
except that the connections of two of the lamps were swapped either at the
generator terminals or the system terminals. In this scheme, when the generator
was in synchronism with the system, one lamp would be dark, but the two with
the swapped connections would be of equal brightness. Synchronizing on
"dark" lamps was preferred over "bright" lamps because it
was easier to discern the minimum brightness. However, a lamp burnout at the
wrong time could cause synchronization errors[why?].
Synchroscope
Another manual method of synchronization relies on observing
an instrument called a "synchroscope", which displays the relative
frequencies of system and generator. The pointer of the synchroscope will
indicate "fast" or "slow" speed of the generator with
respect to the system. To minimize the transient current when the generator
circuit breaker is closed, usual practice is to initiate the close as the
needle slowly approaches the in-phase point. An error of a few electrical
degrees between system and generator will result in a momentary inrush and
abrupt speed change of the generator.
Synchronizing relays
Synchronizing relays allow unattended synchronization of a
machine with a system. Today these are digital microprocessor instruments, but
in the past electromechanical relay systems were applied. A synchronizing relay
is useful to remove human reaction time from the process, or when a human is
not available such as at a remote controlled generating plant. Synchroscopes or
lamps are sometimes installed as a supplement to automatic relays,for possible
manual use or for monitoring the generating unit.
Sometimes as a precaution against out-of-step connection of
a machine to a system, a "synchro check" relay is installed that
prevents closing the generator circuit breaker unless the machine is within a
few electrical degrees of being in-phase with the system. Synchro check relays
are also applied in places where several sources of supply may be connected and
where it is important that out-of-step sources are not accidentally paralleled.
Synchronous operation
When the generator is synchronized, the frequency of the
system will change depending on load and the average characteristics of all the
generating units connected to the grid. Large changes in system frequency can
cause the generator to fall out of synchronism with the system. Protective
devices on the generator will operate to disconnect it automatically.
Synchronous speeds
Synchronous speeds for synchronous motors and alternators
depend on the number of poles on the machine and the frequency of the supply.
The relationship between the supply frequency, f, the number
of poles, p, and the synchronous speed (speed of rotating field), ns is given
by:
In the following table, frequencies are shown in hertz (Hz)
and rotational speeds in revolutions per minute (rpm):
No. of poles Speed
(rpm) at 50 Hz Speed (rpm) at 60 Hz
2 3,000 3,600
4 1,500 1,800
6 1,000 1,200
8 750 900
10 600 720
12 500 600
14 429 514
16 375 450
18 333 400
20 300 360
22 273 327
24 250 300
26 231 277
28 214 257
30 200 240
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