Synchronization (alternating current)
In an alternating current (AC) electric power system, synchronization is the process of matching the frequency, phase and voltage of a generator or other source to an electrical grid in order to transfer power. If two unconnected segments of a grid are to be connected to each other, they cannot safely exchange AC power until they are synchronized.
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Conditions
There are five conditions that must be met before the synchronization process takes place. The source (generator or sub-network) must have equal
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 is correct so that the order of phases (phase sequence) matches the system. Connecting a generator with the wrong phase sequence will result in large, possibly damaging, currents as the system voltages are opposite to those of the generator terminal voltages.[2]
The voltage, frequency and phase angle must be controlled each time a generator is to be connected to a grid.[1]
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
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.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/7c/Synchronoscope_stack.jpg/170px-Synchronoscope_stack.jpg)
Synchronizing lamps
Formerly, three
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 could give a false-positive for successful synchronization.
Synchroscope
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/Synchroscope.jpg/220px-Synchroscope.jpg)
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
While 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.[1] 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 |
See also
- Phase synchronization
- High-voltage direct current: allows power transmission between AC transmission systems that are not synchronized
References
- ^ a b c Soft synchronization of dispersed generators to micro grids for smart grid applications
- ISBN 0-07-013932-6pages 7-45 through 7-49
- ISBN 0-07-020974-Xpp. 3-64,3-65
Sources
- The Electrical Year Book 1937, published by Emmott and Company Limited, Manchester, England, pp 53–57 and 72