Sincronizada
The exact engine speed is sincronizada critical. There are five conditions that must be met before the synchronization process takes place.
Waveform and phase sequence are fixed by the construction of the generator and its connections to 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.
The sequence of events is similar for manual or automatic synchronization. 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. 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. This synchroscope was used to synchronize a factory’s power plant with the utility’s power grid. 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. 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. 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. 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. 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 for synchronous motors and alternators depend on the number of poles on the machine and the frequency of the supply. A beef on weck is a sandwich found primarily in Western New York State, particularly in the city of Buffalo. The sandwich, along with Buffalo wings and sponge candy, is one of the three best-known food specialties of Buffalo. The origin and history of the beef on weck sandwich is not well established but is thought to predate the 1960’s development of Buffalo wings by approximately a century. A local pub owner is said to have used the roll to create the beef on weck, with the thought that the salty top of the roll would encourage his patrons to purchase more drinks. Buffalo-area bakeries are the only commercial source for the roll.
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