Ques. What forms of drive are especially desirable for running alternators in parallel, and why?
Ans. Water turbine or steam turbine because of the uniform torque, thus giving uniform motion of rotation.
With reciprocating engines, the crank effect is very variable during the revolution, resulting in pulsations driving the alternator too fast or too slow, and causing cross current between the alternators.
Ques. Is a sluggish, or a too sensitive governor preferable on an engine driving alternators in parallel?
Ans. A sluggish governor.
Alternators in Series.—Alternators are seldom if ever connected in series, for the reason that the synchronizing tendency peculiar to these machines causes them to oppose each other and fall out of phase when they are joined together in this way. If, however, they be directly connected to each other, or to an engine, so that they necessarily keep in phase at all times, and thus add their respective voltages instead of counteracting them, series operation is possible.
NOTE.—According to the practice of the General Electric Co., 2½ degrees of phase difference from a mean is the limit allowable in ordinary cases. It will, in certain cases, be possible to operate satisfactorily in parallel, or to run synchronous apparatus from machines whose angular variation exceeds this amount, and in other cases it will be easy and desirable to obtain a better speed control. The 2½ degree limit is intended to imply that the maximum departure from the mean position during any revolution shall not exceed 2½ ÷ 360 of an angle corresponding to two poles of a machine. The angle of circumference which corresponds to the 2½ degree of phase variation can be ascertained by dividing 2½ by ½ the number of pole; thus, in a 20 pole machine, the allowable angular variation from the mean would be 2½ ÷ 10 = ¼ of one degree.
Fig. 2,853.—Diagram of connections for synchronizing two compound wound three phase alternators. A and A' are the armatures of the two machines, the fields of which are partly separately excited, the amount of excitation current being controlled by the series compounding rheostats B and B', which form a stationary shunt. It is assumed that the alternator A is connected to the bus bars 1, 2, and 3, by the switch 1S. If an increase make it necessary to introduce the alternator A', it is first run up to speed and excited to standard pressure by its exciter, and then the double plug switch 3S is closed, connecting the primary of the station transformer T and T' with the bus bars through the secondary coil, so that the synchronizing lamps light up when the secondary circuit is closed through the single pole switch 4S. The primary of the station transformer T is thus excited through the double pole switch 5S, connecting it with the outer terminals of the armature A'. The two alternators will now work in opposition to each other upon the synchronizing lamps, the transformer T being operated by the new alternator A' through the switch 2S, and the transformer T' being operated by the working alternator A, from the bus bars. If the new alternator be not in step with the working alternator, the synchronizing lamps will glow, growing brighter and dimmer alternately with greater or lesser rapidity. In this case, the armature speed of the new alternator must be controlled in such a manner that the brightening and dimming will occur more and more slowly, until the lamps cease to glow or remain extinguished for a decided interval of time. The extinction of the light is due to the disappearance of the secondary current, and indicates that the alternators are in step. The switch 2S should now be thrown, thus coupling the two machines electrically, and both of them will continue to operate in step. The double pole equalizer switch 6S should now be closed, connecting the two field windings in parallel and equalizing the compounding, so that any variations of load will affect the two alternators equally. After the alternators have been connected in parallel, the switches 4S and 5S, may be opened leaving the switch 3S closed, to operate the switchboard lamps K, K, as pilot lights from the bus bars.