Re: OT generator question for John T

Posted by NEsota on December 21, 2008 at 11:47:58 from (75.72.251.223):

In Reply to: OT generator question for John T posted by NE IA on December 20, 2008 at 21:15:58:

In the mid sixties I was employed as operator for a village diesel electric plant. The kilowatt capacity of the three phase generators was: 600, 260, 120 and 80. As the plant load varied during my shifts, it was necessary to start up and shut down generators accordingly. Each generator’s speed, once paralleled, was controlled by the load on it, which varied with system load and system generation, as well as its’ own mechanical governor. The synchronization was accomplished with nothing more than the light bulb system Gerald described below.

I would not be reluctant to connect smaller power generators in the same manner as in the above (using light bulbs) but to play safe I would use fusing or circuit breakers equal to each of the generators overload capacity. Because I am in general agreement with what Lou and Gerald posted, I will recycle part of what they said:

“AC generators can be paralleled, having said so it is not easy or cheap, the synchronization is critical. The utility companies constantly have numerous generators in sync. The unloaded unit is brought to speed and the voltage is brought to match and then to a slightly higher freq then the loaded unit, as the unloaded unit syncs to the loaded unit the switch gear is closed to bring it on line, at that time it will start to take on some of the load. In power plants units are brought on line and dropped off to match the load requirements constantly. The governors and synchronizers are expensive, plus all the needed safety gear, back feed relays etc.
Lou”
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“There are fancy synchronizing meters, always backed up by a couple light bulbs. Two 120 volt lamps in series across the open switch indicate by the lamp brightness how much the two circuits are out of phase. At the worst point there is 240 volts across the open switch so it takes the two lamps (240 volt lamps being uncommon, but possible). The operational trick is to adjust the speed of one of the generators to slow the brightness changes to ten or more seconds per light brightness cycle, then pick the middle of the dark period for closing the switch. If you miss zero voltage difference, there will be a grunt from both machines and you might have to stand one or both back up if you miss by too much.

Then you adjust the throttle on the dependent generator to get the two generators to share the load. As the load varies, the load sharing will depend on the quality of the individual voltage regulators and engine governors.
Gerald J.” <>

In the late sixties I did sales engineering work for tie line control instrumentation of systems with considerably more capacity than the one sited above. Nothing that I experienced there contradicted any of the above.
John

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