Parallel operation of shunt generators.
Shunt-wound generators will in general operate very well in parallel and will divide the load well if the machines have similar characteristics. If the machines do not have similar characteristics, one machine will take more than its share of the load and may tend to drive the other as a motor.
When this machine is running as a motor, its direction of rotation will be the same as when it was generating; hence the operator must watch the ammeters closely for an indication of this trouble. Shunt generators are now seldom installed. Figure below shows the connections for shunt generators that are to be operated in parallel.
Parallel operation of compound-wound generators.
It is readily effected if the machines are of the same make and voltage or are designed with similar electrical characteristics (Westinghouse Electric Corp.) The only change that is usually required is the addition of an equalizer connection between machines.
If the generators have different compounding ratios, it may be necessary to adjust the series-field shunts to obtain uniform conditions.
Operation of a shunt and a compound dynamo in parallel is not successful because the compound machine will take more than its share of the load unless the shunt-machine field rheostat is adjusted at each change in load.
Shutting down a shunt- or compound-wound generator operating in parallel with others.
(1) Reduce the load on the machine as much as possible by cutting resistance into the shunt-field circuit with the field rheostat.
(2) Throw off the load by opening the circuit breaker if one is used; otherwise open the main generator switches.
(3) Shut down the driving machine.
(4) Wipe off all oil and dirt, clean the machine, and put it in good order for the next run. Turn on all resistance in the field rheostat. Open the main switch.
Adjusting the division of load between two compound-wound generators.
First adjust the series shunts of both machines so that, as nearly as possible, the voltages of both will be the same at one-fourth, one-half, three-fourths, and full load. Then connect the machines in parallel, for trial.
If, upon loading, one machine takes more than its share of the load (amperes), increase the resistance of the path through its series-field coil path until the load divides between the machines proportionally to their capacities.
Only a small increase in resistance is usually needed. The increase can be provided by inserting a longer conductor between the generator and the busbar, or iron or nickel silver washers can be inserted under a connection lug.
Inasmuch as adjustment of the series-coil shunt affects both machines when the machines are connected in parallel, nothing can be accomplished through making such an adjustment.
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