Parallel Operation of Compound Generators

Author: E.E. Kimberly

The economic generation of direct current sometimes involves the operation of two or more generators supplying current to the same bus bars. The parallel operation of shunt generators is accomplished with no particular difficulty, inasmuch as the reduction of load on one generator for any reason permits a rise of its terminal voltage and it therefore tends to recover the lost load. For this reason, parallel operation of shunt generators is said to be "stable." However, if the load on an over-compounded generator be reduced, the generated voltage will decrease and, therefore, the ability of the generator to carry load decreases. When such a generator is connected in parallel with a similar one, the second generator assumes any load dropped by the first and the additional load causes its voltage to rise still higher. Thus, the loss of load and voltage by one generator and the subsequent gain of load and voltage by the other results in an unstable state of operation, and the underloaded generator will quickly be driven as a differential motor by the other generator, which has also assumed all of the load. This unstable condition can be remedied by connecting a low-resistance conductor, or "equalizer," between the series fields of the two generators at the points where they connect to their respective armatures, as indicated in Fig. 10-23. When such an equalizer is used, the current in either series field is not determined by the armature current of its own generator alone but depends on the division of the load current between the parallel paths of the two fields. In that way the compounding of the two generators is interdependent, and complete stability of operation may be achieved.

Fig. 10-23. Cumulative Compound Generators in Parallel With Equalizer Bus

In order that two over-compounded generators may share a common load in proportion to their respective capacities, their characteristics must be identical and the resistances of their series fields, including the equalizer conductor, must be inversely proportional to their respective capacities.

Calculation of Series-Field Turns for Compounding a Shunt Generator. If space permits, a shunt generator may be converted into a compound generator by the addition of a series-field winding. In order to calculate the correct number of turns to produce flat compounding, it is necessary to know the number of shunt-field turns per pole and to make a simple test. With the generator loaded to full-load current, the field rheostat should be adjusted until the required terminal voltage is obtained, and the field current should be recorded. The difference between this field current and the field current required for no-load voltage will be the necessary increase in the field current. This current increase multiplied by the number of shunt-field turns per pole gives the increase in the number of ampere-turns per pole required to accomplish compounding. The ampere-turn increase divided by the full-load current gives the required number of series turns per pole.