PULLEY GP 2W4874 - Caterpillar

When two or more electric sets are operating in parallel and it is known that the total load will be less than the capacity of all the units and will not cause an overload if the number of paralleled units is reduced by disconnecting one or more units, the load can be transferred to an adequate number of units and the others disconnected from the load. The following discussion is an explanation of the actions of governors and regulators on engine electric sets during load transfer and the disconnection of one or more units from parallel operation. This discussion will cover the operation of two units. If more than two units are operating, the same principles apply to the complete system.If the system were initially started with the units unloaded before being paralleled, the speed of each engine would be rated speed plus speed droop as shown at points A and D of Illustration 1. The voltage of each unit would be rated voltage plus voltage droop as shown by points G and K. With these conditions, the circuit breakers can be closed to place the units in parallel. When the system is loaded, the engine speed will follow the governor sped droop characteristics and the generator voltages will follow the regulator voltage droop characteristics. With 50% load on each unit, the engine speeds (and the frequency of the system) will be as shown by points E and F, and the voltage of the system will be as shown by points L and M. At full load on each unit, the engine speed and the frequency of the system will be as shown by points B and C and the voltage of the system will be as shown by H and J.
Illustration 1 g01059140
Load transfer and disconnections of paralleled generators.If it is known than the steady load on the system for a period of time will be 50% or less of the total capacity of the generators, it may be desirable to transfer the load to one unit and disconnect the other unit from the system. In this case, the unit still operating will be loaded to 100% or less of its rating. Note that in the preceding paragraphs, the two units were paralleled with the governors at their high idle settings. In order to transfer the load to just one unit, for example Unit Two, the governor control of Unit One is moved in the "reduce speed" direction until the k.w. meter for generator one indicates zero k.w. In the event that there are no k.w. meters for the installation, then the ammeter for generator one should show a minimum indication which is not necessarily zero amperes. At this point, Unit Two is carrying all the load and, if this load is 100% of the rating of the unit, the speed of the system will tend to be at point C and the voltage of the system will tend to be at point J.What has occurred in the