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.
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| 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 above paragraph is that the governor characteristic of Unit One has been moved down to the points A' and B' (Illustration 1) and this unit is floating on the line, but carrying no k.w. load; however, as the load was increase on Unit Two, its speed followed the governor characteristic line from point F to point C. AS the load increase on Unit Two, the voltage of the generator tended to follow the voltage droop characteristic from point M to point J. Also the voltage of generator one tended to follow its voltage droop characteristics from point L to point G. This tendency for the voltage of the unloaded unit to rise as the k.w. load on the unit is reduced can result in an indication of amperes on the ammeter for generator one even though the unit is delivering no k.w. to the system. What has occurred is that the voltage droop circuit of the regulator has tended to increase voltage as the load on the generator is reduced. This is accomplished by the exciter furnishing additional magnetizing power to the generator. This magnetizing power is in excess of that required to maintain rated voltage in the system and magnetizing amperes are delivered to the system. These magnetizing amperes are shown by the indication of the ammeter for generator one. If this indication is a fairly small value such as 20% or less of generator rated amperes, the circuit breaker for Unit One can be opened and the unit can be disconnected from the system.
If the amperes indicated on the ammeter for generator one are greater than 20% of the governor rated amperes, or if the device for disconnecting the unit from the system is a knife switch, it may be desirable to reduce the magnetizing amperes to a much smaller value before disconnecting the unit from the system. This is accomplished by reducing the setting of the voltage level control of generator one. Turning the voltage level control in the direction to reduce the voltage will reduce the magnetizing power of the generator and the voltage droop characteristics of generator one has been moved down to the points G' and H'. Under these conditions Unit One can be disconnected with a minimum of disturbance to the system and Unit Two will be carrying the complete load. If the voltage or the frequency of Unit Two is varied from the desired values during this operation, they can be corrected by minor adjustments to the governor and the voltage level controls.