LINER-LH 1740108 - Caterpillar

Figure 1-Paralleling two units with similar speed droop characteristics.If Unit A had been operating alone carrying full load, the system frequency would be 60 cycles as shown by Line "A" at 60 cycles and 100% load. Now, if the frequency of Unit B is adjusted by the engine speed control to be equal to that of Unit A and the circuit breaker of Unit B is closed, the system would be operating under the following conditions:1. Unit A is at 60 cycles and 100% load.2. Unit B is at 60 cycles and 0% load. The characteristic of the governor of Unit B at this time is shown by the dotted line B.In order for Unit B to carry load, it is necessary to advance the speed setting of the governor. If it is advanced to the full load position, the governor characteristic of B will coincide with the characteristic of A. Since the load on the system was 100% of one unit, no change was made in total load, and the available capacity is now 200%, the system will operate at 50% load on each unit, and 60.9 cycles for two units of equal capacity (Point E). For units of unequal capacity, the load will be divided in proportion to the ratio of the capacity of each to the total capacity. The system frequency will be determined by the points on the governor characteristics corresponding to these loads. The frequency will be the same for both units since paralleled alternators must operate at the same speed.If Unit A had been a 60 KW unit fully loaded and a 100 KW Unit B was paralleled with it and the governor adjusted to the full load position, the final load division and frequency would be determined as follows: Figure 2-Paralleling two units, one with a 3% governor and the other with a hydraulic governor set for isochronous operation. Example 2-One 3% Governor and one 0% (Isochronous) Governor.The characteristics of the 3% governor (Unit A) is shown by line A of Figure 2 and the characteristics of the isochronous governor (Unit B) is shown by line B. Only at full load, 60 cycles, do the frequencies of the units have the same value. It is customary to operate a system of this type with a system load greater than the capacity of Unit A. In this way Unit A carries its full load at 60 cycles and the additional load and load swings are handled by Unit B, also at 60 cycles. The system can maintain constant frequency by this method of operation. The system described in example 1 cannot maintain constant frequency with load changes because of the speed droop characteristics of the governors.In the system described in this example, if the load is less than the capacity of Unit A (which has the 3% governor) and can be carried by Unit B, the governor setting of Unit A can be reduced to give a governor characteristic such as A,