SR 4 AND SRCR GENERATORS - OPERATION & MAINTENANCE Caterpillar

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:

37.% of the capacity of either unit. Again, using Figure 1 for the governor characteristic of the 100 KW unit and reading up from the value of load (37.5% to point F), we find the system frequency to be 61.125 cycles.

PERCENT OF LOAD

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, so Unit A will still carry the steady part of the load and Unit B will carry the load swings. If the system load is reduced to the point where Unit A is not operating at the 60 cycle point of its governor characteristic, Unit A will try to motor Unit B and the system frequency may be greater than 60 cycles. The reason for using an isochronous governor in a power system is to maintain constant frequency above approximately 40% load.

Summary

The preceding discussion and examples of governor operation can be summarized as follows:

1. The simplest governor combination for paralleled electric sets is to have a 3% speed droop characteristic for each governor. If a constant frequency from no-load to full-load is required, one governor can be adjusted for isochronous operation. This is called a "lead unit".

2. In order for all paralleled units to accept their full share of the load, the following governor adjustments are required:

a. The same full load speed.

b. The same high idle (no-load) speed in the case of governors adjusted for speed droop operation.

c. Governor controls set to the high idle position so the full governor range is available.

3. Operation of an isochronous governor in parallel with speed droop governors requires the special techniques described in example 2.

4. Any number of electric sets can be operated in parallel. However, only one governor of the group can be adjusted for isochronous operation except in the special cases of electronic governors with automatic load sharing.