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Parallel Plant Operation

If the load of a single generator becomes so large that its rating is exceeded, it becomes necessary to add another generator in parallel to increase the power available for the generating station. Before two ac generators can be paralleled, the following conditions have to be fulfilled:

1. Their terminal voltages have to be equal.

2. Their frequencies have to be equal.

3. Their voltages have to be in phase.

When two generators are operating so that the requirements are satisfied, they are said to be in synchronism. The operation of getting the machines into synchronism is called synchronizing.

Generating plants may be operated in parallel on an isolated bus (two or more generators supplying camp or base load) or on an infinite bus (one or more generators paralleled to a utility grid).

One of the primary considerations in paralleling generator sets is achieving the proper division of load. This can be accomplished by providing the governor of the generator with speed droop. This would result in a regulation of the system. The relationship of REGULATION to LOAD DIVISION is best explained by referring to a speed versus load curve of the governor. For simplicity, we will refer to the normal speed as 100 percent speed and full load as 100 percent load. In the controlled system, we will be concerned with two types of governor operations: isochronous and speed droop.

The operation of the isochronous governor (0 percent speed droop) can be explained by comparing speed versus load, as shown in figure 5-6. If the governor were set to maintain the speed represented by line A and connected to an increasing isolated load, the speed would remain constant. The isochronous governor will maintain the desired output frequency regardless of load changes if the capacity of the engine is not exceeded.

The speed droop governor (100 percent speed droop) has a similar set of curves, but they are slanted, as shown in figure 5-7. If a speed droop governor were connected to an increasing isolated load, the speed would drop (line A, fig. 5-7) until maximum engine capacity is reached.

Now let's imagine that we connect the speed droop governor (slave machine) to a utility bus so large that our engine cannot change the bus frequency (an infinite bus). Remember that the speed of the engine is no longer determined by the speed setting but by the frequency of the

Figure 5-6.\Isochronous governor curve.


Figure 5-7.\Speed droop governor curve.

infinite bus. In this case, if we should change the speed setting, we would cause a change in load, not in speed. To parallel the generator set, we are required to have a speed setting on line A (fig. 5-7), at which the no-load speed is equal to the bus frequency. Once the set is paralleled, if we increase the speed setting to line B, we do not change the speed, but we pick up approximately a half-load. Another increase in speed setting to line C will fully load the engine. If the generator set is fully loaded and the main breaker is opened, the no-load speed would be 4 percent above synchronous speed. This governor would be defined as having 4 percent speed droop.

Paralleling an isochronous governor to an infinite bus would be impractical because any difference in speed setting would cause the generator load to change constantly. A speed setting slightly higher than the bus frequency would cause the engine to go to full-load position. Similarly, if the speed setting were slightly below synchronous speed, the engine would go to no-load position.

Setting speed droop on hydraulic governors is accomplished by adjusting the speed droop knob located on the governor body. Setting the knob to position No. 5 does not mean 5 percent droop. Each of the settings on the knob represents a percentage of the total governor droop. If the governor has a maximum of 4 percent droop, the No. 5 position would be 50 percent of 4 percent droop. Setting speed droops on solid-state electronic governors is accomplished by placing the UNIT-PARALLEL switch in the PARALLEL position. The governor speed droop is factory set, and no further adjustments are necessary.


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