Page 97 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

4.26 CHAPTER FOUR

FIGURE 4.16 Power station ac single-line diagram.

Some plants install the generator breaker before the line feeding power from the output of
the generator back into the plant. In this design, the plant loads are supplied from another
source (e.g., the grid) when the breaker opens. Other plants install the generator breaker after
the line feeding power from the output of the generator back into the plant. When the genera-
tor breaker opens (due to a load rejection), a significant load shedding occurs (about 94 per-
cent of the load is taken off within a fraction of a second). This results in a significant reduction
in countertorque on the generator shaft (about 94 percent reduction). Since the driving torque
from the turbine has not changed, the turbine-generator shaft will accelerate. The governing
system will limit the overspeed normally to 8 percent. However, since the generator is still sup-
plying the plant loads at higher frequency (up to 8 percent higher), the synchronous speed of
the motors inside the plant will increase. The currents pulled by these motors from the power
supply will increase. This results in tripping of these motors in some plants on overload.
Transformers are used inside the plant to provide power to the loads requiring lower
voltage. Some steam power plants use separate gas turbines as backup power (standby gen-

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