Central Plant Machinery And Equipment Loads

Fig. 27-13 Line Diagram of Integrated Generator Switchgear in Peaking or Baseload Application with Provisions for Emergency Power Service. Source: Automatic Switch Company generator switchgear bus. These feeders could provide emergency power to other buildings for emergency purposes. When the utility service is disrupted, the generators would start automatically and feed power to these emergency loads. Emergency power can also be fed to the switchgear bus of the central plant by opening both utility mains and closing the generator main, 52MC.

Example 3

Figure 27-14 illustrates suitable protective relay schemes for the utility and generator mains. In comparison with the previous example, there are some additional devices included in the generator main circuit. Devices 50/51, 32R, 65, and 65C and meters W, V, and A serve the same functions as in the previous example. Because this is a synchronous generator, it generates voltage on its own and does not require an energized bus. Device 59G is a ground overvoltage relay. Its purpose is to sense when there is a ground on one of the phases and to enunciate it. The resistor in parallel with the 59G limits ground current to or from the generator when it is in parallel with the utility system.

Devices 27 and 81 check the generator voltage and frequency before initiating synchronizing operations. The synchronizer, Device 25, compares the voltage and frequency of the generator with that of the bus. This synchronizer takes control of the voltage regulator and governor to cause the generator output to match the voltage and frequency of the bus. The synchronizer then matches the phase angle of the generator to the bus and initiates closing of the general breaker.

As is the case with the induction generator (Example 1), the loading control causes the governor to increase fuel (or steam) to the prime mover so that electrical energy is delivered to the bus. Similarly, the VAR/PF controller causes that voltage regulator to increase excitation to the generator field windings so that the generator produces the proper VARS to run at the desired power factor.

Protective relaying at the utility main provides voltage and frequency functions. These assure that the utility services are at nominal values prior to an attempt to parallel it with the generator bus. This would be required, for example, if the facility were running on the generators alone following a utility outage.

The control strategy for this type of system can

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