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Configuration D is extremely versatile but an even more costly arrangement for providing power during a blackout. It consists of locating a stationary automatic standby generator unit inside or outside the building with permanent wiring to an automatic transfer switch. It also includes the complex protective and control circuits to allow paralleling with the utility source. This type of system must be installed by an electrical contractor after pulling the electric meter. Permission usually is required by the power company for this type of installation and they may have special equipment requirements that have to be met.
This configuration is economically feasible for large establishments that must have backup power and can justify the higher installation expense of the paralleling controls.
Operation is generally as follows. When power is lost, after a brief delay, the
standby generator automatically starts. When it reaches rated speed and voltage, the
automatic transfer switch connects the building electrical system to the running
generator. Various engine and generator parameters are monitored, along with the
utility feed, and if all remain within specified ranges, the building will continue to be
powered. When the utility feed is restored, the automatic controls will switch the
building back to the utility source after a preset time delay. The generator is then
automatically shutdown after a brief period of no-load operation.
The automatic controls also are set up to regularly exercise the standby generator set by starting and running it every few weeks.
Since the paralleling controls allow operation connected to the utility source, this system can actually sell power back to the utility.
Advantages of this configuration include:
Some disadvantages of this arrangement are: