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loss-of-excitation relay usually disconnects the generator when it begins accepting excessive
reactive power from the outside. Automatic Load Shedding. System frequency is a sensitive measure of
discrepancy between load and generation. In the normal operating state, the generation is
adequate to supply the existing load and no equipment will be overloaded. If the system
becomes overloaded and additional generation is not available, the frequency falls below 60 Hz.
Control centers normally have equipment that will automatically disconnect blocks of load when
the system frequency reaches prescribed values below 60 Hz.
6.2.4 Computer Control. Today most generation and load control systems are computer
based. The computer can be programmed to provide format changes, to make comparisons
between desired and actual data measurements, and to correct system operation. These processes
are performed faster using a computer than they are performed by human or hard-wired logic
controllers. The computer can also provide efficient entry, formatting, and storage of data.
Computer control of generating and transmission stations can also greatly improve the security of
the power systems. For example, the computer can:
(a) Improve the monitoring and display of information.
(b) Be programmed to evaluate contingencies and to develop corrective procedures.
6.3 SWITCHBOARDS. This section provides a general discussion of power and control
switchboards, supervisory control equipment, and the automatic control of devices in generating
stations and substations.
6.3.1 Power Switchboards. For small stations or substations, the main switching equipment
and buses may be mounted directly on or adjacent to the board. For larger stations, the switching
equipment and buses are always remotely located in separate buildings or enclosures, or
outdoors. In these cases, the main switching equipment and buses are generally identified as bus
structures. Power switchboards are also identified as:
(a) Direct control.
(b) Remote mechanical control.
(c) Electrically operated.
This section describes the various types of switchboards available to provide different functions. Direct Control. Direct control panel-type switchboards are generally used for
small and medium-capacity installations where a complete installation of only a few panels is
required. They are designed for control of incoming lines, generators, motor-generator sets,
induction and synchronous motors, feeders, light and power supply, control-power supply, and
battery-charging equipment.


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