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 (a) Start-stop.
(b) Change levels.
(c) Switching.
(d) Lines.
(e) Capacitors.
(f) Reactors.
(g) Synchronous condensers.
(h) Power plants.
(i) Transformer LTC tap.
(j) Voltage regulator position.
Supervisory control equipment is often combined in quantities at remote locations such as
substations or switchyards. Circuits and equipment are energized by operating circuit breakers
and switches. For decision making, the operator also receives information concerning equipment
state and electrical loading. The remote supervisory equipment combines the capability of
receiving and executing commands and transmitting associated information to the central
supervisory equipment. The centrally located supervisory equipment conversely incorporates the
capability of receiving information and sending the operator's commands to a number of remote
supervisory equipment. Operators, therefore, may observe and control remotely located
elements of the power system using both remote and centrally located supervisory equipment.
6.2.3 Basic System Control Operations. The control objectives of a power system are related
to the conditional state at which the system is operating. As the conditional state decreases
below an acceptable threshold, restorative measures are initiated. The control system
continuously maintains a balance between load and generation or demand and supply through
basic operations.
6.2.3.1 Load and Frequency Control. The load and frequency control, of interconnected
generators, introduces relatively simple problems to systems having one or two generating
stations. The problems caused by load and frequency control become more difficult in large
interconnected systems having many stations scattered over a wide area.
(a) In small single-station systems, the operator can readily adjust the governors of the
prime movers to divide the load most economically between them. Guided by an accurate
frequency meter and an electric clock (rather than a standard clock), the operator is able to
accurately maintain the station frequency and time.
(b) In large systems, a central load dispatcher is necessary to assign loads to various
stations and units in accordance with a predetermined schedule. This schedule is modified
occasionally as the actual load differs from the predicted load or as emergencies arise owing to
loss of generating units or tie lines. The load dispatching may be by telephone, remote
telemetering and signaling, or both. Load assignment to a particular station varies with the type
6-3
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