After the location and mode of control
is defined, the manner in which the plant is supervised or staffed
should be recognized. Though this aspect is not commonly addressed in
industry recognized labels for plant control, the equipment selection
and its degree of automation is related to the plant supervision.
The greater the degree of
sophistication in the controls, the greater the distance between the
equipment being controlled and the control location.
As the main point of control moves from
the equipment to an off-site locale, the need for
staffing/supervision at the lower levels of control diminishes.
Supervision can, therefore, be described relative to the plant as
either attended or unattended.
Attended
- The plant is staffed 24 hours a day.
The operator is available to perform control actions either locally
or at a centralized area.
Unattended
- The plant is not staffed for the full
24 hours a day. An operator may be present for a single shift or make
a routine visit to the project. With the exception of small hydro,
the policy throughout the industry is to have some form of
supervision or monitoring at a given plant. If the plant’s on-site
control is defined as unattended, then it is implied that the
supervision/monitoring is performed off-site.
Unattended operation is represented by
two predominant examples:
1) Off-Site Supervisory Control
- Here, control of the remote plant
exists for all essential operations and a full complement of
indications for the remote plant are brought to the off-site control
location. Occasional visits by operation and maintenance people are
made to ensure plant security.
2) Off-Site Monitored Control
- All of the controls for the plant are
local. A minimum representation of plant indication is brought to an
off-site location where full attendance exists. The capability exists
at the off-site location to dispatch an operator to the plant if
conditions warrant. Routine maintenance visitations can also be made
to the plant.
The practice is to describe a control
system by location and then mode. It can then be modified by defining
the type of supervision. Since the staffing of a plant varies within
the industry, it is difficult to define this aspect generically.
All combinations of location and mode
are legitimate and more than one combination can exist at a plant.
For example, a multiple unit plant can have local manual controls at
the unit auxiliaries and the unit switchboard.
It can have a control room that would
have both centralized manual and centralized automatic controls. It
may even have equipment at the plant that would allow the capability
of off-site automatic controls.
To complete the scenario, the plant may
be unattended except for routine maintenance visits. A hierarchy of
control can, therefore, be developed, going from those controls
closest to the equipment and the least complex to the controls
located off-site and the most sophisticated. Starting from the lowest
(most basic) echelon in the hierarchy and working to the highest
(most sophisticated), we find
1) Local manual
2) Local automatic
3) Centralized manual
4) Centralized automatic
5) Off-site manual
6) Off-site automatic
It should be kept in mind that the
above combinations can further be modified/described by appending
either attended or unattended to them.
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