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.

- The plant is staffed 24 hours a day. The operator is available to perform control actions either locally or at a centralized area.

- 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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