RIGID AND STRAIN BUS COMPARISON FOR SUBSTATION USES BASIC INFORMATION

A comparison of rigid and strain buses indicates that careful consideration should be given to selection of the proper type of bus to use.

Rigid-bus advantages:

1. Less steel is used, and structures are of a simpler design.

2. Rigid conductors are not under constant strain.

3. Individual pedestal-mounted insulators are more accessible for cleaning.

4. The rigid bus is lower in height, has a distinct layout, and can be definitely segregated for maintenance.

5. Low profile with the rigid bus provides good visibility of the conductors and apparatus and gives a good appearance to the substation.

Rigid-bus disadvantages:

1. More insulators and supports are usually needed for rigid-bus design, thus requiring more insulators to clean.

2. The rigid bus is more sensitive to structural deflections, causing misalignment problems and

possible damage to the bus.

3. The rigid bus usually requires more land area than the strain bus.

4. Rigid-bus designs are comparatively expensive.

Strain-bus advantages:

1. Comparatively lower cost than the rigid bus.

2. Substations employing the strain bus may occupy less land area than stations using the rigid bus.

3. Fewer structures are required.

Strain-bus disadvantages:

1. Strain structures require larger structures and foundations.

2. Insulators are not conveniently accessible for cleaning.

3. Painting of high-steel structures is costly and hazardous.

4. Emergency conductor repairs are more difficult.

The design of station buses depends on a number of elements, which include the following:

1. Current-carrying capacity

2. Short-circuit stresses

3. Minimum electrical clearances

The current-carrying capacity of a bus is limited by the heating effects produced by the current. Buses generally are rated on the basis of the temperature rise, which can be permitted without danger of overheating equipment terminals, bus connections, and joints.

The permissible temperature rise for plain copper and aluminum buses is usually limited to 30°C above an ambient temperature of 40°C. This value is the accepted standard of IEEE, NEMA, and ANSI. This is an average temperature rise; a maximum or hot-spot temperature rise of 35°C is permissible.

Many factors enter into the heating of a bus, such as the type of material used, the size and shape of the conductor, the surface area of the conductor and its condition, skin effect, proximity effect, conductor reactance, ventilation, and inductive heating caused by the proximity of magnetic materials.