Development of detailed, complete plans and specifications is essential to
ensure substation construction and operation at the lowest possible cost commensurate
with the quality of service desired.The drawings accompanying the specifications
should be of sufficient detail and accuracy to avoid any possible construction
delay and/or errors/misinterpretations during the course of the project.The
following partial list of items needs to be considered when preparing substation
drawings.
- One-Line Diagram
This drawing should be very closely reviewed since the general arrangement
and ratings of all major electrical equipment are depicted on this drawing.
Various items which should be included on the one-line diagram include:
- Current transformers, primary and secondary bus, regulators, breakers,
and fuses are adequate for emergency loading.
- All spare current transformers shorted and grounded.
- Station service transformer fused.
- Relative location and ratings of surge arrestors to transformers and other
critical equipment.
- North arrow included to orient the substation.
- Voltage, kVA, and current ratings included for all equipment.
- Overall relay protection scheme.
- All future and existing construction shown.
- Reflects physical arrangement of equipment.
- Adequate personnel protection via isolating and grounding switches, portable
grounds, interlocks, etc.
- Plot Plan
The orientation of the substation structures on the plot plan should be
coordinated with the direction of the incoming and outgoing lines with all
distances between adjacent structures and clearances properly dimensioned.
Other items include:
- Elevations of top of finished subgrade.
- Access roads, culverts, and other drainage surfacing.
- Fence and gate location.
- North arrow.
- Allowance for removal and installation of equipment and access for maintenance.
- Sufficient space allowed for future expansion.
- The entire area inside the fence and including a minimum of 3.3 feet
outside the fence is covered with a minimum layer of 4-6 inches of
gravel.
- Elevations
The elevation drawings should detail all electrical equipment with special
emphasis given to identifying critical clearances. These electrical clearances
should be in accordance with RUS Bulletin 1724E-300, Table 4-7,
with additional allowances made for heavy snow, high altitude, high contamination,
and special problem areas. Other items include:
- Rigid bus vibration dampers, usually conductor inside tubing bus.
- Phase-to-ground clearances.
- Minimum clearance of 8 feet 6 inches from grade level to the
lowest external part of any insulator, bushing, or insulated housing.
- Proper application of rigid bus expansion, slip or fixed joints.
- Lightning masts, static wires, or bayonets.
- Overhead groundwire shield angle less than 45 degrees and preferably
30 degrees.
- Surge arrestor lead length short as possible.
- Surge arrestors not used as bus supports.
- Grounding provisions for disconnect switch handles.
- Mobile substation connection provisions.
- Proper length and BIL of suspension and post-type insulators.
- Foundations
The drawings should include the type and design of the foundation and/or
footings for the various substation structures. The drawings should be detailed
enough to define the required construction as follows:
- Steel reinforcement bar size, spacing, and location.
- Top of concrete elevations.
- Anchor bolt mark number, projection, and number required.
- Number of foundations required for each type.
- Depth of foundation in relation to frost line.
- Cable trench
- Outline and reinforcement
- Gravel fill
- Cover
- Grading
The grading drawings should show the plan and elevation of the finished subgrade
and existing contours of the substation and surrounding area. These drawings
should also give cross-sectional views indicating slopes for cut and fill
areas, berms, access roads, and graveled surfaces. Reference to a north arrow
and a horizontal tie should be shown on the plan. It is desirable for record
purposes to show the location and log of the soil borings.
- Grounding and Fence Details
The grounding calculations will be used to determine whether the overall
substation ground grid will be of adequate size, length, and impedance to
provide for proper operation of protective equipment as well as personnel
safety. Improper grounding may produce such adverse effects as improper relay
operation, transformer insulation failure, and possible serious injury to
substation personnel. The following items are applicable:
- Ground grid buried a minimum of 18 inches below grade.
- Ground grid should extend a minimum of three feet outside the fence with
the gate bypass extending a minimum of 1 foot 6 inches beyond
the opened gate.
- Ground conductor to be either copper, copper clad steel, or steel.
- All below ground connections by exothermic weld process or compression
type connections.
- All above ground connections by bolted or compression connections.
- Two or more ground paths for surge arrestors, transformers, and all other
electrical equipment.
- All structures connected to the ground grid.
- Overhead ground wires terminated directly to the ground grid.
- Adequate grounding conductor buried to allow for expansion and contraction
during freeze/thaw cycles.
- All runs of conduits and cable trays grounded to the grid.
- All disconnect switch handles provided with grounding mats.
- Chain link fence and gate grounded.
- Transformer neutral bushing and surge arrestors connected directly to
the grid.
- Foundation reinforcing bars not used for grounding electrodes.
- Fence minimum of 7 feet high with 1 foot barbed wire extension
(recommended).
- Adjoining metal fences not connected directly to substation fence.
- Structural
Structural drawings should show the design loading requirements, manufacturing
details, and erection of structure components, including embedded materials
such as stub angles and anchor bolts.Major structures used for transmission
line take-off (deadend) and bus (strain) tower should be accompanied by a
drawing indicating design conditions. This drawing serves as the criteria
for the designer and a check to insure that the actual installation does not
exceed design limits. The data shown on the drawings should include the following:
- Basic geometry, height and beam dimensions.
- Wind-ice design conditions.
- Groundwire and conductor maximum tensions.
- Horizontal and vertical spans and line angle.
- Electrical equipment load.
- Loading combinations.
Construction drawings for structure erection and stringing are needed to
control and expedite construction activities at the site. Erection drawings
should indicate the following:
- Plan and elevation orientation of structure, referenced to north arrow.
- Orientation of individual structure members.
- Number, size, length, and torquing of field connection bolts.
- Rake, camber orientation, and base plate grounding.
- Station Service (AC)
Site and utility drawings should include all details of auxiliary substation
equipment which will be installed. This should include details of AC and DC
power panels, yard lighting, and wire sizing and the following items:
- Transfer scheme provided for critical AC loads.
- Cable sized for ultimate as well as present requirements.
- Outdoor receptacles provided with ground fault interrupter (GFI) circuit
breakers.
- Adequate lighting over critical panel boards.
- Minimum of 20 percent spare branch circuits.
- Conduit fill not to exceed NEC requirements.
- Cable will not exceed critical thermal limits under short-circuit.
- Station Service (DC)
The DC system has the most critical loads. The drawing should include:
- Alarm for loss of AC to battery charger.
- Battery properly sized.
- Battery room provided with eye wash facility and ventilator.
- Positive leg of DC branch circuits should be fused.
The construction specifications must ensure that the materials and construction
practices meet and comply with Rural Utilities Service (RUS) standards