DESIGN PROCESS
Typical steps in an overhead distribution line design are shown below. The actual steps and their sequence will depend upon the individual project and the context in which the design is performed.
The process is iterative, with the designer making some initial assumptions, e.g. as to pole height and size, which may later need to be adjusted as the design is checked and gradually refined. The optimum arrangement that meets all constraints is required as the final outcome. Utility Power uses overhead line simulation software to aid the design process.
Determine Design Inputs
Prior to commencing design, it is important to collect and document all relevant design inputs. This may include:
a) planning reports, concept, specification or customer request for supply initiating the project;
b) load details, disturbing loads etc;
c) special requirements of customers or stakeholders (e.g. supply reliability);
d) system planning requirements;
e) information about possible future stages or adjacent developments, road widening or other;
f) applicable relevant standards and statutory requirements;
g) co-ordination with other utilities - 'Dial Before You Dig' results
h) co-ordination with road lighting design;
i) survey plans or base maps;
j) any site constraints identified and
k) environmental factors (as elaborated below)
The designer should take into consideration the environmental factors which could influence the design of the supply arrangement, e.g. selection of and location of equipment, etc.
For example, suppose an overhead MV line is to be constructed to supply a customer remote from a zone substation, and the line route traverses an area of high lightning activity. It would seem prudent for the designer to include an earth-wire system to shield the conductors, in the line design, even though this is not normal practice for distribution lines.
Similar considerations should apply for lines or installations close to the coast, which are subjected to high salt-pollution levels. High pollution insulators may be incorporated in the line design.
Consideration must be given to the location of the equipment or the environment the equipment is to operate in. For example, a pole top transformer may not be entirely suitable for use outside a cement plant or quarry, where the build up of fly-ash or dust on insulators may lead to nuisance tripping or a disproportionately high level of maintenance. Others include mines sites, with open air blasting, etc.
Consideration shall also be given to:
• Cultural Heritage and Native Title;
• Environmental approvals for clearing or removal of native vegetation; and
• Siting of Substations with respect to Noise Control.
Current statutory processes require a range of approvals to be obtained prior to commencement of works. Due to the time taken to obtain these approvals, these issues must be considered at the commencement of a project.
As per the Western Australian Distribution Connections Manual (WADCM Section 6.12) environmental and heritage impacts must be investigated and managed by the applicant for power supply and their agent. Issues may include but are not limited to the following:
a) Aboriginal heritage sites and objects of suspected aboriginal origin;
b) Acid sulphate soils;
c) Bio-security weeds, pests and disease spread (e.g. dieback disease);
d) Declared rare flora and threatened ecological communities;
e) Dust;
f) Erosion;
g) Land entry permits;
h) Native title;
i) Noise;
j) Protected wetlands;
k) Vegetation clearing permits; and
l) Waste management including controlled waste.
The design should be 'traceable' back to a set of design inputs. Persons other than the original designer should be able to review the design and see why it was done a certain way.
