Frequency Control Of a Power System

  Frequency Control Of a Power System

How can the system frequency of a larger power system be adjusted without affecting the power sharing among the system generators?

It can’t unless each generator has room to move up and down in its allowed power output band. The power output band is defined as its maximum capacity minus its minimum capacity. Each generator has a maximum power output based on the unit size and a minimum power output based on both economics and engineering.

The frequency is determined by the ratio of load to generation. If there is too much generation then the frequency will rise. If there is too little generation then the frequency will drop. Assuming the load is fixed, the generation output has to be changed to adjust the frequency. If the balance of generation or the share of total output is to remain the same, then each generator has to be able to move up or down in its power output band to adjust the system frequency. Now this will never happen due to economics.

Each generator has a cost curve which dictates the price at each output range the generator will bid at. In the electric market the system will use the cost curves to find the most economical dispatch. This means some units with a lower price will be 100% committed to their maximum output while other more expensive units may be committed at their lowest output or below maximum.

The cost is dictated by several factors such as startup costs, fuel costs, and operations and maintenance (O&M) costs among others. At the Pmin - minimum power output the cost will be comprised of a startup cost plus fuel costs etc... As the units output rises, the startup cost stays fixed but the fuel and O&M costs rise.

The system frequency will be maintained by bringing generators online and offline in an economical order. As load rises the higher costs units will be brought online as all the lower units are already committed and generating.

Selecting Pole Positions and Pole Top Construction For MV Line

Selecting Pole Positions and Pole Top Construction For MV Line

For pole positioning guidelines.

Firstly, position poles along the route at any key or constrained locations.

Next determine the maximum span length that can be achieved over flat ground given the attachment heights on poles, the sag at the nominated stringing tension and the required ground clearance. Also check the spanning capability of the pole top constructions to be used. Position poles along the route so that this spacing is not exceeded. If there are gullies between poles, the spacing can be increased and if there are 'humps' mid-span, span lengths can be reduced.

Strain Points, Pole Details and Pole Top Constructions have to be determined. Strain point locations need to be determined:

1) To isolate electrically different circuits.

2) To keep very short spans or very long spans mechanically separate, such that all spans in a strain section are of similar length (no span less than half or more than double the ruling span length, and on tight-strung lines, the longest span not more than double the shortest span). Failure to limit span variance can cause excess sagging in longest span at higher design temperature loadings.

3) To isolate critical spans, e.g. spans over a river, major highway or railway line, to help facilitate repairs or maintenance.

4) On line deviation angles too great for intermediate constructions, e.g. Cross-arms with pin insulators.

5) At locations where there are uplift forces on poles.

6) At intervals of approximately 10 spans or so.

The following points also must be considered:

1) Strain section length limitation will be favorable if a line is affected by wires brought down in a storm. Also, the length of conductor on a drum may be a consideration.

2) Span lengths within the strain section must be reasonably similar and poles and pole top construction used must be reasonably consistent, as this gives the line a tidy appearance.

3) When nominating suitable pole top constructions for intermediate poles, adequate capacity must be available for the deviation angle at each site.

4) Pole strengths and foundation types/sinking depths must be nominated as a first pass, as these may need to be amended later once tip loads are determined. Stronger poles will be required at terminations and on large deviation angles. Pole sinking depths can be determined.

Selection of Poles and Pole Tops For MV Line Design

 Selection of Poles and Pole Tops

Typical pole sizes are presented in when selecting poles, potential future sub-circuits and streetlight mounting must be considered, if these are identified / known during the design phase.

Apart from spanning and angular limitations, selecting a suitable pole top configuration should take in to account:

1) Life cycle suitability;

2) Reliability;

3) Suitability for the environment (vegetation, wild life, salt and/or industrial pollution levels); and

4) Ease of construction and maintenance.

Horizontal (flat) construction has the advantage of reduced pole height at the expense of a wider line and corresponding broader easement width.

Flat configurations are preferred in areas frequented by birds. For higher risk spans increasing conductor separation can reduce conductor flash-over due to bird impact. Attaching bird diverts on conductors is also effective as a visual warning to birds.

Delta pin configuration provides for both horizontal and vertical separation and helps reduce conductor clashing.

Overall, more compact pole top configurations are less visually obstructive. It is best to keep to reasonably consistent configurations to maintain visual amenity as well as maintain spanning capability and ease of conductor phasing.

Route Survey and Ground Line Profile for MV Line Design

 Route Survey and Ground Line Profile for MV Line Design

A ‘line route survey’ is carried out to determine:

a) Details of existing electricity infrastructure;

b) Terrain and site features, e.g. trees, access tracks, fences, gullies; and

c) Ground line rise and fall along the route.

Ground line profiling may not be necessary for minor projects in urban areas where the ground is reasonably level or has a consistent slope throughout and there are no on site obstructions.

The designer can check worst case ground clearances by deducting the sag in the span from the height of the supports at either end by taking the following measurements:

a) Conductor temperature

b) Conductor size/type

c) Ambient temperature

d) Conductor attachment point with respect to ground level

e) Strain points

However, ground line profiling is essential where:

1) Poles have to be positioned along an undulating traverse;

2) There is a 'hump' or change in gradient in the ground at mid span;

3) Outside of urban areas where spans are comparatively long-say in excess of 80 m;

4) The designer has doubts about the adequacy of required clearances; and

5) Where uplift on poles is suspected.

The equipment used to obtain measurements will depend on the complexity of the project. For many distribution lines, a simple electronic distance measuring device and inclinometer are adequate. Elsewhere, use of a high end GPS unit or LiDAR may be warranted. The route is broken up into segments, typically corresponding with 'knee points' or changes in gradient. Slope distance and inclination measurements for each segment can be converted to chainage and reduced level (RL) values to facilitate plotting as follows:

Software packages can be used to plot survey data. Apart from the ground line, various features and stations must be shown, including existing poles, gullies, fences, obstacles, roadways. A clearance line is then drawn offset from the ground line, according to the minimum vertical clearances that apply.

Selection of Conductor Size and Type For MV Line Design

 Selection of Conductor Size and Type For MV Design

Factors influencing selection include:

a) Load current and whether the line is 'backbone' or a spur;

b) Line voltage and voltage profile along the line;

c) Fault levels and line rating;

d) Environmental conditions – ambient temperature, vegetation, wildlife, pollution or salt spray;

e) Compatibility with existing adjacent electrical infrastructure;

f) Required span lengths and stringing tension; and

g) Future requirements with respect to distribution system planning.

Selection of Route for MV Line

 Selection of Route

Ideally, the line route should be as short and straight as possible in order to minimize costs, minimize stays and have a tidy appearance. However, some other factors that need to be taken into account are:

a) Land issues, ease of acquisition, rights over private lands etc.;

b) Ease of obtaining necessary approvals;

c) Stakeholder considerations and acceptance;

d) Vegetation clearing, environmental and visual impact, EMF impact;

e) Access for construction, maintenance and operations;

f) Ease of servicing all lots for Low Voltage Lines;

g) Compatibility with future development;

h) Waterways, parks and natural habitat; and

i) Terrain suitability and ground conditions (excavation, pole foundation etc.)

DESIGN PROCESS OF MV LINE

 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.