The principal use for the map will be to provide a base on which to lay out the distribution lines for the mini-grid so that detailed design work can be initiated (sizing of the power system, conductor, and poles).
For this purpose, the next step will be to visit each potential consumer, to assess what design load is to be used during the system peak (the daily coincident peak demand) for that consumer (p. 44), and to indicate this at the proper location on the map. If a motor or other load with atypical characteristics is to be used by any consumer, this should also be indicated.
In addition to assessing initial consumer load, the growth in this load into the future must be estimated as realistically as possible. Also to be included is the expected growth in demand from new consumers, either from existing villagers who are yet unwilling to commit to being electrified or from new
households that have yet to establish themselves. And finally, some thought may already have been given to the establishment of new shops and commercial loads in the near future or new institutional loads like a clinic, school, or government office that are under consideration. The size and location of these new loads must also be considered in planning for a mini-grid if it is expected that this grid will serve them.
Once all the design loads to be served have been estimated, the distribution system can be laid out. This requires finalizing the location of the powerhouse, the placement of the lines, and the pole locations. In large part, these are determined by the layout of the village and the general nature of the loads to be served. Factors affecting this aspect of project design are explained in the following sections.
Once the nature of the power demand and the layout of the distribution system are known, the next steps will be to determine the line configuration (Chapter VI), the conductor type and sizes to adequately supply that demand (Chapter VII), and available pole options and size to ensure adequate line clearance and a safe system (Chapter VIII). While a few comments are made below on the placement of poles, this only serves as initial guidance. Final pole placement can be determined after the steps just mentioned have been completed.
Powerhouse location
The location of the powerhouse will be affected by several factors, but this task is simplified by the fact that there are usually a very limited number of options. These factors include the following:
• Voltage drop. As with much of the design planning for a mini-grid, the location of the powerhouse is determined, to the extent possible, by the need to ensure that voltage drop at the end of each line remains within acceptable limits at minimum cost. To achieve this, the optimum generator location is in the center of the load it is to serve.
• Location of the energy source to be harnessed. If the powerplant relies on hydropower, it must be located at the most efficient location for power-generation purposes. A very limited number of options usually exist. Power must be transmitted from that point to the mini-grid. While this will increase the cost of the distribution system somewhat, this is offset by the other advantages implicit in relying on low-cost hydropower-generated electricity. If the power source is wind-based, the powerplant must usually be located on a ridge or other high point to tap the largest wind resource, even though this may also be outside the load center. If the source of energy is diesel, the powerplant could be located in the center of the load. However, even in this case, if the village is on the flank of a hill on one side of a valley, with the main road below, it might be more advantageous to generate power just off the road, where fuel drums can be more easily delivered, even though it may be on the outskirts of the village.
It is often the case that a mini-grid is privately owned, as a small business. In such cases, the location of the powerplant owner's property may determine the powerplant location.
• Size and nature of the end-use. Irrespective of who owns the mini-grid, if a large load such as a grain mill is to be supplied, it may be most efficient to place the powerplant near that mill to reduce the costs of the heavier line that would otherwise be required to serve that load.
• Noise. Diesel-based electricity generation can be a noisy undertaking. If effective silencing of the exhaust is not possible, this might also force the genset to be located at a more isolated part of the village.
Placing the lines
Once the powerhouse has been located, the distribution line is required to bring the electricity to the vicinity of the consumers. The best layout for the distribution system will be one that meets the criteria for voltage drop while minimizing cost and keeping safety and reliability in mind. In general, the shortest line will minimize cost, because this will reduce the cost of both the conductor and poles. Poles are often the most expensive component of a distribution system, and an important part of the design process is to be economical in their use.
Depending on the layout of the consumers relative to the location of the powerhouse, the best layout may be to extend lines in several directions from the powerhouse. Several factors must be taken into
consideration in deciding where these lines are to be placed. The relative importance of each must be decided in each situation. These factors include the following:
• Location of roads, trails, and paths. The principal reason for locating lines along such arteries is that most present and future consumers typically build their homes along road or trails and these permit easy access for line construction and maintenance. Should street lighting also be a priority, this is facilitated by locating poles along the principal arteries.
Care should be taken if roads carry vehicular traffic. Sufficient clearance under the conductor is required whenever the possibility exists that vehicles will pass underneath. Road crossing should be avoided or minimized whenever possible. The alternative is to use higher, more robust, and therefore costlier poles on either side of the road at each crossing to provide this greater clearance.
In some countries, crossing property lines also causes problems, as many are not eager to have power poles, especially with guys, in the “middle” of their yards, rice paddy, or coconut plantation. Following well-established paths and trails known to be open to the general public minimizes this problem. At other times, in some communities, there is sufficient esprit de corps and interest in electrification for all to join together and accept such inconveniences as one of the costs of electrification.
• Presence of trees. When conventional lines are built, trees are often the first casualties. The right-of-way along lines is generally cleared of trees to prevent them from interfering with the operation of the line: to prevent branches from falling and breaking conductors or from shorting the lines. In some areas, trees represent a source of income for the villagers (from the sale of fruit and nuts) who are loath to destroy them for this reason.
On the other hand, depending of the strength, flexibility, amount of foliage, and age of a tree, they are sometimes used as living power poles that have already withstood the test of time. These
“poles” have the advantage of requiring no treatment to prevent decay, especially of the buried portion at and just below the ground line that is the most susceptible. Lines are sometimes draped over branches, while at other times, they are properly fixed to spool insulators mounted on the main trunk.
• Religious buildings/areas. Buildings or areas of religious or cultural significance must be identified and a clear understanding of what constraints these impose on line routing should be established.
• Topography. Certain areas should be avoided if they will complicate the construction or ongoing upkeep and maintenance of a line. These includes steep slopes, areas susceptible to erosion, swampy areas, and areas prone to flooding.
• Line length. Because poles and conductor are the most costly component of a mini-grid project, the alignment of the line should be selected to minimize their number and length, respectively.
• Minimizing changes in alignment. Whenever there is a bend in the line, the conductor under tension imposes a lateral force on the pole tending to tip it. Depending on the change in alignment at a pole and the tension of the conductor, this lateral force might have to be counteracted by a guy and anchor.* This adds to the cost and effort required in installing the mini-grid. They also pose a safety hazard, as they may be difficult to see, especially in the evening, or simply get in the way. For this reason, where conductor tension is sufficiently large, an effort should be made to minimize deviations of adjacent spans for as long a distance as possible, “concentrating” bends at as few points as possible..
• Loading. If several lines radiate from the powerhouse, the aim should be to equalize the kW·km loading on each line during peak demand times, to the extent possible. This will permit the use of the same size conductor, reducing its cost through quantity discounts and possibly reducing the selection of connection hardware required. This will also make most efficient use of the lines.
However, with three-phase power, another more critical requirement for the proper operation of the generator is that loads on all three phases be as balanced as possible. This requirement becomes more critical as generator capacity is approached and should receive high priority.
• Planning horizon. In laying out and designing a mini-grid, an adequate planning horizon should be used and, to the extent possible, the mini-grid should be designed to permit it to be efficiently used over this period. Both new areas into which the grid might expand or existing customers who might expand their use of electricity should be considered.
The focus of the present effort is to lay out the lines that are part of the distribution system itself and will bring electricity to points relatively close to each consumer. Separate from the distribution line are the service drops that are used to bring the power the remainder of the way from the nearest pole to the consumer. Details for the design of the service drops will be discussed separately in Chapter XII.
Depending on the layout of the village, one question that may need to be answered at this stage is how close the main distribution line must approach each consumer. This answer depends on the peak demand or current required by the consumer(s) served by a service drop, the sizes of the conductor that can be used for the service drop, and the maximum allowable voltage drop. At this point, Fig. 101 (p. 149) can
* When electric utilities build distribution lines, guys are typically used on poles whenever the change in direction of the line exceeds 5 °.
be used to derive the maximum distance for any given values for these parameters. If too many home are off in one direction from the distribution line, then one branch of the distribution line may have to be extended in that direction to bring power closer to those consumers and permit shorter service drops.
Once an initial layout for the distribution line has been established, selecting line configuration (Chapter VI) and the size of the conductor for the distribution system (Chapter VII) can proceed.
Locating poles
Once the general layout for the distribution line has been prepared, poles must be placed along that line to support the conductors with adequate clearance to ensure a line that does not pose any hazard to people or vehicular traffic passing beneath it. Factors affecting pole location include the following:
• At bends in the line. As noted in the discussion of line placement, guying may be needed at each bend in the line. Therefore, to minimize the need for guys and anchors and associated costs, hassles, and safety issues, any significant bends along the line should be concentrated at as few points as possible. Poles must then be located at each of these bends.
• Location of load clusters. As is explained later (p. 150), it is recommended that each service drop supplying a consumer takes off from a pole rather than from mid-span. For this reason, at least one pole will have to be located near each cluster of homes within a certain radius of the pole. In this case, the location of home clusters determines pole location.
• Adequate ground clearance. In areas where homes are less densely located, the type and size of conductor used, the length of available poles, and the required ground clearance will determine the maximum span that is possible. For this reason, pole locations can only be finalized once these parameters have been established.
• Pole strength. Mechanical loading caused by wind on the conductor is transferred to the poles (as is described in Chapter VIII). The poles have to be strong enough to support this load, and the strength of the poles available for the project may limit the maximum spans achievable.
VI. Line configuration
To distribute power around a load center, four basic distribution line configurations are possible: two single-phase configurations and two three-phase configurations. These are illustrated in Fig. 15. All configurations use similar materials and construction techniques. On some occasions, a combination of these configurations can be used to achieve a more cost-effective distribution system design.
For a particular village situation, the attributes of each configurations and a rough sizing and costing of the conductor and poletop hardware for each configuration should be assessed to determine which line configuration is the most cost-effective. The sizing of the conductor for each configuration can be found using Table 8 or Box 5 after an acceptable voltage drop has been established. An example of conductor sizing for a sample line and the impact on line configuration on conductor size are illustrated in Chapter VII and in Appendix 7 (beginning on p. 235).