In distribution power system, indeed we face to gathering the geographical distributed loads and many factors effect to choose of secondary branch numbers. In a radial system primary and secondary circuits can be laid out and regulation, transformer capacity, and circuit capacity can be calculated directly by arithmetical means. This is not true in the case of the network because the loads divide among the various transformers and primary circuits in such a way that calculations of the characteristics of the system by ordinary methods become tedious. There are two general methods of planning a network system. One is to use a network calculator or a miniature system to determine the characteristics of an estimated system arrangement and then make such revisions as are required. The other method is to estimate a plan and then by inspection estimate the division of load for various conditions; the plan is then revised until the estimated load division gives satisfactory conditions. The first method is more satisfactory because it shows accurately the operating characteristics of the plan being considered while the accuracy of the estimation-by-inspection method can be determined only after the system has been installed. The network calculator method is described briefly in the following paragraphs.
Some consideration to secondary distribution planning which noted in reference books is illustrated below:
1-The layout of the secondary grid is made on the basis of the locations of the loads that must be served and the routes of existing secondary mains. In most cases existing secondaries almost completely cover the area and it is only necessary to connect these secondaries into a continuous grid. In some places it may be desirable to add sections of main to provide multiple paths to certain loads for emergencies when adjacent transformers are out of service.
When the loads and secondary plan have been determined the approximate sizes and locations of transformers can be selected. The transformer size will depend on the type of system, size of concentrated loads, number of feeders available, the feeder interlacing obtainable, and spacing of the transformers. In general, the larger the transformer the lower the cost per kva and the wider the spacing between transformers in the network. But as this spacing is increased the secondaries must be larger to keep secondary voltage drop within reasonable limits and to provide adequate carrying capacity. As the number of transformers is reduced less primary cable is required.
The ideal size of transformer then is that which not only handles the loads but also gives a minimum total cost including costs of primary feeders, transformers, and secondary mains. In the initial trial plan for the network the transformers should be located at the major loads and at the various junctions where the concentrated loads are large enough so that the distance between transformers is not greater than about two blocks or 600 to 800 feet.
Usually the spacing is less than this because of the locations of loads. It is generally desirable to select not more than two sizes of network transformers. This permits stocking fewer spare transformers and protectors. Also, interchangeability of units and parts is increased by using only one or two sizes. At points where large concentrated loads are served it is desirable to use multiple installations consisting of two or more transformers rather than one transformer much larger than the rest of the units. This avoids a large number of sizes and the use of a few units of a size that is not interchangeable with any of the predominating sizes. In addition, multiple unit installations improve load distribution and voltage regulation at the large loads for emergency conditions.
After tentatively selecting sizes and locations of transformers a size of secondary main is selected. This size depends on the required carrying capacity, estimated regulation, and size of existing secondary copper. The carrying capacity should be adequate to carry one-half to two thirds of the rated capacity of the predominating size of transformer. The voltage regulation for normal operation can be estimated by calculating the voltage drop to the distributed loads along some sections of main where inspection indicates the worst regulation is likely to prevail.
2-One of the questions often raised during the design of the plant power distribution system is how to make a quantitative comparison of the failure rate and the forced downtime in hours per year for different circuit arrangements, including radial, primary-selective, secondary-selective, simple spot network, and secondary-network circuits. This quantitative comparison could be used in trade-off decisions involving the initial cost versus the failure rate and forced downtime per year. The estimated cost of power interruptions at the various distribution points should be considered in deciding which type circuit arrangement to use. The decisions should be based upon total owning cost over the useful life of the equipment rather than the first cost.
In general, electric power systems are designed on a first contingency basis. The incremental cost to provide such services is typically a relatively small cost as compared to the total facility or plant cost. The risk and cost of a long-term interruption due to system failure far outweighs the added incremental cost required to provide first contingency capacity at the time of installation.
In order to calculate the failure rate and the forced downtime per year, it is necessary to have reliability data on the electric utility supply and each piece of electrical equipment used in the power distribution system. One of the best sources for this type of data are the extensive IEEE surveys on the reliability of electrical equipment in industrial plants and commercial buildings. (See IEEE Std 493-1990.1) While this data may be quite useful, it represents a limited data base; therefore, it may not be representative of an individual company's experience.
In-house data, if available, may be more appropriate in this analysis.
Statistical analysis methods involving probability of failure may be used to make calculations of the failure rate and the forced downtime for the power distribution system. The methods and formulas used in these calculations are given in IEEE Std 493-1980. Data and calculations for determining the cost of power interruptions are also given in IEEE Std 493-1990.