Basic principles

The best size and spacing of substations is that which results in the least annual cost of the sum of fixed charges on substations and feeders, operation and maintenance, and losses.

Increasing the number of substations for a given load density tends to increase total cost. However, increasing the number of substations reduces the cost of feeders and feeder losses. Clearly then, the least total annual cost is a function of substation and feeder cost, capacity of feeder and load density

ِِِِِِِِOptimum Sitting and Sizing of Substations

Substations are the second step in power system after the power planning. Beginning with Substation, the distribution network will begin. Substation will distribute its power using feeders on distribution voltage 11KV.

Due to the increasing demands for electric energy and the increasing scarcity of available substation sites, the location and spacing of substations is becoming a major economic problem. Because of local geography and other conditions beyond control, it may be impractical to select the ideal substation size and spacing. Anyway, these factors should be known so that the ideal conditions may be approached as near as possible.

Our concern in this part is to find the number, the size and the way of distributing the substations on each planning area. The selection of these parameters is just an economic matter. We try to find the optimum number, size and spacing of substations to have minimum cost. We want to minimize the cost of substations and the cost of feeders. 

Forecasting Methodology

Forecasting is simply a systematic procedure for quantitatively defining future loads. Depending on the time period of interest, a specific forecasting procedure may be classified as a short term, intermediate, or long term technique,

Because system planning is our basic concern and because planning for the flew generation, transmission and distribution facilities must begin 4 - 10 year in advance of the actual in service data, we shall be concerned with the methodology of intermediate-range forecasting.

For simplicity, the word “Forecast” will usually imply an intermediate range forecast.

Forecasting techniques may be divided into 3 broad classes. Techniques may be used on extrapolation or correlation or a combination of both. Techniques may be further classified as deterministic, probabilistic, or stochastic.

a)   Extrapolation

Extrapolation techniques involve fitting trend curves to basic historical data                                                                                   adjusted to reflect the growth. It produces reasonable results in many cases.

Such a technique is to be classified as a deterministic extrapolation, since no attempt is made to account for random errors in the data or in the analytical model. Some standard analytical functions are used in trend curves fitting, including:

1.   Straight line        = a + b x

2.   Parabola            = a + b x + cx²

The most common curve - fitting technique for finding coefficients of function in a given forecast is the method of least squares as will be discussed later

b)   Correlation

Correlation techniques are used to relate system loads to various demographic and economic factors. This approach has an advantage of forcing the forecast to understand clearly the interrelationship between load growth patterns and other measurable factors. The most obvious disadvantage, however results from the need to forecast demographic and economic factors, which can be more difficult than forecasting system load. Typically, these factors may be population, employment, building permits saturation and business indicators.

Factors Affecting ElF(electrical load forecasting)

Several factors affect ELF. Among them we mention:

1. Land use: (residential, industrial, commercial, agriculture . . . etc.). Different types of use affect the capacity of the substation, i.e. residential loads is different from industrial loads.

2. Population growth: As the population increases more loads are needed.

3. Historical Data: Historical data plays an important role in forecasting since they can tell how the load will behave in the future.

4. Load Densities (KVA/Km²): Load density must be put into account during load forecasting, we must consider the range of enmities as it differs for different types of loads. Common figures for load densities are 1000 KVA/Km² for agricultural areas; 3000 KVA/Km² for residential areas; 5000 KVA/Km² for city center and 10,000 KVA/Km² for industrial areas.

Lighting Arrangement

Two Way Traffic Roads: 

There are four basic types of lighting arrangements that are recognized as being suitable for these types of roads.

1.                    Single Side

§    This type of arrangement, in which all luminaries are located on one side of the road, is used only when the width of the road is equal to, or less than, the mounting height of the luminaries.

§   The luminance of the road surface at the side opposite the luminaries.

2.                    Staggered

§   This type of arrangement in which the luminaries are Located on either side of the road in a staggered, or zigzag, arrangement is used mainly when the width of the road is between 1 to 1.5 times the mounting height of the luminaries.

§   Very careful attention should be paid to the uniformity of the luminary on the road surface-alternate bright and dark patches can produce an unpleasant effect.

3.                    Opposite

 This type of arrangement, with the luminaries located opposite one another, is used mainly when the width of the road is greater than 1.5 times the mounting height of the luminaries.

4.                    Span wire

     §     This  type  of  arrangement, with  the   luminaries suspended along that axis of the road, is normally only used for narrow roads that have buildings on either side.

     §     The luminaries being suspended from cable strings between the building Combinations of these four basic arrangements are, of course, also used There are also special purpose low-mounted arrangements for the purpose of providing visual guidance.

     §     The road surface luminance is then very low because of shadows cast passing vehicles, and depreciation due to dirt collecting on the luminaries.

Introduction to street lighting

§   The basic purpose of street lighting is to promote safety and convenience on the street at night, through adequate visibility, and to promote civic progress.

§   Statistics show that good street lighting installation bring about material reduction in traffic accidents, act as a deterrent to crime and achieve a higher business throughout in commercial areas.

§   There are a number of factors that contribute to the illumination level required in street installations.  An important factor, common to all streets is highway safety

Considerations, as the volume of traffic increase, traffic interference and exposure to accidents also increases.

Good visibility is more difficult to achieve amidst the Confusion of moving vehicles and pedestrians, for it is against this background that accident hazards must be discerned.

§   In order to make logical recommendations as to illumination levels required in street lighting applications, it is necessary to classify streets with respect to vehicle and pedestrian traffic.

§   The color of light does not affect the level of illumination to be provided. Studies have shown that the visibility of objects does not differ greatly between sodium, mercury, florescent and filament lamps. the comparison has to be on the basis of an equal Amount of light and similar distribution schemes.

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