Building Wiring Calculations

In this chapter it is required to plan the distribution system of the residential area. The planning of this area starts from inside the individual flats of the buildings.

The various types of loads in the house like lighting, normal sockets (N.S), power sockets (P.S) and load break switches (L.B.S) are estimated according to the standard forms. The feeding circuits are defined and the main wiring c.s.a. are calculated.

The mains wiring is generally built using insulated copper cables. The choice of conductor material is a compromise among electrical properties, mechanical properties, and price. From the start, copper has been the material of choice for household branch circuits.

Aluminum is softer than copper and weaker, and a poorer electrical conductor, so is not widely used in small sizes for home wiring. Aluminum cable material is sometimes used (for economical reasons) for thick mains feeder cables coming from electrical utility to the mains distribution panel.

The ratings of the sub-circuits' miniature circuit breakers (M.C.B) and the main circuit breaker of the flat or the villa as well as energy meter are selected.

Any house that has been properly wired will have a circuit breaker panel used to shut circuits off in case they draw too much current. It is the current capacity of circuit breaker (in amperes) that determines how much current a circuit can supply. In case of an overload or a short-circuit on that circuit, the breaker trips and automatically shuts off power to that circuit. Ground fault circuit breakers offer protection against more than just overloads.

After the load of the flat is being calculated, the diversified estimation of the total load of the building is made. The buildings are fed from distribution boxes via cables of suitable sizes, forming a part of the low voltage distribution network. The distribution boxes are fed from 11 KV/380 V distribution transformers, preferably in loops, to secure the continuity of supply to the distribution boxes and hence to the buildings.

The distribution transformers are located in the appropriate sites and connected in loops to the 11KV Distribution points and the 66/11KV substation.

   

Detailed calculations and planning of the 380V low voltage distribution network, the 11KV medium voltage network as well as details of the 66/11KV substation feeding the area, are presented in the following chapters. Before this, the principles of lighting and wring are summarized in the following sections.

Design Aspects

1-    Substations used in the agricultural area are generally outdoor

2-    substations due to the low cost of land while in city center the substations are GIS (Gas Insulated Substations) since the land is very expensive. In other areas, the indoor substations are very suitable.

3-    Over Head Transmission Lines ( OHTL ) are used in  the agricultural area while Under Ground Cables ( UGC ) are used elsewhere for the primary distribution networks ( 11 KV ) due to the following:

·         The lower cost of OHTL makes it more suitable for use in the agricultural areas

·         UGC may be destroyed while digging or due to irrigation in the agricultural areas

4-    The cross-sectional area (c.s.a) of the 3-core cables shouldn't exceed 300 mm²; else single core cables of c.s.a 400 mm² are used.

5-    From the reliability point of view, it is better to replace one large substation with some smaller substations distributed over the planning area, inspite of the increase in the cost; yet, the increase is not that considerable amount.

6-    It is recommended that the number of transformers in each substation doesn't exceed 6 transformers including the reserve.

7-    It is recommended that the bus bars in the substation are sectionalized and doubled to allow maneuver and that the number of transformers and feeders per section is even number.

8-    It is recommended that the number of feeders per transformer in agricultural area shouldn't exceed 6 feeders, while for all the other areas it shouldn't exceed 8 feeders per transformer.

9-    It is recommended that the no. of transformers in each substation doesn't exceed 6 transformers including the reserve.

10- In general, the calculation of the no. of feeders & the loading percent is done considering the total no. of transformers in the substation including the reserve since all the transformers are simultaneously connected from the beginning.

11- The loading percent in each substation shouldn't exceed 80% for safety and continuity of feeding in case of outage of any unit.

12-A recommended current density for the UGC is 1 A/mm² while for OHTL it is generally around 2 A/mm².

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.