Planning of Low Voltage Distribution Network

A realistic view of the power distribution systems should be based on "gathering" functions rather than "distributing" them since the size and locations of the customer demands are not determined by the distribution engineer but by the customers. Customers install all types of energy-consuming devices which can be connected in every conceivable combination and at times chosen by the customers. The concept of distribution starts with the individual customers and loads, and proceeds through several gathering stages where each stage includes various groups of increasing numbers of customers and their loads. Ultimately the generating stations themselves are reached through services, secondaries, distribution transformers, primary feeders, distribution substation, subtransmission networks, bulk power stations and transmission network as will be discussed in the next section.

In designing a system, distribution engineers may find a conflict between fulfilling the requirements of the electrical considerations and the economical considerations in the same time. A good distribution system is the one compromising both considerations in the same time as much as possible. An example of this conflict is the voltage drop on the feeders. For achieving good performance of the system, voltage drop should be eliminated in order to have a flat voltage profile. To achieve this we use cables of larger cross sectional area (c.s.a) in order to have smaller resistance. On the other hand, the economical considerations in some cases permits a certain range of voltage drop so as to fully use the used cables. Yet if the conflict between electrical requirements and economical requirements can't be solved; the priority is always for the electrical requirements since they represent the safe operation which is the main aim of the distribution engineer.

Another example on the conflict between electrical and economical requirements is to increase the service reliability for the critical loads, e.g. hospitals, computer and control centers, critical industrial loads. To do this some back-up systems such as emergency generators and/or batteries with automatic switching devices are used in such places. These extra equipments cost more money, yet the reliability of the service is more important in this case than any money.

In their system design decisions of the secondary distribution network, distribution engineers are primarily motivated by the considerations of economy, coppers losses in the transformer and the secondary circuit, permissible voltage drops and voltage flickers of the system. Of course, there are some other engineering and economic factors affecting the selection of the distribution transformer and the secondary configuration, such as permissible transformer loading, balanced phase loads for the primary system, investment costs of the various secondary system components, cost of labor, capital cost, inflation rates and other factors.

Types of lamps

The lighting design process in its most basic form entails identifying a task and then providing a light source that will provide proper quantity and quality of light for the task. The fixture protects the light source, connects it to the power source and distributes its light. 

 

The light source is the actual light-producing component of the lighting system. It may operate simply as a lamp (incandescent/halogen) or as a lamp powered by a ballast (fluorescent and high-intensity discharge [HID]).

I-Incandescent Lamps:

Incandescent light sources are the cheapest light sources.

.

·         Do not require a ballast

·          It is based on the fact that current is passed through a filament, which heats until it glows

·         Less efficacious light source

·         Shorter service life than other light sources in most cases

·         Filament is sensitive to vibrations   

·         Bulb can get very hot during operation

·         Must be properly shielded because incandescent lamps can produce direct glare as a point source

·         Require proper line voltage as line voltage variations can severely affect light output and service life

An example of incandescent lamps is given in figure 4.1

Efficiency of incandescent is 14 lumen/watt.                                

ii-Fluorescent Lamps:

These lamps rely on the gaseous discharge method.

·         Require a ballast

·         Low surface brightness compared to point sources 

·         More efficacious compared to incandescent

·         Ambient temperatures and convection currents can affect light output and life

·         Options for starting methods and lamp current loadings

·         Requires compatibility with ballast

·         Low temperatures can affect starting unless"cold weather" ballast is specified.  

An example of fluorescent lamps is given in figure 4.2

Efficiency of fluorescent lamps is 46 lumen/wat

iii- Compact Fluorescent Lamps (CFL):

·         It is a new and advanced lighting technology

·         More efficient than incandescent lamps

·         CFL use 70 - 75% less energy than their incandescent equivalents. When replacing a 100 watt incandescent lamp a 28 watt CFL is used.

·         CFL last approximately 10,000 hours, which is 10 to 13 times the life of an incandescent lamp (expected life approximately 750 hours).

·         Compact fluorescents are most cost-effective when used at least 2-3 hours per day.

·         Although CFL may appear different than the common incandescent, they fit most standard fixtures found in homes today. The screw-in base is the same on both lamps.

·         The typical incandescent lamp wastes 90% of the energy it uses, producing heat rather than light.

·         CFL will provide the same amount of light (or lumens) at a fraction of the electricity used.

Designing the lighting system

To produce a new lighting system in a construction, it must be designed. The designer must determine the desired light levels for tasks that are to be performed in a given space, then determine the light output that will be required to meet those objectives consistently, taking into account all the factors that degrade both light output and light levels over time. Equipment must then be chosen and placed in a layout to produce the desired light distribution.

Requirements of a good lighting scheme

A good lighting scheme should fulfill the following:

1. Provide adequate illumination.
2. Provide uniform illumination allover the working plan.
3. Provide light of suitable color.
4. Avoid glare and hard shadows.

Lighting background

Importance of light:

    Light is the prime factor in the human life as all activities of human beings ultimately depend upon the light. Where there is no natural light, use of artificial is made. Lighting increases production and reduce accidents.

 Basic Definitions:

Candela
International unit (SI) of  luminous intensity; term evolved from considering a standard candle, similar to a plumber's candle, as the basis of evaluating the intensity of other light describe the relative intensity of a source .

Candlepower Distribution Curve

A graphic presentation of the distribution of light intensity of a lamp or luminaire.

Illuminance (E)

The quantity of light (measured in foot-candles, Lux, etc) at a point on a surface.

Inverse Square Law

Formula stating that illumination at a point on a surface varies directly with the intensity of a point source, and inversely as the square of the distance between the source and the point; it illustrates how the same quantity of light flux is distributed over a greater area as the distance from the source to the surface is increased.

Light Loss Factor

The product of all considered factors that contribute to a lighting system's depreciated light output over a period of time, including dirt and lamp lumen depreciation.

Lumen
The international unit of luminous flux or quantity of light.

Luminaire

A complete lighting unit consisting of a lamp (or lamps) together with the parts designed to distribute the light, position and protect the lamps, and connect them to the power supply. This is sometimes referred to as a "fixture".

Lamp efficiency

It is the amount of output lumen per watt.

Lux (lumen/m²)

SI (international system) unit of illumination. One lumen uniformly distributed over an area of one square meter.

Mounting Height

Distance from the bottom of the fixture to either the floor or work plane, depending on usage.

Spacing to Mounting Height Ratio

Ratio of fixture spacing (distance apart) to mounting height above the work plane. Sometimes it is called spacing criterion.  A normal range is 1 à 1.5

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.