ELECTRICAL DISTRIBUTION SYSTEM PLANNING AND SECURITY STANDARDS (Part 1)

This paper was contributed by:

Engr. Kenneth Onako Ajayi (MNSE, PMP), Consultant Engineer / Systems Analyst, Al-Rosi Consult.

INTRODUCTION:

The design of power distribution system is done, particularly to ensure that subject to the availability of adequate generating and transmitting capacity, the system is capable of providing consumers with a safe, reliable, economical and efficient supply of electricity.

The design should be done to conform to the statutory requirements of applicable local and international codes and standards such as ISO, IEC, BS, NERC, etc. 

The distribution system planning goal is to assure that a demand growth can be satisfied in an optimal way from the Primary distribution feeders to the substations from where energy must be delivered to the final client economically while complying with several technical specifications.

When designing distribution planning, careful consideration should be given to energy consumption, their geographical location, laws regarding the use of soil plus other aspects to come up with the substations dimensioning and location, the maximum efficiency routes, while minimizing the energy loss in the feeders and deployment costs, plus satisfying the reliability of service constraints.

Planning Procedure and Network Development:

The Distribution System planning involves setting of sub-station, routing of feeders and many other decisions relating to both location and amounts of capacity additions.

The following procedures can be adopted when planning the design

1. Standardization of sizes and ratings of lines and substation equipments
2. Standardization of substation layout
3. Load forecasting

4.      Design of the project at hand, using a power flow model with suitable characteristics, so as to accurately simulate system performance. This includes:

                                i.            Distribution lines

                              ii.            Service lines design

                            iii.            Substation design

                             iv.            Consideration of coordination of protection against over-currents.

5.      Estimation of project costs.

6.       Presentation of the project in a sufficiently well-grounded and documented manner, so as to ensure that the technical aspects enumerated are dully considered.

The following standards should also be taken into consideration when planning the design of a distribution network.

• Distribution Code

• Electricity Safety, Quality and Continuity Regulations

• Environmental standards

Standardization of Sizes and Ratings

For each voltage class of application such as 230/415Volts, 11kV, and 33kV, conductors, insulators, lightning arresters, transformers, switchgear, etc. used in the Distribution System should be standardized with the objective of reducing the inventory. Specifications for these materials shall at least conform to relevant Nigeria/International standards in general. The Electricity Distribution Authority usually develops a study of the economic selection of conductors, to establish standard conductor sizes for use in electrification projects.

Standardization of Sub-Station Layouts
Substation layouts should be design to conform to minimum requirements as stipulated by the Electricity Distribution Authority for that location. It is here recommended for the design engineer to consult the applicable distribution codes laid out by the relevant regulatory authorities (NERC in case of Nigeria)

Some of this minimum requirements needed to be fulfilled are detailed below.

33/11kV Sub-Station

                                i.            Independent Circuit Breaker control of 33kV Feeders and Transformers as applicable.

                              ii.            Group circuit Breaker control of Transformers as applicable.

                                i.            Independent Circuit Breaker control of 11KV Feeders.

                              ii.            Provision of Tariff and Operational metering in accordance with Distribution Code as applicable.

                            iii.            Single bus sectionalized as applicable.

                            iii.            Single Bus as applicable.

11/0.4kV 3-phase Distribution Transformer Centres

1. Transformers up to 200 KVA capacity other than those meant for indoor application can be pole mounted.
2. The layout of the distribution transformers should generally conform to relevant Construction Standards of the Electricity Distribution Authority.
3. The distribution transformers should be located as close to the load centres as possible. Transformers above 100 KVA capacity other than those meant for indoor installations shall be outdoor plinth mounted.
4. Moulded Case Circuit Breakers (MCCB) or Air Break Switches of suitable ratings should be provided on the secondary side of transformers of capacity above 100 KVA for protecting the transformers from over load and short circuits. Fuse units of suitable ratings should be provided on the secondary side of transformers of capacity up to and including 100 KVA for protecting the transformers from overload and short circuits.

INFORMATION COMPILATION/ANALYSIS

It is very important to start the electric design of a project with information from the Electrical Distribution Authority/company that will undertake the role of operator upon completion of the work, to obtain all existing data available on the project area and to take into account the rules of the company that is to maintain and operate the lines.

The following information should be obtained from the supply Authority:

1. Standards and materials used (poles structures, conductors, etc)
2. Plans for network extension in the project area if intended
3. Point of origin or supply for the project
4. Voltage level of the existing distribution lines
5. Number of phases available
6. Distance of the substation from the initial point of the project (this can help to determine the distance from the substation to the initial point of the project for power flow analysis and voltage drop)
7. Existing conductor size from substation to the project (needed in the load flow analysis)
8. Load in the existing line (needed in the load flow analysis to determine the voltage drop in the existing line)
9. Average energy consumption in the last electrified area (can be used to estimate energy consumption in the area to be electrified examining the energy consumption in the nearby areas that already has electric service)
10. Existing penetration rate in the electrified area (how many users out of a total of potential users will be connected to the project in the first year, and the period over which the rest are likely to begin service)
11. Substation characteristics such as:

1. Source impedance
2. Capacity of the substation
3. Available capacity at the substation
4. Voltage on both sides of the transformer
5. Available taps in the transformer
6. Existence of automatic voltage regulation
7. Impedance of the transformer and ground connection
8. Transformer connections
9. Characteristics of over current protection devices (the type of device, brand, pick-up current, relay settings, the current transformer taps
10. Characteristics of other substation equipments

FIELD INSPECTION:

On the completion of the compilation of the available information from the supply service authority, it is often necessary to confirm and complement the information so obtained by visiting the project area to establish the geographic relationship between the loads to be electrified.

WAY POINTS:

During this visit, by way of GPS, map, or vehicle odometer, distances and locations of towns and probable loads should be established. The following points of interest can be marked or noted on either the map of the area, or the architectural layout plan or the GPS.

1. Location of the substation,
2. The end point of the three-phase line,
3. The initial point and
4. The center of each area/zone/community as the case may be to be considered in the project.

Each such point should be recorded with an indicative name, which could be the complete name of the community/street or a simpler indicative name. In any case, keep a written file of all waypoints with the indicative name, the real name of the community, and the additional characteristics of each point. There may be other equipment in the substation that could have a great influence on the power flow model, such as capacitors and reactors, so be sure to record their capacity and form of connection to the system.

RECORD DISTANCES AND CONSUMERS:

Records of distances between all points, as well as the accumulated distance so far should be recorded, to facilitate the calculation of distances later. Also record the number of users in each street/area/zone/community in the same record. See the table below of an example of record of points of interest.

To be continued.

Engr. Kenneth O. Ajayi (MNSE, PMP) is an Electrical Engineer with vast experience in Electrical Engineering practice (power systems: design and analysis, modeling/simulation, automation and maintenance; electricity distribution: industrial, commercial and residential building installations). It is worthy of note that he is highly proficient in the use of engineering software tools for analysis and designs.