From an engineering trainee’s diary _Transmission line planning

The following article is based on my experience as a trainee electrical engineering undergraduate in “Lighting Sri Lanka-Hambanthota Project (LSHP)” under Ceylon Electricity Board (CEB).

Transmission and distribution lines are the live veins of a country’s power system. They simply interconnect generating stations, grid substations and distribution substations. In Sri Lankan context, we have generation voltages around 13.5 kV, transmission voltages of 132 kV and 220 kV and distribution voltages of 33 kV (CEB) and 11 kV (LECO and small portion of CEB). The ultimate objective of a power system utility such as CEB is to keep the above mentioned “live veins” as healthy as possible while keeping the generation cost and other operating costs at a minimum while maximizing customer satisfaction. In order to achieve those objectives designing, planning and construction of transmission lines along with timely and proper maintenance, must be carried out with extreme care.

Planning is the initial stage of any transmission or distribution line development project. Planning process can be divided into three stages namely, long term planning, medium term planning and operation planning.

Long term planning

Long term planning emphasizes on topics such as need of construction of new transmission lines, development and introduction of new technologies for transmission (e.g. introducing HVDC system to Sri Lanka) and distribution (e.g. introduction of equipment such as “Fuse saver” to improve reliability of distribution system). Basically Transmission and generation planning branch of CEB carries out the above mentioned planning functions. Long term transmission planning is carried out as a rolling plan with a time horizon of ten years. This plan will cater the growing demand for electricity while incorporating new technologies to enhance the performance of the existing system.

Medium term planning

Medium term planning consists of defining characteristics of system voltages, transmission and distribution lines and substations etc. The time horizon is much shorter than that of long term planning.

Short term planning

Operational or short term planning involves in maintaining the quality and availability of the power system. Regular maintenance and unintended interruption handling falls under operation planning. Generally, this segment of planning is done by area engineers.

During my time in LSHP, I got the opportunity to study the planning process of 33 kV medium voltage lines. During the planning, answers to the following questions need to be obtained.

• When the new distribution line or upgrading of the existing line is required?
• What will be the capacity of the line? 
• How many circuits are needed?
• What is the quality of the supply and the reliability level?

In order to find successful and acceptable answers for the above questions, a large amount of data is required. The data requirement can be fulfilled by using the already available data and through surveys.

Concerns on planning

Accurate technique(s) of forecasting is a must for a successful transmission planning because the time period required to complete a transmission project, from planning to commissioning, may sometimes extend up to a decade. But within that time, the society, or in other words the beneficiaries of the transmission project, will change drastically. The living standards of the people will rise, new industries may be established and rapid development of infrastructures may attract more and more people to the area (in Hambanthota district, the electrification level was 66% at 2006 and now it is almost 100%). When forecasting, all such dynamic facts should be taken into account. One must not forget that such transmission line, plan and design at present, constructing in another two years and commissioning in another five years must  at least last for another three or four decade in operation.

Preparation for Planning

Generally an electrical study is carried out in planning stages for a proposed transmission line. The areas such as power flow study,system stability and dynamic performance,selection of voltage levels (generally, the standard voltage levels are used in Sri Lanka as a practice),voltage and reactive power flow control (mostly for transmission lines),insulation and over voltage design,conductor selection,loss calculation,and protection scheme design should be carefully analysed for reliable planning.

Financial viability

Even a sound electrical design might be rejected by final decision makers, if the project cost is not financially feasible. Financial and economic aspects play a vital role in any engineering development. and the designers should be able to justify the expenditure for the project against the expected benefits. Thus, a proposed design should be checked for both economical and financial feasibility using analytical tools such as Net Present Value (NPV), Internal Rate of Return (IRR) and Cost to Benefit Ratio (CBR) etc.
Finally, the optimum design is selected based on the economic and technical analysis. But the factors such as capacity and the prior experiences of the electricity utility and local constraints also govern the decision.

Transmission line design and construction 

My training diary is further filled with notes on transmission line designing and construction as well. I’m more than happy to share some insight from those areas in the upcoming articles.

Expressing my gratitude

I would be obliged to Mr. S. Bogahawatta, the project director of lighting Sri Lanka Hambantota project for his immeasurable support extended to my team during our internship in the project.

Terminology

HVDC -  High voltage direct current (HVDC) is a electric power transmission system uses direct current for the bulk transmission of electrical power.

Fuse saver - Fuse saver is a new class of intelligent, compact and low cost single phase circuit breaker that minimizes interruptions by protecting spur line fuses from blowing on transient faults

Article By: Ayantha Sampath

Posted in: Power System