Transmission Planning

The purpose of long-term transmission system planning is to determine the type and timing of new transmission facilities that will be required to transmit power from the new generating plants to the main load centres at least cost, with an acceptable level of reliability. The transmission planning process relies upon an adequate definition of the locations of the new generation and the expected growth in demand at the major load demand centres. The degree of precision that can be assigned to each of these factors declines rapidly the further into the future the analyses are pursued, so that transmission planning becomes more conceptual and less precise in the latter part of the planning period.

Comilla-Baroaulia Transmission Lines

The Long-Term Transmission Planning Process

The overall planning process for the transmission system proceeds from the development of a conceptual system through to a least-cost sequence by successive analysis and adjustment. The first step is to estimate and assess the principal power flows that would need to be transported across the country. This assessment is carried out by calculating the range of power flows at major interfaces in the system during several key years in the planning period. Of prime importance is power flow requirements between the Eastern and Western Zones. Generation expansion in the two zones affect these power flows and the timing at which a second, and perhaps third, interconnector will be required.

The ranges of major power flows across the interfaces between geographic subsystems is determined from the preliminary generation options and locations, the system demand forecast, different daily and seasonal load levels, and known operating constraints.

Using judgement and approximate methods such as transmission line loadability, preliminary transmission expansion alternatives are postulated. The conceptual transmission schemes include the definition of several variables:

•  the primary system voltage level (132 KV, 230 KV, 400 KV)

•  the principal tower type (single or double circuit)

•  the approximate routing

•  a limited range of conductor sizes

•  a limited range of transformer ratings

•  reactive compensation requirements.

The costs of schemes that satisfy the design criteria is then estimated to establish the least-cost transmission alternative for each generation sequence. Considerations taking into account less tangible factors, such as flexibility and reliability, are used to choose between competing transmission schemes.

Load Flow Analysis

System load cases flow are established for key years like FY 1995, 2000, 2005, 2010 and 2015. For these years, maximum power flow requirements are established by considering combinations of load level and generation scenarios.

Various transmission alternatives are postulated to achieve a reasonable load flow situation for normal and outage conditions, while minimizing the transmission additions.

Transient Stability Analysis

The principal transmission additions required is determined by testing the transient stability performance of a selected number of the proposed transmission alternatives. This performance represents the ability of the system to withstand sudden short circuits and outage of the associated faulted equipment, without collapsing due to loss of synchronism between generators on the system.