Transmission Planning is purposed to define a transmission system or its expansions as to comply with the electric energy demand at specified quality and reliability criteria at a minimum cost. Planning process should be continuous, should have an interactive structure as to make an optimized future evolution use.

The planning studies take a leading role in the definition of an electric system or its expansion. The planning activities have to start several years before the installation of new or expansion of existing transmission lines is to be implemented.

Planning Stages of Transmission Lines

1. Long Term Planning - It defines the basic future structure of an electric system. It comprises a long term horizon for the system planning usually in the range of 15-30 years.

2. Medium Term Planning - Its target horizon is in the range of 10-15 years. It usually defines the basic characteristic of the system such as voltage, main transmission lines  and substations.

3. Operation or Short Term Planning - The horizon to be analyzed is usually below three years and urgent requirements of the system. These includes but not limited to the anticipation of operation dates of new facilities, needs to uprate or upgrade existing lines, or load transfers in scheduled interruptions.

Note: Planning should be continuously revised depending on the variation that can occur in the economic environment, the energy market, energy industry, or in the generation program.

Planning Aspects Regarding Transmission Lines

Planning Studies should answer the following questions:
When will a new transmission line, or uprating, or upgrading of existing lines will be required?
Where is it required and what quality of supply or reliability is required?
What normal and emergency ratings are required?
What type of transmission should be used? (Overhead, underground, AC or DC?)
What voltage and how many circuits will be needed?

Transmission Lines Planning Criteria

The basic criteria that should be established in a system planning is that no load can be lost under occurrence of a simple contingency in the system being studied or in another neighboring interconnected system.

Steady State Condition Criteria
The system must be tested for heavy load and light load conditions. It should support the outage of any of its components, also known as n-1 criterion.

Load flow studies parameters are as follow:
Voltage range should be between 0.95 and 1.05 p.u.
Transformer loads: Normal conditions: no overload
                             Loss of a transmission line or generator: 20% overload
                             Loss of a transformer: 40% overload

Temporary and Transient Condition Criteria
System stability is required under any load condition in case of phase to ground short circuit without reclosing, considering the loss of one of the system components.

Temporary over voltages should not cause damage to any system equipment. The maximum allowable temporary over voltages are in the range of 140% in points with saturable equipment and 150% in other points.

Short Circuit powers and currents have to be assessed as accurately as possible in order to prevent exceeding the equipment capacity of the system and installations.

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