Transmission Lines And Cables – Electrical Engineering Notes – For W.B.C.S. Examination.

Overhead transmission line and underground cable segments (or right-of-ways) are represented in two parts:  The definition of the transmission segment itself, which can include the admittance/impedance data or the conductor and insulation properties, ground impedance data, and geometric position of all towers and conductors; and the interface to the rest of the electrical system, through electrical interface components (one at each end).Continue Reading Transmission Lines And Cables – Electrical Engineering Notes – For W.B.C.S. Examination.

If the transmission line is configured in Direct Connection mode, the interface components are not required.

Transmission segments considered to be electrically short in length (i.e. less than 15 km for a 50 ms time step), may also be represented using an equivalent p-section representation.  This is accomplished by either:  Using the p-section component in the master library, where only the admittance and impedance data of the line segment is required, or by using the p-section equivalent component creator feature in the Line Constants Program (LCP).  P-sections are essentially a network of passive elements, and hence do not represent propagation delay.

NOTE:  A 15 km line length to a 50 ms time step is derived assuming that waves propagate through the line at the speed of light.  In general however, wave propagation speed is less than the speed of light.

Using the data provided by the cross-sectional definition of the segment, the transmission lines and cables are modeled using one of two distributed (travelling wave) models:

• Bergeron (Single Frequency)
• Frequency Dependent (Phase) (Multiple Frequencies)

Transmission line is the long conductor with special design (bundled) to carry bulk amount of generated power at very high voltage from one station to another as per variation of the voltage level.

Types of Transmission Line

In transmission line determination of voltage drop, transmission efficiency, line loss etc. are important things to design. These values are affected by line parameter R, L and C of the transmission line. Length wise transmission lines are three types.

A short transmission line is classified as a transmission line with:

• A length less than 80km (50 miles)
• Voltage level less than 69 kV
• Capacitance effect is negligible
• Only resistance and inductance are taken in calculation capacitance is neglected.

Medium Transmission Line

A medium transmission line is classified as a transmission line with:

• A length more than 80 km (50 miles) but less than 250 km (150 miles)
• Operational voltage level is from 69 kV to approx 133 kV
• Capacitance effect is present
• Distributed capacitance form is used for calculation purpose.

Long Transmission Line

A long transmission line is classified as a transmission line with:

• A length more than 250 km (150 miles)
• Voltage level is above 133 kV
• Line constants are considered as distributed over the length of the line.

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