An electrical energy transmitted from generating power stations to substations or grid through overhead transmission lines. A transmission lines has three constants resistance R, inductance L and capacitance C. These constants are distributed uniformly along the whole length of the transmission line.

The line resistance cause voltage drop (IR) and power loss (I^{2}R) in the line, the inductance also causes voltage drop due to inductive reactance (IX_{L}). The resistance and inductance form the series impedance. The capacitance produces charging current (2∏fcv) in the line, which quadrature with the voltage. This constant existing between conductor and neutral for single phase and or three phase line forms shunt path through out the length of the line. The capacitance effects make the calculations complex. The capacitance effects are more predominant in case of underground cables. The overhead transmission lines are classified as.

- Short transmission lines
- Medium Transmission lines
- Long transmission lines

**Short transmission Lines**

Transmission lines having length about 50km and operating voltage is comparatively low about 20KV, it is usually considered as a short transmission lines. Due to smaller distance and lower operating voltage, the capacitance effects are extremely small and hence can be neglected. Hence, performance of short transmission lines depends upon resistance and inductance only. Therefore, while studying the performance of such lines, only resistance and inductance of the transmission line are taken into account. In short transmission lines, Constants are assumed to be lumped at one phase.

**Medium Transmission Lines**

Transmission lines having length is about 50 to 150km and operating voltage is more than 20KV and less than 100KV, it is considered as medium transmission line. Due sufficient length and operating voltage of the line, capacitance effects are taken into account. The capacitance is distributed uniformly over the entire length of the line. It is assumed to be shunted across the line at one or more points.

**Long Transmission Lines**

Transmission lines having length more than 100KM and operating voltage more than 100KV (that is very high), it is considered as a long transmission lines. In case of long transmission lines constants are considered uniformly distributed over the entire length of the transmission line.