JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year POWER SYSTEM - II Previous Question Paper (September - 2021 - R18)

The Ferranti effect is a phenomenon in long transmission lines where the receiving-end voltage (V_r) exceeds the sending-end voltage (V_s) when the line is operating at no-load or very light load. This occurs primarily due to the charging current flowing through the line capacitance.

The effect is caused by the line capacitance drawing a leading current from the source. As this current flows through the line inductance, it creates a voltage drop that is in phase opposition to the sending-end voltage, leading to a rise in voltage at the receiving end. The effect is more pronounced in long lines with high capacitance and low load.

In the phasor diagram, the sending-end voltage V_s is represented as the vector sum of V_r and the voltage drops across the resistance and inductance. Under no-load conditions, the charging current I_c leads V_r by 90 degrees. The voltage drop across the inductor (I_cX_L) leads I_c by 90 degrees, meaning it is in phase opposition to V_r, causing the magnitude of V_s to be less than V_r.

Transmission lines are classified based on their length and operating voltage, as these factors determine the significance of line parameters (resistance, inductance, and capacitance) in performance calculations:

1. Short Transmission Lines: Lines with a length up to 80 km or operating at voltages below 20 kV. In these lines, the shunt capacitance is negligible, and only resistance and inductance are considered. The line is represented by a simple series impedance.
2. Medium Transmission Lines: Lines with a length between 80 km and 200 km, operating at voltages between 20 kV and 100 kV. Here, the shunt capacitance is significant and is typically lumped at one or both ends (Nominal-T or Nominal-π circuits).
3. Long Transmission Lines: Lines exceeding 200 km in length, operating at voltages above 100 kV. In these lines, parameters are considered to be uniformly distributed along the entire length. Rigorous mathematical analysis using hyperbolic functions is required to account for the distributed nature of resistance, inductance, and capacitance.